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; ICC2,1) 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) ICC2,1 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.

Reproducibility and robustness of motor cortical stimulation to assess muscle relaxation kinetics

“…The intrarater (within‐subject) repeatability and reliability are in accordance with our previous study on TMS‐induced muscle relaxation in finger flexor muscles (Molenaar et al, 2018 ). Recently, repeatability of TMS‐induced muscle relaxation was also studied in knee‐extensor muscles which also demonstrated high repeatability and reliability (Vernillo et al, 2021 ). The demonstrated high repeatability (small within‐subject variability) and reliability in these previous studies and our current study lead to high statistical power when studying the effect of an intervention on muscle relaxation or to detect differences in relaxation rate between groups for a given sample size (Bartlett & Frost, 2008 ).…”

“…Muscle relaxation in healthy subjects has been studied previously using this technique, example, in elbow flexors (Molenaar et al, 2013 ; Todd et al, 2007 ), finger flexors (Molenaar et al, 2018 ), knee‐extensors (Vernillo et al, 2021 ), dorsiflexors, and plantar flexors (McNeil et al, 2013 ). Furthermore, physiological slowing effects of muscle cooling and fatigue on muscle relaxation have been demonstrated, as well as slower muscle relaxation in female compared to male and the elderly compared to younger adults (Hunter et al, 2008 ; McNeil et al, 2013 ; Molenaar et al, 2013 ; Todd et al, 2007 ).…”

Sex differences in neuromuscular and biological determinants of isometric maximal force

“…Knee extensors were chosen as target muscle group since they play a key role during ambulatory, functional, and sporting activities. 97,98 The first three visits were separated by 7 days, and each participant performed all tests at the same time of day. Because the neuromuscular function of the knee extensors can be influenced by the different phases of the menstrual cycle, 99 the first day of menstruation was considered as day 1 of the cycle and females visited the lab on day 14 ± 2 of their menstrual cycle.…”

Neural and muscular contributions to the age-related loss in power of the knee extensors in men and women

“…Absolute (Nm·s -1 ) and normalized (s -1 ) peak rates of torque relaxation were also determined from the TMS delivered during the MVC contractions (23). When TMS is delivered to the motor cortex during an MVC, there is a brief transient withdrawal of the descending neural drive following the stimulus that causes the muscle to involuntarily relax.…”

“…For example, neural drive from the motor cortex during maximal voluntary contractions can be estimated with the interpolated twitch technique by delivering single magnetic pulses to the cortical motor neurons via transcranial magnetic stimulations (TMS) (19-21). In addition, the contractile properties of the muscle can be evaluated with TMS by measuring the involuntary relaxation rates following the depolarization elicited by the magnetic pulse (22, 23) or by measuring the kinetics of the potentiated resting twitch elicited by supramaximal electrical stimulations to the peripheral nervous system. Limitations with current stimulation paradigms, however, restrict measurements of voluntary activation with an interpolated stimulus to isometric or slow velocity contractions (24); thus, integrating measures of surface electromyography (EMG) obtained during the moderate-to high-velocity contractions required to elicit peak power along with measures of TMS and electrical stimulation will help localize the primary sites along the motor pathway contributing to power loss with aging.…”