Percent voluntary inactivation and peak force predictions with the interpolated twitch technique in individuals with high ability of voluntary activation

No acute effects of placebo or open-label placebo treatments on strength, voluntary activation, and neuromuscular fatigue

PURPOSE

Recent evidence suggests that deception may not be necessary for placebos to improve clinical outcomes. We tested the hypothesis that placebo and open-label placebo (OLP) treatments would acutely improve strength and voluntary activation, as well as minimize neuromuscular fatigue, in untrained participants.

METHODS

Twenty-one males (n = 11) and females (n = 10) visited the laboratory on three occasions (placebo, OLP, control) to receive each treatment in a randomized, counter-balanced manner. Trials involved a pretest, a 15-min intervention, and posttests. For the placebo trial, participants were informed that they would be ingesting a capsule that would improve their performance and make them feel more energetic. For the OLP intervention, participants were told that the capsules would have no effects. In "Experiment #1", knee extensor maximal voluntary contraction (MVC) peak torque and percent voluntary activation were evaluated. In "Experiment #2", participants performed 20 consecutive MVCs while surface electromyographic signals were detected from the vastus lateralis. Subjective assessments of energy and perceived exertion were examined.

RESULTS

The interventions had no effect on strength or voluntary activation, but energy levels increased following treatments (p = 0.016, η² = 0.257). Neither treatment influenced neuromuscular fatigue. Though some variables showed moderate-to-large effect sizes, these results were consistent for individuals with lower voluntary activation.

CONCLUSION

Placebo and OLP treatments had minimal influence on strength, voluntary activation, and fatigue resistance. As these findings differ from recent reports, we speculate that placebos and OLPs are more likely to enhance muscle function in patient populations seeking medical care.

Maximal contraction methods influence the magnitude and reliability of global electromyographic signal characteristics

OBJECTIVE

The purpose was two-fold: (1) to examine differences in maximal voluntary isometric torque (MVIT) production, and electromyographic signal amplitude (EMG_AMP) and mean power frequency (EMG_MPF) values obtained during traditional (MVIC_TRAD), rapid (MVIC_RAPID), and ramp (MVIC_RAMP) maximal voluntary isometric contractions, and (2) to determine if there were differences in the reliability of MVIT, EMG_AMP and EMG_MPF among the three MVIC types.

APPROACH

Twenty-two young males and females completed MVIC_TRAD, MVIC_RAPID, and MVIC_RAMP muscle actions on two separate visits separated by 48 h. During all MVICs, MVIT and EMG_AMP and EMG_MPF of the vastus lateralis (VL) and rectus femoris (RF) were quantified.

MAIN RESULTS

MVIT was greater during MVIC_TRAD and MVIC_RAPID than during MVIT_RAMP (both p < 0.001). VL and RF EMG_AMP were greater during MVIC_RAMP than during MVIC_RAPID (p = 0.02 and 0.004). For EMG_MPF, there were no significant differences among MVIC types. Although all MVIC types generally resulted in reliable measurements of MVIT and EMG_AMP, reliability was stronger for EMG_MPF quantified during the MVIC_RAMP.

SIGNIFICANCE

Investigators may choose MVIC type based on preference or equipment availability. However, investigators should note that MVIC_RAMP contractions will likely yield the greatest EMG_AMP values and more reliable measurements of VL and RF EMG_MPF.

Re-evaluation of EMG-torque relation in chronic stroke using linear electrode array EMG recordings

The objective was to re-evaluate the controversial reports of EMG-torque relation between impaired and non-impaired sides using linear electrode array EMG recordings. Ten subjects with chronic stroke performed a series of submaximal isometric elbow flexion tasks. A 20-channel linear array was used to record surface EMG of the biceps brachii muscles from both impaired and non-impaired sides. M-wave recordings for bilateral biceps brachii muscles were also made. Distribution of the slope of the EMG-torque relations for the individual channels showed a quasi-symmetrical "M" shaped pattern. The lowest value corresponded to the innervation zone (IZ) location. The highest value from the slope curve for each side was selected for comparison to minimize the effect of electrode placement and IZ asymmetry. The slope was greater on the impaired side in 4 of 10 subjects. There were a weak correlation between slope ratio and strength ratio and a moderate to high correlation between slope ratio and M-wave ratio between two sides. These findings suggest that the EMG-torque relations are likely mediated and influenced by multiple factors. Our findings emphasize the importance of electrode placement and suggest the primary role of peripheral adaptive changes in the EMG-torque relations in chronic stroke.

The reliability of the interpolated twitch technique during submaximal and maximal isometric muscle actions

The reliability of the interpolated twitch technique during submaximal and maximal isometric muscle actions. J Strength Cond Res 27(10): 2909-2913, 2013-The purpose of this study was to examine the test-retest reliability of the percent voluntary activation (%VA) vs. force relationships. Fourteen healthy men (mean ± SD age = 21 ± 2.6 years) and 8 women (age = 21 ± 1.8 years) completed 4 maximal voluntary contractions (MVCs) and 9 randomly ordered submaximal isometric plantar flexions from 10 to 100% of the MVC. Transcutaneous electrical stimuli were delivered to the tibial nerve using a high-voltage constant-current stimulator (DS7AH; Digitimer, Herthfordshire, United Kingdom). The %VA was calculated for each maximal and submaximal MVC. Paired-samples t-tests were used to quantify systematic variability, whereas the intraclass correlation coefficients (ICCs), standard error of the mean (%SEM), and minimum differences (%MD; expressed as a percentage of the means) were used for test-retest reliability. Systematic variability was not present at any of the contraction intensities (p > 0.05). The ICCs ranged from 0.52 to 0.84, whereas the %SEM ranged from 6.75 to 38.45%, and the %MD ranged from 18.71 to 106.58%. The ICCs were ≥0.74 at contraction intensities ranging from 40 to 100% MVC (6.75-16.78% SEM), whereas the ICCs were ≤0.65 (20.95-38.45% SEM) for the contraction intensities ≤30% MVC. Although not statistically tested, the ICCs tended to be higher, whereas the %SEMs lower for contractions ≥40% MVC. Future research using %VA during submaximal contraction intensities to predict a true maximal force may want to exclude contraction intensities <40% MVC. In addition, caution is warranted when interpreting the changes in the %VA during MVCs after an experimental intervention.

Activation deficit correlates with weakness in chronic stroke: evidence from evoked and voluntary EMG recordings

OBJECTIVE

To use evoked (M-wave) and voluntary (during maximal voluntary contraction (MVC)) EMG recordings to estimate the voluntary activation level in chronic stroke.

METHODS

Nine chronic hemiparetic stroke subjects participated in the experiment. M-wave (EMGM-wave) and MVC (EMGMVC) EMG values of the biceps brachii muscles were recorded.

RESULTS

Peak torque was significantly smaller on the impaired than non-impaired side. EMGM-wave was also significantly smaller on the impaired than non-impaired side. However, the normalized EMGM-wave/TorqueMVC ratio was not significantly different between two sides. In contrast, both absolute EMGMVC and normalized EMGMVC/TorqueMVC were smaller on the impaired than non-impaired side. The voluntary activation level, EMGMVC/M-wave, was also smaller on the impaired than non-impaired side. The voluntary activation level on the impaired side was highly correlated with weakness (R=0.72), but very low (R=0.32) on the non-impaired side.

CONCLUSION

Collectively, our findings suggest that both peripheral and central factors contribute to post-stroke weakness, but activation deficit correlates most closely with weakness as estimated from maximum voluntary torque generation.

SIGNIFICANCE

These findings serve to highlight the potential benefit from high-intensity exercises to enhance central activation for facilitation of motor recovery.