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). DISCUSSION: 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. Muscle Nerve 57: 279-286, 2018.

Cross-correlation between head acceleration and stabilograms in humans in orthostatic posture.

This work aims at evaluating the role of the vestibular system in the postural sway control using the cross correlation function. A sample of 19 young, healthy male adults was monitored with a three axial accelerometer placed over the head during a stabilometric test, standing on a force platform during 3 min in four conditions: eyes closed and open, and feet apart and together. The normalized cross correlation (NCCF) function and the Monte Carlo simulation were used to correlate changes in body sway with head accelerations. Significant NCCF was rarely observed in conditions with opened eyes, and occurred in six subjects with eyes closed and reduced support basis. These results are inconclusive. As no delayed response was observed, the classical negative feedback appears to be absent, and either phasic displacements of the center of pressure and the head or anticipatory control could be occurring.

Role of vestibular sensor on body sway control: coherence between head acceleration and stabilogram.

This work aims at evaluating the role of the vestibular system in the postural sway control using the coherence function. A sample of 19 young, healthy male adults was monitored with a three axial accelerometer placed over the head during a stabilometric test, standing on a force platform during 3 min in four conditions: eyes closed and open, and feet apart and together. The magnitude squared coherence (MSC) function and Monte Carlo simulation was used to correlate changes in body sway with head accelerations. Significant MSC values were found in the frequency range 0.1-0.5 Hz, mainly in conditions of larger oscillations: eyes closed and feet together. These results may be related to utricular otoliths responses and ankle strategy.

Principal components of frequency domain electromyograms for muscular fatigue analysis.

The parameters commonly employed for muscular fatigue (MF) analysis, the mean frequency (F(mean)) and median frequency (F(median)) of surface EMG (SEMG) spectra, did not present consistent results for exercises performed at low to moderate intensities. For overcoming this limitation, the present study proposes the use of principal component (PC) analysis of SEMG spectra for MF monitoring. The SEMG from vastus lateralis muscle of 24 young male subjects were recorded during a maximal effort test in cycle ergometer. Epochs of 200 ms SEMG, extracted from each period of muscle activation of each subject were used for estimating the amplitude spectra, which were employed to obtain the first two PCs. The PC coefficients from the first 40 spectra were used as a reference for calculating the standard distance. This index presented a consistent increase along the exercise, while the values of F(mean) and F(median) did not present any pattern. Comparisons between initial and final values from all subjects show no significant changes in F(mean) and F(median) (Student t test, P > 0.05) and a significant increase (P < 10(-3)) in standard distance, indicating this index as an alternative to MF analysis in cyclical exercises.

Localized fatigue effects on quiet standing control by fractional Brownian motion.

In this work, the anterior-posterior displacement of the center of pressure was modeled as a fractional Brownian motion to analyze the effect of fatigue of plantar flexor muscles by isometric contraction. A sample of 17 young, healthy adults was evaluated by stabilometric test, 2 min before and after a plantar flexion, sustained until exhaustion. For each test, the model was applied to four consecutive periods of 30 s and then averaged to estimate the parameters. The fatigue increased the stochastic activity in both persistent and antipersistent controls, and also increased the Hurst exponent of the long-term mechanism. As a conclusion, the used model suggests that peripheral fatigue increases the body sway and reduces the gain of the antipersistent mechanism. However, these changes are detectable only when more than one 30 s data segment is considered for analysis.

Effects of maximal oxygen uptake test and prolonged cycle ergometer exercise on sway density plot of postural control.

This work aims at testing the influence of the maximal oxygen uptake test and prolonged cycle ergometer exercise on sway density plot (SDP) parameters of postural control. Sixteen healthy male subjects were submitted to stabilometric tests with eye open and closed, before and after two different exercises. The maximal oxygen uptake test caused decrease of the mean duration of peaks in SDP, decreasing the stability level, without modify the rates of central and muscular torque controls. Conversely, 60 min exercise increased the mean time interval between two consecutive peaks in SDP, thus decreasing the control rate but not changing the stability level. Visual privation had a greater effect on body sway than these exercises, which were applied to muscles that are not the main actuators in body sway control. Concluding, the changes in postural control are dependent on the intensity and duration of exercise.

Detection of ventilatory threshold by an automatic parabolic model.

This work aims at presenting a method for automatic detection of the ventilatory threshold by modeling ventilatory equivalent of oxygen by a parabolic function. Thirty healthy male subjects were submitted to a single maximal oxygen uptake test, being monitored the ventilatory gas exchange signals. Ventilatory threshold by visual inspection, automatic v-slope and parabolic function methods was compared. The automatic methods showed no significant differences with visual inspection, but the parabolic technique presented higher consistence than v-slope. Since the parabolic method have low computation cost and is a simple method, it can be recommended as an alternative choice to v-slope method.

Digital Butterworth filter for subtracting noise from low magnitude surface electromyogram.

This work presents a digital filter designed to delimitate the frequency band of surface electromyograms (EMG) and remove the mains noise and its harmonics, focusing the signal analysis during reduced muscle activity. A Butterworth filter was designed as the frequency-domain product of a second order, high-pass filter with cutoff frequency 10 Hz, an eighth order low-pass filter, with cutoff at 400 Hz and six stop-band filters, second order, centered at the 60 Hz mains noise and its harmonics until 360 Hz. The resulting filter was applied in both direct and reverse directions of the signals to avoid phase distortions. The performance was evaluated with a simulated EMG signal with additive noise in multiples of 60 Hz. A qualitative assessment was made with real EMG data, acquired from 16 subjects, with age from 20 to 32 years. Subjects were positioned in orthostatic position during 21s, being only the last second analyzed to assure stationarity. EMG were collected by Ag/AgCl electrodes on right lateral gastrocnemius, amplified with gain 5000, filtered in the band from 10 Hz to 1 kHz, and thus digitized with 2ksamples/s. The filter effectively removed the mains noise components, with attenuations greater than 96.6%. The attenuation of the simulated signal at frequencies below 15 Hz and at 60 Hz caused only a small reduction of total power, preserving the original spectrum. Thus, the filter resulted suitable to the proposed application.

Anticipation mechanism in body sway control and effect of muscle fatigue.

The aim of this work is to quantify the occurrence of an anticipatory mechanism in the control of quiet standing by measuring the lag between the myoelectric activity of the lateral gastrocnemius muscle and the stabilometric signal, as well as to determine the influence of the muscle fatigue on this process. Stabilometric and electromyographic (EMG) signals were synchronously collected from 22 subjects. Gastrocnemius fatigue was induced by a sustained plantar flexed posture until muscle failure. The data acquisition lasted for 120 s before and after the induced fatigue. After mean removal, the root mean square values of the EMG (RMS-EMG) were calculated for each 20 ms period. The normalized cross-correlation function was estimated to find the time delay between RMS-EMG and stabilometric signals. Anticipation values up to 1.62 s were found both before and after fatigue conditions (p<0.05), indicating that this mechanism plays an important role in body sway control. The fatigue caused a significant increase in the latency between the myoelectric activity of the gastrocnemius muscle and the movements of the center of pressure (p<0.05).

Detection of the anaerobic threshold by surface electromyography.

This work aims at presenting a method for automatic detection of the anaerobic thresholds by surface electromyography (RMS-slope) and compare with respective values identified by the analysis of ventilatory gas exchange. Thirteen healthy adults were submitted to a single maximal oxygen uptake test, being monitored the ventilatory parameters V(O2), V(CO2), and V(E), and the surface EMG (SEMG) of the right vastus lateralis muscle. Each cycle of myoelectric activity was automatically detected and the time series of the mean RMS values of these cycles used for determining the SEMG threshold (SEMG-T) by linear regression. The method was successful to determine SEMG-T for all subjects, with consistent results when compared to respective anaerobic thresholds visually estimated in the ventilatory equivalent V(E)/VO(2).