Functional Electrical Stimulation of the Soleus Redistributes Lower-Limb Joint Work Distally in Young and Older Adults
Older adults walk with reduced ankle and greater hip mechanical output compared to young adults. This "distal-to-proximal redistribution" likely contributes to the greater metabolic energy expenditure during walking in older versus young adults. Due to the inverse relationship between ankle and hip use, functional electrical stimulation (FES) of the ankle extensors may increase ankle mechanical work and indirectly decrease hip mechanical work. Although FES increases stimulated muscle metabolism, bilateral soleus stimulation may restore more youthful walking kinetics without a detectable change in whole-body metabolism because ankle extension requires less metabolic energy than hip extension. Ten young adults and 10 older adults walked on a treadmill at 1.25 m/s with and without FES bilaterally applied over the respective legs soleus when the anterior-posterior ground reaction force exceeded 10% body weight. FES use altered walking mechanics and metabolic power similarly across age groups (all FES condition and age-group interactions p[≥]0.214). Across age groups, FES increased ankle mechanical power (p=0.041) and redistributed mechanical work production to occur relatively more at the ankle and less at the hip (p=0.010). The lower-limb joint redistribution ratio of older adults walking with FES was not different to that of young adults during baseline (p=0.785). Moreover, walking with FES increased metabolic power by 2% (p=0.037). FES attenuated older adult distal-to-proximal redistribution and modestly increased whole-body metabolic rate. FES applied to soleus muscles during the late stance of walking affects users similarly across the lifespan, indicating that FES interventions ought to consider a persons functional needs, regardless of age.