Background and objectives: Sedentary behavior and physical activity are important determinants of musculoskeletal health, yet their relationship with gait biomechanics, explosive strength, and speed remains understudied in healthy, young populations relative to older and clinical groups. This study primarily aimed to examine the relationship between skeletal muscle mass and explosive strength in a healthy, young population, while secondarily exploring its association with sedentary behavior, physical activity, and gait biomechanical parameters. Methods: A cross-sectional study included 37 healthy young adults. Physical activity and sedentary behavior were assessed using the long-form IPAQ, body composition using bioelectrical impedance analysis, and gait, explosive strength, and speed using a wearable inertial sensor system (countermovement and plyometric jumps, parkour test). Muscle mass–strength associations were also examined with partial correlation adjusted for sex. Results: Skeletal muscle mass percentage was positively associated with countermovement jump height and power and plyometric jump height, but these associations did not remain significant after adjustment for sex. Muscle mass was not associated with total IPAQ score and was only nominally associated with IPAQ activity category, an association that did not survive correction for multiple testing. Physical activity was not associated with any gait parameter, nor did gait asymmetry differ by activity category. Sedentary behavior was not associated with any gait, strength, or speed parameter, though men reported more sitting time than women, a difference that did not remain significant after correction for multiple testing. Conclusions: In healthy young adults, the unadjusted muscle mass–strength association largely reflected shared sex differences rather than an independent relationship, underscoring the importance of accounting for sex in this population. These preliminary findings support combining questionnaire-based, body-composition, and instrumented biomechanical assessment, with statistical adjustment, in future, larger, sex-balanced research.