Efficacy of unilateral strength training for enhancing load carriage performance

Introduction: Load carriage has been shown to have a significant impact on postural control. Falls associated with load carriage are a significant concern for the military. Risk of falling is compounded during operational scenarios combining load carriage with varying and arduous terrain. As the magnitude of load is increased, loss of balance contributes to increased risk of falling. This is especially important for recruits during basic training, when exposed to increasing magnitudes of load. Appropriate training should be implemented to mitigate the challenge placed on the postural control system during load carriage. The demands of training should mimic the demands of the task/occupation to be most effective. Therefore, implementing training that improves strength while simultaneously challenging the postural control system may reduce any deleterious effects of load carriage on postural stability. The purpose of the study was to compare the effects of 4-week unilateral (UL) and bilateral (BL) strength training (ST) programs on postural control when carrying additional load in recruit-aged males.Methods: 15 participants were assigned to UL (n = 7) or BL (n = 8) ST, performing ST 3 times a week for 4 weeks. Participants completed the following pre-test and post-test assessments: one repetition maximum squat in bilateral (1RM-BL) and unilateral (1RM-UL) stance positions, bilateral and unilateral balance tasks with eyes open and eyes closed. Balance tasks were performed in 3 conditions: body weight (BW), BW + 50%, and BW + 70%. Sway variability was quantified by calculating the root mean square (RMS) and sample entropy (SE) of the center of pressure (COP) in the anterior-posterior and medial-lateral directions.Results: The UL group showed significant improvement in 1RM-UL and 1RM-BL, while the BL group only showed significant improvement in 1RM-BL. There was a significant main effect of load on RMS across all tasks with RMS increasing as load increased and on SE in the unilateral eyes open and bilateral stance tasks with SE decreasing with increasing load. However, no effects of ST on COP signal complexity were observed.Conclusion: Increased strength alone is not enough to mitigate the effects of load carriage on postural control, even when training is performed in stance positions that are posturally challenging. This may be a consequence of not undertaking any specific balance training, demonstrating that increasing strength may not necessarily lead to changes in postural control. Longer-term ST or repeated exposure to the postural task could lead to control changes or “refinement” of the strategy, as has been shown for other balance tasks