Cardiovascular Demand Differences Between Male and Female US Marine Recruits During Progressive Loaded Hikes.

ABSTRACT: Schram, B, Orr, R, Niederberger, B, Givens, A, Bernards, J, and Kelly, KR. Cardiovascular demand differences between male and female US Marine recruits during progressive loaded hikes. J Strength Cond Res XX(X): 000-000, 2024-Despite having to carry the same occupational load, female soldiers tend to be lighter than male soldiers. The aim of this study was to determine the differences in cardiovascular load between female and male US Marine recruits during progressive load carriage hikes. United States Marine Corps recruits (565 male recruits; 364 female recruits) completed 6 loaded hikes over 6 weeks (1: 10 kg, 30 minutes; 2: 10 kg, 45 minutes; 3: 15 kg, 30 minutes, 4: 15 kg, 45 minutes; 5: 20 kg, 30 minutes; 6: 20 kg, 45 minutes) during which cardiovascular response was measured. Average heart rate (HRavg), HR maximum (HRmax), and pace were measured via a wrist-worn physiological monitor. Independent sample t-tests were conducted to compare between sexes, with significance set at 0.008 after adjusting for multiple comparisons. The average female recruit had significantly lower body mass (BM) compared with the average male recruit (p < 0.001) and thus carried a significantly heavier relative load. (10 kg ∼17%, 15 kg ∼25%, 20 kg ∼33%, p < 0.001). There were no significant differences in pace in any hike, and no significant differences were found in HRavg or HRmax when comparing female and male Marines during Hike 1. For female Marines, HRavg was significantly higher compared with male Marines during Hike 2 (+6.5 b·min-1, p < 0.001) and Hike 3 (+7.4 b·min-1, p < 0.001), and both HRavg and HRmax were significantly higher in Hike 4 (+11.9 b·min-1, +8.4 b·min-1, p < 0.001), Hike 5 (+7.7 b·min-1, +7.9 b·min-1, p < 0.001), and Hike 6 (+6.9 b·min-1, +7.1 b·min-1, p < 0.001), respectively. Female Marines endured greater cardiovascular demand compared with male Marines during load carriage events when carrying loads greater than 15 kg (∼25% BM).

Quantification of daily workload, energy expenditure, and sleep of US Marine recruits throughout a 10-week boot camp.

BACKGROUND: During periods of high-volume vigorous exercise, United States Marine Corps recruits often experience musculoskeletal injuries. While the program of instruction (POI) for basic training is a defined training volume, the total workload of boot camp, including movements around the base, is unknown. OBJECTIVE: The present study aimed to quantify the daily total workload, energy expenditure, and sleep during basic recruit training at Marine Corps Recruit Depot (MCRD) San Diego. METHODS: Eighty-four male recruits from MCRD San Diego wore wrist wearable physiological monitors to capture their complete workload (mileage from steps), energy expenditure, and sleep throughout the 10-week boot camp. RESULTS: Marine recruits traveled an average of 11.5±3.4 miles per day (M±SD), expended 4105±823 kcal per day, and slept an average of 5 : 48±1 : 06 hours and minutes per night. While the POI designates a total of 46.3 miles of running and hiking, the actual daily average miles yielded approximately 657.6±107.2 miles over the 10-week boot camp. CONCLUSION: Recruit training requires high physical demand and time under tension due to the cumulative volume of movements around base in addition to the POI planned physical training.

Profiling injuries sustained following implementation of a progressive load carriage program in United States marine corps recruit training.

BACKGROUND: Load carriage tasks during United States Marine Corps (USMC) recruit training can cause injury. Load carriage conditioning, if optimized, can reduce injury risk. OBJECTIVE: To compare injuries sustained by USMC recruits following participation in either the Original Load Carriage (OLC) program or a Modified Load Carriage (MLC) program. METHODS: Retrospective musculoskeletal injury data were drawn from the USMC San Diego Sports Medicine injury database for recruits completing the OLC (n = 2,363) and MLC (n = 681) programs. Data were expressed as descriptive statistics and a population estimate of the OLC:MLC relative risk ratio (RR) was calculated. RESULTS: The proportion of injuries sustained in the MLC cohort (n = 268; 39% : OLC cohort, n = 1,372 : 58%) was lower, as was the RR (0.68, 95% CI 0.61- 0.75). The leading nature of injury for both cohorts was sprains and strains (OLC n = 396, 29%; MLC n = 66; 25%). Stress reactions were proportionally higher in MLC (n = 17, 6%; OLC n = 4, 0.3%), while stress fractures were proportionately lower (MLC n = 9, 3%; OLC n = 114, 8%). Overuse injuries were lower in MLC (- 7%). The knee, lower leg, ankle, and foot were the top four bodily sites of injuries and the Small Unit Leadership Evaluation (SULE), Crucible, overuse-nonspecific, running, and conditioning hikes were within the top five most common events causing injury. The prevalence rates of moderate severity injury were similar (MLC = 23%; OLC = 24%), although MLC presented both a higher proportion and prevalence of severe injuries (MLC = 6%; OLC = 3%, respectively). CONCLUSION: A periodized load carriage program concurrently increased exposure to load carriage hikes while reducing injuries both during the load carriage hikes and overall.

Validation of Polar Grit X Pro for Estimating Energy Expenditure during Military Field Training: A Pilot Study.

Wearables are lightweight, portable technology devices that are traditionally used to monitor physical activity and workload as well as basic physiological parameters such as heart rate. However recent advances in monitors have enabled better algorithms for estimation of caloric expenditure from heart rate for use in weight loss as well as sport performance. can be used for estimating energy expenditure and nutritional demand. Recently, the military has adopted the use of personal wearables for utilization in field studies for ecological validity of training. With popularity of use, the need for validation of these devices for caloric estimates is needed to assist in work-rest cycles. Thus the purpose of this effort was to evaluate the Polar Grit X for energy expenditure (EE) for use in military training exercises. Polar Grit X Pro watches were worn by active-duty elite male operators (N = 16; age: 31.7 ± 5.0 years, height: 180.1 ± 6.2 cm, weight: 91.7 ± 9.4 kg). Metrics were measured against indirect calorimetry of a metabolic cart and heart rate via a Polar heart rate monitor chest strap while exercising on a treadmill. Participants each performed five 10-minute bouts of running at a self-selected speed and incline to maintain a heart rate within one of five heart rate zones, as ordered and defined by Polar. Polar Grit X Pro watch had a good to excellent interrater reliability to indirect calorimetry at estimating energy expenditure (ICC = 0.8, 95% CI = 0.61-0.89, F (74,17.3) = 11.76, p < 0.0001) and a fair to good interrater reliability in estimating macronutrient partitioning (ICC = 0.49, 95% CI = 0.3-0.65, F (74,74.54) = 2.98, p < 0.0001). There is a strong relationship between energy expenditure as estimated from the Polar Grit X Pro and measured through indirect calorimetry. The Polar Grit X Pro watch is a suitable tool for estimating energy expenditure in free-living participants in a field setting and at a range of exercise intensities.

Deriving pQCT-Type Spatial Density from DXA Images.

INTRODUCTION: Musculoskeletal overuse injuries are a serious problem in the military, particularly in basic combat training, resulting in hundreds of millions of dollars lost because of limited duty days, medical treatment, and high rates of reinjury. Injury risk models have been developed using peripheral computed tomography (pQCT)-based injury correlates. However, pQCT image capture on large number of recruits is not practical for military settings. Thus, this article presents a method to derive spatial density pQCT images from much lower resolution but more accessible dual-energy X-ray absorptiometry (DXA). MATERIALS AND METHODS: Whole-body DXA images and lower leg pQCT images for 51 male military recruits were collected before a 40-day School of Infantry. An artificial neural network model was constructed to relate the DXA density profiles to spatial pQCT density at the 38% and 66% tibial locations. RESULTS: Strong correlation, R2 = 0.993 and R2 = 0.990 for the 38% and 66% pQCT slices, respectively, was shown between spatial density predicted by the artificial neural network model and raw data. CONCLUSIONS: High potential exists to create a practical protocol using DXA in place of pQCT to assess stress fracture risk and aid in mitigating musculoskeletal injuries seen in military recruits.

Effect of a load distribution system on mobility and performance during simulated and field hiking while under load.

This study was conducted to test a modular scalable vest-load distribution system (MSV-LDS) against the plate carrier system (PC) currently used by the United States Marine Corps. Ten Marines engaged in 1.6 km load carriage trials in seven experimental conditions in a laboratory study. Kinematic, kinetic, and spatiotemporal gait parameters, muscle activity (electromyography), heart rate, caloric expenditure, shooting reaction times, and subjective responses were recorded. There was lower mean trapezius recruitment for the PC compared with the MSV-LDS for all conditions, and muscle activity was similar to baseline for the MSV-LDS. Twenty-seven Marines carrying the highest load were evaluated in the field, which measured an increase in energy expenditure with MSV-LDS; however, back discomfort was reduced. The field evaluation showed significantly reduced estimated ground reaction force on flat-ground segments with the MSV-LDS, and the data suggest both systems were comparable with respect to mobility and energy cost. Practitioner summary: This study found that a novel load distribution system appears to redistribute load for improved comfort as well as reduce estimated ground reaction force when engaged in hiking activities. Further, hiking with a load distribution system enables more neutral walking posture. Implications of load differences in loads carried are examined. Abbreviations: AGRF: anterior-posterior ground reaction forces; CAREN: Computer Assisted Rehabilitation Environment; GRF: ground reaction forces; HR: heart rate; ML-GRF: mediolateral ground reaction forces; MOLLE: Modular Lightweight Load-carrying Equipment; MSV-LDS: modular scalable vest-load distribution system; NHRC: Naval Health Research Center; PC: plate carrier; PPE: personal protective equipment; RPE: rating of perceived exertion; SAPI: small arms protective insert; sEMG: surface electromyography; USMC: United States Marine Corps; VGRF: Ground reaction forces in the vertical.

The effect of training on lumbar spine posture and intervertebral disc degeneration in active-duty Marines.

Military training aims to improve load carriage performance and reduce risk of injuries. Data describing the lumbar spine (LS) postural response to load carriage throughout training are limited. We hypothesised that training would reduce the LS postural response to load. The LS posture of 27 Marines was measured from upright MR images: with and without load (22.6 kg) at the beginning, middle, and end of School of Infantry (SOI) training. Disc degeneration was graded at L5-S1. No changes in posture and disc degeneration were found throughout training. During load carriage the LS became less lordotic and the sacrum more horizontal. Marines with disc degeneration had larger sacral postural perturbations in response to load. Our findings suggest that the postural response to load is defined more by the task needs than by the physical condition of the Marine. Practitioner Summary: The effect of military training on lumbar spine posture is unknown. The lumbar posture of 27 Marines was measured from upright MR images, with and without load throughout infantry training. No changes in posture or IVD degeneration were found across training. Marines with degeneration at the L5-S1 level had larger sacral postural perturbations in response to load.

Exercise training with blood flow restriction has little effect on muscular strength and does not change IGF-1 in fit military warfighters.

OBJECTIVE: Aerobic exercise with blood flow restriction (aBFR) has been proposed as an adjunctive modality in numerous populations, potentially via an enhanced growth factor response. However, the effects of aBFR on highly trained warfighters have yet to be examined. The purpose of this study was to determine if adjunctive aBFR as part of a regular physical training regimen would increase markers of aerobic fitness and muscle strength in elite warfighters. In addition, we sought to determine whether the changes in blood lactate concentration induced by aBFR would be associated with alterations in the insulin-like growth factor (IGF) axis. DESIGN: Active-duty US Naval Special Warfare Operators (n=18, age=36.8 ± 2.2 years, weight=89.1 ± 1.2 kg, height=181.5 ± 1.4 cm) from Naval Amphibious Base Coronado were recruited to participate in 20 days of adjunctive aBFR training. Peak oxygen consumption (VO2 peak), ventilatory threshold (VT), and 1-repetition max (1-RM) bench press and squat were assessed pre- and post-aBFR training. Blood lactate and plasma IGF-1 and IGF-binding protein-3 (IGFBP-3) were assessed pre-, 2 min post-, and 30 min post-aBFR on days 1, 9, and 20 of aBFR training. RESULTS: Following aBFR training there were no changes in VO2 peak or VT, but there was an increase in the 1-RM for the bench press and the squat (5.0 and 3.9%, respectively, P<0.05). Blood lactate concentration at the 2-min post-exercise time point was 4.5-7.2-fold higher than pre-exercise levels on all days (P<0.001). At the 30-min post-exercise time point, blood lactate was still 1.6-2.6-fold higher than pre-exercise levels (P<0.001), but had decreased by 49-56% from the 2-min post-exercise time point (P<0.001). Plasma IGF-1 concentrations did not change over the course of the study. On day 9, plasma IGFBP-3 concentration was 4-22% lower than on day 1 (P<0.01) and 22% lower on day 9 than on day 20 at the 30-min post-exercise time point (P<0.001). CONCLUSIONS: Our data suggest that aBFR training does not lead to practical strength adaptations or alterations in the IGF axis in a population of highly trained warfighters.

CK-MM Polymorphism is Associated With Physical Fitness Test Scores in Military Recruits.

OBJECTIVE: Muscle-specific creatine kinase is thought to play an integral role in maintaining energy homeostasis by providing a supply of creatine phosphate. The genetic variant, rs8111989, contributes to individual differences in physical performance, and thus the purpose of this study was to determine if rs8111989 variant is predictive of Physical Fitness Test (PFT) scores in male, military infantry recruits. METHODS: DNA was extracted from whole blood, and genotyping was performed in 176 Marines. Relationships between PFT measures (run, sit-ups, and pull-ups) and genotype were determined. RESULTS: Participants with 2 copies of the T allele for rs8111989 variant had higher PFT scores for run time, pull-ups, and total PFT score. Specifically, participants with 2 copies of the TT allele (variant) (n = 97) demonstrated an overall higher total PFT score as compared with those with one copy of the C allele (n = 79) (TT: 250 ± 31 vs. CC/CT: 238 ± 31; p = 0.02), run score (TT: 82 ± 10 vs. CC/CT: 78 ± 11; p = 0.04) and pull-up score (TT: 78 ± 11 vs. CC/CT: 65 ± 21; p = 0.04) or those with the CC/CT genotype. CONCLUSION: These results demonstrate an association between physical performance measures and genetic variation in the muscle-specific creatine kinase gene (rs8111989).

Changes in combat task performance under increasing loads in active duty marines.

UNLABELLED: U.S. Marines perform mission tasks under heavy loads which may compromise performance of combat tasks. However, data supporting this performance decrement are limited. PURPOSE: The aim of this study was to determine the effects of load on performance of combat-related tasks. METHODS: Subjects (N=18) ran a modified Maneuver Under Fire ([MANUF], 300 yards [yd] total: two 25-yd sprints, 25-yd crawl, 75-yd casualty drag, 150-yd ammunition can carry, and grenade toss) portion of the U.S. Marine Corps Combat Fitness Test under 4 trial conditions: neat (no load), 15%, 30%, and 45% of body weight, with a shooting task pre- and post-trial. RESULTS: There was a significant increase in total time to completion as a function of load (p<0.0001) with a relationship between load and time (r=0.592, p<0.0001). Pre- to post-MANUF shot accuracy (p=0.005) and precision (p<0.0001) was reduced. CONCLUSION: Short aerobic performance is significantly impacted by increasing loads. Marksmanship is compromised as a function of fatigue and load. These data suggest that loads of 45% body weight increase time to cover distance and reduce the ability to precisely hit a target.

Effect of load carriage on lumbar spine kinematics.

STUDY DESIGN: Feasibility study on the acquisition of lumbar spine kinematic data from upright magnetic resonance images obtained under heavy load carrying conditions. OBJECTIVE: To characterize the effect of the load on spinal kinematics of active Marines under typical load carrying conditions from a macroscopic and lumbar-level approach in active-duty US Marines. SUMMARY OF BACKGROUND DATA: Military personnel carry heavy loads of up to 68 kg depending on duty position and nature of the mission or training; these loads are in excess of the recommended assault loads. Performance and injury associated with load carriage have been studied; however, knowledge of lumbar spine kinematic changes is still not incorporated into training. These data would provide guidance for setting load and duration limits and a tool to investigate the potential contribution of heavy load carrying on lumbar spine pathologies. METHODS: Sagittal T2 magnetic resonance images of the lumbar spine were acquired on a 0.6-T upright magnetic resonance imaging scanner for 10 active-duty Marines. Each Marine was scanned without load (UN1), immediately after donning load (LO2), after 45 minutes of standing (LO3) and walking (LO4) with load, and after 45 minutes of side-lying recovery (UN5). Custom-made software was used to measure whole spine angles, intervertebral angles, and regional disc heights (L1-S1). Repeated measurements analysis of variance and post hoc Sidak tests were used to identify significant differences between tasks (α = 0.05). RESULTS: The position of the spine was significantly (P < 0.0001) more horizontal relative to the external reference frame and lordosis was reduced during all tasks with load. Superior levels became more lordotic, whereas inferior levels became more kyphotic. Heavy load induced lumbar spine flexion and only anterior disc and posterior intervertebral disc height changes were observed. All kinematic variables returned to baseline levels after 45 minutes of side-lying recovery. CONCLUSION: Superior and inferior lumbar levels showed different kinematic behaviors under heavy load carrying conditions. These findings suggest a postural, lumbar flexion strategy aimed at centralizing a heavy posterior load over the base of support.