Effects of Stroboscopic Vision on Depth Jump Motor Control: A Biomechanical Analysis.

Researchers commonly use the 'free-fall' paradigm to investigate motor control during landing impacts, particularly in drop landings and depth jumps (DJ). While recent studies have focused on the impact of vision on landing motor control, previous research fully removed continuous visual input, limiting ecological validity. The aim of this investigation was to evaluate the effects of stroboscopic vision on depth jump (DJ) motor control. Ground reaction forces (GRF) and lower-extremity surface electromyography (EMG) were collected for 20 young adults (11 male; 9 female) performing six depth jumps (0.51 m drop height) in each of two visual conditions (full vision vs. 3 Hz stroboscopic vision). Muscle activation magnitude was estimated from EMG signals using root-mean-square amplitudes (RMS) over specific time intervals (150 ms pre-impact; 30-60 ms, 60-85 ms, and 85-120 ms post-impact). The main effects of and interactions between vision and trial number were assessed using two-way within-subjects repeated measures analyses of variance. Peak GRF was 6.4% greater, on average, for DJs performed with stroboscopic vision compared to full vision (p = 0.042). Tibialis anterior RMS EMG during the 60-85 ms post-impact time interval was 14.1% lower for DJs performed with stroboscopic vision (p = 0.020). Vastus lateralis RMS EMG during the 85-120 ms post-impact time interval was 11.8% lower for DJs performed with stroboscopic vision (p = 0.017). Stroboscopic vision altered DJ landing mechanics and lower-extremity muscle activation. The observed increase in peak GRF and reduction in RMS EMG of the tibialis anterior and vastus lateralis post-landing may signify a higher magnitude of lower-extremity musculotendinous stiffness developed pre-landing. The results indicate measurable sensorimotor disruption for DJs performed with stroboscopic vision, warranting further research and supporting the potential use of stroboscopic vision as a sensorimotor training aid in exercise and rehabilitation. Stroboscopic vision could induce beneficial adaptations in multisensory integration, applicable to restoring sensorimotor function after injury and preventing injuries in populations experiencing landing impacts at night (e.g., military personnel).

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.

A method to assess response inhibition during a balance recovery step.

BACKGROUND: Correlations between falls and individual differences in inhibitory control, suggest the ability to suppress automatic, but unwanted, action is important in fall prevention. Response inhibition has been a topic of considerable interest in the cognitive neuroscience community for many decades, bringing a wealth of techniques that could potentially inform assessment of reactive balance. For example, the stop signal task is a popular method to quantify inhibitory control ability. RESEARCH QUESTION: Can we apply the stop signal task to measure response inhibition in a balance recovery task? METHODS: Twenty healthy, young adults completed a novel reactive balance test that required occasional suppression of a balance recovery step. Participants were released from a supported lean ('Go' cue) requiring them to quickly step forward to regain balance. On some trials, a tone ('Stop' cue) instructed participants to suppress a step and relax into a harness. Step trials were more frequent (80%) than stop trials (20%) to bias a rapid stepping response. The stop tone was presented at various delays following cable release, to manipulate task difficulty (i.e., longer delays make step suppression difficult). Individual differences in inhibitory control were determined using lift off times from force plates, and by contrasting muscle activation in failed compared to successful stop trials. RESULTS: Most participants were able to successfully suppress a balance recovery step on occasion, allowing for accurate estimation of individual differences in inhibitory control. The successful suppression of a balance recovery step was more likely in the group (n = 10) where shorter stop signal delays were used (i.e., the task was easier). SIGNIFICANCE: While balance assessments often stress reflexive action, there is a need for methods that evaluate response inhibition. The present study leveraged a well-established cognitive test of inhibitory control to develop a method to quantify stopping ability in a reactive balance context.

The Bilateral Deficit Phenomenon in Elbow Flexion: Explanations for Its Inconsistent Occurrence and Detection.

The underlying mechanism(s) of the Bilateral Deficit (BLD) phenomenon is without consensus. Methodological inconsistencies across prior works may be an important source of equivocal results and interpretations. Based on repeatability problems with the BLD measure and maximal force definition, the presence or absence of the BLD phenomenon is altered, shifting conclusions of its mechanistic cause. Our purpose in this study was to examine methodological inconsistencies in applying the BLD measure to establish optimal methods for evaluating the underlying mechanism. Eleven healthy participants engaged in one familiarity and five test sessions, completing bilateral and unilateral elbow maximal voluntary isometric contractions. We defined maximal force by averaged and absolute peak and plateau values. BLD was evident if the bilateral index (BI), the ratio of the bilateral over summed unilateral forces, was statistically different from zero. We addressed interclass correlations (ICC), Chronbach's α, standard error of the mean, and minimal detectable change between and within sessions for all force measures and BI. We evaluated all combinations of sessions (i.e., 1-2, 3-5, 5-6) and maximal forces to establish the optimal number of sessions to achieve reliability. BLD was present for test sessions, but not for familiarization. All measures of maximal force were highly reliable between and within sessions (ICC(2,1) ≥ .895). BI was only considered significantly reliable in sessions 3-5 (p < .027), defined by absolute and average plateau forces, but reliability was still quantifiably poor (absolute: ICC(2,1) = .392; average: ICC(2,1) = .375). These results demonstrate that high force reliability within and between sessions does not translate to stable and reliable BI, potentially exposing the lack of any defined BLD mechanism.

Promoting Generalized Learning in Balance Recovery Interventions.

Recent studies have shown balance recovery can be enhanced via task-specific training, referred to as perturbation-based balance training (PBT). These interventions rely on principles of motor learning where repeated exposure to task-relevant postural perturbations results in more effective compensatory balance responses. Evidence indicates that compensatory responses trained using PBT can be retained for many months and can lead to a reduction in falls in community-dwelling older adults. A notable shortcoming with PBT is that it does not transfer well to similar but contextually different scenarios (e.g., falling sideways versus a forward trip). Given that it is not feasible to train all conditions in which someone could fall, this limited transfer presents a conundrum; namely, how do we best use PBT to appropriately equip people to deal with the enormous variety of fall-inducing scenarios encountered in daily life? In this perspective article, we draw from fields of research that explore how general learning can be promoted. From this, we propose a series of methods, gleaned from parallel streams of research, to inform and hopefully optimize this emerging field where people receive training to specifically improve their balance reactions.

Differences in brain structure and theta burst stimulation-induced plasticity implicate the corticomotor system in loss of function after musculoskeletal injury.

Traumatic musculoskeletal injury (MSI) may involve changes in corticomotor structure and function, but direct evidence is needed. To determine the corticomotor basis of MSI, we examined interactions among skeletomotor function, corticospinal excitability, corticomotor structure (cortical thickness and white matter microstructure), and intermittent theta burst stimulation (iTBS)-induced plasticity. Nine women with unilateral anterior cruciate ligament rupture (ACL) 3.2 ± 1.1 yr prior to the study and 11 matched controls (CON) completed an MRI session followed by an offline plasticity-probing protocol using a randomized, sham-controlled, double-blind, cross-over study design. iTBS was applied to the injured (ACL) or nondominant (CON) motor cortex leg representation (M1LEG) with plasticity assessed based on changes in skeletomotor function and corticospinal excitability compared with sham iTBS. The results showed persistent loss of function in the injured quadriceps, compensatory adaptations in the uninjured quadriceps and both hamstrings, and injury-specific increases in corticospinal excitability. Injury was associated with lateralized reductions in paracentral lobule thickness, greater centrality of nonleg corticomotor regions, and increased primary somatosensory cortex leg area inefficiency and eccentricity. Individual responses to iTBS were consistent with the principles of homeostatic metaplasticity; corresponded to injury-related differences in skeletomotor function, corticospinal excitability, and corticomotor structure; and suggested that corticomotor adaptations involve both hemispheres. Moreover, iTBS normalized skeletomotor function and corticospinal excitability in ACL. The results of this investigation directly confirm corticomotor involvement in chronic loss of function after traumatic MSI, emphasize the sensitivity of the corticomotor system to skeletomotor events and behaviors, and raise the possibility that brain-targeted therapies could improve recovery.NEW & NOTEWORTHY Traumatic musculoskeletal injuries may involve adaptive changes in the brain that contribute to loss of function. Our combination of neuroimaging and theta burst transcranial magnetic stimulation (iTBS) revealed distinct patterns of iTBS-induced plasticity that normalized differences in muscle and brain function evident years after unilateral knee ligament rupture. Individual responses to iTBS corresponded to injury-specific differences in brain structure and physiological activity, depended on skeletomotor deficit severity, and suggested that corticomotor adaptations involve both hemispheres.

Action Boundary Proximity Effects on Perceptual-Motor Judgments.

INTRODUCTION: Designed as a more ecological measure of reaction times, the Perception-Action Coupling Task (PACT) has shown good reliability and within-subject stability. However, a lengthy testing period was required. Perceptual-motor judgments are known to be affected by proximity of the stimulus to the action boundary. The current study sought to determine the effects of action boundary proximity on PACT performance, and whether redundant levels of stimuli, eliciting similar responses, can be eliminated to shorten the PACT.METHODS: There were 9 men and 7 women who completed 4 testing sessions, consisting of 3 familiarization cycles and 6 testing cycles of the PACT. For the PACT, subjects made judgments on whether a series of balls presented on a tablet afford "posting" (can fit) through a series of apertures. There were 8 ratios of ball to aperture size (B-AR) presented, ranging from 0.2 to 1.8, with each ratio appearing 12 times (12 trials) per cycle. Reaction times and judgment accuracy were calculated, and averaged across all B-ARs. Ratios and individual trials within each B-AR were systematically eliminated. Variables were re-averaged, and intraclass correlation coefficients (ICC) and coefficients of variation (CVTE) were calculated in an iterative manner.RESULTS: With elimination of the 0.2 and 1.8 B-ARs, the PACT showed good reliability (ICC = 0.81-0.99) and consistent within-subject stability (CVTE = 2.2-14.7%). Reliability (ICC = 0.81-0.97) and stability (CVTE = 2.6-15.6%) were unaffected with elimination of up to 8 trials from each B-AR.DISCUSSION: The shortened PACT resulted in an almost 50% reduction in total familiarization/testing time required, significantly increasing usability.Johnson CD, LaGoy AD, Pepping G-J, Eagle SR, Beethe AZ, Bower JL, Alfano CA, Simpson RJ, Connaboy C. Action boundary proximity effects on perceptual-motor judgments. Aerosp Med Hum Perform. 2019; 90(12):1000-1008.

Reliability and Validity of a Pool-Based Maximal Oxygen Uptake Test to Examine High-Intensity Short-Duration Freestyle Swimming Performance.

Nagle, EF, Nagai, T, Beethe, AZ, Lovalekar, MT, Zera, JN, Connaboy, C, Abt, JP, Beals, K, Nindl, BC, Robertson, RJ, and Lephart, SM. Reliability and validity of a pool-based maximal oxygen uptake test to examine high-intensity short-duration freestyle swimming performance. J Strength Cond Res 33(5): 1208-1215, 2019-A modality-specific swimming protocol to assess maximal oxygen uptake (V[Combining Dot Above]O2maxsw) is essential to accurately prescribe and monitor swimming conditioning programs. Consequently, there is a need for a reliable and valid graded intensity swimming pool test to accurately assess V[Combining Dot Above]O2maxsw using indirect calorimetry. The purpose of this study was to assess (a) reliability of an intensity self-regulated swimming pool test of V[Combining Dot Above]O2maxsw and (b) validity of a V[Combining Dot Above]O2maxsw test using performance swim (PS) time as the criterion. Twenty-nine men (n = 15) and women (n = 14) (age, 23 ± 6.4 years; body mass index, 23.5 ± 3.0 kg·m) performed 2 swimming pool V[Combining Dot Above]O2maxsw trials (V[Combining Dot Above]O2maxsw A and V[Combining Dot Above]O2maxsw B), and 2 PS tests (45.7 m [31.20 ± 4.5 seconds] and 182 m [159.2 ± 25.5 seconds]). For test-retest reliability (trials A vs. B), strong correlations (p < 0.05) were found for V[Combining Dot Above]O2maxsw (ml·kg·min) (r = 0.899), O2 pulse (ml O2·beat) (r = 0.833), and maximum expired ventilatory volume (L·min) (r = 0.785). For performance validity, moderately strong correlations (p < 0.05) were found between V[Combining Dot Above]O2maxsw A and 45.7-m (r = -0.543) and 182-m (r = -0.486) swim times. The self-regulated graded intensity swimming pool protocol examined presently is a reliable and valid test of V[Combining Dot Above]O2maxsw. Studies should consider the suitability of a V[Combining Dot Above]O2maxsw test for military personnel, clinical populations, and injured athletes.

Prediction of exertional lower extremity musculoskeletal injury in tactical populations: protocol for a systematic review and planned meta-analysis of prospective studies from 1955 to 2018.

BACKGROUND: Musculoskeletal injuries (MSI) represent more than half of all injuries in tactical populations (i.e., military service and public safety workers including police, firefighters, emergency medical services (EMS)). Most lower extremity MSIs result from physical exertion during training, occupational tasks, and recreation. Such exertional lower extremity injuries (ELEI) produce a significant human and financial cost. Accordingly, significant efforts have been made to identify sensitive, specific, and reliable predictors of ELEI. There is a need to synthesize and evaluate the predictive value of risk factors for ELEI while addressing the influence of occupation, sex, exposure, injury characteristics, and study quality. Therefore, the purpose of this systematic review and planned meta-analysis is to evaluate risk factors for ELEI in tactical populations. METHODS: After the development of a search strategy, comprehensive searches will be conducted in MEDLINE, EMBASE, Cochrane, and CINAHL databases. Articles will be screened with a multi-user process and delimited to prospective comparative cohort studies that directly measure injury occurrence in the target population(s). Extracted data will be synthesized and assessed for reporting bias, meta-bias, and overall quality, with subgroup analyses to determine the influence of participant, injury, and exposure characteristics in addition to study quality. DISCUSSION: This systematic review and planned meta-analysis will comprehensively evaluate ELEI risk factors. Information gained will inform injury prevention protocols, facilitate the use of improved measurements, and identify requirements for future research. TRIAL REGISTRATION: The systematic review protocol was registered with the International Prospective Register of Systematic Reviews (PROSPERO) on 3 Jan 2018 (registration number CRD42018056977 ).

Improvement of Flutter-Kick Performance in Novice Surface Combat Swimmers With Increased Hip Strength.

PURPOSE: To examine strength, range of motion, anthropometric, and physiological contributions to novice surface-combat-swimming (sCS) performance and establish differences from freestyle-swimming (FS) performance to further understand the transition of FS to sCS performance. METHODS: A total of 13 competitive swimmers (7 male and 6 female; 27.7 [2.3] y; 176.2 [2.6] cm; 75.4 [3.9] kg) completed 8 testing sessions consisting of the following: physiological land-based measurements for maximal anaerobic and aerobic capacity and upper- and lower-extremity strength and range of motion, an sCS anaerobic capacity swim test measuring peak and mean force and fatigue index, 2 aerobic capacity tests measuring maximal aerobic capacity for both FS and sCS, and four 500-m performance swims for time, 1 FS, and 3 sCS. Separate multiple linear-regression analysis was used to analyze predictors of both sCS and FS performance models. RESULTS: FS performance was predicted by the final FS maximal oxygen uptake with an R2 of 42.03% (F1,10 = 7.25; P = .0226), whereas sCS performance was predicted by isometric hip-extension peak strength with an R2 of 41.46% (F1,11 = 7.79; P = .0176). CONCLUSIONS: Results demonstrate that different physiological characteristics predict performance, suggesting that an altered strategy is used for novice sCS than FS. It is suggested that this may be due to the added constraints as mandated by mission requirements including boots, weighted gear, and clandestine requirements with hips lowered beneath the surface. Further research should examine the kinematics of the sCS flutter kick to improve performance by developing training strategies specific for the task.

Prevention of exertional lower body musculoskeletal injury in tactical populations: protocol for a systematic review and planned meta-analysis of prospective studies from 1955 to 2018.

BACKGROUND: Exertional lower body musculoskeletal injuries (ELBI) cost billions of dollars and compromise the readiness and job performance of military service and public safety workers (i.e., tactical populations). The prevalence and burden of such injuries underscores the importance of prevention efforts during activities necessary to sustain core occupational competencies. Attempts to synthesize prevention techniques specific to tactical populations have provided limited insight on the comparative efficacy of interventions that do not modify physical training practices. There is also a need to assess the influence of sex, exposure, injury classification scheme, and study design. Thus, the primary purpose of the systematic review and planned meta-analysis detailed in this protocol is to evaluate the comparative efficacy of ELBI prevention strategies in tactical populations. METHODS: A systematic search strategy will be implemented in MEDLINE, EMBASE, Cochrane, and CINAHL. A multi-tiered process will be used to capture randomized controlled trials and prospective cohort studies that directly assess the prevention of ELBI in tactical population(s). Extracted data will be used to compare prevention strategies and assess the influence of heterogeneity related to occupation, sex, exposure, injury characteristics, and study quality. In addition, individual risk of bias, meta-bias, and the quality of the body of evidence will be rigorously tested. DISCUSSION: This systematic review and planned meta-analysis will comprehensively evaluate ELBI mitigation strategies in tactical populations, elucidate factors that influence responses to treatment, and assess the overall quality of the body of research. Results of this work will guide the prioritization of ELBI prevention strategies and direct future research efforts, with direct relevance to tactical, health and rehabilitation science, and human performance optimization stakeholders. SYSTEMATIC REVIEW REGISTRATION: The systematic review protocol was registered with the International Prospective Register of Systematic Reviews (PROSPERO) on 3 Jan 2018 (registration number CRD42018081799 ).