People are able to fully activate their quadriceps at mid-length during a brief isometric contraction but it is uncertain whether this is the case at other muscle lengths. With the twitch superimposition technique for determining levels of voluntary activation (VA), the muscle may be stimulated through the intramuscular branches of the nerve or via the nerve trunk itself. The former technique is easier to use, but different populations of motor units may be stimulated if the joint position is changed to alter muscle length. The purpose of this study was to investigate quadriceps VA at a range of knee joint angles using both magnetic stimulation of the motor nerve and percutaneous electrical stimulation over the muscle belly. Eight healthy subjects (six females, mean age 29 years) performed maximal voluntary contractions of the quadriceps at knee joint angles at 10-110 degrees of flexion. Surface electromyography (EMG) of quadriceps and hamstrings was recorded as an indication of the amount of muscle activity. Nearly all subjects showed >95% VA at all joint angles. VA did not vary with joint angle nor were there significant differences between the two stimulation techniques. Similarly, there was no significant effect of knee joint angle on the EMG activity of either muscle group. These findings indicate that VA of the quadriceps during a brief isometric contraction is not affected by muscle length and can be measured by either stimulation technique.

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