Variations in the spatial distribution of the amplitude of surface electromyograms are unlikely explained by changes in the length of medial gastrocnemius fibres with knee joint angle

This study investigates whether knee position affects the amplitude distribution of surface electromyogram (EMG) in the medial gastrocnemius (MG) muscle. Of further concern is understanding whether knee-induced changes in EMG amplitude distribution are associated with regional changes in MG fibre length. Fifteen surface EMGs were acquired proximo-distally from the MG muscle while 22 (13 male) healthy participants (age range: 23-47 years) exerted isometric plantar flexion at 60% of their maximal effort, with knee fully extended and at 90 degrees flexion. The number of channels providing EMGs with greatest amplitude, their relative proximo-distal position and the EMG amplitude averaged over channels were considered to characterise changes in myoelectric activity with knee position. From ultrasound images, collected at rest, fibre length, pennation angle and fat thickness were computed for MG proximo-distal regions. Surface EMGs detected with knee flexed were on average five times smaller than those collected during knee extended. However, during knee flexed, relatively larger EMGs were detected by a dramatically greater number of channels, centred at the MG more proximal regions. Variation in knee position at rest did not affect the proximo-distal values obtained for MG fibre length, pennation angle and fat thickness. Our main findings revealed that, with knee flexion: i) there is a redistribution of activity within the whole MG muscle; ii) EMGs detected locally unlikely suffice to characterise the changes in the neural drive to MG during isometric contractions at knee fully extended and 90 degrees flexed positions; iii) sources other than fibre length may substantially contribute to determining the net, MG activation

Changes in supramaximal M-wave amplitude at different regions of biceps brachii following eccentric exercise of the elbow flexors

Purpose Previous evidence from surface electromyograms (EMGs) suggests that exercise-induced muscle damage (EIMD) may manifest unevenly within the muscle. Here we investigated whether these regional changes were indeed associated with EIMD or if they were attributed to spurious factors often afecting EMGs. Methods Ten healthy male subjects performed 3×10 eccentric elbow fexions. Maximal voluntary contraction (MVC), muscle soreness and ultrasound images from biceps brachii distal and proximal regions were measured immediately before (baseline) and during each of the following 4 days after the exercise. Moreover, 64 monopolar surface EMGs were detected while 10 supramaximal pulses were applied to the musculocutaneous nerve. The innervation zone (IZ), the number of electrodes detecting largest M-waves and their centroid longitudinal coordinates were assessed to characterize the spatial distribution of the M-waves amplitude. Results The MVC torque decreased (~25%; P<0.001) while the perceived muscle soreness scale increased (~4 cm; 0 cm for no soreness and 10 cm for highest imaginable soreness; P<0.005) across days. The echo intensity of the ultrasound images increased at 48 h (71%), 72 h (95%) and 96 h (112%) for both muscle regions (P<0.005), while no diferences between regions were observed (P=0.136). The IZ location did not change (P=0.283). The number of channels detecting the greatest M-waves signifcantly decreased (up to 10.7%; P<0.027) and the centroid longitudinal coordinate shifted distally at 24, 48 and 72 h after EIMD (P<0.041). Conclusion EIMD consistently changed supramaximal M-waves that were detected mainly proximally from the biceps brachii, suggesting that EIMD takes place locally within the biceps brachii

Electrodes positioning and gastrocnemius architecture.

<p>A schematic illustration of the relative position of surface electrodes on the medial gastrocnemius (MG) muscle is shown. The parameters considered to characterise architectural differences between the MG proximal and distal regions are further illustrated in the figure; pennation angle, fibre length and fat thickness. Proximal and distal MG regions were respectively defined as the proximal and distal half of the distance between the distal extremity of the superficial aponeurosis and the most proximal electrode. Only the surface EMGs detected by electrodes positioned in correspondence of the superficial aponeurosis were retained for analysis.</p

Is the firing rate of motor units in different vastus medialis regions modulated similarly during isometric contractions?

Introduction: Previous evidence suggests the fibers of different motor units reside within distinct vastus medialis (VM) regions. It remains unknown whether the activity of these motor units may be modulated differently. Herein we assess the discharge rate of motor units detected proximodistally from the VM to address this issue. Methods: Surface electromyograms (EMGs) were recorded proximally and distally from the VM while 10 healthy subjects performed isometric contractions. Single motor units were decomposed from surface EMGs. The smoothed discharge rates of motor units identified from the same and from different VM regions were then cross-correlated. Results: During low-level contractions, the discharge rate varied more similarly for distal (cross-correlation peak; interquartile interval: 0.27-0.40) and proximal (0.28-0.52) than for proximodistal pairs of VM motor units (0.20-0.33; P = 0.006). Conclusions: The discharge rates of motor units from different proximodistal VM regions show less similarity in their variations than those of pairs of units either distally or proximally

Displacement of innervation zone with knee flexion.

<p>Short epochs (250 ms) of the 15 single-differential EMGs collected from a single participant are shown. Signals in the left and right panels were obtained during knee extended and knee flexed positions, respectively. Propagating potentials are observed in the most distal channels, which were covering the most distal MG fibres. The channel in the array positioned most closely to the innervation zone of the muscle distal fibres is indicated with grey, shaded rectangles. Note the innervation zone moved distally from knee extended to knee flexed position.</p

Regional changes in gastrocnemius architecture with knee position.

<p>Mean values and standard deviation (whiskers) are shown for the fat thickness (panel <i>a</i>), the MG fibre length (panel <i>b</i>), and their pennation angle (panel <i>c</i>). These values were obtained from panoramic ultrasound images (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0126888#pone.0126888.g005" target="_blank">Fig 5</a>), separately for the proximal (dark, shaded bars) and distal (light, shaded bars) muscle regions. Asterisks denote statistical differences at <i>P</i> < 0.05.</p

Changes in the spatial distribution of RMS values with knee position.

<p>Median values and inter-quartile intervals are shown for the RMS amplitude (<i>a</i>), the <i>active channels</i> (<i>b</i>) and the barycentre coordinate (<i>c</i>). These variables were respectively normalised with respect to the maximal RMS value obtained at 100% MVC attempts performed during knee extended position, the total number of channels located over the MG superficial aponeurosis and the distance between the femoral condyle and the distal extremity of the superficial aponeurosis (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0126888#pone.0126888.g001" target="_blank">Fig 1</a>). Asterisks denote statistical significance at <i>P</i><0.05.</p

Changes in the surface EMGs with changes in knee position.

<p>A short epoch of raw, surface EMGs is shown during plantar flexion contractions exerted with the knee fully extended (<i>a</i>) and the knee flexed at 90 deg (<i>b</i>). Only nine of the 15 channels in the array were positioned on skin regions covering the MG superficial aponeurosis. The RMS amplitude computed from EMGs detected by each of these nine channels is shown on the right side of each panel, with black circles denoting the channels providing RMS amplitudes greater than 70% of the maximum. Dashed lines indicate the barycentre coordinate computed for these channels.</p