BACKGROUND:Assessment of voluntary activation is useful in the study of neuromuscular impairments, particularly after spinal cord injury (SCI). Measurement of voluntary activation with transcranial magnetic stimulation (VATMS) is limited by technical challenges, including the difficulty in preferential stimulation of cortical neurons projecting to the target muscle and minimal stimulation of antagonists. Thus, the motor evoked potential (MEP) response to TMS in the target muscle compared to its antagonist may be an important parameter in the assessment of VATMS. OBJECTIVE:The purpose of this study was to evaluate the effect of isometric elbow flexion angle on two metrics in individuals with tetraplegia following SCI: 1) the ratio of biceps/triceps MEP amplitude across a range of voluntary efforts, and 2) VATMS. METHODS:Ten individuals with tetraplegia and ten nonimpaired individuals were recruited to participate in three sessions wherein VATMS was assessed at 45°, 90°, and 120° of isometric elbow flexion. RESULTS:In SCI participants, the biceps/triceps MEP ratio was not modulated by elbow angle. In nonimpaired participants, the biceps/triceps MEP ratio was greater in the more flexed elbow angle (120° flexion) compared to 90° during contractions of 50% and 75% MVC, but VATMS was not different. VATMS assessed in the more extended elbow angle (45° flexion) was lower relative to 90° elbow flexion; this effect was dependent on the biceps/triceps MEP ratio. In both groups, VATMS was sensitive to the linearity of the voluntary moment and superimposed twitch relationship, regardless of elbow angle. Linearity was lower in SCI relative to nonimpaired participants. CONCLUSIONS:Increasing the MEP ratio via elbow angle did not enable estimation of VATMS in SCI participants. VATMS may not be a viable approach to assess neuromuscular function in individuals with tetraplegia.

中文翻译：

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四肢瘫痪患者经颅磁刺激二头肌自主激活的评估

背景：自愿激活的评估在神经肌肉损伤的研究中很有用，特别是在脊髓损伤 (SCI) 后。用经颅磁刺激 (VATMS) 测量自愿激活受到技术挑战的限制，包括难以优先刺激投射到目标肌肉的皮质神经元和对拮抗剂的最小刺激。因此，与其拮抗剂相比，目标肌肉对 TMS 的运动诱发电位 (MEP) 反应可能是评估 VATMS 的重要参数。目的：本研究的目的是评估等距肘屈曲角度对脊髓损伤后四肢瘫痪患者的两个指标的影响：1）在一系列自愿努力中二头肌/三头肌 MEP 振幅的比率，以及 2）VATMS。方法：招募了 10 名四肢瘫痪患者和 10 名未受损的患者参加三个疗程，其中 VATMS 在 45°、90° 和 120° 等长肘屈曲时进行评估。结果：在 SCI 参与者中，二头肌/三头肌 MEP 比率不受肘部角度的调节。在未受损的参与者中，与 50% 和 75% MVC 收缩期间的 90° 相比，更弯曲的肘部角度（120° 屈曲）的二头肌/三头肌 MEP 比率更大，但 VATMS 没有不同。相对于 90° 肘屈曲，在更伸展的肘部角度（45° 屈曲）中评估的 VATMS 较低；这种效果取决于二头肌/三头肌 MEP 比率。在两组中，无论肘部角度如何，VATMS 都对随意力矩和叠加抽搐关系的线性敏感。相对于未受损的参与者，SCI 的线性度较低。结论：通过肘部角度增加 MEP 比率不能估计 SCI 参与者的 VATMS。VATMS 可能不是评估四肢瘫痪患者神经肌肉功能的可行方法。