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The in vivo force-velocity relationship of the knee flexors and extensors

Neuromuscular Research Laboratory, Division of Orthopaedics, Department of Surgery, College of Medicine, University of California, lrvine, California

Vincent J. Caiozzo, MS

Neuromuscular Research Laboratory, Division of Orthopaedics, Department of Surgery, College of Medicine, University of California, lrvine, California

Previous studies have compared the torque production of the knee flexors and extensors and developed a "hamstring-quadricep" ratio. Collectively, these studies indicate that the ratio of strength between the knee flexors and extensors is a function of test speed, with the ratio increasing at the faster test speeds. Unfortu nately, these studies have employed the peak torque isokinetic method for making measurements. This tech nique suffers from a number of problems that are biomechanical and physiological in nature. Conse quently, in this study, we employed comparisons of the in vivo force-velocity relationship using an angle-specific method and a gravitational correction factor to more accurately determine flexor-extensor ratios and to bet ter describe any variations occurring as a function of test speed. Nine male subjects volunteered to partici pate in this study. Each of the subjects participated in two test sessions. Subjects were tested at six angular velocities of 0, 48, 96, 144, 192, 240 deg/sec. At these respective test speeds, the mean flexor torques were 95.0, 94.8, 84.1, 66.2, 56.2, and 49.2 Nm, respectively. The corresponding values for the knee extensors were 252.2, 246.5, 220.7, 184.5, 161.9, and 146.4 Nm. Normalization of these torque-velocity relationships demonstrated that the in vivo force-velocity relation ships of the two muscle groups are similar at slow speeds, but become disparate at higher velocities. When examining flexor-extensor torque ratios at the various test speeds, it was found that there were significant differences (P < 0.01). The mean values for the flexors expressed as a percent of knee extensor torque were 37.5%, 38.2%, 38.2%, 35.9%, 34.5%, and 33.4% at speeds of 0, 48, 96, 144, 192, and 240 deg/ sec, respectively.