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At least 253 records · Page 14Linked to original sources
PubMed2002

Electromechanical assessment of ankle stability.

The goal of this study was to approach ankle instability by measuring the electromechanical delay of the peroneal muscles (foot pronators). For that purpose, supramaximal electrical stimulation of common peroneal nerve was applied when the subject was standing on a force plate in bipedal stance or monopedal stance, postures requiring greater ankle stabilization. The electromechanical delay (EMD) was defined as the time interval between the onset of the peroneus longus (PL) electromyogram detected by surface electrodes and the onset of the lateral ground reaction force (Fy) measured on a force plate. Ten healthy subjects (control group, C-G) and ten subjects with functional ankle instability ("FAI" group, FAI-G) performed the tests. In C-G, the mean EMD values decreased significantly ( P<0.001) from the bipedal [10.54 (0.71) ms; mean (SD)] to the monopedal stance [8.67 (0.63 ms)]. Since a monopedal stance is known to require higher leg muscle tone resulting in a higher ankle stiffness, these results indicate that PL EMDs are sensitive to musculo-tendinous stiffness. They validate the choice of a PL EMD measurement as an indirect index of musculo-tendinous stiffness at the ankle. In both bipedal and monopedal stance conditions, EMD values were significantly higher in FAI subjects. They ranged from 12.64 (1.14) ms in the bipedal stance to 10.85 (1.07) ms in the monopedal stance. This suggests a lower musculo-tendinous stiffness at the ankle, which may contribute to the ankle instability.

Adult↗
PubMed2004

Anticipatory control related to the upward propulsive force during the rising on tiptoe from an upright standing position.

The purpose of the present investigation was to explore whether and how the amplitude and/or duration parameters of anticipatory postural adjustments (APAs) affect the upward propulsive force (Delta FZamp) during rising on tiptoes from upright standing position. Seven subjects initiated the movement of rising on tiptoe from a strictly regulated initial body position at various velocities at the subjects' own spontaneous pace. In this experiment, electromyographic (EMG) activities of the muscles in the tibialis anterior (TA) and soleus, the goniogram in the ankle joint, and the anteroposterior and vertical ground reaction forces were simultaneously recorded. The average amplitude and the duration of the EMG activities and ground reaction force were measured, and the displacement of the center of body mass and the center of foot pressure (CP) were calculated and reported as the mean value plus its standard deviation. Delta FZamp correlated positively and significantly with the maximum backward CP shift, the average amplitude of the backward loading and the average EMG amplitude in the TA in the anticipation phase, whereas Delta FZamp did not correlate positively or significantly with the anticipatory duration of the backward CP shift of the backward loading and of the EMG activity in the TA. These results suggest that the amplitude parameters of the APAs associated with rising on tiptoes are dependent on the upward propulsive force, and that APAs may be directly involved not only with compensation of postural disturbance but also with increasing performance of the focal movement.

Adaptation, Physiological↗
PubMed2004

Are changes in pain induced by myofeedback training related to changes in muscle activation patterns in patients with work-related myalgia?

The objective of this explorative study was to investigate to what extent changes in perceived pain, induced by myofeedback training, are correlated to changes in muscle activation patterns. Thirty subjects with work-related myalgia received myofeedback training. Before (T(0)), directly after (T(1)) and 4 weeks or, in a subset of patients, 3 months after (T(2)) this training, surface electromyography (sEMG) measurements of the upper trapezius muscle were performed during standardized computer tasks; a typing and a stress task. Besides this, visual analogue scales (VAS) were filled in to assess the levels of pain in the neck and shoulders. From the sEMG, root mean square (RMS) and relative rest time (RRT, i.e. the percentage of time RMS is below a certain threshold) were used for data analysis. The relationships between RRT, RMS and VAS at T(0) as well as for the changes between T(1)-T(0) and T(2)-T(0) were investigated using Spearman correlation coefficients. The results revealed no significant correlations between VAS and RMS both at baseline (range R = -0.22 to 0.17) and for the observed changes (range R = -0.33 to 0.32). Also, for VAS and RRT, low correlations were found for baseline (range R = -0.27 to 0.21) and for changes between T(1)-T(0) (range R = -0.02 to 0.38). However, for the changes between T(2)-T(0), correlation coefficients for the VAS for the shoulder and the RRT of the right trapezius during both the typing and stress tasks were significant at the P = 0.05 level, whereas the correlation coefficients for the VAS for the neck and both the left and right trapezii during the stress task approached significance (P = 0.05 and P = 0.1, respectively). These results suggest that decreases in pain observed at long term follow up after myofeedback training might occur as a result of an increased ability to relax but not as a result of decreased muscle activation level. However, the largest correlation found was 0.6. This means that the maximal explained variance (R (2)) is low (36%), and that there are also other processes than the changes in muscle activation that contribute to changes in perceived pain.

Adolescent↗
PubMed2003

Muscle synergies during shifts of the center of pressure by standing persons: identification of muscle modes.

When a standing person performs a movement such that the center of gravity shifts, the activity of postural muscles adjusts to keep the balance. We assume that such adjustments are controlled using a small set of central variables, while each variable induces changes in the activity of a subgroup of postural muscles. The purpose of this study has been to identify such muscle groups (muscle modes or M-modes) and compare them across tasks and subjects. Four tasks required the subjects to release a load from extended arms leading to a center of pressure (COP) shift prior to the load release. The fifth task required an explicit COP shift by voluntary sway. Electromyographic activity of 11 postural muscles on one side of the body was integrated over a 100-ms interval corresponding to the early stage of the COP shift, and this integrated EMG activity was subjected to a principal component (PC) analysis across multiple repetitions of each task. Three PCs were identified and associated with a "push-back M-mode," a "push-forward M-mode," and a "mixed M-mode." Cluster analysis of the PC vectors across tasks and across subjects confirmed the existence of distinctive push-forward and push-back muscle groups. PC vectors were also compared across tasks and across subjects using cosines as a measure of colinearity between pairs of vectors. In general, M-modes were similar across both tasks and subjects. We conclude that shifts of the COP, whether implicit or explicit, are controlled using a small set of central variables associated with changes in the activity of robust subsets of postural muscles. These results can be used for future analysis of muscle synergies associated with postural tasks.

Adult↗
PubMed2005

Adjustment of the human arm viscoelastic properties to the direction of reaching.

The viscoelastic properties of the human arm were measured by means of short force perturbations during fast reaching movements in two orthogonal directions. A linear spring model with time delay described the neuromuscular system of the human arm. The obtained viscoelastic parameters ensured movement stability in spite of the time delay of 50 ms. The stiffness and viscosity ellipses appeared to be predominantly orthogonal to the movement direction, which reduced the effect of force perturbation in the direction orthogonal to the reaching movement. Thus, it can be argued that the viscoelastic properties of the neuromuscular system of the human arm are adjusted to the direction of movement according to a "path preserving" strategy, which minimizes the deviation of the movement path from a straight line, when exposed to an unexpected external force.

Adaptation, Physiological↗
PubMed1998

Cross-spectral analysis of physiological tremor and muscle activity. II. Application to synchronized electromyogram.

We investigated the relationship between synchronized muscle activity and tremor time series in (enhanced) physiological tremor by cross-spectral analysis. Special attention was directed to the phase spectrum and its potential to clarify the contribution of reflex mechanisms to physiological tremor. The phase spectra are investigated assuming that the electromyogram (EMG) synchronization was caused by a reflex or a central oscillator. Comparing these results to phase spectra of measured data, we found a significant contribution of reflexes. But reflexes only modify existing peaks in the power spectrum. The main agents of physiological tremor are an efferent pace and the resonant behavior of the biomechanical system.

Biomechanical Phenomena↗
PubMed1994

The effects of inhibitory controls triggered by heterotopic noxious stimuli on a jaw reflex evoked by perioral stimuli in man.

Electromyographic recordings (EMGs) were made from the active masseter muscle of the inhibitory reflex evoked by application of electrical stimuli to the skin of the upper lip in 11 human subjects. In control sequences, the reflex had a mean latency and duration of 41 +/- 1.7 and 44 +/- 2.6 ms, respectively. The magnitude of the reflex (measured by integration of the EMG) was significantly reduced by the application of cold (3 degrees C) or hot (47-48 degrees C) but not warm (38-46 degrees C) water to a hand or foot. The strongest stimuli (3 or 48 degrees C) produced mean reductions of the reflex magnitude in the range of 62-85%. These effects occurred regardless of whether the background activity in the masseter was raised or lowered during the application of the thermal stimuli. Thus, activity in nociceptive nerves from widespread areas of the body can modulate jaw reflexes in man by exerting an influence on the reflex pathway at a point before the motor neurones. This may involve the system of 'diffuse noxious inhibitory controls', which have been shown to depress limb flexion reflexes and neuronal activity in the spinal dorsal horn and trigeminal nuclear complex.

Cold Temperature↗
PubMed1996

Sensitivity of the jaw-jerk reflex in patients with myogenous temporomandibular disorder.

Changes in the activity of human jaw-elevator muscles related to the mandibular stretch (jaw-jerk) reflex could be involved in the aetiology of temporomandibular disorders (TMD). In order to investigate whether there are differences in the sensitivity of the jaw-jerk reflex between myogenous TMD patients (n = 10) and gender- and age-matched controls (n = 10), jaw-jerk reflexes were elicited under standardized conditions. By measuring the reflex with bipolar surface electromyography (EMG), reflex sensitivity was determined from relations between reflex amplitude and jaw displacement from the masseter and the anterior temporalis muscles. Reflex amplitude and background EMG activity were normalized with respect to the maximal voluntary contraction (MVC) to correct for differences in the thickness of soft tissues overlying the muscle or in electrode placement. In addition to normalization with respect to MVC, for the patients, normalization was also applied with respect to a MVC that was scaled by multiplying values by the ratio of the mean MVC of controls to the mean MVC of patients. At a constant level of background EMG activity, the reflex sensitivity can be determined from the slope (reflex gain) and x-intercept (reflex threshold) of the reflex amplitude-jaw displacement relation. No significant differences between patients and controls were found for the gain or threshold values of either the masseter or the anterior temporalis muscles with a univariate analysis of variance. It is concluded that jaw-jerk reflex sensitivity is not significantly changed in myogenous TMD patients. Therefore, the fusimotor system probably does not play a part in the perpetuating myogenous TMD.

Action Potentials↗
PubMed1980

Modulation of the myotatic reflex gain in man during intentional movements.

Human subjects were asked to perform sinusoidal tracking movements (0.5--3.0 Hz) with their forearms while external torque disturbances were applied at the elbow. The changes in angular position, velocity, and acceleration produced by these disturbances were found to be represented in the reflex changes in EMG activity of both biceps and triceps muscles. The gain of each of these reflex components varied during the tracking task, their maximal being about the same as those measured when the torque disturbances were applied in the absence of movements and the subjects attempted to maintain a constant forearm position. Such changes in gain were found to be centrally regulated since they were shown not to depend on the movement itself, being also present during force tracking, i.e. under nearly isometric conditions. Also their minima and maxima did not coincide with those of the EMG activity. These results suggest that an internal plan (or model) of the learned task is present, whereby reflex gains can be regulated independently from the motion and alpha-motoneuron activity. Such regulation effectively uncouples the reflex motor output from the intentionally controlled motion and maintains spindle sensitivity to external disturbances independent of large changes in muscle length. These conclusions are discussed in the context of the functional role of gamma-motoneurons in the control of movements.

Adult↗
PubMed1985

Peripheral inputs and early unit activity in area 5 of the monkey during a trained forelimb movement.

Previous studies on area 5 in the monkey showed that an early neuronal activity up to 300 ms before the onset of movement was encountered in this cortical area after deafferentation of the trained forelimb (C1-T7). This present work indicates that none of the EMG activity recorded in other parts of the body may account for these early changes and lends weight to the hypothesis that their activation is purely central.

Animals↗
PubMed1985

'Fusimotor set': new evidence for alpha-independent control of gamma-motoneurones during movement in the awake cat.

The discharge activity of single muscle spindle receptors was recorded in freely moving cats. Large changes in responsiveness to length variations was observed in different types of movement. In separate stimulations on anaesthetised cats, the activity of the gamma-fusimotor neurones responsible for these changes was reconstructed. The results suggested that fusimotor action on a given spindle afferent during the movements studied was not rigidly alpha-linked, but 'set' by the CNS to steady levels, and that it could switch from largely static (gamma s) to largely dynamic (gamma d) according to the motor tasks performed.

Animals↗
PubMed1991

Instability of motor unit firing rates during prolonged isometric contractions in human masseter.

The firing patterns of up to 4 concurrently active masseter motor units were studied with intramuscular electrodes during a continuous isometric contraction of 15 min duration, in which the subject maintained the mean firing rate of one selected unit at 10 Hz. With this paradigm the net excitation (i.e. mean firing rate) of one unit in the muscle was controlled. This served as the reference for the functional state of other active units during the prolonged contraction. With the mean firing rate of one unit in the muscle fixed, 58% of other active units showed a slow, statistically-significant change in mean firing rate over the 15 min. The initial firing rate of the units did not influence the change in rate. The original firing rate hierarchy, which in short-term contractions reflects the recruitment order, was altered during the prolonged contraction. The explanation for these differential changes in motoneuron net excitation is not clear; they could be intrinsic to the motoneurons or perhaps mediated by reflex pathways. The selective facilitation or suppression of some motor units with continuous activation means that the original size-structured combination of motor units can be modified during a prolonged contraction.

Adolescent↗