Role of the tendon in the dynamic performance of three different load-moving muscles

The effect of the tendon's viscoelastic properties on the dynamic performance of three different load-moving muscles was determined. The frequency response models of the cat's medial gastrocnemius (MG), extensor digitorum longus (EDL), and tibialis anterior (TA) with and without their tendons were derived under sinusoidal shortening-lengthening, manipulated by orderly recruitment and derecruitment of motor units together with firing rate increase and decrease. The passive load sizes applied to the muscles were approximately 30%-40% of each muscle's maximal isometric force. It was shown that the tendon has a moderate effect on the dynamic response of muscles while moving loads of fixed mass. The MG and EDL without their tendons show a decrease in high frequency gain (2-5 dB) and increasing phase lag angles (7 degrees-9 degrees). In contrast, the TA without its tendon shows an increase in high frequency gain (2 dB) and decreasing phase lag angles (20 degrees) compared with the same muscle with the tendon. It was concluded that tendon's viscoelastic properties have a moderate effect during load-moving contractions, influencing the dynamic performance of different muscles in a different manner.