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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. [ABSTRACT FROM AUTHOR]

The specific neuromuscular mechanisms for compromised muscle strength with PD, and the improvement that occurs with medication, have not been clearly delineated. This study assessed knee extension and flexion strength of PD patients whilst on and off medication and examined the neural mechanisms responsible for any changes. Ten idiopathic PD patients were assessed whilst on and off medication (&ges; 12-h after drug withdrawal), ∼ 7 days apart. Isometric strength of the knee extensors and flexors was assessed, and the interpolated twitch technique used to measure activation of the knee extensors. Surface EMG was also used to measure neural drive to the agonists and antagonists. Without medication isometric strength of the knee extensors (7%) and flexors (11%) was impaired and the interpolated twitch technique revealed activation of the knee extensors was reduced (8%, P=0.005). Maximum agonist amplitudes for nkee extension and flexion were unchanged off-medication (0.59 P< 0.77). The agonist and antagonist EMG-force relationships, and the maximum antagonist EMG, were unaffected by medication withdrawal. The decrease in knee extension strength when PD patients were off medication was due to reduced activation of the agonist muscle, rather than any change in antagonist co-activation, and these changes were associated with reduced locomotory performance. [ABSTRACT FROM AUTHOR]