Effect of Stroke and Aging on Plantarflexor Strength and Voluntary Activation

Plantarflexor strength and voluntary activation are key metrics for characterizing lower extremity function in stroke survivors. However, the extent to which stroke and/or aging affects these neuromuscular properties remains unclear. This study addressed this gap by testing plantarflexor strength and voluntary activation in fifty-two (stroke: 19, older: 15, young: 18) participants. Testing was done bilaterally in stroke survivors and on the dominant leg of the control participants using the central activation ratio (CAR) and interpolated twitch technique (ITT) with triplets. Stroke survivors demonstrated significantly reduced raw and mass-normalized plantarflexor strength on both legs compared with controls, and on their more affected leg compared with the less affected leg (all p's ≤ 0.02). Regardless of technique, voluntary activation was significantly lower only in the more affected leg compared with the less affected leg and control leg (all p's ≤ 0.02). Older adults also demonstrated lower plantarflexor strength (p ≤ 0.01), but not voluntary activation, compared with young adults. These findings indicate that both stroke and aging affect plantarflexor strength; however, voluntary activation is only affected by stroke. Additionally, quantification technique influenced voluntary activation estimates, with CAR consistently demonstrating higher activation relative to ITT. Collectively, these findings highlight the need for targeting plantarflexor strength and voluntary activation during post-stroke rehabilitation.