The effect of muscle length on transcranial magnetic stimulation-induced relaxation rate in the plantar flexors

Abstract

Transcranial magnetic stimulation (TMS) of the motor cortex during a maximal voluntary contraction (MVC) permits functionally relevant measurements of muscle group relaxation rate (i.e., when muscles are actively contracting under voluntary control). This study's purpose was twofold: (1) to explore the impact of muscle length on TMS‐induced plantar flexor relaxation rate; and (2) to incorporate ultrasonography to measure relaxation‐induced lengthening of medial gastrocnemius (MG) fascicles and displacement of the muscle–tendon junction (MTJ). Eleven males (24.8 ± 7.0 years) performed 21 brief isometric plantar flexor MVCs. Trials were block‐randomized every three MVCs among 20° dorsiflexion (DF), a neutral ankle position, and 30° plantar flexion (PF). During each MVC, TMS was delivered and ultrasound video recordings captured MG fascicles or MTJ length changes. Peak relaxation rate was calculated as the steepest slope of the TMS‐induced drop in plantar flexor torque or the rate of length change for MG fascicles and MTJ. Torque relaxation rate was slower for PF (−804 ± 162 Nm·s−1) than neutral and DF (−1896 ± 298 and −2008 ± 692 Nm·s−1, respectively). Similarly, MG fascicle relaxation rate was slower for PF (−2.80 ± 1.10 cm·s−1) than neutral and DF (−5.35 ± 1.10 and −4.81 ± 1.87 cm·s−1, respectively). MTJ displacement rate showed a similar trend (P = 0.06), with 3.89 ± 1.93 cm·s−1 for PF compared to rates of 6.87 ± 1.55 and 6.36 ± 2.97 cm·s−1 for neutral and DF, respectively. These findings indicate muscle length affects the torque relaxation rate recorded after TMS during an MVC. Comparable results were obtained from muscle fascicles, indicating ultrasound imaging is suitable for measuring evoked contractile properties during voluntary contraction.

Myths and Methodologies: How loud is the story told by the transcranial magnetic stimulation‐evoked silent period?

“…However, it not clear how this hypothesis would apply to the SPs elicited during MVCs, where TMS-induced twitches are significantly smaller compared with submaximal contractions (Todd, Taylor, & Gandevia, 2016), making tendon organs less likely to discharge and muscle spindles to be unloaded to a lesser extent. It seems more likely that extra firing of muscle spindles would occur during the muscle relaxation phase at the time of the SP as a result of muscle lengthening (Butler, Petersen, Herbert, Gandevia, & Taylor, 2012;Yacyshyn, Nettleton, Power, Jakobi, & McNeil, 2017). On this point, it is worth noting that the SP can also be interrupted by small bursts of EMG activity or low-level of EMG activity before recommencement of voluntary EMG (Butler et al, 2012).…”

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

“…This interplay presents a slow phase followed by a fast (almost mono-exponential) phase [for a comprehensive review see Poggesi et al ( 2005 )]. Previous studies reported faster mean relaxation rates to the present one in healthy young men for finger flexors [− 14.1 s −1 (Molenaar et al 2018 )], elbow flexors [− 13.5 s −1 (Hunter et al 2006 ), − 12.9 s −1 (Hunter et al 2008 ), − 14.3 s −1 (Molenaar et al 2013 )], and plantarflexors [− 13.1 s −1 (Yacyshyn et al 2017 )]. These faster relaxation rates could be due to a greater proportion of fast-twitch muscle fibres in the above-mentioned muscles compared to KE (Johnson et al 1973 ).…”

“…To account for differences in both voluntary strength and evoked twitch amplitude within and between participants, normalized rates of relaxation were calculated by dividing the absolute rates of relaxation by the peak force which preceded the relaxation. This value reflects the relative peak relaxation rate of all knee-extensor muscles that contribute to the measured force (voluntary plus evoked) and that are suppressed by the inhibitory effects of TMS (Todd et al 2005 , 2007 ; Hunter et al 2006 , 2008 ; McNeil et al 2013 ; Yacyshyn et al 2017 ). Furthermore, time to peak relaxation was assessed as the time from TMS stimulus until the moment of peak relaxation (Molenaar et al 2013 ).…”

“…Accordingly, it has been proposed to analyze the rate of muscle relaxation during the silent period elicited by TMS delivered to the motor cortex (Todd et al 2005 ). This method has been applied to the finger flexors (Molenaar et al 2018 ), elbow flexors (Todd et al 2005 , 2007 ; Hunter et al 2006 , 2008 ; Molenaar et al 2013 ), plantar flexors (McNeil et al 2013 ; Yacyshyn et al 2017 ), and dorsiflexors (McNeil et al 2013 ), either in an unfatigued or fatigued state. Results have shown that TMS can be used to measure relaxation rates in the above-mentioned muscle groups.…”