Comparing EMG amplitude patterns of responses during dynamic exercise: polynomial vs log-transformed regression

The purposes of this study were to determine if (a) the log-transformed model can be applied to dynamic exercise and (b) the slope and y-intercept terms can provide additional information above and beyond the polynomial regression analyses. Eleven physically active individuals performed incremental cycle ergometry on a single occasion. Electromyographic electrodes were placed on the three superficial quadriceps muscles to record muscle activation during the exercise test. The patterns of responses for electromyographic amplitude vs power output were analyzed with polynomial and log-transformed regression models. The results of the polynomial regression for the composite data indicated that the best-fit model for the vastus lateralis muscle was linear (R(2) = 0.648, P < 0.0001), whereas the best-fit model for the rectus femoris (R(2) = 0.346, P = 0.013) and vastus medialis (R(2) = 0.764,P = 0.020) muscles was quadratic. One-way repeated measures analyses indicated no significant differences(P > 0.05) across the three superficial quadriceps muscles for the slope and y-intercept terms. These findings suggest that the log-transformed model may be a more versatile statistical approach to examining neuromuscular responses during dynamic exercise.

Validating the EMG(FT) from a single incremental cycling test

The purposes of this study were to (1) identify the EMG(FT) from a single incremental cycle ergometry test and (2) validate this fatigue threshold by having participants perform constant workload rides at 70, 100 and 130% of the estimated EMG(FT). 11 healthy college-age participants performed incremental cycle ergometry on the initial visit. The EMG amplitude was recorded from the vastus lateralis muscle for each power output and fitted with linear regression which provided the estimated EMG(FT). In subsequent visits, participants exercised at 3 percentages of their EMG(FT) with the EMG amplitude recorded for each condition. The results indicated no significant (p>0.05) increases in EMG amplitude vs. time for the 70% and 100% workloads, respectively. In addition, the participants were able to maintain these exercise intensities for over 40 min. For the 130% workload, however, EMG amplitude vs. time increased significantly (p<0.001) and the participants were able to maintain the exercise condition for less than 12 min. These findings indicate that the EMG(FT) estimated from a single incremental cycle ergometry test is a valid measure of neuromuscular fatigue and may potentially be useful in assessing the efficacy of rehabilitative interventions.

The effect of pedaling cadence and power output on mechanomyographic amplitude and mean power frequency during submaximal cycle ergometry

The purpose of this study was to examine the effects of power output and pedaling cadence on the amplitude and mean power frequency (MPF) of the mechanomyographic (MMG) signal during submaximal cycle ergometry. Nine adults (mean age +/- SD = 22.7 +/- 2.1 yrs) performed an incremental (25 W increase every min) test to exhaustion on an electronically braked cycle ergometer to determine VO2Peak and Wpeak. The subjects also performed three, 8 min continuous, constant power output rides (randomly ordered) at 35%, 50%, and 65% Wpeak. The continuous 8 min workbouts were divided into 4 min epochs. The subjects pedaled at either 50 or 70 rev x min(-1) (randomized) during the first 4 min epoch, then changed to the alternate cadence during the second 4 min epoch. The MMG signal was recorded from the vastus lateralis during the final 10 s of each minute. Two separate two-way [cadence (50 and 70 rev x min(-1)) x %Wpeak (35, 50, and 65)] repeated measures ANOVAs indicated that MMG amplitude followed power output, but not pedaling cadence, whereas MMG MPF was not consistently affected by power output or pedaling cadence. Furthermore, these findings suggested that power output was modulated by motor unit recruitment and not rate coding.

Mechanomyographic responses of the superficial quadriceps femoris muscles to incremental isometric muscle actions

The purpose of this study was to compare the composite and individual relationships for mechanomyographic (MMG) amplitude vs. torque during incremental isometric muscle actions. Ten women and six men (mean age +/- SD = 21.8 +/- 1.6 y) performed isometric leg extension muscle actions at 10-100% of peak isometric torque. Accelerometers were placed over the superficial quadriceps femoris muscles to detect the MMG signals. Polynomial regression analyses were used to determine the composite and individual patterns for MMG amplitude vs. isometric torque. The results indicated that the composite MMG amplitude vs. isometric torque relationships for all subjects combined were quadratic for the vastus lateralis and vastus medialis, and linear for the rectus femoris. The results of the individual analyses for the vastus lateralis revealed that the best fit model was linear for six subjects, quadratic for five subjects, cubic for three subjects, while two subjects exhibited no significant relationship. For the rectus femoris, eight subjects demonstrated linear patterns, six were quadratic, and two were cubic. For the vastus medialis, six subjects demonstrated linear patterns, four were quadratic, four were cubic, and two demonstrated no significant relationship. Examination of the patterns between muscles for each individual subject demonstrated that twelve of the sixteen subjects demonstrated different patterns for MMG amplitude vs. isometric torque for at least two of the three muscles examined. The results indicated there were differences in the patterns of responses for the composite MMG amplitude vs. isometric torque relationships for the three muscles. Furthermore, individual patterns for MMG amplitude frequently differed from the composite patterns, as well as between muscles. The results suggested that, in addition to using composite results of MMG responses, individual subject responses should be examined. Furthermore, caution should be used in generalizing the MMG responses of the quadriceps femoris muscles when examining a single muscle.

Comparison of Fourier and wavelet transform procedures for examining mechanomyographic and electromyographic frequency versus isokinetic torque relationships

The purpose of this study was to compare the isokinetic torque-related patterns for mechanomyographic (MMG) and electromyographic (EMG) center frequency [wavelet center frequency (CF), mean power frequency (MPF), and median frequency (MDF)] determined by the fast Fourier transform (FFT) and discrete wavelet transform (DWT). Ten adults [mean +/- SD age = 22.0 +/- 3.4 yrs] performed submaximal to maximal, isokinetic muscle actions of the biceps brachii on a Cybex II dynamometer. For both MMG and EMG, the CF, MPF, and MDF values were intercorrelated at (r = 0.91-0.98). Quadratic models provided the best fit for the absolute and normalized CF, MPF, and MDF versus isokinetic torque relationships for MMG (R2 = 0.67-0.83) and EMG (R2 = 0.72-0.90). The similarities among the CF, MPF, and MDF patterns suggested that Fourier or wavelet transform procedures can be used to examine the patterns of MMG and EMG responses during dynamic muscle actions.

Mechanomyographic and electromyographic amplitude and frequency responses during fatiguing isokinetic muscle actions of the biceps brachii

The purpose of this investigation was to examine the mechanomyographic (MMG) and electromyographic (EMG) amplitude and mean power frequency (MPF) patterns during fatiguing isokinetic muscle actions of the biceps brachii. Ten adults [three women (mean +/- SD age = 20 +/- 2 yrs) and seven men (mean +/- SD age = 23 +/- 3 yrs) ] volunteered to perform 50 consecutive maximal, concentric isokinetic muscle actions of the biceps brachii at 180 degrees x s(-1). The percent decline (mean +/- SD) in isokinetic peak torque (PT) was 70 +/- 17% and polynomial regression analyses indicated a cubic relationship (R2 = 0.994) between PT and repetition number. Both MMG amplitude and MMG MPF decreased linearly (r2 = 0. 774 and 0.238, respectively) across repetitions. The results for EMG amplitude demonstrated a cubic (R2 = 0.707) pattern across repetitions, where EMG amplitude increased during repetitions 1-20, remained stable during repetitions 20-40, and increased during repetitions 40-50. There was a quadratic (R2 = 0. 939) reduction in EMG MPF throughout the test. The decreases in MMG amplitude and MMG MPF may have been due to de-recruitment of fast fatiguing motor units, a reduction in muscular compliance, or the effects of "muscle wisdom." The results for EMG amplitude may have reflected nonmaximal efforts by the subjects and/or peripheral fatigue. The factor(s) determining the decrease in EMG MPF are unclear, although a reduction in muscle fiber action potential conduction velocity may have been partially responsible.

Mechanomyographic time and frequency domain responses of the vastus medialis muscle during submaximal to maximal isometric and isokinetic muscle actions

The purpose of this study was to examine the patterns for the mechanomyographic (MMG) amplitude and mean power frequency (MPF) versus torque relationships during isometric and isokinetic muscle actions. Ten adults (mean age +/- SD = 22 +/- 1 y) volunteered to perform isometric and isokinetic leg extension muscle actions at 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100% of peak torque on a Cybex II dynamometer. A piezoelectric crystal contact sensor was placed on the vastus medialis to detect the MMG signal. Regression analyses indicated that for the isometric muscle actions, the relationships for MMG amplitude (R2 = 0.998) and MPF (R2 = 0.987) versus torque were cubic. For the isokinetic muscle actions, the relationships for MMG amplitude (r2 = 0.927) and MPF (r2 = 0.769) versus torque were linear. The different patterns for MMG amplitude and frequency may reflect differences in the motor control strategies that modulate torque production for isometric versus dynamic muscle actions.