Allostatic Load Is Associated with Overuse Musculoskeletal Injury during US Marine Corps Officer Candidates School.

INTRODUCTION: Overuse musculoskeletal injuries (MSKIs) remain a significant medical challenge in military personnel undergoing military training courses; a further understanding of the biological process leading to overuse MSKI development and biological signatures for injury risk are warranted. The purpose of this study was to determine the association between overuse MSKI occurrence and physiological characteristics of allostatic load (AL) characterized as maladaptive biological responses to chronic stress measured by wearable devices in US Marine Corps officer candidates during a 10-week training course. METHODS: Devices recorded energy expenditure (EE), daytime heart rate (HR), sleeping HR, and sleep architecture (time and percent of deep, light, REM sleep, awake time, total sleep). Flux was calculated as the raw or absolute difference in the average value for that day or night and the day or night beforehand. Linear mixed-effect model analysis accounting for cardiorespiratory fitness assessed the association between overuse MSKI occurrence and device metrics (α = 0.05). RESULTS: Sixty-nine participants (23 females) were included. Twenty-one participants (eight females) sustained an overuse MSKI. Overuse MSKI occurrence in male participants was positively associated with daytime HR (ß = 5.316, p = 0.008), sleeping HR (ß = 2.708, p = 0.032), relative EE (ß = 8.968, p = 0.001), absolute flux in relative EE (ß = 2.994, p = 0.002), absolute EE (ß = 626.830, p = 0.001), and absolute flux in absolute EE (ß = 204.062, p = 0.004). Overuse MSKI occurrence in female participants was positively associated with relative EE (ß = 5.955, p = 0.026), deep sleep time (ß = 0.664, p < 0.001), %deep sleep (ß = 12.564, p < 0.001) and negatively associated with absolute flux in sleeping HR (ß = -0.660, p = 0.009). CONCLUSIONS: Overuse MSKI occurrences were associated with physiological characteristics of AL including chronically elevated HR and EE and greater time in restorative sleep stages, which may serve as biological signatures for overuse MSKI risk.

Recruit Profiles Across the U.S. Armed Forces: Implications for Increasing Gender Integration in Recruit Training.

INTRODUCTION: This article uses recent survey data from a study on gender integration at recruit training across the U.S. Armed Forces to examine service, gender, and training differences in recruit's gender attitudes, beliefs, and cohesion metrics. In addition to providing a descriptive understanding of our recruit sample, this article examines factors germane to gender integration at recruit training such as gender attitudes, sexist beliefs, and unit cohesion. MATERIALS AND METHODS: This article uses data from a survey of U.S. Marine Corps (USMC), U.S. Army, U.S. Air Force, U.S. Navy, and U.S. Coast Guard recruits as part of a USMC-commissioned study to develop alternate models and recommendations for increasing gender integration in USMC recruit training. At all sites but the USMC, recruits completed the survey 1 to 2 weeks before graduation. At the Marine Corps Recruit Depots, recruits participated in the survey at the beginning (week 2) and end (week 11) of their 13-week training cycle. A 19-question survey captured sociodemographic information, perspectives and experiences during recruit training, and gender attitudes and beliefs. Analyses were conducted on the sample with complete data on the measures reported, including those USMC recruits observed at both week 2 and week 11 (n = 629). Descriptive statistics (percentage) were calculated for all categorical outcome variables. Outcome variables (cohesion, gender, and sexism attitudes) were compared between groups using chi-squared tests or Fisher's exact tests, as appropriate. RESULTS: Recruits differed significantly across services in perceptions of unit cohesion on 3 of the 4 indicators: Platoon members are cooperative, know they can depend on each other, and really respect each other (P < .05). These differences appear to be driven by female recruit difference across service. They reported significant differences in gender role attitudes across the services, with USMC recruits being most likely to endorse traditional gender roles compared to other service recruits. Male USMC recruits were significantly more likely than their female peers to agree that men should achieve outside the home and women should take care of the home and family (21.3% versus 6.3%, P < .05). The only statistical differences in attitudes about equality of treatment and opportunity among recruits were between male and female USMC recruits. All items capturing sexist attitudes elicited statistically significant differences between male USMC recruits and male recruits in other services (P < .05). There were significant differences in all sexism measures between male and female USMC recruits (P < .05). There is a significant decline in some sexist attitudes between weeks 2 and 11 of training among male USMC recruits (P < .05). Nearly one-half to three-fourths of male USMC recruits hold sexist attitudes, even near the end of recruit training. CONCLUSIONS: The disparate gender and sexism attitudes of male USMC recruits compared with their peers in other services, and their fellow female USMC recruits suggest efforts to increase gender integration at entry-level training are needed, but also may be challenging since male USMC recruits report the highest levels of sexist attitudes among all recruits.

Unsupervised Machine Learning in Countermovement Jump and Isometric Mid-Thigh Pull Performance Produces Distinct Combat and Physical Fitness Clusters in Male and Female U.S. Marine Corps Recruits.

INTRODUCTION: Several challenges face the U.S. Marine Corps (USMC) and other services in their efforts to design recruit training to augment warfighter mobility and resilience in both male and female recruits as part of an integrated model. Strength and power underpin many of the physical competencies required to meet the occupational demands one might face in military. As the military considers adopting force plate technology to assess indices of strength and power, an opportunity presents itself for the use of machine learning on large datasets to deduce the relevance of variables related to performance and injury risk. The primary aim of this study was to determine whether cluster analysis on baseline strength and power data derived from countermovement jump (CMJ) and isometric mid-thigh pull (IMTP) adequately partitions men and women entering recruit training into distinct performance clusters. The secondary aim of this study is then to assess the between-cluster frequencies of musculoskeletal injury (MSKI). MATERIALS AND METHODS: Five hundred and sixty-five males (n = 386) and females (n = 179) at the Marine Corps Recruit Depots located at Parris Island and San Diego were enrolled in the study. Recruits performed CMJ and IMTP tests at the onset of training. Injury data were collected via medical chart review. Combat fitness test (CFT) and physical fitness test (PFT) results were provided to the study team by the USMC. A k-means cluster analysis was performed on CMJ relative peak power, IMTP relative peak force, and dynamic strength index. Independent sample t-tests and Cohen's d effect sizes assessed between-cluster differences in CFT and PFT performance. Differences in cumulative incidence of lower extremity %MSKIs were analyzed using Fisher's exact test. Relative risk and 95% confidence intervals (CIs) were also calculated. RESULTS: The overall effects of cluster designation on CMJ and IMTP outcomes ranged from moderate (relative peak power: d = -0.68, 95% CI, -0.85 to -0.51) to large (relative peak force: d = -1.69, 95% CI, -1.88 to -1.49; dynamic strength index: d = 1.20, 95% CI, 1.02-1.38), indicating acceptable k-means cluster partitioning. Independent sample t-tests revealed that both men and women in cluster 2 (C2) significantly outperformed those in cluster 1 (C1) in all events of the CFT and PFT (P < .05). The overall and within-gender effect of cluster designation on both CFT and PFT performance ranged from small (d > 0.2) to moderate (d > 0.5). Men in C2, the high-performing cluster, demonstrated a significantly lower incidence of ankle MSKI (P = .04, RR = 0.2, 95% CI, 0.1-1.0). No other between-cluster differences in MSKI were statistically significant. CONCLUSIONS: Our results indicate that strength and power metrics derived from force plate tests effectively partition USMC male and female recruits into distinct performance clusters with relevance to tactical and physical fitness using k-means clustering. These data support the potential for expanded use of force plates in assessing readiness in a cohort of men and women entering USMC recruit training. The ability to pre-emptively identify high and low performers in the CFT and PFT can aid in leadership developing frameworks for tailoring training to enhance combat and physical fitness with benchmark values of strength and power.

A Sex Comparison of the Physical and Physiological Demands of United States Marine Corps Recruit Training.

INTRODUCTION: The U.S. Marine Corps (USMC) recruit training is a 13-week preparatory period for military service men and women. Differences in absolute performance capabilities between sexes may impact physical and physiological responses to the demands of recruit training. The purpose of this study was to monitor U.S. Marine Corps recruits throughout recruit training to comparatively assess workload, sleep, stress, and performance responses in men and women. MATERIALS AND METHODS: A total of 281 recruits (men = 182 and women = 99; age = 19 ± 2 years) were monitored and tested. Workload, sleep, and stress assessments occurred at week 2, week 7/8, and week 11 of training. Workload (energy expenditure per kg body mass [EEREL], distance [DIS], steps) and sleep (continuity and duration) were tracked over 72-hour periods using wearable accelerometry and heart rate technology. Stress responses were determined through salivary cortisol analyses. Performance testing, consisting of countermovement vertical jump (CMJ) and isometric mid-thigh pull (IMTP) performance relative to body mass, occurred at weeks 2 and 11. Linear mixed models were used to test for sex, time, and sex-by-time interactions (α < .05). RESULTS: On average, recruits covered 13.0 ± 2.7 km/day, expended 3,762 ± 765 calories/day, and slept 6.2 ± 1.1 hours/night. Sex-by-time interactions were found for DIS, steps, sleep duration, cortisol, and CMJREL performance (P < .05). Planned contrasts revealed that men covered more DIS than women at week 7/8 (P < .001). Women experienced greater step counts compared to men at week 11 (P = .004). Women experienced no significant change in sleep duration (P > .05), whereas men increased sleep duration from week 2 to week 7/8 (P = .03). Women experienced greater sleep duration at week 2 (P = .03) and week 11 (P = .02) compared to men. Women exhibited higher cortisol levels than men at week 2 (P < .001) and week 11 (P < .001). Women experienced declines in cortisol at week 7 compared to week 2 (P < .001). Men experienced no changes in cortisol response at any timepoint (P > .05). Both sexes experienced declines in CMJREL from week 2 to week 11 (P > .001). Sex main effects were observed for EEREL, DIS, CMJREL, and IMTPREL (P < .05) with men experiencing greater overall workloads and producing greater strength and power metrics. Sex main effects were also found for sleep continuity and cortisol (P < .05), for which men experienced lower values compared to women. Time main effects were observed for EEREL, DIS, steps, cortisol, CMJREL, and IMTPREL (P < .05). CONCLUSIONS: This study not only highlights the known sex differences between men and women but also sheds light on the different physical and physiological responses of each sex to military training. Interestingly, the greatest physical demands incurred earlier in the training cycle. Despite declining workloads, the stress response was maintained throughout the training, which may have implications for adaptation and performance. In addition, average sleep duration fell notably below recommendations for optimizing health and recovery. Effectively monitoring the demands and performance outcomes during recruit training is essential for determining individual fitness capabilities, as well as establishing the effectiveness of a training program. Individual performance assessments and adequately periodized workloads may help to optimize recruit training for both men and women.

Recruit Perspectives on Gender Integration at Recruit Training: A Comparison by Service and Gender.

INTRODUCTION: Recruit training is the initial entry for enlisted personnel in the military. The Services execute gender-integrated recruit training differently. The U.S. Marine Corps (USMC) maintains same-gender platoons led by same-gender drill instructors in integrated companies; further integration occurs at select training events. The other Services train recruits in gender-integrated units with mixed-gender drill instructor teams. We examine recruits' experiences and perceptions of gender integration at recruit training, their desired level of integration, and preferences for increasing gender integration, comparing by Service and gender. MATERIALS AND METHODS: Recruit perspectives and experiences were captured in a 19-question survey (n = 632) and 90-min focus groups (n = 260) near graduation from recruit training. Data were collected from June to November 2021. Because of the Coronavirus disease (COVID-19) pandemic restrictions, the Navy and Air Force were not conducting gender-integrated recruit training during data collection. Outcome variables were compared cross-Service by gender and within Service by gender using chi-squared tests or Fisher's exact tests, as appropriate; focus group data were analyzed using initial and secondary coding schemes. Three USMC training models, varied by level of integration, were also analyzed (Male-Only, Series Track, and Integrated Company). RESULTS: Significant gender differences across and within Service emerged in recruits' experience being trained by an opposite-gender instructor. Male recruits had significant differences by Service (P < .001), and USMC female recruits reported being trained by male instructors more than their male peers by female instructors (Series Track P = .002; Integrated Company P < .001). In the focus groups, recruits described common differences with how men and women embodied being a drill instructor. Significant gender differences across (both male and female P < .001) and within Service were reported for closeness of training with opposite-gender peers (Air Force P = .028; U.S. Marine Corps (USMC) Integrated Company P = .010; Army P = .048), an expected finding given varied integration during data collection. Male and female recruits had significant differences by Service in their preference for integration at the lowest unit level (both male and female P < .001), with those who experienced integrated training showing higher levels of endorsement. In the focus groups, recruits articulated benefits and challenges of gender-integrated recruit training. Significant gender differences across Service emerged in preferences for more integration in specific training activities. Within Service, female USMC Integrated Company recruits wanted more integration in tactical/field, physical fitness, and classroom training than their male peers (P < .001 for all). In the focus groups, USMC recruits of both genders desired more integrated training events, particularly those involving combat and tactical skill development. CONCLUSIONS: This study provided an opportunity to examine recruit perspectives on gender-integrated training. Services valuing opposite-gender instructor exposure in recruit training must ensure that male recruits are being taught and led by female instructors given disproportionate demographics. Recruits who experienced integrated training were more supportive of integration, indicating that this experience may increase their support for gender-integrated training units and environments. Today's recruits understand that they are entering gender-integrated working environments, and our results indicate that they expect recruit training to mirror that reality.

Descriptive Epidemiology of Musculoskeletal Injuries During Marine Corps Recruit Training in Gender-Integrated and Male-Only Training Units.

INTRODUCTION: Musculoskeletal injuries (MSIs) occur frequently in military personnel. U.S. Marine Corps (USMC) recruits participate in an intense 13-week training program designed to transform civilians into basically trained Marines, during which they are susceptible to MSIs. Previous injury epidemiology research with USMC recruits was conducted in a non-gender-integrated training. Data for the current study were derived from a larger study, the USMC Gender-Integrated Recruit Training study, that was initiated to provide data-driven recommendations for gender integration during USMC recruit training. The purpose of the current analysis was to describe the unique profile of MSIs during USMC recruit training and compare MSIs between female and male recruits in gender-integrated training. MATERIALS AND METHODS: Medical record-reviewed MSI data were obtained for recruits in three models of USMC recruit training: two models of gender-integrated recruit training at Marine Corps Recruit Depot (MCRD) Parris Island-the older Series Track (ST) model and the newer Integrated Company (IC) model, and a Male-Only (MO) cohort at MCRD San Diego. Incidence, anatomic region and subregion, event at the time of MSI, MSI type and onset, and disposition following MSI were described for each model. Group comparisons were conducted using Fisher's exact tests or independent samples t tests, as appropriate. RESULTS: MSI data were available for 584 recruits (ST: 98 female recruits, 95 male recruits; IC: 85 female recruits, 106 male recruits; MO: 200 male recruits). The cumulative incidence of MSIs was significantly higher among female compared to male recruits in the ST (59.2% vs. 29.5%, P < .001) and in the IC (25.9% vs. 12.3%, P = .023) cohorts. The most frequent anatomic location for MSIs was the lower extremity (female recruits: ST: 76.5% of MSIs, IC: 88.6%; male recruits: ST: 81.1%, IC: 80.0%, MO: 83.8%). The most frequent body part affected was the hip among female recruits (ST: 26.5% of MSIs, IC: 37.1%). The most frequent body part affected by MSIs among the male recruits was the knee in the ST (32.4%) and IC (53.3%) cohorts and the lower leg (27.0%) in the MO cohort. A significantly greater percentage of female compared to male recruits sustained a hip MSI in the ST (23.5% vs. 2.1%, P < .001) and IC (12.9% vs. 0.0%, P < .001) cohorts. There was no significant difference in knee MSI incidence between sexes in the ST (P = .323) or IC (P = .757) cohorts. A large percentage of MSIs resulted in light duty (female recruits: ST: 69.4% of MSIs, IC: 74.3%; male recruits: ST: 64.9%, IC: 73.3%, MO: 94.6%). CONCLUSIONS: This was the first study to assess the burden of MSIs concurrently among female and male USMC recruits in gender-integrated training. MSIs, especially those affecting the lower extremity, continue to occur frequently in this population. Female recruits are more susceptible to MSIs during USMC recruit training compared to male recruits and are especially prone to hip MSIs. Future research should focus on identifying modifiable risk factors for MSIs in this population, with a focus on reducing lower-extremity MSIs in all recruits and hip MSIs in female recruits.

Design Considerations for a Multidisciplinary Approach to Provide Policy Recommendations on Gender-Integrated Recruit Training in the Marine Corps.

INTRODUCTION: Recruit training is designed to transform civilians into physically fit military service members, who embody their service's core values and possess military discipline and skills. At the time this research began, the U.S. Marine Corps (USMC) remained the only service that segregated recruits by gender at the lowest unit level (e.g., platoon) and employed gender-segregated drill instructor teams. USMC's Marine Corps Recruit Depots (MCRD) must comply with a 2020 Congressional Mandate to not segregate training by gender in Parris Island by 2025 and San Diego by 2028. In turn, USMC requested an independent scientific study to analyze current approaches to gender integration at recruit training to propose alternate models and other policy recommendations that increase gender integration while maintaining current USMC standards. The Marine Corps is currently evaluating alternate models and recommendations to optimize entry-level training. This article outlines considerations for choosing the optimal research study design, research methods, and types of data collected in a study intended to provide policy recommendations on gender-integrated recruit training for the USMC. MATERIALS AND METHODS: Research data were collected during visits to the MCRDs and selected recruit training locations for the Army, Air Force, Navy, and Coast Guard. Data collection on USMC recruits from three cohorts involved social science assessments (focus groups and surveys) and human performance testing (countermovement jumps and isometric mid-thigh pulls, sleep and activity wearables, and cortisol data) at multiple points in the training cycle. Data on recruits from the sister services were limited to social science assessments. Approximately 600 recruits between the two MCRDs and 160 recruits from the sister services participated in the study during a 7-month timeframe in 2021. The research team conducted extensive ethnographic observations of recruit training at all selected research sites and interviewed training cadre, drill instructors, and service leadership responsible for recruit training (∼90 interviews). Additionally, the research team interviewed 20 experts on gender integration or recruit training who possessed alternate viewpoints from the current USMC practice. RESULTS: The mixed methods study was designed to assess the current gender integration practices at recruit training across the services to generate alternative models of gender integration for USMC. The research team developed a set of multidisciplinary objectives and research questions serving as the foundation of the research study design and data collection process. The study was designed to collect qualitative, quantitative, and administrative data informed by social science and human performance disciplines. To ensure that all aspects and implications relevant to gender integration were considered, select data were collected across services and with stakeholders at all levels. CONCLUSIONS: This multidisciplinary research approach provided a comprehensive picture of the current USMC recruit training models. The research team captured multiple perspectives and data points for analysis through an expansive view on gender integration across all services, by interacting with participants at all levels of the institutions in varied ways. The information and data gathered enabled the research team to establish objective, data-driven alternate models, and recommendations for enhancing gender integration at recruit training for the USMC.

Using the Capture-Recapture Technique to Estimate the Ascertainment-Corrected Incidence of Musculoskeletal Injuries During Marine Corps Recruit Training.

INTRODUCTION: Injury epidemiology research with military populations typically utilizes data obtained through medical chart review (MCR) or injury self-reports (ISRs). MCR data will not capture musculoskeletal injury (MSKI) data for which medical care was not sought, which is common during military recruit training. Injury self-report is affected by issues with recall, especially for MSKIs perceived as less severe. U.S. Marine Corps (USMC) recruits participate in an intense 13-week recruit training program during which they are susceptible to MSKIs. The purpose of the current analysis was to utilize a novel statistical method, the capture-recapture (CRC) technique, to account for the undercounting inherent in MSKI data sources and estimate the ascertainment-corrected cumulative incidence of MSKIs during USMC recruit training. MATERIALS AND METHODS: Data for the current study were derived from a larger study, the USMC Gender-Integrated Recruit Training Study, which was initiated to provide data-driven recommendations to increase gender integration in USMC recruit training. The estimated cumulative incidence of MSKIs during 13-weeks of USMC recruit training was calculated from the 2 sources of MSKI data (MCR, ISR) and using CRC analysis. Medical charts were reviewed to extract data about MSKIs that occurred during recruit training. Self-reported MSKI data for the same period were obtained from recruits at the end of recruit training. MSKIs were classified according to their anatomical location and type. The Chapman modification of the Lincoln-Peterson estimator was utilized to conduct the CRC analysis. RESULTS: Medical chart review and ISR MSKI data were available for 464 USMC recruits (age: 19.1 ± 1.9 years; gender: men 70.0%). The observed 13-week cumulative incidence of MSKI in the sample was 21.8% in the MCR and 28.4% in the ISR, while the CRC incidence was much higher (62.0%). The MCR and ISR ascertainment were 35.1% and 45.9%, respectively, while the overall ascertainment or completeness of MSKI data when 2 sources were used was moderate (65.0%). When stratified by MSKI anatomical location, the overall ascertainment varied by anatomical location of the MSKI. It was highest for lower extremity MSKIs (64.8%), but lower for upper extremity (38.9%) and spine (33.3%) MSKIs. The overall ascertainment also varied by MSKI type; it was highest for sprain (55.1%), followed by strain (54.8%), and the pain/spasm/ache (43.3%). CONCLUSIONS: This was the first study to utilize the CRC technique to access the ascertainment-corrected incidence of MSKIs among USMC recruits. There was significant undercounting in both sources of the data analyzed, and the extent of undercounting varied by both MSKI anatomical location and type. When 2 sources of data were utilized simultaneously, the percent of CRC-estimated MSKIs observed from 2 sources of data was more complete. There is a need for further application of the CRC technique to MSKI data in military populations to provide a more complete assessment of MSKIs. Identification of modifiable factors that influence completeness of MSKI data obtained during military recruit training is also warranted.

Neuromuscular Strength and Power Predict Musculoskeletal Injury and Attrition During Marine Corps Recruit Training.

INTRODUCTION: Elevated rates of musculoskeletal injuries (MSIs) and attrition are documented in military recruit training. By identifying and addressing modifiable risk factors, the rate of successful training completion and military readiness can be enhanced. Despite their impact, the causes of MSIs and attrition among U.S. Marine Corps (USMC) recruits remain underexplored. This study investigates demographic, psychological, and physiological predictors of MSIs and attrition among USMC recruits. MATERIALS AND METHODS: In this prospective cohort study, we evaluated USMC recruits at Marine Corps Recruit Depot, Parris Island and San Diego. Recruits were briefed and invited to volunteer. All recruits who were medically cleared to participate in recruit training were eligible to participate in the study. We gathered baseline data on potential predictors at the start of training, with follow-up data on MSIs and attrition collected post-training. Analyzed predictors encompassed dynamic and static strength measures from countermovement jumps, isometric mid-thigh pulls; and participant surveys. We employed multiple logistic regression to discern risk factors for MSI and attrition. RESULTS: Our study comprised 584 USMC recruits (183 female recruits, 19.49 ± 1.88 years, 160.10 ± 7.17 cm, 61.19 ± 8.05 kg; 401 males, 18.94 ± 1.92 years, 172.97 ± 7.26 cm, 73.86 ± 11.04 kg). We observed 193 MSIs in 135 recruits, with 80.31% affecting the lower extremity (LE). Notably, lower relative peak power (odds ratio [OR] 0.91 [0.89, 0.94], P < .001) and shorter eccentric deceleration duration (OR 0.99 [0.99, 1.00], P = .005) were significant predictors of MSIs. Specifically, for LE MSIs, similar trends were noted for relative peak power and eccentric deceleration duration, with additional risks associated with lower body mass index (OR 0.93 [0.86, 0.99], P = .036) and previous LE MSIs (OR 2.25 [1.18, 4.27], P = .013). Attrition was more likely with a reduced eccentric deceleration impulse (OR 0.98 [0.97, 0.99], P < .001) and prolonged time to peak force (OR 1.36 [1.17, 1.59], P < .001) and cigarette use (OR 2.12 [1.01, 4.43], P = .046). CONCLUSIONS: MSIs and attrition during USMC recruit training significantly undermine force readiness and escalate costs. Our research has pinpointed several modifiable risk factors, chiefly reduced muscular power and cigarette smoking. We advocate for neuromuscular training programs to bolster strength and power, integrated nutrition and exercise strategies for optimal body composition, and support for smoking cessation to alleviate the incidence of MSIs and curtail attrition. Initiating training with a gradual increase in activity intensity can provide a critical window to correct pre-existing neuromuscular imbalances and weaknesses, particularly those stemming from prior MSIs. Effectively addressing these risk factors is pivotal for diminishing the rates of MSIs and attrition among recruits, thereby enhancing overall military readiness and operational efficiency.

United States Marine Corps Recruit Training Demands Associated With Performance Outcomes.

INTRODUCTION: United States Marine Corps' (USMC) recruit training is a 13-week program designed to maximize physical and mental performance adaptations. The purpose of this study was to evaluate the training demands and characteristics that are associated with performance outcomes during USMC recruit training. MATERIALS AND METHODS: A total of 196 recruits (M = 97 and W = 99) were monitored and tested throughout training. Laboratory-based performance testing occurred at the start of weeks 2 and 11 and consisted of body mass assessments, countermovement vertical jump, and isometric mid-thigh pull. Military-specific performance testing occurred twice within the first 8 weeks of training and included the physical fitness test (PFT) and combat fitness test (CFT) implemented by the USMC. Resilience data were collected at week 2 using the Connor-Davidson Resilience Scale. Workload, sleep, and stress responses were monitored at weeks 2, 7, and 11. Recruits were provided with a wearable tracking device which utilized heart rate and accelerometry-based technology to determine energy expenditure (EE), distances (DIS), and sleep metrics. Data were averaged over a 3-day period. Salivary cortisol testing occurred at the start of each monitoring week. Change scores were calculated for performance tests, and body mass was calculated from data obtained at week 2 to week 11. Area under the curve was calculated for the workload, sleep metrics, and cortisol responses using the trapezoidal method. Pearson product-moment correlations (r) were used to assess the relationships between training demands and performance. An α level of 0.05 was used to establish significance. RESULTS: A moderate positive correlation was found between changes in body mass and peak power (P < .001; r = 0.43). Weak positive correlations were found between changes in body mass and peak force (P = .002; r = 0.28), as well as body mass and resilience (P = .03; r = 0.19). A moderate negative correlation was observed between changes in body mass and PFT (P < .001; r = -0.49). A weak negative correlation was found between changes in body mass and EE (P = .003; r = -0.24). A weak negative correlation was found between changes in peak power and EE (P = .001; r = -0.29). A weak positive correlation was found between changes in peak power and changes in CFT (P = .05; r = 0.19) A weak negative correlation was found between changes in sleep continuity and CFT (P = .02; r = -0.20). A weak negative correlation was found between cortisol and changes in PFT (P = .05; r = -0.20). A weak negative correlation was found between cortisol and both EE (P = .001; r = -0.27) and DIS (P = .045; r = -0.16). A weak negative correlation was found between EE and sleep continuity (P < .001; r = -0.34). Weak negative correlations were found between sleep duration and both DIS (P = .01; r = -0.18) and steps (P = .003; r = -0.21). CONCLUSIONS: Increases in body mass throughout training were positively associated with strength and power changes, but negatively related to PFT scores. Changes in peak power related to improvements in CFT scores; however, higher workloads (i.e., EE) were negatively associated with peak power. The identification of the USMC physical and physiological training demands that are associated with performance outcomes may be a valuable resource to guide conditioning efforts to boost military readiness.

Increasing Gender Integration in U.S. Marine Corps Recruit Training: Policy Recommendations and Rationales From an Interdisciplinary Study.

INTRODUCTION: This article describes alternate models and policy recommendations created by an interdisciplinary team of researchers to increase gender integration at U.S. Marine Corps (USMC) recruit training. The USMC requested a study to analyze current approaches to gender integration at recruit training and provide alternate models that maximize integration, while continuing to train marines to established standards. USMC remains the only service that segregates recruits by gender at the lowest unit level (e.g., platoon) in recruit training and maintains gender-segregated drill instructor teams (i.e., same-gender teams train platoons of same-gender recruits). MATERIALS AND METHODS: A mixed-method, interdisciplinary approach was used to capture multiple perspectives and informed recommendations and alternate models for gender integration. The team studied select USMC, army, navy, air force, and coast guard recruit training locations, between June and November 2021. At each site, the team collected qualitative, quantitative, and administrative data as well as physical performance metrics and human performance outcomes. Study participants included recruits, drill instructors, training cadre, service leaders, and subject matter experts on gender integration in military services. Each alternate model was designed to maximize the feasibility of implementation within current USMC training conditions. RESULTS: The research team developed three alternate models and several policy recommendations. Alternate model 1 proposed a methodology for mixed-gender drill instructor teams within the Integrated Company model, USMC's current integrated training model. Alternate model 2 proposed an Integrated Company Plus model which increases the number and types of gender-integrated training events at or below the platoon level within the Integrated Company model. Alternate model 3 outlined an integrated platoon model where recruits fall out into integrated training platoons after their morning routine, with two options of varying integration based on USMC priorities for integration. The team also proposed a set of policy recommendations independent of the three alternate models to support current and future gender integration efforts at the Marine Corps Recruit Depots. CONCLUSIONS: Gender-integrated military training has been shown to positively alter perceptions and evaluations of women in military settings over detrimental aspects developed by gender-segregated training. The study team recommended USMC train recruits in the Integrated Company model with mixed-gender drill instructor teams (alternate model 1) and integrate more training events following the priority tiers outlined in the Integrated Company plus model (alternate model 2). The combined execution of these two alternate models would provide USMC recruits increased exposure to direct, sustained training from opposite-gender drill instructors and deliver intentional training opportunities for male and female recruits to work together and interact in meaningful ways. The integrated platoon model (alternate model 3) would offer USMC recruits the most direct exposure to training and working with members of the opposite gender, but it requires substantial changes to current logistics, accountability, and training procedures.

Load carriage changes tibiofemoral arthrokinematics during ambulatory tasks in recruit-aged women.

The introduction of women into U.S. military ground close combat roles requires research into sex-specific effects of military training and operational activities. Knee osteoarthritis is prevalent among military service members; its progression has been linked to occupational tasks such as load carriage. Analyzing tibiofemoral arthrokinematics during load carriage is important to understand potentially injurious motion and osteoarthritis progression. The study purpose was to identify effects of load carriage on knee arthrokinematics during walking and running in recruit-aged women. Twelve healthy recruit-aged women walked and ran while unloaded (bodyweight [BW]) and carrying additional + 25%BW and + 45%BW. Using dynamic biplane radiography and subject-specific bone models, tibiofemoral arthrokinematics, subchondral joint space and center of closest contact location between subchondral bone surfaces were analyzed over 0-30% stance (separate one-way repeated measures analysis of variance, load by locomotion). While walking, medial compartment contact location was 5% (~ 1.6 mm) more medial for BW than + 45%BW at foot strike (p = 0.03). While running, medial compartment contact location was 4% (~ 1.3 mm) more lateral during BW than + 25%BW at 30% stance (p = 0.04). Internal rotation was greater at + 45%BW compared to + 25%BW (p < 0.01) at 30% stance. Carried load affects tibiofemoral arthrokinematics in recruit-aged women. Prolonged load carriage could increase the risk of degenerative joint injury in physically active women.

Skeletal muscle adaptations to high-intensity, low-volume concurrent resistance and interval training in recreationally active men and women.

This study compared the structural and cellular skeletal muscle factors underpinning adaptations in maximal strength, power, aerobic capacity, and lean body mass to a 12-week concurrent resistance and interval training program in men and women. Recreationally active women and men completed three training sessions per week consisting of high-intensity, low-volume resistance training followed by interval training performed using a variety upper and lower body exercises representative of military occupational tasks. Pre- and post-training vastus lateralis muscle biopsies were analyzed for changes in muscle fiber type, cross-sectional area, capillarization, and mitochondrial biogenesis marker content. Changes in maximal strength, aerobic capacity, and lean body mass (LBM) were also assessed. Training elicited hypertrophy of type I (12.9%; p = 0.016) and type IIa (12.7%; p = 0.007) muscle fibers in men only. In both sexes, training decreased type IIx fiber expression (1.9%; p = 0.046) and increased total PGC-1α (29.7%, p < 0.001) and citrate synthase (11.0%; p < 0.014) content, but had no effect on COX IV content or muscle capillarization. In both sexes, training increased maximal strength and LBM but not aerobic capacity. The concurrent training program was effective at increasing strength and LBM but not at improving aerobic capacity or skeletal muscle adaptations underpinning aerobic performance.

Effect of acute resistance exercise on bone turnover in young adults before and after concurrent resistance and interval training.

Weight-bearing physical activity can stimulate bone adaptation. This investigation explored the effect of an acute bout of resistance exercise before and after resistance+interval training on circulating biomarkers of bone metabolism and muscle-bone crosstalk. Healthy young male and female participants (n = 21 male, 28 ± 4 years; n = 17 female, 27 ± 5 years) performed a 6 × 10 squat test (75% 1RM) before and after a 12-week resistance+interval training program. Before and after completion of the training program, blood samples were collected at rest, immediately postexercise, and 2 h postexercise. Blood samples were analyzed for ßCTX, P1NP, sclerostin, osteocalcin, IGF-1, and irisin. Significant effects of acute exercise (main effect of time) were observed as increases in concentrations of IGF-1, irisin, osteocalcin, and P1NP from rest to postexercise. A sex*time interaction indicated a greater decline in ßCTX concentration from rest to 2 h postexercise and a greater increase in sclerostin concentration from rest to immediately postexercise in male compared with female participants. Sex differences (main effect of sex) were also observed for irisin and P1NP concentrations. In summary, changes in concentrations of biochemical markers of bone metabolism and muscle-bone crosstalk were observed in males and females after an acute bout of resistance exercise and following 12 weeks of resistance+interval training.

Utility of HR-pQCT in detecting training-induced changes in healthy adult bone morphology and microstructure.

Healthy bone adjusts its traits in an exceptionally coordinated, compensatory process. Recent advancements in skeletal imaging via High-Resolution Peripheral Quantitative Computed Tomography (HR-pQCT) allows for the in vivo 3-dimensional and longitudinal quantification of bone density, microarchitecture, geometry, and parameters of mechanical strength in response to varying strain stimuli including those resulting from exercise or military training. Further, the voxel size of 61 microns has the potential to capture subtle changes in human bone in as little as 8 weeks. Given the typical time course of bone remodeling, short-term detection of skeletal changes in bone microstructure and morphology is indicative of adaptive bone formation, the deposition of new bone formation, uncoupled from prior resorption, that can occur at mechanistically advantageous regions. This review aims to synthesize existing training-induced HR-pQCT data in three distinct populations of healthy adults excluding disease states, pharmacological intervention and nutritional supplementation. Those included are: 1) military basic or officer training 2) general population and 3) non-osteoporotic aging. This review aims to further identify similarities and contrasts with prior modalities and cumulatively interpret results within the scope of bone functional adaptation.

Hydrodynamic Flow Characteristics of a Recirculating Pool: Examining the Ecological Validity for Training and Testing.

ABSTRACT: Krajewski, KT, Beethe, AZ, Dever, DE, Johnson, CD, Nindl, BC, Lovalekar, MT, Flanagan, SD, and Connaboy, C. Hydrodynamic flow characteristics of a recirculating pool: examining the ecological validity for training and testing. J Strength Cond Res 37(10): 2023-2031, 2023-Recirculating swimming flumes (RSFs) with elliptical multifeature designs have grown in popularity due to their multifunctionality for rehabilitation and training. Because of their smaller footprint, laboratories have adopted their use to investigate swimming and underwater treadmill running. However, little is known about the hydrodynamic characteristics of these RSFs and how they might influence outcomes. The purpose was to determine hydrodynamic flow characteristics of an RSF at the manufacturers' set "speeds" around the centroid of flow projection. Hydrodynamic velocity profiles were collected through a 3D profiling velocimeter, sampling at 200 Hz in an RSF. Data were collected 0.5 and 1.5 m from the projection channel at designated flume "speeds" of 30-95 (+99) in 5-unit increments. Velocity data were collected for 1 minute per trial (location × speed) to determine mean flow velocity (MFV) for 10, 20, 30, and 40 cm2 cross-sectional areas (CSAs). A two-way ANOVA was conducted comparing CSAs from the surface by distance from the current channel (4 × 2). Separate ANOVAs were conducted to assess differences in MFV across each CSA. Significant differences between flow CSAs indicated that MFV is less for a larger area at the same speed, indicative of variable and turbulent flow characteristics across the respective CSAs. Mean flow velocity was further diminished by distance from the flow channel as supported by the main effect, thus exposing an individual to variant flow velocities simultaneously. Limited stability of the flow velocity centroid could affect swim mechanics making the movement pattern no longer analogous to traditional pool and open water swimming, rather resembling swimming upstream in a river with turbulent flow.

Association of clinically-measured and dynamic ankle dorsiflexion assessed by markerless motion capture during the drop-jump task on landing biomechanics and risk of ankle injury in military personnel undergoing 10 weeks of physical training.

OBJECTIVES: Determine the influence of clinically-measured maximum dorsiflexion, dynamic peak dorsiflexion and percent of clinically-measured maximum dorsiflexion used during a drop-jump task on landing biomechanics and risk of ankle injury in military personnel. DESIGN: Prospective cohort study. METHODS: 672 participants (122 women) enrolled. The weightbearing lunge test assessed clinically-measured maximum dorsiflexion averaged across limbs (degrees). Markerless motion capture and force plates collected lower extremity kinematic and kinetic data during a drop-jump task. Percent of clinically-measured maximum dorsiflexion used during landing was calculated as dynamic peak dorsiflexion divided by clinically-measured value, multiplied by 100 (%). De-identified injury data was derived from military physical therapists. Simple linear regression analysis determined the association between dorsiflexion measures and landing biomechanics. Simple binary logistic regression analyses identified predictors of ankle injuries. Statistical significance was set at α = 0.05. RESULTS: Eighteen participants sustained a traumatic ankle injury from a landing. All measures of dorsiflexion were associated with movement patterns that countered the stiff-legged landing strategy with dynamic measures showing a higher predictive value. Protective factors against ankle injury included height (odds ratio: 0.818, p = 0.006) and weight (odds ratio: 0.824, p = 0.023) for women. Relative braking impulse was a risk factor for men (odds ratio: 1.890, p = 0.001). CONCLUSIONS: Greater clinically-measured and dynamic measures of dorsiflexion were associated with movement patterns that countered the stiff-legged landing strategy but neither measure of dorsiflexion predicted ankle injury risk. Resultant biomechanics and anthropometrics influenced ankle injury risk to warrant recognition for injury prevention initiatives.

Twelve weeks of concurrent resistance and interval training improves military occupational task performance in men and women.

In the British Army, ground close combat roles have opened to women, however, they must pass the newly developed, gender-neutral Role Fitness Tests for Soldiers (RFT(S)). Due to physiological differences between sexes, training that optimally prepares both sexes for military occupational demands and the RFT(S) is needed. The purpose of this study was to determine the efficacy of a 12-week periodized strength and power programme with concurrent interval training on RFT(S) performance and determine if performance adaptations differed between sexes. 39 recruit-aged (18-35 yrs) participants, including 21 men (29 ± 1 yrs) and 18 women (27 ± 1 yrs), completed the study. Participants performed 3 training sessions per week that included strength and power resistance training followed by interval training. Pre- to post-training, improvements were observed for seated medicine ball throw (4.5%, p < 0.001), casualty drag (29.8%, p < 0.001), single lift (8.9%, p < 0.001), water can carry (13.8%, p = 0.012), repeated lift and carry (6.5%, p < 0.001), 2-km load carriage (7.2%, p < 0.001) and 2-km run (3.2%, p = 0.021). Pre- to post-training improvements were also observed for maximal squat (27.0%, p < 0.001), bench press (8.9%, p < 0.001) and deadlift (24.6%, p < 0.001) maximal strength, but not upper body power or aerobic capacity. No differences in RFT(S) improvements were observed between sexes, however men performed better than women in all RFT(S) and physical performance measures. Concurrent resistance and interval training improves military occupational performance in men and women; however, women may need more training than men to pass the gender-neutral RFT(S).

Twelve weeks of concurrent resistance and interval training improved seated medicine ball throw, casualty drag, single lift, water can carry, repeated lift and carry, 2-km load carriage and 2-km run performance, military occupational performance measures that comprise the British Army Role Fitness Test for Soldiers (RFT(S)).Men and women demonstrated similar military occupational performance improvements from pre- to post-training, however, men performed better than women in all measures.Simple linear regression analyses between improvements in RFT(S) tasks and measures of physical fitness (one-repetition maximal strength, upper body power, lower body power, aerobic capacity) demonstrated limited significant associations suggesting that military occupational performance improvement relies on simultaneous development of multiple fitness domains.

Sex differences in musculoskeletal injury epidemiology and subsequent loss of tactical readiness during Marine Corps Officer Candidates School.

INTRODUCTION: The US Marine Corps (USMC) Officer Candidates School (OCS) is a 10-week training course for Marine Officer Candidates (MOCs). OCS training is rigorous and demanding, which results in a high risk of musculoskeletal injuries (MSIs). The objective of this analysis was to describe MSIs among women and men during the USMC OCS at Quantico, Virginia, from September 2020 to November 2021. METHODS: This prospective cohort study assessed MSIs that occurred among 736 MOCs (women: 17.8% of sample, men: 82.2%). Data for the study were derived from routinely collected injury data by athletic trainers and physical therapists embedded within the training units. Injury incidence, event at the time of injury occurrence, anatomic location, injury type and disposition following injury were described. Fisher's exact tests were used to compare proportions of injured women and men. RESULTS: The cumulative injury incidence was higher among women (39.7%) compared with men (23.1%, p<0.001). When specific events associated with injuries were reported, most frequent events were the obstacle course (women: 20.9% of injuries, men: 12.9%) and the conditioning hike (women: 11.6%, men: 6.9%). Most injures affected the lower body (women: 67.4%, men: 70.8%). The most frequent body part injured was the lower leg (18.6%) in women and the knee (23.3%) in men. The most frequent injury type was strain (women: 39.5%, men: 24.3%), followed by sprain (women: 16.3%, men: 14.9%). A greater percentage of female (92.3%) compared with male MOCs (69.3%; p<0.001) were assigned light duty status following MSIs. CONCLUSIONS: Mitigation of injuries during OCS events such as the obstacle course and the conditioning hike needs further investigation. The high risk of overuse lower leg injuries among women and the higher incidence of injuries among women compared with men underscore the need for further investigation of modifiable sex-specific injury risk factors.

Military tactical adaptive decision making during simulated military operational stress is influenced by personality, resilience, aerobic fitness, and neurocognitive function.

Laboratory-based studies designed to mimic combat or military field training have consistently demonstrated deleterious effects on warfighter's physical, cognitive, and emotional performance during simulated military operational stress (SMOS). Purpose: The present investigation sought to determine the impact of a 48-h simulated military operational stress (SMOS) on military tactical adaptive decision making, and the influence of select psychological, physical performance, cognitive, and physiological outcome measures on decision making performance. Methods: Male (n = 48, 26.2 ± 5.5 years, 177.7 ± 6.6 cm, 84.7 ± 14.1 kg.) subjects currently serving in the U.S. military were eligible to participate in this study. Eligible subjects completed a 96-h protocol that occurred over five consecutive days and four nights. Day 2 (D2) and day 3 (D3) consisted of 48-h of SMOS wherein sleep opportunity and caloric needs were reduced to 50%. Differences in SPEAR total block score from baseline to peak stress (D3 minus D1) were calculated to assess change in military tactical adaptive decision making and groups were stratified based on increase (high adaptors) or decrease (low adaptors) of the SPEAR change score. Results: Overall, military tactical decision-making declined 1.7% from D1 to D3 (p < 0.001). High adaptors reported significantly higher scores of aerobic capacity (p < 0.001), self-report resilience (p = 0.020), extroversion (p < 0.001), and conscientiousness (p < 0.001). at baseline compared to low adaptors, while low adaptors reported greater scores in Neuroticism (p < 0.001). Conclusion: The present findings suggest that service members whose adaptive decision making abilities improved throughout SMOS (i.e., high adaptors) demonstrated better baseline psychological/self-reported resilience and aerobic capacity. Further, changes in adaptive decision-making were distinct from those of lower order cognitive functions throughout SMOS exposure. With the transition of future military conflicts placing higher priority on enhancing and sustaining cognitive readiness and resiliency, data presented here demonstrates the importance of measuring and categorizing baseline measures inherent to military personnel, in order to change and train one's ability to suffer less of a decline during high stress conditions.