Relation between EMG & Joint Moment

from Univ. of Vienna Dept. of Physical Medicine & Rehabilitation.

Technical Details

Normal male subject (33 yo) standing with weight over the ball of right foot on a narrow wooden support (stool leg). Ankle angle measured by Penny & Giles strain-gauge goniometer and medial gastrocnemius electromyography monitored by Myotrac T-4000 biofeedback amplifier (showing the rectified filtered EMG). Calibration rule in 100 mm steps (yellow dots).


Since the foot is pivoted on a narrow based support, and no weight is taken by the left foot nor upper-limbs, the force passing through the ball of the foot must equal body weight. The moment arm of this force would appear to get shorter as the subject rises higher, so the moment exerted by the gastrocnemius to balance it should be lower. However, the EMG recorded from the muscle gets larger!

  • How can you explain this paradox?

  • What are its implications for clinical gait analysis?

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    Supplied by Dr. Josef Kollmitzer
    Last modified on 2-Feb-97.