Differences in Eccentric Force-velocity Characteristics between Isotonic and Isokinetic Contractions

Abstract

The relationship between torque and velocity of the elbow flexor muscles was studied with isotonic and isokinetic dynamometers in the same subjects (males; n=26; aged 18-31 years). Simultaneously, the electromyographic activities (EMG) of the biceps brachii (BB), brachioradialis (BRD) and, triceps brachii (TB) muscles were recorded during concentric, eccentric, and isometric contractions. In the isotonic condition, plotted data covered almost entire force region, i.e. 0-1.0 P₀ where P₀ is the maximal isometric torque, and were well described by Hill equation. The maximum angular velocity of elbow flexion (V_max), the maximum isometric torque, and the maximal eccentric torque were 943.78±312.48°s^－1, 59.07±11.65 Nm, and 1.40±0.18 P₀, respectively. In the isokinetic condition, the measureable ranges of torque and angular velocity were limited between 0.5 and 1.0 P₀, and 0 and 0.35 V_max, respectively. In concentric condition, the shape of torque-angular velocity curve obtained from isokinetic measurements was well coincided with that obtained from isotonic measurements. On the other hand, the maximal eccentric torque obtained from isokinetic measurements was 1.12±0.17 P₀, which was considerably smaller than that obtained from isotonic measurements. Electrtomyographic (EMG) measurements with surface electrodes showed that the activation level of the elbow flexor muscles during eccentric contraction was significantly lower in the isokinetic condition than in the isotonic condition. These results suggest that muscle-fiber recruitment during eccentric contraction is suppressed in isokinetic condition when compared with isotonic condition. One of the possible reasons for this may be the difference between proprioceptive input to the feedback system.