The purpose of this study was to investigate the effect of teeth clenching on isokinetic knee extension force during repeated voluntary contractions. We assessed isokinetic muscle strength in association with teeth clenching during 100 consecutive knee extensions at 60 degrees per second (deg/s) using a Cybex 6000 isokinetic dynamometer. In this study, 8 healthy male volunteers (28.4 ± 3.89 years) were asked to perform isokinetic contractions in an extended cycle of five contractions without teeth clenching followed by five contractions with the teeth clenching. The peak torque per body weight was statistically analyzed. In our results, the peak torque per body weight with teeth clenching were significantly greater than those without teeth clenching in the first 70 cycles; however, no significant differences were shown at 71-100 cycles. There was a significant negative correlation between the number of knee extensions and the difference in peak torque per body weight derived from with and without teeth clenching (r=-0.475, p<0.0001) . Our findings reveal that the effect of teeth clenching on the isokinetic muscle strength of knee extension is dependent on muscular fatigue, and that the improving effect of teeth clenching on isokinetic muscle strength declines with increasing muscular fatigue.

Previous research has demonstrated that the concentric muscle strength of knee extension exerted at slow to medium angular velocities is increased by teeth clenching. This study evaluated the isokinetic eccentric strength of knee extensors at 4 angular velocities (60, 120, 180 and 240 degrees per second) with and without teeth clenching on the Cybex 6000 Extremity Testing and Rehabilitation System. Twelve healthy adult male (26.8±1.8 years) volunteered for the study. Peak torque per body weight and average power per body weight were analyzed. The results demonstrated that peak torque per body weight accompanied by teeth clenching was significantly increased at 60, 120, 180 and 240 deg/s by 9.5%, 8.2%, 8.2% and 9.8%, respectively (p<0.01) . Similarly average power per body weight during teeth clenching was significantly higher at 60, 120, 180 and 240 deg/s by 9.1%, 9.5%, 9.4% and 7.7%, respectively (p<0.05) . These findings suggest that teeth clenching is a factor that leads to augmented eccentric strength of knee extensors. Its effect appears to be independent of angular velocity.