AbstractThis 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.

The objective is to examine the predictors of attrition in male and female candidates undergoing a 10-week early career military training program. 1006 candidates (79.5% male, 24.7 ± 3.2 years) consented to participating in a larger study examining predictors of injury during US Marine Corps Officer Candidates School (OCS). Participants completed a blood draw, demographic and psychological characteristics questionnaires, and two fitness tests. Participants were then grouped based on successful completion of OCS or not. Associations between potential predictors and attrition were analyzed using simple logistic regression analyses, followed by a backward stepwise elimination method. Area under the curve (AUC) of the receiver operating characteristic (ROC) curve was used to determine the accuracy of the attrition prediction model. 260 candidates (25.8%) attritted over the 10-week training, with the highest number of discharges during week 5. Musculoskeletal injury (MSKI) was the most common cause of attrition (30%), followed by non-MSKI medical (21.5%), and volitional withdrawals (19.6%). Sex, body mass index (BMI), resilience, initial physical fitness test score, combat fitness test (CFT) score, and prior military service were all significantly associated with attrition from OCS (all p < .05). The final prediction model of attrition included CFT score (p = .027) and resilience (p = .018). Multiple demographic, psychological, and fitness characteristics are associated with attrition from an early career military training course (OCS) and may be utilized as part of early screening procedures to identify and provide guidance for individuals at risk for not completing OCS.

Challenges for some women meeting the physical employment standards (PES) for ground close combat (GCC) roles stem from physical fitness and anthropometric characteristics. The purpose of this study was to identify the modifiable and nonmodifiable characteristics predictive of passing GCC-based PES tasks and determine the modifiable characteristics suitable to overcome nonmodifiable limitations. 107 adults (46 women) underwent multiday testing assessing regional and total lean mass (LM), percent body fat (BF%), aerobic capacity (V̇O2peak), strength, power, and PES performance. Predictors with p-value <0.200 were included in stepwise logistic regression analysis or binary logistic regression when outcomes among sexes were insufficient. Relative and absolute arm LM (OR: 4.617-8.522, p < 0.05), leg LM (OR: 2.463, p < 0.05), and upper body power (OR: 2.061, p < 0.05) predicted medicine ball chest throw success. Relative and absolute arm LM (OR: 3.734-11.694, p < 0.05), absolute trunk LM (OR: 2.576, p < 0.05), and leg LM (OR: 2.088, p < 0.05) predicted casualty drag success. Upper body power (OR: 3.910, p < 0.05), absolute trunk LM (OR: 2.387, p < 0.05), leg LM (OR: 2.290, p < 0.05), and total LM (OR: 1.830, p < 0.05) predicted maximum single lift success. Relative and absolute arm LM (OR: 3.488-7.377, p < 0.05), leg LM (OR: 1.965, p < 0.05), and upper body power (OR: 1.957, p < 0.05) predicted water can carry success. %BF (OR: 0.814, p = 0.007), V̇O2peak (OR: 1.160, p = 0.031), and lower body strength (OR: 1.059, p < 0.001) predicted repeated lift and carry success. V̇O2peak (OR: 1.540, p < 0.001) predicted 2-km ruck march success. Modifiable characteristics were the strongest predictors for GCC-based PES task success to warrant their improvement for enhancing PES performance for women.

INTRODUCTION:Overuse musculoskeletal injuries (MSKI) remain a significant medical challenge in military personnel undergoing military training courses; 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 characterized as maladaptive biological responses to chronic stress measured by wearable devices in US Marine Corps officer candidates during a 10-wk training course. METHODS:Devices recorded energy expenditure (EE), daytime heart rate (HR), sleeping HR, and sleep architecture (time and percentage of deep, light, rapid eye movement 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 overuse MSKI. Overuse MSKI occurrence in male participants was positively associated with daytime HR ( β = 5.316, P = 0.008), sleeping HR ( β = 2.708, P = 0.032), relative EE ( β = 8.968, P = 0.001), absolute flux in relative EE ( β = 2.994, P = 0.002), absolute EE ( β = 626.830, P = 0.001), and absolute flux in absolute EE ( β = 204.062, P = 0.004). Overuse MSKI occurrence in female participants was positively associated with relative EE ( β = 5.955, P = 0.026), deep sleep time ( β = 0.664, P < 0.001), and percent deep sleep ( β = 12.564, P < 0.001) and negatively associated with absolute flux in sleeping HR ( β = -0.660, P = 0.009). CONCLUSIONS:Overuse MSKI occurrences were associated with physiological characteristics of allostatic load, including chronically elevated HR and EE and greater time in restorative sleep stages, which may serve as biological signatures for overuse MSKI risk.

Habitual weight-bearing physical activity can improve bone mineral density (BMD), structure, and strength, but it remains unclear if bone adaptations to prolonged physical training are similar between elite male and female endurance athletes. PURPOSE: To compare the effect of 6 months of sport training on areal BMD (aBMD) and tibial volumetric BMD (vBMD), microarchitecture, and strength between elite male and female distance runners. METHODS: Twenty-one male (n = 12; 19.4 ± 0.5 yr; 21.4 ± 0.4 kg/m2) and female (n = 9; 18.9 ± 0.4 yr; 20.9 ± 0.4 kg/m2) Division I cross-country runners completed high-resolution peripheral quantitative computed tomography (HR-pQCT; XtremeCTII) scans at 4% (metaphysis) and 30% (diaphysis) of total tibial length and DXA (Lunar iDXA) scans of the total body, lumbar spine, and hip prior to and following the competitive fall season. Generalized linear mixed effects modeling was used to compare changes over time between men and women, adjusting for baseline BMI. Data are presented as estimated marginal mean ± SEM, α = 0.05. RESULTS: At the tibial diaphysis, training increased (main effects of time) cortical area (286.8 ± 5.4, 290.3 ± 4.9 mm2, p = 0.016) and perimeter (77.7 ± 0.6, 78.2 ± 0.6 mm, p = 0.007), stiffness (307.2 ± 6.2, 310.8 ± 5.6 kN/mm, p = 0.021), and failure load (17.5 ± 0.4, 17.7 ± 0.3 kN, p = 0.040). Training also increased (main effect of time) total body aBMD (1.238 ± 0.014, 1.248 ± 0.016 g/cm2, p = 0.001). No significant main effects of time were observed for the tibial metaphysis (4%). Men had lower total and cortical vBMD, but greater cortical area, perimeter, and thickness, stiffness, and failure load than women at the tibial diaphysis (main effects of sex, p ≤ 0.032). Men had greater trabecular area and thickness, stiffness, and failure load than women at the tibial metaphysis (main effects of sex p ≤ 0.003). Men had greater aBMD for the total body, femoral neck, and total hip (main effects of sex p ≤ 0.037). No significant sex*time interaction effects were observed. CONCLUSION: Six months of cross-country training elicited positive bone adaptations for the total body and at the diaphyseal tibia, which were similar between men and women. Sex-differences in parameters of bone health may also explain differences in risk for bone stress injury between men and women. Funding: USAMRDC W81XWH2110542

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 biomechan-ics 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 lin-ear 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 alpha = 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 mea-sures 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.(c) 2023 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved.

Allostatic load (AL) reflects the cumulative physiological demands that trigger recurrent autonomic responses elicited by heightened stress and engender a gradual disruption of biological systems if unmanaged. Overnight cardiac chronotropy (OCC) reflects the basal autonomic activity during sleep assessed by heart rate that responds to AL. Understanding the suitability of OCC on markers of AL assessed by wrist-worn device in Marines may help manage AL in this populace. PURPOSE Assess the relation between OCC and markers of AL from a wrist-worn device in Marines enduring a military training course. METHODS 82 Marines (men: n = 59, 22 [21, 26] y, 177.5 [172, 183.2] cm, 81.7 [72.7, 87.2] kg; women: n = 23, 24 [21, 27] y, 166 [163.4, 171.4] cm, 68.7 [59.8, 73.3] kg) participated. Wrist-worn devices (Garmin Instinct Solar, Garmin, Ltd., Olathe, KS) assessed OCC and markers of AL over 30 d including active kilocalories (kcal), steps, distance (m), moderate intensity (min), vigorous intensity (min), total sleep (min), deep sleep (min), light sleep (min), REM sleep (min), awake time during sleep (min) and sleep phase (%). Wear time ≥ 10 h/d over ≥1 wk was analyzed. Data were shown as median [Q1, Q3]. Spearman’s correlations (rs) assessed associations between OCC and AL. Strength of correlation coefficients was based on 0.8-1.0 very strong, 0.6-0.79 moderate, 0.3-0.59 fair and 0.1-0.29 poor. RESULTS 64 Marines (19 women) were included. Wear time was 24 [23.8, 24] h/d over 21 [16, 23] d. OCC (50 [44, 55] bpm) showed poor to fair correlations with AL: kcal (rs = 0.173 [0.12, 0.23] p < 0.001), steps (rs = -0.127 [-0.18, -0.07] p < 0.001), distance (rs = -0.180 [-0.24, -0.12] p < 0.001), moderate intensity (rs = 0.319 [0.25, 0.39] p < 0.001), vigorous intensity (rs = 0.198 [0.12, 0.27] p < 0.001), deep sleep (rs = 0.010 [0.02, 0.17] p = 0.011), % deep sleep (rs = 0.128 [0.05, 0.20] p < 0.001), REM sleep (rs = -0.149 [-0.22, -0.07] p < 0.001), % REM sleep (rs = -0.140 [-0.21, -0.06] p < 0.001), % awake time during sleep (rs = 0.090 [0.01, 0.16] p = 0.018) and total sleep (rs = -0.068 [-0.13, -0.01] p = 0.018). Light sleep (p = 0.328), % light sleep (p = 0.639) and awake time during sleep (p = 0.063) were not correlated. CONCLUSION OCC captured by a wrist-worn device may be unsuitable to assess AL in Marines as more sensitive markers or devices are warranted. ONRN00014-20-C2020

Bone geometry and microarchitecture vary between athletes with different habitual loading patterns because bone adapts to withstand loading demands. As gymnastics involves infrequent high impact loading and running involves repetitive medium impact loading, differences in the tibial structure are expected between these athletes. PURPOSE: Investigate differences in geometry and microarchitecture of the distal tibial metaphysis between collegiate female athletes competing in gymnastics and cross-country. METHODS: High resolution peripheral quantitative computed tomography was used to assess the distal tibia of NCAA Division I female cross-country runners (n = 17, age = 19.0 ± 0.9 yrs, BMI = 20.6 ± 1.4 kg/m2) and gymnasts (n = 16, age = 19.5 ± 1.4 yrs, BMI = 23.3 ± 1.8 kg/m2). Scans were taken at 4% of tibial length and evaluation software measured bone parameters. Finite element analysis estimated stiffness and failure load. Unadjusted group comparisons were conducted using independent samples t tests, followed by analysis of covariance adjusting for baseline BMI. Data are presented as mean ± SD, α = 0.05, two-sided. RESULTS: Unadjusted group comparisons showed that gymnasts exhibited greater total area (1067.4 ± 100.3mm2, 950.8 ± 65.9mm2, p < .001), trabecular area (995.5 ± 103.2mm2, 880.7 ± 64.5mm2, p < .001), and trabecular number (2.1 ± 0.2 mm-1, 1.9 ± 0.2 mm-1, p = .015) than runners. Stiffness (228.7 ± 47.5Nmm-1, 190.5 ± 46.9Nmm-1, p = .027) and failure load (12.2 ± 2.4 N, 10.3 ± 2.4 N, p = .026) were also greater in gymnasts than runners. Group differences analyzed after adjusting for BMI remained significant for total area (p = .030), trabecular area (p = .020), and trabecular number (p = .008); whole bone stiffness and failure load were no longer significant (p ≥ .381). Cortical volumetric bone mineral density, area, and thickness were not significantly different between groups in either analysis (p > .05). CONCLUSION: Gymnasts presented with more favorable bone structure than runners, possibly due to higher forces experienced during training and competition. Differences in tibial metaphysis bone structure between gymnasts and runners, which persist after controlling for BMI, indicate that the adaptive bone formation response to sport training is specific to demands of the sport. USAMRDC W81XWH2110542