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.
Highlights
Voluntary force production is dependent on both central and peripheral elements working in concert within the neuromuscular system (Bigland-Ritchie et al 1983; Gandevia 2001)
This study was designed to investigate the rate of torque and fascicle relaxation as well as muscle–tendon junction (MTJ) displacement following high-intensity transcranial magnetic stimulation (TMS) during maximal voluntary contractions at three muscle lengths
Our hypotheses were confirmed as muscle length influenced the relaxation rate of plantar flexor torque and medial gastrocnemius (MG) fascicles
Summary
Voluntary force production is dependent on both central and peripheral elements working in concert within the neuromuscular system (Bigland-Ritchie et al 1983; Gandevia 2001). Muscle Group and Single Fascicle Relaxation contractile properties, transcranial magnetic stimulation (TMS) during a maximal voluntary contraction (MVC) (e.g., Todd et al 2005, 2007) has numerous advantages for the assessment of muscle relaxation. These advantages have been identified (Todd et al 2007; McNeil et al 2013) but the principal benefit of TMS during an MVC over electrical stimulation of a relaxed muscle is the functional relevance; that is, the measurement of relaxation when the central nervous system is actively driving the muscle. It is well established that muscle properties are dependent on the contractile state and this is clear when one considers the marked age-related slowing of elbow flexor relaxation rate shown with TMS (Hunter et al 2008; Molenaar et al 2013) which was not observed with an electrically induced twitch response in relaxed muscle (Doherty et al 1993; Allman and Rice 2001; Dalton et al 2010)
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