Coactivation of the ankle musculature during maximal isokinetic dorsiflexion at different angular velocities.

The objectives of this study were to determine the differences in the level of coactivation among the ankle agonist and antagonist muscles during maximal isokinetic dorsiflexion and to determine whether velocity alters the activation levels. Raw surface electromyograms (EMG) were recorded from six muscle sites--two over the agonist tibialis anterior (upper and lower sites) and four over the antagonist (lateral and medial gastrocnemius, plus lateral and medial soleus) muscles--in ten healthy subjects (two males and eight females) as they performed maximal dorsiflexor efforts against an isokinetic dynamometer at each of three angular velocity settings (30, 90 and 150 degrees/s). The root-mean-square amplitude (RMS) was calculated over a 35 degree angular displacement for each EMG recording, then was normalized (RMS(Nji)) to the amplitude measured during maximal voluntary isometric contractions (MVIC) of the dorsifexors and plantarflexors (i.e. RMS(MVICj)). A two-factor repeated-measures analysis of variance (ANOVA) tested the muscle site by velocity interaction and the main effects of muscle site and velocity separately. The raw EMG signals were then full-wave rectified and low-pass filtered (6 Hz) and ensemble average curves of the sample were calculated, for each muscle site, at each velocity. The ANOVA revealed a statistically significant muscle-by-velocity interaction (P < 0.05). The RMS(Nji) values for the two tibialis anterior sites were not significantly different from each other for any of the three velocity settings, nor for the two gastrocnemius muscles at 30 degrees/s velocity. All other between-muscle comparisons were statistically significant (P < 0.05). The RMS(Nji) means were 28, 23, 59, 52, 98 and 98% MVIC for the lateral and medial gastrocnemius, the lateral and medial sites on the soleus, and for the upper and lower sites on the tibialis anterior, respectively. The RMS(Nji) from the lower site on the tibialis anterior yielded the only statistically significant difference (P < 0.05) among velocities; at this site, the RMS(Nji) at 150 degrees/s was higher than at the other two velocities. The ensemble-average curves revealed that not all muscle sites are activated to a consistent level throughout the entire movement. The interaction between agonist and antagonist activation during isokinetic dorsiflexion has implications for interpreting the results of isokinetic dynamometry for strength assessment and for understanding the neuromuscular control strategies used in this exercise modality.