Use of transcranial magnetic stimulation to assess relaxation rates in unfatigued and fatigued knee-extensor muscles

Abstract

We examined whether transcranial magnetic stimulation (TMS) delivered to the motor cortex allows assessment of muscle relaxation rates in unfatigued and fatigued knee extensors (KE). We assessed the ability of this technique to measure time course of fatigue-induced changes in muscle relaxation rate and compared relaxation rate from resting twitches evoked by femoral nerve stimulation. Twelve healthy men performed maximal voluntary isometric contractions (MVC) twice before (PRE) and once at the end of a 2-min KE MVC and five more times within 8 min during recovery. Relative (intraclass correlation coefficient; ICC_2,1) and absolute (repeatability coefficient) reliability and variability (coefficient of variation) were assessed. Time course of fatigue-induced changes in muscle relaxation rate was tested with generalized estimating equations. In unfatigued KE, peak relaxation rate coefficient of variation and repeatability coefficient were similar for both techniques. Mean (95% CI) ICC_2,1 for peak relaxation rates were 0.933 (0.724-0.982) and 0.889 (0.603-0.968) for TMS and femoral nerve stimulation, respectively. TMS-induced normalized muscle relaxation rate was - 11.5 ± 2.5 s^-1 at PRE, decreased to - 6.9 ± 1.2 s^-1 (- 37 ± 17%, P < 0.001), and recovered by 2 min post-exercise. Normalized peak relaxation rate for resting twitch did not show a fatigue-induced change. During fatiguing KE exercise, the change in muscle relaxation rate as determined by the two techniques was different. TMS provides reliable values of muscle relaxation rates. Furthermore, it is sufficiently sensitive and more appropriate than the resting twitch evoked by femoral nerve stimulation to reveal fatigue-induced changes in KE.

Keywords: Fatigue; Knee extensors; Muscle relaxation rate; Transcranial magnetic stimulation.

Fig. 1

Peak muscle relaxation rates before the 2-min maximal MVC (PRE) and at the end of the 2-min MVC. After the sustained contraction, a neuromuscular function evaluation was performed as an extension of the 2-min MVC (POSTimm) and additional evaluations were performed after 5 s of relaxation (POSTrelax) and 1 (POST 1), 2 (POST 2), 4 (POST 4), and 8 (POST 8) min after the end of the 2-min MVC. Peak muscle relaxation rates were calculated from the decrease in force during the silent period during maximal voluntary contractions (a), and from the falling phase of the resting twitch evoked by femoral nerve stimulation (b). Stimuli were delivered at time 0 ms. Peak rate of relaxation was calculated as the negative slope over a 10-ms interval (5 ms either side of the steepest instantaneous slope). To account for differences in both voluntary strength and evoked twitch amplitude, normalized rates of relaxation were calculated by dividing the absolute rates of relaxation by the peak force which preceded the relaxation. EMG traces for rectus femoris (black traces) and vastus lateralis (grey traces) show muscular responses evoked by TMS (a) and femoral nerve stimulation (b). Force and EMG traces are from a single participant (33-year-old man). Arrows indicate the time at which the peak relaxation rate occurred. Different scales have been used for y-axes for illustrative purposes

Fig. 2

Comparison of coefficient of variation (a), and repeatability coefficient (b) of peak muscle relaxation rates determined from the falling phase of the resting twitch evoked by femoral nerve stimulation (PNS), and the decrease in force during the silent period during maximal voluntary contractions (TMS). Circles represent individual data, black squares means, and error bars 95% confidence intervals. Different scales have been used for y-axes for illustrative purposes

Fig. 3

Changes in maximal voluntary contraction (MVC) force. The neuromuscular function evaluation was performed before (PRE) and at the end of the 2-min MVC. After the sustained contraction, a neuromuscular function evaluation was performed as an extension of the 2-min MVC (POSTimm) and additional evaluations were performed after 5 s of relaxation (POSTrelax) and 1 (POST 1), 2 (POST 2), 4 (POST 4), and 8 (POST 8) min after the end of the 2-min MVC. The shaded box indicates the sustained 2-min MVC and time ‘zero’ corresponds to the beginning of the recovery period. Values are means ± SD. For differences between time-points ^‡P < 0.001

Fig. 4

Changes in absolute and normalized peak relaxation rates (as determined from the TMS-induced decrease in force) during maximal voluntary contractions. The neuromuscular function evaluation was performed before (PRE) and at the end of the 2-min MVC. After the sustained contraction, a neuromuscular function evaluation was performed as an extension of the 2-min MVC (POSTimm) and additional evaluations were performed after 5 s of relaxation (POSTrelax) and 1 (POST 1), 2 (POST 2), 4 (POST 4), and 8 (POST 8) min after the end of the 2-min MVC. The shaded box indicates the sustained 2-min MVC and time ‘zero’ corresponds to the beginning of the recovery period. Values are means ± SD. For differences between time-points ^†P < 0.01; ^‡P < 0.001