Stroke is a leading cause of disability in United States and each year over 795,000 Americans experience new or recurrent strokes. Stroke severely degrades lower extremity muscle function and results in slower walking speeds and deteriorated quality of life. Following stroke, a variety of gait rehabilitation modalities are available to improve community mobility but produce mixed results. Several studies have identified pre-swing as a critical region of the gait cycle and have associated deficits in plantar flexor contribution to walking subtasks with slower walking speeds. Body Weight Supported Treadmill Training (BWSTT) is a training program that consists of walking on a treadmill while a portion of body weight is supported by a harness. The Active Leg Exoskeleton (ALEX) is a 6 DOF orthosis using a force-field controller with an assist as needed paradigm designed for gait retraining of stroke victims. To compare the effects of BWSTT and ALEX training, a previous intervention study with 12 stroke survivors revealed that only the ALEX group exhibited significant changes in over ground walking speed and peak knee flexion angle. The objective of this study was to use musculoskeletal modeling to investigate the changes in muscle coordination of patients that underwent BWSTT and ALEX training. Three dimensional musculoskeletal simulations for one full gait cycle were built for each patient before and after training. Analyses were performed on lower extremity muscles during paretic pre-swing to calculate average muscle force, time integral of individual muscles to support and accelerate the center of mass forward as well as individual muscle contributions to knee angle acceleration. Moreover muscle potential analyses were performed to quantify the role of posture, independent of muscle force, to accelerate the center of mass. ALEX trained subjects exhibited an increase in plantar flexor force, soleus and iliopsoas propulsion and less vastus braking of the center of mass. Results also indicated that plantar flexors and vastus contributed less to knee extension. BWSTT subjects exhibited a decrease in vastus breaking but plantar flexor propulsion showed close to zero change. Vastus also contributed less to knee extension similar to ALEX, but the net soleus contribution to extension increased post training.

Increases in speed and peak knee flexion angles were only found to be significant for the ALEX group. Our results indicate decreased vastus braking and increased paretic propulsion was the main reason for this change following ALEX training. The BWSTT group also exhibited decreased braking but paretic plantar flexors did not contribute more to propulsion. To explain the increase in peak knee flexion after ALEX training, the plantar flexors and vastus showed net decreased contribution to knee extension, but the net contribution to knee extension increased for soleus after BWSTT training.

Despite the heterogeneity among stroke survivors, ALEX appears to induce important changes in pre-swing muscle function related to observed increases in speed and knee flexion.