Impact of Redistributing Torso-borne Loads on Soldier Muscular Cost

Abstract

Previous studies have identified significant reductions in lumbar strength measures following prolonged torso-borne load carriage. A proposed solution to mitigate the negative impact of bearing mass on the shoulders and upper back is to utilize a device designed to redistribute the load to the pelvic region. PURPOSE: To assess the impact of redistributing torso-borne military loads from the shoulders to the pelvic region with a load distribution device (LDD) on lumbar muscle strength of the load carrier after a prolonged march. METHODS: Fourteen Soldiers conducted 2-h prolonged marches at 1.34 m·s-1 under two conditions: a baseline condition consisting of a 29.7-kg fighting load with no redistribution and a condition utilizing the LDD to alter the distribution of the fighting load to a high level (70-90%) of shoulder offloading. Flexible pressure sensors were placed on the shoulders to establish and verify the level of offloading relative to the baseline condition. Isokinetic lumbar muscle flexion and extension data were collected immediately before and after the marches for 5 maximal muscle contractions at 20°·s-1. Total work (N·m) and average power (W) were calculated over the 5 repetitions and the delta (pre- minus post-march) was determined. A one-way, repeated measures ANOVA (p < 0.05) was performed to assess the effects of redistributing the torso-borne load on lumbar total work delta and average power delta. RESULTS: Lumbar pre- minus post-march muscle strength scores revealed significant differences in isokinetic lumbar flexion, reflecting smaller decreases in both total work performed (2.3 N·m, p=0.012) and average power generated (0.274 W, p=0.046) after the prolonged march for the high offloading condition versus the no offloading condition. No statistical differences were found for lumbar extension. CONCLUSIONS: Redistributing load from the shoulders to the pelvic region with an LDD reduces the muscular cost of the lumbar flexor muscles associated with prolonged load carriage. Preserving this localized strength may improve Soldier mission effectiveness following long marches. Supported by NSRDEC.