Search Results

This study aimed to identify areas of reduced surface EMG amplitude and changed frequency across the phase space of a maximal dynamic knee extension task. The hypotheses were that (1) amplitude would be lower for eccentric contractions compared with concentric contractions and unaffected by fiber length and (2) mean frequency would also be lower for eccentric contractions and unaffected by fiber length. Joint torque and EMG signals from the vasti and rectus femoris were recorded for eight athletic subjects performing maximum knee extensions at 13 preset crank velocities spanning ±300°⋅s⁻¹. The instantaneous amplitude and mean frequency were calculated using the continuous wavelet transform time–frequency method, and the fiber dynamics were determined using a muscle model of the knee extensions. The results indicated that (1) only for the rectus femoris were amplitudes significantly lower for eccentric contractions (p = .019) and, for the vasti, amplitudes during eccentric contractions were less than maximal but this was also the case for concentric contractions due to a significant reduction in amplitude toward knee extension (p = .023), and (2) mean frequency increased significantly with decreasing fiber length for all knee extensors and contraction velocities (p = .029). Using time–frequency processing of the EMG signals and a muscle model allowed the simultaneous assessment of fiber length, velocity, and EMG.

The purpose of this study was to investigate if force enhancement (FE) in vivo is influenced by stretch-induced changes of muscle architecture. Therefore, 18 subjects performed maximum voluntary isometric (100° knee flexion angle) and isometric-eccentric-isometric stretch contractions (80°–100° ω = 60°s⁻¹) whereby pennation angle and fascicle length of vastus lateralis was determined using ultrasonography. We found significant (2-way repeated ANOVA; α = 0.05) enhanced torque of 5–10% after stretch as well as significant passive FE but no significant differences in muscle architecture between isometric and stretch contractions at final knee angle. Furthermore, EMG recordings during a follow-up study (n = 10) did not show significant differences in activation and mean frequency of contraction conditions. These results indicate that FE in vivo is not influenced by muscle architectural changes due to stretch.

Temporal and spatial movement characteristics are often seen as controlled separately, although they are not independent. Even in the case of simple oscillations mean frequency and mean amplitude covary when one or the other is changed intentionally. The present experiment revealed that in rapid finger oscillations there is also a cycle-to-cycle covariation so that smaller amplitudes are associated with locally increased frequency and (the associated) earlier electromyographic (EMG) bursts. Both globally and locally the observed covariations are consistent with modeling rhythmic movements as output of a driven damped oscillator. The existence of local spatio-temporal covariations suggests limitations for models of timing and reasons for the observation that spatio-temporal movement characteristics cannot be chosen arbitrarily even in uniarticular movements.

The purpose of study was to demonstrate age-related changes during gait initiation (GI). Therefore, displacement, velocity, total power, mean and median frequency of COP trajectories were measured during phases of GI in anteroposterior (AP) and mediolateral (ML) directions. The older group demonstrated the slower and lesser displacement in comparison with the younger group during anticipatory phase in AP direction and during locomotor phase in AP and ML directions. In addition, the median and mean frequency were greater in the older relative to the younger group during anticipatory phase in AP direction, while these were lesser in older than younger group during locomotor phase in AP and ML directions. Moreover, total power was greater among older than younger adults during the anticipatory phase in ML direction and during all phases in AP direction. This study suggests that COP-related parameters extracted from time and frequency domains have the ability to demonstrate age-related changes.

This study examined whether features of the built environment and residents’ perceptions of neighborhood walkability, safety, and social cohesion were associated with self-reported physical activity (PA) and community-based activity among a sample of 190 older adults (mean age 74) residing in 8 Denver neighborhoods. Neighborhood walking audits were conducted to assess walkability and social capital. In multilevel analyses, a few walkability variables including curb cuts, crosswalks, and density of retail predicted greater frequency of walking for errands (p < .05), but mean frequency of walking for errands for this sample was low (<1/wk). Contrary to expectations, total PA and community-based activity were highest in neighborhoods with fewer walkability variables but higher respondent perceptions of safety and social cohesion (p < .01). For seniors, the importance of characteristics of the built environment in promoting PA and general activity engagement might be secondary to attributes of the social environment that promote safety and social cohesion.

This study examined the peak torque and mechanomyographic (MMG) amplitude and mean frequency (MNF) responses during fatiguing isokinetic muscle actions. On four separate occasions, twenty men (mean ± SD age = 23 ± 3 years) performed 25, 50, 75, and 100 repeated maximal concentric isokinetic muscle actions of the dominant forearm flexors. During each muscle action, the MMG signal was detected from the biceps brachii with an accelerometer. The data were examined with linear regression and one-way repeated measures analyses of variance. The results indicated that the mean percent decline in peak torque value for the 25 repetition trial (25.6%) was significantly less than that for the 50 repetition trial (45.2%). Furthermore, the mean linear slope coefficient for the peak torque versus repetition number relationship for the 50 repetition trial was significantly less than that for the 100 repetition trial. There were no mean differences among the trials for the linear slope coefficients and y-intercepts for the MMG amplitude and MNF versus repetition number relationships. When detected with an accelerometer, the linear slope coefficients and y-intercepts for the MMG amplitude and MNF versus repetition number relationships were not sensitive enough to track the decline in muscle function during fatigue.