1998 Volume 2 Issue 1 Pages 19-27
The purpose of this study is to investigate the behavior of the triceps surae muscle-tendon complex during repetitive ankle extension-flexion bending exercises (with knee and hip joints fixed) at various frequencies. During the exercises, ground reaction forces, ankle angles, and electromyograms of gastrocnemius, soleus and tibialis anterior were measured. Using inverse dynamics, triceps surae forces were calculated, and these were used to calculate the series elastic component (SEC) elongation ΔLSEC. In combination with ankle angle changes (from which total length changes of the muscle-tendon complex (ΔLOI) was calculated), this allowed for calculation of the contractile component (ΔLCC). At low frequencies, ΔLOI and ΔLCC were calculated to be in phase, and EMG was high when the force was high. At high frequencies, however, ΔLOI and ΔLCC were calculated to be in the opposite phase. The CC was shortening in the dorsiflexion phase against increasing tension. This suggests the muscles were very active in this phase as was confirmed by the high EMG activities. On the contrary, in the plantarflexion phase the total length of the muscle-tendon complex of the triceps surae became shorter, but the CC was elongating with the tension decreasing. The EMG activities disappeared in this phase. At intermediate frequencies, the CC acted almost isometrically, although the total length changed about the same as at the other two frequencies. Using a model of an externally forced mass-spring system, the phase reversal between CC and SEC behavior is explained: the low frequencies are below the natural frequency of the subject, and the high frequencies are above it. This assumption suggests that a strong relationship must exist between the natural frequency (ω0) and the frequency at which large phase shift occurs between the joint movements and EMG activities (ω0′). Ankle bending exercises were done with two legs and with one leg. For both exercises ω0 and ω0′ were calculated and no significant differences were observed between the two values, which support the assumption.
