Voluntary activation of the ankle plantar flexors: A systematic review and meta-analysis

Voluntary activation is a measure of neural drive, typically measured during maximal contractions, and provides insight into motor function. This systematic review examined voluntary activation assessment of the ankle plantar flexors in healthy and pathological populations, and the association of participant age and positioning (knee and ankle joint angles) with voluntary activation level.

A systematic review and meta-analyses were conducted. Six electronic databases were systematically searched for studies that assessed voluntary activation of the ankle plantar flexors using the interpolated twitch technique or the central activation ratio. Meta-analyses were performed using an inverse variance, random-effects maximal likelihood model of continuous outcomes within SPSS Statistics, and subsequent meta-regression performed for age, knee angle and ankle angle. Methodological quality was assessed using the Analytical Cross-Sectional Studies Checklist from the Joanna Briggs Institute.

A total of 74 studies were included, 5 included participants with pathological conditions (2 Achilles tendinopathy and 3 stroke) and 69 included only healthy participants. The meta-analysis demonstrated plantar flexion voluntary activation levels for healthy populations of 91 %, 90 % for Achilles tendinopathy and 35 % for stroke. Older age was associated with lower voluntary activation (β ​= ​-0.072; p ​= ​0.035) and greater knee flexion angle was associated with higher voluntary activation (β ​= ​-0.033; p ​= ​0.045). No association of voluntary activation and ankle position was demonstrated (β ​= ​-0.070, p ​= ​0.488). Majority of the included studies were judged to have low methodological quality (97 %).

Voluntary activation was comparable between healthy participants and Achilles tendinopathy, and significantly lower in people following stroke. Age and knee joint position but not ankle joint position was associated with the level of plantar flexor voluntary activation.