Muscle Contributions to Trunk Forward Progression and Support in Walking

Felix E. Zajac, PhD; Richard R. Neptune, PhD; Steven A Kautz, PhD

Objectives: Our long-term goal is to enhance walking performance in individuals with hemiparesis post-stroke. However, uncertainty exists with how best to target rehabilitation treatment because the dynamics of walking is extremely complex (multijoint; multimuscle control). A scientific framework is needed to better understand the relative importance of neural control and musculoskeletal factors affecting walking performance. Our short-term goal was to develop the framework methodology and apply it to understand the role of individual leg muscles to trunk forward progression and support in normal walking.

Methods: The framework is based on analyzing computer simulations of walking dynamics that emulate gait lab measurements. Muscle coordination of healthy subjects (five young healthy males walking at their self-selected pace, 1.5 m/s) has been studied to date. Because the energetics of trunk progression is dominated by sagittal plane motion in these subjects, forward dynamic computer simulations were generated from a sagittal plane musculoskeletal model having a trunk and two legs. Each leg was modeled by a thigh, patella, shank, foot, and 15 muscles. Foot/ground contact was modeled by 30 viscoelastic elements. An optimization algorithm determined muscle excitations with onset and offset timing compatible with recorded EMGs that minimized to within # ±2SD the average difference between the simulated kinetic and kinematic trajectories and the trajectories obtained from gait lab measurements (e.g., ground reaction forces, net joint muscle moments and powers computed from inverse dynamics, leg segmental motion). A muscle's contribution to the segment powers was determined at each instant from knowledge of the current kinematics and the accelerations of the body segments induced by that muscle, including its contribution to the ground reaction force and center-of-pressure.

Results: The biarticular (gastrocnemii, GAS) and uniarticular (e.g., soleus, SOL) plantarflexors provide support throughout single-leg stance and pre-swing. However, their contributions to leg and trunk energetics are often in opposition. In late stance, while both are concentric and also releasing elastic energy stored from their eccentric activity during early stance , GAS accelerates the leg and SOL the trunk. In mid stance, while isometric, their different and about equal contributions to leg and trunk energetics are synergistic so the body can be supported and its forward motion maintained. The quadriceps (vasti group particularly) in the beginning of stance act as accelerators of the trunk by redistributing much of the leg energy caused by their braking (deceleration) of the leg, irrespective of the muscle state (eccentric, isometric, concentric). Hip extensors also accelerate the trunk then.

Conclusions: Force generation by the ankle plantarflexors, quadriceps, and uniarticular hip extensors are critical to trunk progression and support. Walking requires some muscles to produce energy through concentric activity (e.g., hamstrings; SOL/GAS in late stance) and others to redistribute that energy to specific segments through their production of force while isometric, concentric or eccentric (e.g., vasti in early stance). This framework will be applied to study the mechanisms by which impaired motor control after neurological injury affects walking performance.

Funding Acknowledgments: VA Rehabilitation Research and Development Service (project # B2252F), the VA Palo Alto Rehabilitation R&D Center, and NIH grant NS17662.