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Abstract
The purpose of this study was to develop and validate a deterministic model for calculation of ankle and knee moments from EMG and kinematic data. The full model developed used: the instantaneous amplitude of the EMG after being rectified and processed using a second order low pass digital filter at an optimal cut-off frequency; the joint angle as a correlate of muscle length differences, and the angular velocity of the joint in question, as a correlate of velocity of muscle contraction. These were all derived from walking trials. The muscles selected as representative of their respective groups were: tibialis anterior for ankle dorsiflexors, soleus for ankle plantarflexors, vastus lateralis and rectus femoris as knee extensors, and medial hamstrings and medial gastrocnemius as knee flexors. Walking trials included simultaneous collection of 16 mm cine film, force plate and EMG data. The film and force plate data were processed and analyzed to yield instantaneous angles, angular velocities and moments. The EMG was processed using the optimal cut-off frequency obtained from calibration trials. Calibration trials were performed in the following manner. Each subject performed cycled isometric contractions simulating the EMG characteristics of his gait while his limb was firmly fixed in force-recording apparatus. Linear regression of the moment on processed EMG yielded constants for the basic moment-EMG relationship. Inserting these constant values into the model two more constants were simultaneously iterated. The first constant increased the predicted moment if the muscle was longer than it had been during calibration contractions, and decreased it if it was shorter. The second constant reduced the predicted moment if the velocity of contraction was positive and increased it if the muscle was lengthening. The predicted moments were compared with those obtained using standard link segment mechanics.
The plots of moments predicted using the full model and those calculated from link segment mechanics followed each other quite closely. The range of mean R.M.S. errors were: for the ankle, optimized on the stance phase, 3.2-9.5 N.m; for the knee, optimized on stance, 4.7-13.0 N.m; and for the knee, optimized on the full gait cycle, 8.7-15.9 N.m.
