The relationship between isotonic jaw-opening and jaw-closing muscle function was studied using a newly developed apparatus which enables load and velocity to be detected simultaneously. The following results were obtained from 17 male adults (age range 22-32 years) without any occlusal dysfunction. (i) The force-velocity relationship in jaw-opening and jaw-closing muscles was represented by a hyperbolic curve, which fitted well with Hill's equation. (ii) The theoretical maximum force obtained by extrapolation from regression was 32.55 +/- 4.98 kg for jaw opening and 35.74 +/- 4.52 kg for jaw closing. (iii) The theoretical maximum velocity obtained by extrapolation from regression was 456.70 +/- 183.27 mm s-1 for jaw opening and 372.77 +/- 141.67 mm s-1 for jaw closing. (iv) The maximum mechanical power (Pmax) calculated from the product of the force and velocity was 772.20 +/- 182.65 kg.mm s-1 for jaw opening and 708.68 +/- 128.14 kg.mm s-1 for jaw closing. (v) The Pmax exerted by individual subjects was approximately 12-34% of the maximum possible force (Fmax) calculated from the force and velocity, in both jaw opening and jaw closing. There were no statistically significant differences between jaw opening and jaw closing with regard to any isotonic muscle functions. In other words, the results of this study strongly indicated a substantial balance between isotonic jaw-opening and jaw-closing muscle function in the subjects who were investigated.