Muscle proton T₂ relaxation times and work during repetitive maximal voluntary exercise

We studied the effects of progressive maximal voluntary handgrip contractions (MVCs) on muscle proton spin-spin (T₂) relaxation times and work, measured as the integrated force vs. time curve (FTI). Six healthy volunteers performed 10, 20, 40, and 80 MVCs in a 0.35-T magnet on four separate occasions. Repeated measures analyses of variance of increases in T₂ and FTI during successive bouts were significant (P < 0.005 and P < 0.001, respectively). FTI increased with successive bouts to a greater extent than did muscle T₂ (P < 0.05). For T₂, the Helmert contrast judged the 10- MVC response lower than the mean of the remaining responses (P < 0.005), and the differences between all others compared with the means of subsequent responses were not significant, indicating a 'flattening' of the T₂ response after the increase from 10 to 20 repetitions. For FTI, all the single degree of freedom Helmert contrasts were significant (P < 0.001), indicating a continual increase in response over increased MVCs. The curved nature of the T₂ response conformed best to a hyperbolic function, suggesting that a limit of ~32% exists for the change in T₂ during progressively longer bouts of MVCs. A limit in the T₂ response is consistent with the existence of a limit in the amount of water that muscle can take up from the vasculature during exertion.