Abstract
The purpose was to evaluate neuromuscular fatigue’s effect on unloaded and loaded countermovement jump (CMJ) force-time characteristics during high-intensity tactical training. Eighteen male and sixteen female Marines completed two maximal effort CMJs, in unloaded (PVC pipe) and loaded (10 kg weight vest and 20 kg barbell) conditions, prior to, and 24, 48, and 72 h after starting the 4-day event. The top three variables from three principal components (PC) were analyzed using mixed-effects modeling (PC1—concentric mean power, eccentric peak force, and modified reactive strength index; PC2—countermovement depth, eccentric mean power, and eccentric mean velocity; PC3—braking duration, jump height, peak power). Metrics from PC1 and PC3 were reduced across training and from both loading conditions. Metrics from PC2 were similarly affected by external loading but were less influenced by training-induced fatigue. Jump performances with the barbell and with shallower countermovement depths did not change throughout training. Thus, 20 kg loaded CMJs are stable neuromuscular measures suitable for tracking chronic training adaptations. Monitoring unloaded and 10 kg loaded CMJ performances, along with movement strategies (i.e., countermovement rates and depth), may help identify moments of accumulated fatigue to inform training and recovery adjustments and improve the sustainability of personnel.
Highlights
Military physical readiness training is intended to adequately prepare military personnel for the physical demands of various occupational tasks, which may be performed for long durations while bearing required equipment [1,2]
The longitudinal models in the current study found a significant reduction in all force-time metrics of interest, which never returned to the baseline, while countermovement depth was reduced at 48 h and returned to the baseline by 72 h following the start of the events
According to a principal component analysis (PCA), the variance of the countermovement jump (CMJ) performances due to fatigue and external loading was predominately explained by the ability to utilize the stretch-shortening cycle, followed by the strategy used to execute the jumps and the outcome of the jump itself
Summary
Military physical readiness training is intended to adequately prepare military personnel for the physical demands of various occupational tasks (e.g., endurance marches across rugged terrain, swift maneuvers under fire, jumping over and landing from obstacles, parachuting), which may be performed for long durations while bearing required equipment (i.e., external loads) [1,2]. The implementation of high training volumes and intensities to reach the required levels of various muscular fitness domains (e.g., strength, power, and endurance) may result in overaccumulations of neuromuscular fatigue (NMF). Consequentially, high incidences of musculoskeletal injuries (MSKI) [3,4,5,6]. In active operational forces and schoolhouse trainees, physical training may account for up to 80% of sustained MSKI, which occur most commonly to the ankles, knees, and lumbar spine in the form of muscle sprains, joint strains, or stress fractures [7,8,9,10,11]. MSKI rates persist through increased financial burdens and restricted operational capacities [10,12]. Due to the influence of physical training-induced NMF on injury risks [3,13], objective monitoring of NMF indicative metrics throughout preparatory training periods assists in confirming whether or not training regimens are eliciting positive (i.e., enhanced physical capacities) or negative (i.e., poor movement mechanics, heightened injury risk) adaptations over time [14].
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