Non-uniform recruitment along human rectus femoris muscle during transcutaneous electrical nerve stimulation

Novel Insights Into Biarticular Muscle Actions Gained From High-Density Electromyogram

Biarticular muscles have traditionally been considered to exhibit homogeneous neuromuscular activation. The regional activation of biarticular muscles, as revealed from high-density surface electromyograms, seems however to discredit this notion. We thus hypothesize the regional activation of biarticular muscles may contribute to different actions about the joints they span. We then discuss the mechanistic basis and methodological implications underpinning our hypothesis.

Effect of taping and its conditions on electromyographic responses of knee extensor muscles

INTRODUCTION

The present study investigated the effect of stretchable characteristics of elastic therapeutic tape and its elongation on surface electromyography (EMG) of knee extensor muscles during knee extension movements.

METHODS

Nine healthy men performed knee extension movement with the application of normal elastic tape or highly stretchable tape and without the tapes (control). Tapes were applied on the anterior thigh to cross the knee joint with no elongation and elongation of 50 and 75% of the maximum stretchability. Surface EMG was recorded from the vastus lateralis (VL) muscle and proximal (RFp) and distal (RFd) sites of the rectus femoris muscle.

RESULTS

Under the no-elongation conditions, decreases in the surface EMG amplitude of the VL and RFd muscles were observed with normal tape during the isometric contraction phase and with highly stretchable tape during isometric and eccentric contraction phases, compared with the control (p < 0.05). There were no significant differences in surface EMG among the different elongation conditions in any muscles (p > 0.05).

DISCUSSION

These results suggest that the stretchable characteristics of tapes change the effect of elastic tape application on neuromuscular activation of the applied muscles and these effects are not dependent on the elongation of the tape.

Additional insight into biarticular muscle function: The influence of hip flexor fatigue on rectus femoris activity at the knee

We evaluated the compensatory adaptations in muscle regionalization and synergist activity after fatiguing a biarticular muscle at one joint with different muscle lengths. Eleven men (mean ± SD age = 23 ± 3 years) performed 50 maximal concentric isokinetic contractions of the dominant hip flexors on two occasions. For one trial, the knee joint was fully extended. For the other, the knee joint was fixed at 70°. Maximal voluntary contractions of the knee extensors were performed immediately before and after the hip flexion fatigue protocol while bipolar surface electromyographic signals were detected from the vastus lateralis and at five points along the length of the rectus femoris. Regardless of knee joint angle during the hip flexion fatigue protocol, knee extension peak torque was unchanged following hip flexion fatigue. Electromyographic amplitude for the vastus lateralis (p = .047, η² = .338) and rectus femoris (p < .001, η² = .667) showed main effects for time, indicating higher and lower post-fatigue values, respectively. There was no evidence of region-specific rectus femoris adaptations during extension at the knee following fatigue of the hip flexors. These data suggest that synergistic adaptations were involved in maintaining knee extension peak torque following hip flexion fatigue.

Region-specific modulation of tendon reflex along human rectus femoris muscle

INTRODUCTION

We investigated regional differences in amplitude modulation of the spinal reflex along the human rectus femoris (RF) muscle to test the hypothesis that this muscle is regionally regulated at the spinal cord or a higher level.

METHODS

Surface electromyography was conducted at six different sites along the RF muscle during the conditioned patellar tendon reflex in eight healthy young men.

RESULTS

A significant difference in the reflex amplitude among the channels was observed during 20% of the maximal voluntary contraction (MVC) and there was a significant difference in normalized reflex amplitude between 10 and 20% of the MVC at most proximal channel (p < 0.05), but not at the other channels (p > 0.05), during knee flexion of the ipsilateral leg.

DISCUSSION

From the results in the present study, we infer that the amplitude modulation of the tendon reflex within the RF muscle is regionally regulated, and that this regulation is dependent on the performed tasks.

Effect of aging on regional neuromuscular regulation within human rectus femoris muscle during stair ascent and descent

Our recent studies showed the rectus femoris (RF) muscle is regionally regulated during the level walking and this unique neuromuscular activation pattern is influenced by aging (Watanabe et al., 2014, 2016 J Biomech). We aimed to investigate and compare regional neuromuscular activation patterns along the RF muscle between the young and elderly during the stair walking. Fourteen young men (age: 20.4±1.0years) and 14 elderly men (age: 73.8±5.9years) performed the stair ascent and descent. Fifteen trials of three steps were performed for both stair ascent and descent. The spatial distribution of surface electromyography (EMG) within the RF muscle was assessed by central locus activation (CLA), which is calculated from 18 surface electrodes along the longitudinal line of the muscle. CLA was significantly moved along the muscle during the stair ascent and descent in both young and elderly (p<0.05). Significant differences in CLA were showed at the stance phase of the ascent (12.5±0.7 and 11.4±1.7cm from most proximal electrodes for the young and elderly, p<0.05) and at the swing phase of the descent (11.4±1.5 and 10.3±1.5cm from most proximal electrodes for the young and elderly, p<0.05). These results suggest that the regional neuromuscular activation within the RF muscle is affected by aging during the stair walking.