Measurement and reproducibility of strength and voluntary activation of lower-limb muscles.

Accurate measurement of muscle strength and voluntary muscle activation is important in the assessment of disorders that affect the motor pathways or muscle. We designed a multipurpose system to assess the variability and reproducibility of isometric torque measurements obtained during maximal voluntary efforts of the knee flexor, knee extensor, ankle dorsiflexor, and ankle plantarflexor muscles on each side. It used two isometric myographs mounted on an adjustable frame. Measurements of maximal voluntary torque (range, 25-188 Nm) displayed low variability within a testing session and over five testing sessions (coefficient of variation range, 5-11%). We used the same equipment to measure voluntary activation of the triceps surae muscles. Voluntary activation, measured with a sensitive twitch interpolation method, increased with increasing voluntary contraction torque (P < 0.001) and was very high during maximal efforts (mean, 97.8 +/- 2.1%; median, 98.5%). Furthermore, measurements of voluntary activation during maximal efforts were reproducible across testing sessions with very little variability (coefficient of variation, <2%). The myograph system and the testing procedures should allow accurate measurement of strength and voluntary drive in longitudinal patient studies.