The force-velocity relation of muscle plays an important role in determining the performance of human movements. Studies on single-joint movements such as elbow flexion have shown that the relation between joint torque and angular velocity is described well with a classical, hyperbolic function (Hill equation). However, it remains unclear whether multi-joint movements, which are more closely related to our daily activities, are also characterized in a similar manner. The present study aimed: (1) to develop a dynamometer for human knee-hip extension movement, which can control precisely force and displacement, and (2) to determine the force-velocity relation of knee-hip extension movement in an isotonic condition.
Forty-one subjects (age 32.0 ± 15.1; height 160.8 ± 8.0; mass 54.1 ± 8.0, means ± S.D.) participated in the study. Written informed consent was obtained from all of the participants. This study was approved by the Ethical Committee for Human Experiments, University of Tokyo. The dynamometer consisted of a vertically placed tri-axial force plate, a servomotor and a computer-assisted control unit. Seated subjects kicked either bilaterally or unilaterally the force plate, the horizontal position of which was servo-controlled so that the measured force is matched with a force command at a time resolution of 2 ms (force clamp). The force command was made from the relation between maximal isometric force and foot position within the range between 70 and 90 % of Êleg lengthË (longitudinal distance between sole of foot and hip joint), so that the same force relative to isometric force was consistently applied regardless of foot position (isotonic in terms of relative force). Electromyographic (EMG) recordings were made by using surface electrodes from seven major muscles: vastus medialis, vastus lateralis, rectus femoris, biceps femoris, semitendinosus, gluteus maximum, and gastrocnemius. EMG data were analysed using a one-way ANOVA with the Tukey post hoc test.
The force-velocity relation obtained was described by a linear function (r2 = 0.996) more appropriately than a hyperbola within a range of force between ~0.1 to ~0.8 F0 (F0, maximal isometric force). The maximal force extrapolated from the linear regressions (Fext) coincided with F0 (F0/Fext = 0.982 ± 0.132). Also, the velocity at zero force (Vext) was obtained from the extrapolation. Compared to the bilateral movements, unilateral movements gave rise to the smaller Fext but the same Vext, suggesting that Vext is independent of force and therefore represents the proper unloaded velocity. The mean integrated electromyogram (mEMG) in each muscle was unchanged with force in the isotonic condition, whereas it was significantly smaller at F0 than in the isotonic condition (P < 0.05) in knee extensor muscles. This suggests that some inhibitory mechanisms operate in knee extensor muscles when they generate large force (Westing et al. 1991). Such an inhibition may have an effect of moving the force-velocity relation downward as the force is large, thereby making the relation linear.