Effect of load carriage on lumbar spine kinematics.

STUDY DESIGN Feasibility study on the acquisition of lumbar spine kinematic data from upright magnetic resonance images obtained under heavy load carrying conditions. OBJECTIVE To characterize the effect of the load on spinal kinematics of active Marines under typical load carrying conditions from a macroscopic and lumbar-level approach in active-duty US Marines. SUMMARY OF BACKGROUND DATA Military personnel carry heavy loads of up to 68 kg depending on duty position and nature of the mission or training; these loads are in excess of the recommended assault loads. Performance and injury associated with load carriage have been studied; however, knowledge of lumbar spine kinematic changes is still not incorporated into training. These data would provide guidance for setting load and duration limits and a tool to investigate the potential contribution of heavy load carrying on lumbar spine pathologies. METHODS Sagittal T2 magnetic resonance images of the lumbar spine were acquired on a 0.6-T upright magnetic resonance imaging scanner for 10 active-duty Marines. Each Marine was scanned without load (UN1), immediately after donning load (LO2), after 45 minutes of standing (LO3) and walking (LO4) with load, and after 45 minutes of side-lying recovery (UN5). Custom-made software was used to measure whole spine angles, intervertebral angles, and regional disc heights (L1-S1). Repeated measurements analysis of variance and post hoc Sidak tests were used to identify significant differences between tasks (α = 0.05). RESULTS The position of the spine was significantly (P < 0.0001) more horizontal relative to the external reference frame and lordosis was reduced during all tasks with load. Superior levels became more lordotic, whereas inferior levels became more kyphotic. Heavy load induced lumbar spine flexion and only anterior disc and posterior intervertebral disc height changes were observed. All kinematic variables returned to baseline levels after 45 minutes of side-lying recovery. CONCLUSION Superior and inferior lumbar levels showed different kinematic behaviors under heavy load carrying conditions. These findings suggest a postural, lumbar flexion strategy aimed at centralizing a heavy posterior load over the base of support.