Pattern analysis of electromyographic linear envelopes exhibited by subjects with uninjured and injured knees during free and fast speed walking.

We wished to determine whether anterior cruciate ligament (ACL) injury caused changes in patterns of activity of individual or groups of muscles that control the knee joint. The electromyographic (EMG) patterns of six muscles in 26 individuals with uninjured knees and 20 individuals with ruptured ACL were studied during free and fast speed walking. A previously developed clustering technique was used for analysis. This technique involves making the Fourier transform of the average linear envelope (LE) of each muscle for each subject and using the amplitude and phase angles of the lower frequency harmonics as features to describe a pattern. These features are then grouped to subdivide the population of EMG patterns into different types. The results of analyses on single muscles showed that there exists a typical or "normal" pattern for each muscle which most uninjured and some injured subjects exhibit as well as several atypical patterns which mostly injured subjects exhibit, and that the atypical patterns are much more evident at fast walking speed. The characteristics of atypical patterns with respect to normal include time shifts in the peak of major phases of activity, the absence of a second phase of activity, and the existence of additional phases of activity. Synergy analysis showed that if one muscle has an atypical pattern, then several do; i.e., ACL injury induces major changes in the control strategy of the knee. The implications are that for rehabilitation programs one must focus on the training and strengthening of more than one muscle, and that for reconstructive procedures the changes in mechanics of the joint can possibly induce a significant change in its control strategy.