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The effect of muscle length on transcranial magnetic stimulation induced-relaxation rate in the plantar flexors Yacyshyn, Alexandra F.


Muscle relaxation rate is an important intrinsic contractile property that influences the neural drive necessary to achieve a desired force. Most relaxation data are obtained at rest through evoked contractions; however, the application of transcranial magnetic stimulation (TMS) during a maximal voluntary contraction (MVC) allows for a more functionally-relevant measurement of muscle relaxation. PURPOSE: The purpose of this study was to determine if TMS is a sufficiently sensitive technique to detect changes in relaxation rate with changes in muscle length. A secondary purpose was to qualitatively compare the relaxation rate of whole muscle torque to relaxation rates of single fascicles and the muscle-tendon junction (MTJ) acquired using ultrasound imaging. METHODS: Eleven males (24.8 ± 7.0 years; mean ± SD) performed 21 brief (~3s) isometric plantar flexor MVCs in a prone position at full knee extension, pseudo-randomized between 20° dorsiflexion (DF), a neutral ankle position, and 30° plantar flexion (PF). During each MVC, high-intensity TMS (80% stimulator output) was delivered to the motor cortex; ultrasound video recordings captured medial gastrocnemius (MG) fascicle and MTJ length changes. Peak relaxation rate was calculated to be the steepest slope of the TMS-induced drop in torque or rate of length change for MG fascicles and MTJ. RESULTS: MVC torque was greater in DF (182.0 ± 40.7 Nm) compared to neutral (147.7 ± 18.9 Nm) and in the neutral compared to PF position (62.2 ± 9.6 Nm). Plantar flexor relaxation rate was slower at PF (-804.02 ± 161.91 Nm/s) compared to neutral and DF (-1895.90 ± 298.31 and -2007.62 ± 692.11 Nm/s, respectively). Similarly, MG fascicle relaxation rate was slower at PF (-2.80 ± 1.10 cm/s) compared to neutral and DF (-5.35 ± 1.10 and -4.81 ± 1.87 cm/s, respectively). Relaxation rate of the MTJ did not differ with ankle angle (p = 0.06). CONCLUSIONS: Absolute relaxation rate was markedly slower when the plantar flexor muscles were shortened, indicating the TMS-induced relaxation technique is sufficiently sensitive to detect changes in muscle length. Comparable results were obtained from single fascicles, indicating that ultrasound imaging is suitable for the measurement of evoked contractile properties during voluntary contraction.

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