Wavelet-based intensity analysis of the mechanomyograph and electromyograph during the H-reflex.

PURPOSE: The relationship between mechanomyography (MMG) and electromyography (sEMG) during electrically evoked muscle contraction was examined using the von Tscharner "intensity analysis," which describes the power of a non-stationary signal as a function of both frequency and time. METHOD: Data for 8 college-aged participants (3 males; 5 females) with measurable H-reflexes were analyzed. Recruitment curves for H-reflex (H), M-wave (M) using sEMG, and peak-to-peak MMG (MMGp-p) were elicited through incremental tibial nerve stimulation. The maximum peak-to-peak values for H and M for each sample were summed (HM); and maximum intensity values were measured for MMG, H, and M following the intensity analysis and computation of total intensity (MMGCvT, HCvT, MCvT). HCvT and MCvT were subsequently added together (HMCvT) for comparisons. RESULTS: Correlations of HM:MMGp-p, HM:MMGCvT, HMCvT:MMGCvT, HMCvT:MMGp-p, were low (r = 0.34, 0.33, 0.09, and 0.12, respectively, p < 0.001); and correlations of HM:HMCvT, MMGCvT:MMGp-p, were moderate-to-high (r = 0.69 and 0.97, respectively, p < 0.001). Correlations for individuals ranged from 0.61 to 0.99 across comparisons. The time at which maximal intensities occurred reflected the transition from a predominant H-reflex to the onset of the M-wave and declining lag times were noted with increasing intensity. CONCLUSIONS: The intensity analysis provides insight into the frequency characteristics of the H-reflex and M-wave not seen in traditional analysis of the H-reflex. The intensity analysis may be a useful tool in studying individual variations and changes in the contraction velocities of skeletal muscle.

Lower extremity fatigue increases complexity of postural control during a single-legged stance.

BACKGROUND: Non-linear approaches to assessment of postural control can provide insight that compliment linear approaches. Control entropy (CE) is a recently developed statistical tool from non-linear dynamical systems used to assess the complexity of non-stationary signals. We have previously used CE of high resolution accelerometry in running to show decreased complexity with exhaustive exercise. The purpose of this study was to determine if complexity of postural control decreases following fatiguing exercise using CE. METHODS: Ten subjects (5 M/5 F; 25 ± 3 yr; 169.4 ± 11.7 cm; 79.0 ± 16.9 kg) consented to participation approved by Western Oregon University IRB and completed two trials separated by 2-7 days. Trials consisted of two single-legged balance tests separated by two Wingate anaerobic tests (WAnT; PreFat/PostFat), or rest period (PreRest/PostRest). Balance tests consisted of a series of five single-legged stances, separated by 30 s rest, performed while standing on the dominant leg for 15-s with the participant crossing the arms over the chest and flexing the non-dominant knee to 90 degrees. High resolution accelerometers (HRA) were fixed superficial to L3/L4 at the approximate center of mass (COM). Triaxial signals from the HRA were streamed in real time at 625 Hz. COM accelerations were recorded in g's for vertical (VT), medial/lateral (ML), and anterior/posterior (AP) axes. A newly developed statistic (R-test) was applied to group response shapes generated by Karhunen Loeve (KL) transform modes resulting from Control Entropy (CE) analysis. RESULTS: R-tests showed a significant mean vector difference (p < .05) within conditions, between axes in all cases, except PostFat, indicating the shape of the complexity response was different in these cases. R-test between conditions, within axis, differences were only present in PostFat for AP vs. PreFat (p < .05). T-tests showed a significantly higher overall CE PostFat in VT and ML compared to PreFat and PostRest (p < .0001). PostFat CE was also higher than PostRest in AP (p < .0001). CONCLUSIONS: These data indicate that fatiguing exercise eliminates the differential complexity response between axes, but increases complexity in all axes compared to the non-fatigued condition. This has implications with regard to the effects of fatigue on strategies of the control system to maintain postural control.

Wavelet-based intensity analysis of mechanomyographic signals during single-legged stance following fatigue.

The von Tscharner (2000) "intensity analysis" describes the power of a non-stationary signal as a function of both frequency and time. The present study applied a version of this intensity analysis that utilizes Morlet wavelets as a means of gaining insight into the application of this technique as alternative to power spectral analysis for the evaluation of postural control strategy during the single-legged stance and to examine the effects of fatigue. Ten subjects (gender balanced, age: 25±3 years; height: 169.4±11.7 cm; weight: 79.0±16.9 kg) participated in two trials consisting of five 15-s dominant-leg stances. Three-uniaxial accelerometers were fixed to the surface of the dominant leg corresponding to VM, VL, SOL, and MMG was recorded at a sampling rate of 1000 Hz. Signals were later analyzed using a variation of the von Tscharner intensity analysis consisting of a filter bank of 11 Morlet wavelets (range: 2.1-131.1Hz). Two Wingate anaerobic tests (WAnT) separated by a 2-min rest were performed to introduce fatigue. Repeated measures ANOVAs showed significant effects for time, gender, trial, and wavelet (p<0.001) and significant interactions for muscle by wavelet, gender by trial, trial by wavelet, and gender by trial by wavelet (p<0.001). Peak total MMG intensity (mean±SD) was higher in males than females and higher following fatiguing exercise preWAnT (squared ms(-2)): 42.6±4.5 vs. 19.2±2.3; postWAnT (squared ms(-2)): 90.4±9.1 vs. 28.4±2.8. Peak total MMG intensity was compressed to the lower frequencies surrounding ∼12 Hz, corresponding to what might be considered physiologic tremor, and a lower peak at ∼42 Hz was most prominent in SOL. The intensity analysis is a useful tool in exploring postural control and in studying the effects of fatigue on the mechanical properties of skeletal muscle.

The acute effect of whole-body vibration on the vertical jump height.

To determine the effectiveness of a single, 1-minute bout of whole-body vibration (WBV) as a viable warm-up activity, 90 subjects (30 men; 60 women, mean age = 19 ± 1 years) were recruited and randomly assigned to either a nonvibration control group or 1 of 8 WBV treatments (4 frequencies × 2 AMplitudes). Subjects stood with the feet shoulder width apart and the knees flexed 10° on a Next Generation Power Plate for 1 minute with the frequency (30, 35, 40, or 50 Hz) and amplitude (2-4 or 4-6 mm) settings at the assigned levels. Before, 1, 5, 10, 15, 20, 25, and 30 minutes after the WBV or control treatment, subjects performed a series of countermovement vertical jumps (CMJs) measured using a Vertec vertical jump tester. Comparisons were made of changes in the countermovement vertical jump height (CMJH) over time and between groups, frequencies, and amplitudes using repeated measures analysis of variance (α ≤ 0.05). There were significant differences in CMJH over time (p = 0.008); however, these were similar for all groups, frequencies, and amplitudes (p > 0.88). Some athletes may benefit from using WBV as a warm-up activity, if the timing of WBV is optimized. The effect of WBV on performance is likely variable and minimal, with a small window of effectiveness. Gender differences were not examined, and the optimal duration, intensity, and postural position are still unclear and warrant further study.

Reliability of mechanomyography and triaxial accelerometry in the assessment of balance.

Reliability of high-resolution accelerometery (HRA) and mechanomyography (MMG) was evaluated for the assessment of single-leg balance. Subjects (5M/5F, 25+/-3 yr; 169.4+/-11.7 cm; 79.0+/-16.9 kg) participated in fifteen (three randomized bouts of five repetitions) 15-s dominant leg stances. A single HRA was fixed superficial to L3/L4 segment to capture motions relative to the center-of-mass, and three-uniaxial accelerometers were fixed on the surface of the dominant leg correspondent to the vastus medialis (VM), vastus lateralis (VL), and soleus (SOL) muscles to record MMG. Triaxial signals from the HRA (s.r.=625 Hz) were streamed to a base station, simultaneously with MMG (s.r.=1000 Hz). Signals were sampled, recorded and later analyzed. HRAs were recorded in g's for vertical (VT), medial/lateral (ML), anterior/posterior (AP) directions, and resultant (RES) scalar. Intraclass correlation coefficients (ICC) were computed for each and Pearson's r was calculated for the relationships between MMG and HRA (alpha < or =0.05). Except for RES (ICC=0.36), all measures demonstrated moderately strong reliability (ICC=0.75, 0.73, 0.63, 0.87, 0.89, and 0.86 for VM, VL, SOL, VT, ML, and AP, respectively). HRA and MMG provide reliable information pertaining to balance, and may have application in evaluating postural control and stability.

The acute effect of whole-body vibration on the hoffmann reflex.

The extent to which motoneuron pool excitability, as measured by the Hoffmann reflex (H-reflex), is affected by an acute bout of whole-body vibration (WBV) was recorded in 19 college-aged subjects (8 male and 11 female; mean age 19 +/- 1 years) after tibial nerve stimulation. H/M recruitment curves were mapped for the soleus muscle by increasing stimulus intensity in 0.2- to 1.0-volt increments with 10-second rest intervals between stimuli, until the maximal M-wave and H-reflex were obtained. After determination of Hmax and Mmax, the intensity necessary to generate an H-reflex approximately 30% of Mmax (mean 31.5% +/- 4.1%) was determined and used for all subsequent measurements. Fatigue was then induced by 1 minute of WBV at 40 Hz and low amplitude (2-4 mm). Successive measurements of the H-reflex were recorded at the test intensity every 30 seconds for 30 minutes post fatigue. All subjects displayed a significant suppression of the H-reflex during the first minute post-WBV; however, four distinct recovery patterns were observed among the participants (alpha = 0.50). There were no significant differences between genders across time (P = 0.401). The differences observed in this study cannot be explained by level or type training. One plausible interpretation of these data is that the multiple patterns of recovery may display variation of muscle fiber content among subjects. Future investigation should consider factors such as training specificity and muscle fiber type that might contribute to the differing H-reflex response, and the effect of WBV on specific performance measures should be interpreted with the understanding that there may be considerable variability among individuals. Recovery times and sample size should be adjusted accordingly.