Reliability of maximal muscle force and voluntary activation as markers of exercise-induced muscle damage

The loss of the ability of skeletal muscle to generate force is one of the most appropriate and valid means to quantify muscle damage. Routine measurements of maximal muscle force, however, include many potential sources of error, the most important of which may be a possible lack of central drive to the muscles. The aim of the present study was to determine the reliability of maximal isometric quadriceps muscle force and voluntary activation over a timescale that is typically employed to examine the aetiology of exercise-induced muscle damage. We also attempted to characterise the reliability of several twitch interpolation variables including the size of the interpolated twitch and the state (i.e. unpotentiated vs potentiated) and size of the resting twitch. Over a 7-day period, eight healthy active males performed repeated maximal voluntary isometric contractions (MVC) of the quadriceps (baseline and 2 h, 6 h, 24 h, 48 h, 72 h and 7 days post). Systematic variations in maximal muscle force, voluntary activation, interpolated twitch, unpotentiated twitch and potentiated twitch were not statistically significant (P>0.05) and 95% repeatability coefficients of ±76.03 N, ±4.42%, ± 8.44 N, ±25.92 N and ±43.58 N were observed, respectively. These data indicate that young healthy well-familiarised male subjects can reproduce their perceived maximal efforts both within and between days where activation levels of >90% are routinely achieved. Providing activation remains within these limits in the 7 days following an acute bout of exercise, the researcher would be 95% certain that exercise-induced muscle damage is present in individual subjects (taken from similar subject populations) if MVC force falls outside these limits.