Article Figures & Data
Figures
- Figure 1.
Changes in the responses to corticospinal stimulation (CMEPs) delivered at rest after voluntary contractions of three strengths. A, Data for biceps brachii (top), brachioradialis (middle), and the profile of voluntary forces (bottom). The area of the CMEPs was expressed relative to control values obtained before the conditioning contraction and is shown as the group mean and SEM. Timing of the conditioning contraction is shown at the bottom. Conditioning contractions were of three strengths: 50% MVC (filled squares), 75% MVC (open triangles), and 100% MVC (filled circles). All responses were obtained with subjects relaxed. The solid and dotted horizontal lines indicate the mean ± SEM of the control responses, respectively. Data were obtained in single sessions in five subjects. At least 45 min separated the contractions. The CMEPs are depressed after the 10 sec contractions, with the depression being greatest for the strongest contraction and least for the weakest one. ANOVA of responses in both muscles revealed significant depression for 90 sec after the 100% MVC, 45 sec after the 75% MVC, and 2 sec after the 50% MVC (p < 0.05). B, Individual traces from a single subject. Top traces are superimposed CMEPs in the control period before a 10 sec 100% MVC. Traces below depict the CMEPs as a raster sequentially from 2 to 180 sec after the conditioning contraction. The broken lines depict the average control response.
- Figure 2.
Responses from a single subject to corticospinal stimulation (CMEPs) after a 10 sec MVC when the stimuli were delivered during a continuous weak voluntary contraction (5% MVC). A, Experimental protocol. Corticospinal stimuli (CMEP) and maximal motor nerve stimuli (Mmax) were delivered (arrowheads) during a 5% MVC before and after a 10 sec conditioning MVC. B, Superimposed CMEPs and maximal motor responses (Mmax, produced by stimulation of the motor nerve) recorded under control conditions before and after the conditioning MVC. C, Area of Mmax plotted relative to initial control before the conditioning MVC. D, Area of the CMEP plotted relative to initial control before the conditioning MVC. E, Area of the CMEP corrected for the small increase in Mmax shown in B. In C-E, the horizontal lines depict means ± SEM for control responses.
- Figure 3.
Responses in biceps to corticospinal stimulation (CMEPs) after a 10 sec MVC when the stimuli were delivered at rest and also during brief voluntary contractions. A, Experimental protocol. Stimuli during rest were delivered initially (precontrols). Then, stimuli were delivered during brief voluntary contractions and at rest (controls). The subject next performed the usual 10 sec conditioning MVC, and responses were measured after it. We compare the postcontraction depression of the CMEPs measured during brief contractions at 5% MVC (B) or 100% MVC (C) with those obtained at rest. The CMEPs obtained in the 90 sec period after the conditioning contraction were grouped in 30 sec periods, and the size of each CMEP was expressed relative to that obtained in the control period immediately before the conditioning contraction. Responses obtained during a voluntary contraction are shown as filled symbols and those during rest as open symbols. Mean ± SEM (n = 3). B, The brief 5% MVCs did not significantly alter the size of the following resting CMEPs. After the conditioning MVC, CMEPs were depressed when tested during contractions or rest. C, Before the conditioning contraction, there was some depression of the CMEPs because of the repeated brief MVCs, illustrated by the precontrols that have a value higher than one. However, after the conditioning contraction, the CMEPs elicited at rest were depressed and recovered over ∼60 sec, whereas the responses obtained during the brief intermittent MVCs were not depressed. *p < 0.05, statistically significant differences from control.
- Figure 4.
Typical matching forces produced in consecutive “identical” contractions on the left and right sides in one subject. A, Data for the left side (10% MVC) as the top trace and for the right side as the bottom traces in each pair. Correlation coefficient shown for each contraction pair. Vertical lines mark some force increments occurring bilaterally. B, Corresponding x-y plot of the forces produced on the two sides from A.
- Figure 5.
Psychophysical experiment that reveals a reduced output on the right side after a 10 sec MVC on that side. A, One set of contractions in a single subject. The subject produced simultaneous elbow flexion contractions to a target force on the left (10% MVC). Traces show left and right elbow flexion forces and EMG activity from right biceps brachii and brachioradialis. After a 10 sec MVC on the right, lower force and EMG activity were produced on the right. B, Pooled data (mean ± SEM; n = 10) for the matching study. Data are shown for the nine contractions before and after a 10 sec MVC on the right. Force, biceps brachii EMG activity, and brachioradialis EMG activity are plotted as a ratio between the right and left (control) side, with the horizontal lines showing ±1 SD. The values were normalized to the mean for the nine control contractions before the MVC. The ratio for brachioradialis EMG activity was depressed after the conditioning MVC, and it recovered over ∼40 sec. This depression of the EMG activity occurred despite the likely increase in the muscle fiber action potential in active fibers (Fig. 2C) (Cupido et al., 1996; Gandevia et al., 1999). Significant changes from control at times after the MVC are shown as filled circles (p < 0.05). The open circles after the conditioning contraction indicate data not significantly changed.
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