In this study, subjects performed wrist flexion in isokinetic manner by using CYBEX dynamometer while their right forearm was attached in a MR magnet. ³¹P-MR spectra were obtained from wrist flexor muscles before and throughout the exerclse. Intracellular pH of muscle was calculated from the chemical shift between phosphocreatine (PCr) and inorganic phosphate (Pi). Fifteen Japanese males volunteered as subjects. They performed two experi-mental protocols, i. e. constant load test (CT) and stepwise incremental load test (IT). In CT, 10 of the subjects were participated. After 2 minutes rest, wrist flexion of 10, 20, 30, 40 or 50 % of maximal voluntary contraction force (MVC was performed at 2 seconds intervals for 15 minutes. In IT, 14 including 9 of CT were participated. They performed wrist flexion in incremental contraction force of 5 steps from 10 % to 50 % of MVC for 3 minutes in each step. Changes in intracellular pH of muscle against contraction level show non·Iinear relationships both in CT and IT. From this relationship we calculated the contraction level at which pH was 6.9 (%MVC_6.9). Mean values of obtained %MVC_6.9 by CT and by IT were 29.3 and 30.0 % of MVC, respectively. And, significant correlation was found between %MVC_6.9 by CT and by IT This result shows that %MVC_6.9 was a reproducible index independent of the type of exercise test, i. e. constant or incremental load test, and might reflect the physiological characteristics of the muscle.