**Background:** Load carriage is a major component of military occupational tasks and a significant source of noncombat musculoskeletal injuries (MSI). Previous research showed that low-magnitude load carriage negatively alters variability distribution and decreases spatiotemporal complexity in women during short gait bouts. However, the effects of heavier military-relevant loads (up to +55% body weight) during prolonged tasks, and potential sex differences, remained unclear. The study aimed to determine the interactive effects of load magnitude and locomotion pattern on motor variability, stride regulation, and spatiotemporal complexity during prolonged gait tasks in recruit-aged adults, and to examine the role of sex and task duration.
**Methods:** Twenty-six healthy, physically active recruit-aged adults (18–35 years) participated; 20 were analyzed (11 men, 9 women) after exclusions. Participants completed 10-minute trials of running and forced-marching under three load conditions: unloaded (BW), +45% body weight (+45%BW), and +55% body weight (+55%BW). Spatiotemporal parameters were captured at 100 Hz using 12 infrared cameras (Vicon) and an instrumented split-belt treadmill at 1000 Hz. Gait transition velocity (GTV) was determined for each load condition, and trials were performed at 10% above mean GTV. Goal equivalent manifold (GEM) decomposition generated tangential (δT) and perpendicular (δP) coordinates. Relative variability was calculated as σδT/σδP. Detrended fluctuation analysis (DFA) was used to compute scaling exponents (α) for stride regulation and spatiotemporal complexity. Statistical analyses included repeated-measures ANOVA with partial eta squared (η²p) effect sizes.
**Key Results:** Relative variability increased with load magnitude independent of locomotion pattern (p=0.01, η²p=0.29), and forced-marching had greater relative variability than running (p<0.001, η²p=0.43). Tangential variability (null space) significantly increased with load (p=0.004, η²p=0.46), while perpendicular variability (task space) decreased with load (p=0.003, η²p=0.48). For stride regulation, forced-marching had greater persistence (looser control) in tangential variability than running (p=0.05, η²p=0.19). Perpendicular variability regulation became stricter as load increased (p=0.04, η²p=0.16), and forced-marching had stricter control than running (p=0.02, η²p=0.24). Spatiotemporal complexity remained optimal across conditions (stride length α=0.68–0.92; stride time α=0.79–0.90). Women exhibited greater stride length complexity than men at +55%BW (p=0.009, η²p=0.32) and greater stride time complexity at both loaded conditions (+45%BW: p=0.05, η²p=0.17; +55%BW: p<0.001, η²p=0.41). RPE increased with load (p<0.001, η²p=0.79) and was greater for running than forced-marching (p=0.02, η²p=0.22). No significant effects of time on relative variability were found, but perpendicular variability was greater in the final 30% of trials compared to the first 30% (p<0.001, η²p=0.53).
**Clinical Implications:** The findings suggest that healthy recruit-aged adults can adapt to loads up to +55%BW and forced-marching for short periods (~10 min) in controlled settings, maintaining optimal spatiotemporal complexity and preferentially weighting the task goal of maintaining velocity. However, the greater relative variability (>2.0) observed during loaded forced-marching conditions, particularly at +55%BW, may indicate state-space exploratory behavior that comes at the expense of other cost functions (e.g., mechanical stress, balance). This excessive variability, even when optimally regulated, may portend increased MSI risk in military settings where load carriage tasks persist for hours. The lack of meaningful sex differences suggests similar locomotor control strategies between men and women, though women showed greater spatiotemporal complexity under loaded conditions. The results highlight the need for motor-specific training to improve recruit populations' locomotor function during loaded gait tasks and caution against generalizing laboratory findings to real-world military environments.