Effects of back muscle fatigue and modified trunk inertia on lumbar kinematics, kinetics and muscle activity during walking.

Back endurance is a strong predictor of back pain, but the mechanisms underlying this relationship are not clear. Fatigue reduces muscles' force-generating capacity, so greater fatigability may increase lumbar motion and loading and trunk muscle co-contraction. Using a novel pack to modify inertia, we tested the effect of back fatigue and increased trunk inertia on lumbar kinematics, kinetics and muscle activity during walking. Lumbar kinematic and kinetic amplitudes and maximum muscle activity were measured per stride across four conditions: pre- and post-fatigue, with and without increased trunk inertia. The pack caused increases in maximum lumbar erector spinae (ES) activity by 3.19 times the average value calculated during the pre-fatigue no-pack trial (P<0.001), amplitude of lumbar flexion-extension moment by 0.0189 N m (kg g m)-1 (P<0.001), lumbar lateral bending moment by 0.0028 N m (kg g m)-1 (P=0.019) and lumbar axial rotation moment by 0.0203 N m (kg g m)-1 (P<0.001), and decreases in the amplitude of roll angle by 1.31 deg and yaw angle by 6.65 deg (both P<0.001). Back endurance is positively associated with change in maximum lumbar ES activity (r=0.69, P=0.013) and negatively associated with change in maximum rectus abdominus (RA) activity (r=-0.72, P=0.008) and lumbar flexion-extension moment amplitude (r=-0.62, P=0.031). Overall, individuals with less back endurance had increased maximum RA activity and sagittal kinetics post-fatigue whereas individuals with higher back endurance showed the opposite response. Increased RA activity with less back endurance may be a protective mechanism for stabilizing the trunk in response to increased sagittal lumbar loading due to fatigue.