Utility of preinduction tests as predictors of attrition in infantry recruits: a prospective study.

INTRODUCTION: Infantry recruit attrition wastes resources and can affect combat readiness. The purpose of this study was to examine the utility of preinduction tests as a predictor of attrition among conscripts in the first year of infantry training. METHODS: 303 infantry conscripted recruits participated in a prospective study. Before their service, recruits received health profile and Quality Group Scores (QGSs). Recruits were screened at induction using questionnaires, by functional movement screening (FMS) and by upper and lower quarter Y-balance, dynamic and anthropometric tests. They were followed for musculoskeletal injuries and attrition during the first year of training. RESULTS: 165/303 (54.5%) recruits were diagnosed with musculoskeletal injury or pain during the first year of their training. 15.2% did not complete their first year of service as combatants and 5.9% were discharged. On multivariable binary stepwise logistic regression analysis for attrition, protective factors were higher QGSs (OR 0.78, 95% CI 0.69 to 0.89) and recruits diagnosed with musculoskeletal injuries or pain (OR 0.20, 95% CI 0.09 to 0.48). Pain in the balance test performed at the beginning of training was a risk factor (OR 3.31, 95% CI 1.44 to 7.61). These factors explained only 15.4% of the variance in attrition. CONCLUSIONS: FMS was not a significant predictor of infantry attrition. Measuring the three variables found to be associated with infantry attrition would seem to be valuable when the number of infantry candidates greatly exceeds the number of infantry positions. Transferring infantry attriters to non-combatant roles and not discharging them is a way to manage the problem of attrition.

Physical and psychological stressors linked with stress fractures in recruit training.

This study aimed to measure ambulation in infantry army basic training, and to evaluate if covering more distance can explain stress fractures in a stressor-stress model. Forty-four male combat recruits (18.7 ± 0.7 years) participated in a 6-month rigorous high intensity combat training program. Baseline data included anthropometric measurements, VO(2)max, and psychological questionnaires. Actual distance covered was measured using a pedometer over an 11-week training period. Psychological questionnaires were repeated after 2 months. Sixteen recruits were diagnosed with stress fractures by imaging (SFi = 36.4%). Statistical analysis included comparing measured variables between SFi and those without stress fractures (NSF). The recruits covered 796 ± 157 km, twofold the distance planned of 378 km (P < 0.001). The SFi group covered a distance 16.4% greater than that of the NSF group (866 ± 136 and 744 ± 161 km, respectively, P < 0.01), and also demonstrated greater psychological stress. These data reveal the importance of adherence to or enforcement of military training programs. In the light of these data, the Israeli Defense Forces program needs reappraisal. A stressor-stress response might explain the susceptibility of certain recruits for injury. Using advanced technology, monitoring ambulation may prevent stress fracture development by limiting subjects exceeding a certain level. Psychological profile may also play a role in predicting stress fracture development.

The effect of high versus low loading on bone strength in middle life.

While bone mass and geometry are largely genetically determined, mechanical loading is considered to be an important additional determinant. This study investigates to what extent very high mechanical loading begun at a young age and sustained afterward can affect tibia bone mass and geometry in middle age. Cohorts from a common ethnic background, with a history of very high and very low tibia bone loading based on an assessment of their activities according their strain levels were compared. The study hypothesis was that the tibia bone density and geometric strength parameters would be greater in the high bone loading cohort. Subjects from a group of elite infantry recruits who sustained a 31% incidence of stress fractures during their basic training in 1983, were reviewed 25 years later. The tibia bone strength of 25 of these soldiers, 11 of whom had sustained stress fractures, was compared to a group of 20 subjects who received exemption from military service in 1982-5 because they were religious scholars and who continued these studies afterwards. Anthropometric measurements were made. The bone density and geometric strength of the tibia was assessed by quantitative computerized tomography (QCT). The average daily dietary intake and metabolic expenditure of subjects were assessed by questionnaires. At the 25 year follow-up soldiers were on an average 3 cm taller than the religious scholars (p=0.02) and had lower abdominal girths (p=0.03). There was no difference in the tibia cortical density between cohorts in spite of the fact that the religious scholars had lower daily calcium intakes (p=0.02). Soldiers had stronger tibias based on geometric engineering criteria. The mean area moments of inertia (p=0.02, p=0.04) and polar moments of inertia (p=0.02) were 16% larger in the soldier cohort. By multivariate regression analysis greater height, weight and daily energy expenditure were related to larger bone geometric strength parameters. According to semipartial eta-square analysis, between 39% to 45% of the variance in the area moments of inertia between the cohorts was attributable to these three parameters. The religious scholars burned less calories daily, principally because they did no sport activity (p=0.001). There was no difference in tibia bone strength parameters between soldiers who did and did not sustain stress fractures in their 1983 basic training. In conclusion, in a middle age population with a common ethnic origin, the high bone loading cohort had stronger tibias than the low bone loading cohort based on larger geometric strength properties and not because of higher cortical density. In spite of being at the extremes of the bone loading spectra, the tibia area moment of inertia of the two cohorts in this study differed by only 16%, with part of this difference attributable to factors other than bone loading. We do not know for sure if the difference in the geometric properties is related to high bone loading or whether people with stronger bones are more likely to engage in high bone loading. Healthy male subjects who sustained stress fractures at a young age do not have weaker tibias at middle age according to QCT measurements.

Bracing in external rotation for traumatic anterior dislocation of the shoulder.

We undertook a prospective study in 51 male patients aged between 17 and 27 years to ascertain whether immobilisation after primary traumatic anterior dislocation of the shoulder in external rotation was more effective than immobilisation in internal rotation in preventing recurrent dislocation in a physically active population. Of the 51 patients, 24 were randomised to be treated by a traditional brace in internal rotation and 27 were immobilised in external rotation of 15 degrees to 20 degrees. After immobilisation, the patients undertook a standard regime of physiotherapy and were then assessed clinically for evidence of instability. When reviewed at a mean of 33.4 months (24 to 48) ten from the external rotation group (37%) and ten from the internal rotation group (41.7%) had sustained a further dislocation. There was no statistically significant difference (p = 0.74) between the groups. Our findings show that external rotation bracing may not be as effective as previously reported in preventing recurrent anterior dislocation of the shoulder.

A prospective biomechanical study of the association between foot pronation and the incidence of anterior knee pain among military recruits.

Excessive foot pronation has been considered to be related to anterior knee pain. We undertook a prospective study to test the hypothesis that exertional anterior knee pain is related to the static and dynamic parameters of foot pronation. Two weeks before beginning basic training lasting for 14 weeks, 473 infantry recruits were enrolled into the study and underwent two-dimensional measurement of their subtalar joint displacement angle during walking on a treadmill. Of the 405 soldiers who finished the training 61 (15%) developed exertional anterior knee pain. No consistent association was found between the incidence of anterior knee pain and any of the parameters of foot pronation. While a statistically significant association was found between anterior knee pain and pronation velocity (left foot, p = 0.05; right foot, p = 0.007), the relationship was contradictory for the right and left foot. Our study does not support the hypothesis that anterior knee pain is related to excessive foot pronation.

Exercise-induced strain and strain rate in the distal radius.

Strains applied to bone can stimulate its development and adaptation. High strains and rates of strain are thought to be osteogenic, but the specific dose response relationship is not known. In vivo human strain measurements have been performed in the tibia to try to identify optimal bone strengthening exercises for this bone, but no measurements have been performed in the distal radial metaphysis, the most frequent site of osteoporotic fractures. Using a strain gauged bone staple, in vivo dorsal metaphyseal radial strains and rates of strain were measured in ten female patients during activities of daily living, standard exercises and falls on extended hands. Push-ups and falling resulted in the largest compression strains (median 1345 to 3146 microepsilon, equivalent to a 0.1345% to 0.3146% length change) and falling exercises in the largest strain rates (18582 to 45954 microepsilon/s). On the basis of their high strain and/or strain rates these or variations of these exercises may be appropriate for distal radial metaphyseal bone strengthening.

A comparison of bone strain measurements at anatomically relevant sites using surface gauges versus strain gauged bone staples.

Instrumented bone staples were first introduced as an alternative to surface-mounted strain gauges for use in human in vivo bone strain measurements because their fixation to bone is secure and requires not only minimally invasive surgery. Bench-top bone bending models have shown that the output from strain gauged bone staples compares favorably to that of traditional mounted gauges. However their within- and across-subject performance at sites typically instrumented in vivo has never been examined. This study used seven human cadaver lower extremities with an age range of 23-81 years old and a dynamic gait simulator to examine and compare axial strains in the mid tibial diaphysis and on the dorsal surface of the second metatarsal as measured simultaneously with strain gauged bone staples and with traditional surface-mounted gauges. Rosette configurations were used at the tibial site for deriving principal compression and tension, and shear strains. Axial outputs from the two gauge types demonstrated strong linear relationships for the tibia (r(2)=0.78-0.94) and the second metatarsal (r(2)=0.96-0.99), but coefficients (slopes) for the relationship were variable (range 7-20), across subjects and across sites. The apparent low reliability of strain gauged staples may be explained by the fact that both strain gauged staples and surface strain gauges are inexact to some degree, do not measure strains from exactly the same areas and strain gauged staples reflect surface strains as well as deformations within the cortex. There were no relationships for the principal tibia compression, tension or shear strain measurements derived from the two rosette gauge types, reflecting the very different anatomical areas measured by each of the constructs in this study. Strain gauged bone staples may be most useful in comparing relative axial intra-subject differences between activities, but inter-subject variability may require larger sample sizes to detect differences between populations.

Cold weather training: a risk factor for Achilles paratendinitis among recruits.

In a prospective study of risk factors for Achilles tendinopathy among four induction cycles of infantry recruits, 95 out of 1405 recruits, (6.8%) were found to suffer from Achilles tendinopathy. In more than 94% of the cases, the tendinopathy was considered to be paratendinitis. Training season had a statistically significant effect on the incidence of Achilles paratendinitis with 3.6% suffering in the summer and 9.4% in the winter (p = .001). This increased risk for Achilles paratendinitis in cold weather outdoor training can be explained by a fall in temperature of the Achilles paratenon whose membranes are rich in mucopolysaccharides and serve as a lubricant for gliding of the tendon and epitenon. Decreased temperature may increase the viscosity of the lubricant and thereby increases friction and risk for Achilles paratendinitis. This same relationship illustrates why "warming up" before exercising may be important in lowering the incidence of Achilles paratendinitis.

Are overground or treadmill runners more likely to sustain tibial stress fracture?

BACKGROUND: Repetitive high bone strain and/or strain rates, such as those that occur during running, contribute to stress fractures as well as promoting maintenance of or increase in bone mass. Kinematic differences are known to exist between overground and treadmill running and these may be reflected in different bone strains and strain rates during the two running techniques. AIM: To measure in vivo strains and strain rates in human tibia during treadmill and overground running and determine if there are significant differences in strain and strain rate levels between the two running techniques. METHODS: A strain gauged bone staple was mounted percutaneously along the axial direction in the mid diaphysis of the medial tibia in three subjects, and in vivo tibial strains were measured during treadmill and overground running at 11 km/h. RESULTS: Axial compression strains (p<0.0001), tension strains (p<0.001), compression strain rates (p<0.0001), and tension strain rates (p<0.0001) were 48-285% higher during overground running than during treadmill running. CONCLUSIONS: On the basis of lower in vivo strains and strain rates, treadmill runners are at lower risk of developing tibial stress fractures, but less likely to achieve tibial bone strengthening, than overground runners.

Metatarsal strains are sufficient to cause fatigue fracture during cyclic overloading.

Human in vivo tibial strains during vigorous walking have not been found to exceed 1200 microstrains. These values are below those found in ex vivo studies (>3000 microstrains) to cause cortical bone fatigue failure, suggesting that an intermediate bone remodeling response may be associated with tibial stress fractures. Metatarsal stress fractures, however, often develop before there is time for such a response to occur. Simultaneous in vivo axial strains were measured at the mid diaphysis of the second metatarsal and the tibia in two subjects. Peak axial metatarsal compression strains and strain rates were significantly higher than those of the tibia during treadmill walking and jogging both barefoot and with running shoes and during simple calisthenics. During barefoot treadmill walking metatarsal compression strains were greater than 2500 microstrains. During one- and two-leg vertical jumps and broad jumping, both metatarsal compression and tension strains were >3000 microstrains. Compression and tension strains in the metatarsus unlike those of the tibia may be sufficiently high even during moderate exertional activities to cause fatigue failure of bone secondary to the number of loading cycles without an intermediate bone remodeling response.

The effect of shoe sole composition on in vivo tibial strains during walking.

To test the hypothesis that shoe sole composition can affect the level of bone strain and strain rates that contribute to the development of stress fractures, in vivo tibial strain measurements were made during treadmill walking while wearing four shoes which differed only in their sole composition. Soles of 65 Shore A polyurethane with an embedded heel air cell had significantly lower compression and shear strains and shear strain rates than soles of 65, 75 and a composite of 40/65 Shore A polyurethane with no embedded air cells. A sole composed of 65 Shore A polyurethane with an embedded air cell can potentially be protective against stress fracture in a walking shoe.

A home exercise program for tibial bone strengthening based on in vivo strain measurements.

OBJECTIVE: To compare the strain and strain rates generated during lower limb calisthenics with walking, an exercise that has been found to have only minimal effect on bone mass. Strengthening of bone, while it still has adaptive ability, can be achieved by exercise. Mechanical loading during physical activity produces strains and strain rates within the bones. It is thought that strain and strain rates higher than the usual provide the stimulus for the bones' adaptation. DESIGN: Three strain-gauged bone staples were inserted percutaneously in a 30 degrees rosette pattern in the medial aspect of the midtibial diaphysis of two volunteers. The principal compression, tension, shear strains, and strain rates were measured during various lower limb calisthenics and compared with those of jogging and walking. RESULTS: Zig-zag hopping was in the grouping of exercises with the highest principal compression, tension, and shear strains and compression strain rates, whereas walking was in the lowest or next-to-the-lowest grouping for all principal strain or strain rates. CONCLUSION: Zig-zag hopping, based on the high strain and strain rates that it produces, may be an optimal tibial bone-strengthening exercise.

[Pre-induction sport activity in prevention of stress fractures in elite infantry recruits].

Pre-induction sports participation of 392 elite infantry recruits was evaluated for correlation with incidence of stress fractures (SF) during 14 weeks of basic training. 23.7% developed lower extremity stress fractures. 72% of the recruits had participated in sports on a regular basis during the 2 years prior to induction. Their fitness as examined by the Bar-Or induction fitness test, was significantly better than that of those who had not trained. 14.9% of the soldiers who had previously participated in ball games (primarily basketball) as an only sport suffered stress fractures, compared to 31.0% of those whose only sport was running (p < 0.005). Training for only 6 months prior to induction had no effect on the incidence of SF. The reason for the difference between ball games and running is probably related to the higher strains and strain rates developed during ball games and to their multidirectional nature, as compared to running. These findings suggest that participation in a pre-induction program that includes activities that create strains, such as basketball, can reduce incidence of SF in infantry recruits. The training period has to last at least 2 years.

Do high impact exercises produce higher tibial strains than running?

BACKGROUND: Bone must have sufficient strength to withstand both instantaneous forces and lower repetitive forces. Repetitive loading, especially when bone strain and/or strain rates are high, can create microdamage and result in stress fracture AIM: To measure in vivo strains and strain rates in human tibia during high impact and moderate impact exercises. METHODS: Three strain gauged bone staples were mounted percutaneously in a rosette pattern in the mid diaphysis of the medial tibia in six normal subjects, and in vivo tibial strains were measured during running at 17 km/h and drop jumping from heights of 26, 39, and 52 cm. RESULTS: Complete data for all three drop jumps were obtained for four of the six subjects. No statistically significant differences were found in compression, tension, or shear strains with increasing drop jump height, but, at the 52 cm height, shear strain rate was reduced by one third (p = 0.03). No relation was found between peak compression strain and calculated drop jump energy, indicating that subjects were able to dissipate part of the potential energy of successively higher drop jumps by increasing the range of motion of their knee and ankle joints and not transmitting the energy to their tibia. No statistically significant differences were found between the principal strains during running and drop jumping from 52 cm, but compression (p = 0.01) and tension (p = 0.004) strain rates were significantly higher during running. CONCLUSIONS: High impact exercises, as represented by drop jumping in this experiment, do not cause higher tibial strains and strain rates than running and therefore are unlikely to place an athlete who is accustomed to fast running at higher risk for bone fatigue.

In-vivo strain measurements to evaluate the strengthening potential of exercises on the tibial bone.

Mechanical loading during physical activity produces strains within bones. It is thought that these forces provide the stimulus for the adaptation of bone. Tibial strains and rates of strain were measured in vivo in six subjects during running, stationary bicycling, leg presses and stepping and were compared with those of walking, an activity which has been found to have only a minimal effect on bone mass. Running had a statistically significant higher principal tension, compression and shear strain and strain rates than walking. Stationary bicycling had significantly lower tension and shear strains than walking. If bone strains and/or strain rates higher than walking are needed for tibial bone strengthening, then running is an effective strengthening exercise for tibial bone.

Using bone's adaptation ability to lower the incidence of stress fractures.

In three prospective epidemiologic studies of the effect of pre-military-induction sport activities on the incidence of lower extremity stress fractures during infantry basic training, recruits who played ball sports (principally basketball) regularly for at least 2 years before basic training had a significantly lower incidence of stress fractures (13.2%, 16.7%, and 3.6% in the three studies, respectively) than recruits who did not play ball sports (28.9%, 27%, and 18.8%, respectively). Preinduction running was not related to the incidence of stress fracture. To assess the tibial strain environment during these sport activities, we made in vivo strain measurements on three male volunteers from the research team. Peak tibial compression and tension strain and strain rates during basketball reached levels 2 to 5.5 times higher than during walking and about 10% to 50% higher than during running. The high bone strain and strain rates that occurred in recruits while playing basketball in the years before military induction may have increased their bone stiffness, according to Wolff's Law. The stiffer bone could tolerate higher stresses better, resulting in lower strains for a given activity and a lower incidence of stress fractures during basic training.

Prevention of stress fractures using custom biomechanical shoe orthoses.

In a prospective study of stress fractures the hypothesis that training with custom made biomechanical shoe orthoses could lessen the incidence of stress fractures in infantry recruits was tested. Recruits were assigned randomly to groups and given soft biomechanical orthoses or semirigid biomechanical orthoses and compared with a control group that did not train in biomechanical orthoses. All recruits wore infantry boots with soles designed like those of basketball shoes. Recruits were examined biweekly during 14 weeks of basic training. The incidence of stress fractures was 15.7% for the recruits with the semirigid biomechanical orthoses, 10.7% for the recruits with the soft biomechanical orthoses, and 27% for the control group. The soft biomechanical orthoses were tolerated better by the recruits than were the semirigid devices. Among trainees at high risk for stress fractures, prophylactic use of custom made biomechanical orthoses may be warranted.

Effect of cane use on tibial strain and strain rates.

The effect of cane ambulation on hip biomechanics has been well studied, but its effect on tibial strains and strain rates is not known. To test the hypothesis that cane use may lower tibial strain and strain rates during walking, percutaneous axial extensometers were mounted on the right medial cortex of the midtibial diaphysis in seven male volunteers. In vivo peak-to-peak axial tibial strains and strain rates were measured for ipsilateral and contralateral cane usage and compared with a no cane control. Cane-assisted ambulation was not found to significantly lower strain magnitudes; however, tibial strain rates were significantly lowered by both ipsilateral and contralateral cane usage. We conclude that either ipsilateral or contralateral cane usage may be beneficial when lowering tibial strain rate is desired, such as in the treatment of tibia stress fracture or osteoarthrosis of the knee.