Title: Changes in Co-Activation and Lower Limb Stiffness during Prolonged Load Carriage

Program: Kinesiology MS

Committee Chair: Tyler Brown

Committee: Tyler Brown, Erin Mannen, Shuqi Zhang

Abstract: Introduction: Overuse musculoskeletal injuries are a common, costly challenge for the armed services. A stiffer lower limb during routine prolonged load carriage tasks may be necessary to attenuate elevated ground reaction forces but further increases musculoskeletal overuse injury risk. Quantifying leg and joint stiffness provides comprehensive and joint level insight into the attenuation of ground reaction forces, and subsequent injury risk. But, it is currently unknown how prolonged load carriage affects leg, knee and ankle stiffness, and knee and ankle muscular activation. Purpose: This study sought to quantify leg, knee, and ankle stiffness, and the activation and coordination of associated joint musculature during prolonged load carriage. Methods: Sixteen participants had leg, knee, and ankle stiffness quantified while they walked (1.3 m/s) 60-minutes with three body borne loads (0, 15, and 30 kg). Surface electromyography quantified average muscle activation, co-activation, and coordination of the vastus lateralis (VL), lateral hamstrings (LH), tibialis anterior (TA), and gastrocnemius (GAS) during the overground walk. Statistical Analysis: Leg, and knee, and ankle joint stiffness, and peak and average VL, LH, TA and GAS muscle activation as well as VL:LH and TA:GA co-activation and coordination during pre-activity (100 ms prior to heel strike) and weight acceptance phases were submitted to analysis. Each dependent variable was submitted to a linear mixed model to test the main effects and interactions between body-borne load (0, 15, and 30 kg) and time (0, 15, 30, 45, and 60
minutes). Results: Body borne load increased leg, knee, and ankle stiffness (all: p<0.001), while time did not impact any lower limb stiffness measure (p>0.05). Body borne load decreased average VL and LH amplitude (p<0.001; p=0.006), and VL:LH co-
activation (p<0.001) and coordination (p=0.011) during pre-activity as well as average VL amplitude (p=0.002), VL:LH co-activation (p<0.001), and VL-only coordination during weight acceptance. Time decreased average VL and LH amplitude (both: p=0.006) and VL:LH co-activation (p=0.002) during pre-activity, and average GAS activity (p=0.021), VL:LH co-activation and coordination index (p=0.004; p=0.008) as well as VL-only coordination (p=0.037) during weight acceptance. Conclusion: Prolonged walking with a heavy body-borne load increased lower limb biomechanics linked to musculoskeletal overuse injuries. Specifically, the stiffer lower limb exhibited with the addition of body borne load may limit the ability of the lower limb musculature to attenuate the larger, faster vertical GRFs, and increase injury risk. While the concurrent decrease in knee and ankle muscle activation may be a neuromuscular strategy utilized by individuals to improve efficiency of prolonged walking with load. In particular, the decreased in VL:LH muscle activity, co-activation, and coordination throughout the load carriage task may be a shift of mechanical energy management strategies used by the participants to minimize the cost of prolonged walking.