Changes in subjective muscle fatigue sensation and blood lactate and their relationships with decreasing force during sustained handgripping using various target values and its recovery stage.

The purpose of this study was to examine the relationship between the level of subjective muscle fatigue sensation (SMS) and the physiological responses to decreasing force during and after sustained and constant effort static gripping (SSG) at 50, 75 and 100% of the maximal voluntary contraction (MVC). The subjects were 10 healthy males without upper extremity impairments. Each subject performed SSG for 6 min with three target forces. SMS of the forearm by Borg's CR 10 scale was measured every 30 sec during and after the SSG. The blood lactate concentration (La) and MVC were measured before the SSG and at 0, 4 and 7 min after the SSG. There were no significant differences in the average grip force after 30 sec from the SSG onset between the target forces. The SMS for 75% MVC during the SSG increased markedly, and was the highest at 30 sec after the onset (p < 0.05). Although the La at 4 min was higher than that before the SSG, the La recovered to 80% at 7 min after the SSG, and the SMS recovered almost completely. The cross correlation coefficients between the increasing SMS and decreasing force during the SSG were very high in all conditions (r(xy) = -0.819 to -0.979, -0.930 to -0.988, -0.789 to -0.985, for 50%, 75% and 100%, respectively; all coefficients: lag = 0), but the relationships between the recovery ratio of the SMS and that of the maximal grip force at 4 and 7 min after the SSG were low. In addition, the relationships between the recovery ratio of the La and the maximal grip force for 75% MVC were high at 0 and 4 min after the SSG (r = -0.814, -0.744, respectively). In the case of sustained muscle contraction for a long time, there were no significant differences between the target forces for the SMS, La and maximal grip force after SSG, and the degree of muscle fatigue was considered to be similar at each target force. However, the SMS during SSG, especially before the gripping force decreased to the almost steady state, may differ at each target force.