Yacyshyn Alexandra F, Nettleton Jane, Power Geoffrey A, Jakobi Jennifer M, McNeil Chris J
School of Health and Exercise Sciences, University of British Columbia, Kelowna, British Columbia, Canada.
Centre for Heart, Lung and Vascular Health, Faculty of Health and Social Development, University of British Columbia, Kelowna, British Columbia, Canada.
Physiol Rep. 2017 Sep;5(18). doi: 10.14814/phy2.13442.
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) than neutral and DF (-1896 ± 298 and -2008 ± 692 Nm·s, respectively). Similarly, MG fascicle relaxation rate was slower for PF (-2.80 ± 1.10 cm·s) than neutral and DF (-5.35 ± 1.10 and -4.81 ± 1.87 cm·s, respectively). MTJ displacement rate showed a similar trend ( = 0.06), with 3.89 ± 1.93 cm·s for PF compared to rates of 6.87 ± 1.55 and 6.36 ± 2.97 cm·s 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.
在最大自主收缩（MVC）期间对运动皮层进行经颅磁刺激（TMS），能够对肌肉群放松速率进行功能相关测量（即当肌肉在自主控制下主动收缩时）。本研究目的有二：（1）探究肌肉长度对TMS诱发的跖屈肌放松速率的影响；（2）结合超声检查来测量放松诱发的腓肠肌内侧头（MG）肌束延长以及肌腱结合处（MTJ）的位移。11名男性（24.8±7.0岁）进行了21次短暂的等长跖屈MVC。试验以每三次MVC为一组进行区组随机化，分别在20°背屈（DF）、踝关节中立位和30°跖屈（PF）状态下进行。在每次MVC期间，施加TMS并通过超声视频记录MG肌束或MTJ的长度变化。峰值放松速率计算为TMS诱发的跖屈肌扭矩下降的最陡斜率，或MG肌束和MTJ的长度变化率。PF状态下的扭矩放松速率（-804±162 Nm·s）比中立位和DF状态下（分别为-1896±298和-2008±692 Nm·s）更慢。同样，PF状态下MG肌束的放松速率（-2.80±1.10 cm·s）比中立位和DF状态下（分别为-5.35±1.10和-4.81±1.87 cm·s）更慢。MTJ位移速率呈现类似趋势（P = 0.06），PF状态下为3.89±1.93 cm·s，而中立位和DF状态下分别为6.87±1.55和6.36±2.97 cm·s。这些发现表明肌肉长度会影响MVC期间TMS后记录的扭矩放松速率。从肌肉肌束获得了类似结果，表明超声成像适用于测量自主收缩期间诱发的收缩特性。
