We examined whether transcranial magnetic stimulation (TMS) delivered to the motor cortex 13 allows assessment of muscle relaxation rates in unfatigued and fatigued knee extensors (KE). 14 We assessed the ability of this technique to measure time course of fatigue-induced changes 15 in muscle relaxation rate and compared relaxation rate from resting twitches evoked by 16 femoral nerve stimulation. Twelve healthy men performed maximal voluntary isometric 17 contractions (MVC) twice before (PRE) and once at the end of a 2-min KE MVC and five 18 more times within 8 min during recovery. Relative (intraclass correlation coefficient; ICC2,1) 19 and absolute (repeatability coefficient) reliability and variability (coefficient of variation) 20 were assessed. Time course of fatigue-induced changes in muscle relaxation rate was tested 21 with generalized estimating equations. In unfatigued KE, peak relaxation rate coefficient of 22 variation and repeatability coefficient were similar for both techniques. Mean (95% CI) 23 ICC2,1 for peak relaxation rates were [0.933 (0.724-0.982)] and [0.889 (0.603-0.968)] for 24 TMS and femoral nerve stimulation, respectively. TMS-induced normalized muscle 25 relaxation rate was -11.5 ± 2.5 s-1 at PRE, decreased to -6.9 ± 1.2 s-1 (-37 ± 17%, P < 0.001), 26 and recovered by 2 min post-exercise. Normalized peak relaxation rate for resting twitch did 27 not show a fatigue-induced change. During fatiguing KE exercise, the change in muscle 28 relaxation rate as determined by the two techniques was different. TMS provides reliable 29 values of muscle relaxation rates. Furthermore, it is sufficiently sensitive and more 30 appropriate than the resting twitch evoked by femoral nerve stimulation to reveal fatigue-31 induced changes in KE
