The Effects of Variation in Physical Activity and Spinal Loading on Trunk Muscle Endurance and Lumbar Spine Function

Abstract

The physical activity transition as people shift from rural agricultural to urban industrial lifestyles typically involves reduced levels of physical activity. In turn, reduced physical activity likely has consequences for musculoskeletal function, particularly with regards to trunk muscle endurance, spinal loading, and biomechanical responses to fatigue and perturbations during walking. This dissertation combines fieldwork and laboratory experiments to test how variation in physical activity affects trunk muscle endurance, spinal loading, and lumbar spine biomechanics during walking. Chapter 1 investigates intra-individual associations and group and sex differences in trunk muscle endurance, strength, and flexibility among physically active subsistence farmers and sedentary urban individuals in western Kenya. I show that high levels of physical activity among subsistence farmers, particularly head carrying among women, is associated with higher trunk muscle endurance and strength. Chapter 2 tests the accuracy of wearable sensor-based estimates of trunk kinematics and kinetics during walking and running against the gold standard optical motion capture approach. I show that signals from both methods are similar in magnitude and trend, that forces in the trunk are higher during running than walking, and that wearable sensors tend to overestimate joint moments and underestimate joint reaction forces. Chapter 3 applies this wearable sensor approach to a natural experiment in Rwanda. Specifically, I investigate to what extent typical occupational activities of rural subsistence farmers demand higher magnitudes and increased variability of back muscle activity and spinal loading compared to occupational activities of urban office workers, and whether these differences are associated with trunk muscle endurance. I show that during occupational work, subsistence farmers activate their back muscles and load their spines to higher magnitudes and with greater variability than office workers, and that this is positively associated with back muscle endurance. Finally, Chapter 4 uses a back fatigue protocol combined with a novel backpack-based perturbation of the trunk to test the effect of back muscle endurance on lumbar kinematics, kinetics, and trunk muscle activity during walking. Within participants, the trunk inertia-increasing perturbation causes back muscle activity and trunk yaw stiffness to increase, and trunk roll angle, trunk yaw angle, the amplitude of lumbar shear forces, and trunk pitch and yaw damping to decrease relative to unperturbed walking. I also show that back muscle endurance is positively associated with maximum back muscle activity, negatively associated with maximum abdominal muscle activity, and positively associated with amplitudes of lumbar compression, shear force, and flexion-extension moments post-fatigue. This suggests less endurance is associated with reduced kinetic amplitudes and increased abdominal muscle activity due to fatigue, responses which may augment lumbar spine stability. Altogether, these studies provide a novel evolutionary perspective on the relationships between physical activity, spinal loading, trunk muscle endurance, and lumbar spine function.