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Recruitment Threshold (recruitment + threshold)

Distribution by Scientific Domains

Life Sciences	85%

Selected Abstracts

Motor unit recruitment and bursts of activity in the surface electromyogram during a sustained contraction

MUSCLE AND NERVE, Issue 6 2008
Zachary A. Riley MS
Abstract Bursts of activity in the surface electromyogram (EMG) during a sustained contraction have been interpreted as corresponding to the transient recruitment of motor units, but this association has never been confirmed. The current study compared the timing of trains of action potentials discharged by single motor units during a sustained contraction with the bursts of activity detected in the surface EMG signal. The 20 motor units from 6 subjects [recruitment threshold, 35.3 ± 11.3% maximal voluntary contraction (MVC) force] that were detected with fine wire electrodes discharged 2,9 trains of action potentials (7.2 ± 5.6 s in duration) when recruited during a contraction that was sustained at a force below its recruitment threshold (target force, 25.4 ± 10.6% MVC force). High-pass filtering the bipolar surface EMG signal improved its correlation with the single motor unit signal. An algorithm applied to the surface EMG was able to detect 75% of the trains of motor unit action potentials. The results indicate that bursts of activity in the surface EMG during a constant-force contraction correspond to the transient recruitment of higher-threshold motor units in healthy individuals, and these results could assist in the diagnosis and design of treatment in individuals who demonstrate deficits in motor unit activation. Muscle Nerve, 2008 [source]

Differences in the control of breathing between Himalayan and sea-level residents

THE JOURNAL OF PHYSIOLOGY, Issue 9 2010
M. Slessarev
We compared the control of breathing of 12 male Himalayan highlanders with that of 21 male sea-level Caucasian lowlanders using isoxic hyperoxic (= 150 mmHg) and hypoxic (= 50 mmHg) Duffin's rebreathing tests. Highlanders had lower mean ±s.e.m. ventilatory sensitivities to CO2 than lowlanders at both isoxic tensions (hyperoxic: 2.3 ± 0.3 vs. 4.2 ± 0.3 l min,1 mmHg,1, P= 0.021; hypoxic: 2.8 ± 0.3 vs. 7.1 ± 0.6 l min,1 mmHg,1, P < 0.001), and the usual increase in ventilatory sensitivity to CO2 induced by hypoxia in lowlanders was absent in highlanders (P= 0.361). Furthermore, the ventilatory recruitment threshold (VRT) CO2 tensions in highlanders were lower than in lowlanders (hyperoxic: 33.8 ± 0.9 vs. 48.9 ± 0.7 mmHg, P < 0.001; hypoxic: 31.2 ± 1.1 vs. 44.7 ± 0.7 mmHg, P < 0.001). Both groups had reduced ventilatory recruitment thresholds with hypoxia (P < 0.001) and there were no differences in the sub-threshold ventilations (non-chemoreflex drives to breathe) between lowlanders and highlanders at both isoxic tensions (P= 0.982), with a trend for higher basal ventilation during hypoxia (P= 0.052). We conclude that control of breathing in Himalayan highlanders is distinctly different from that of sea-level lowlanders. Specifically, Himalayan highlanders have decreased central and absent peripheral sensitivities to CO2. Their response to hypoxia was heterogeneous, with the majority decreasing their VRT indicating either a CO2 -independent increase in activity of peripheral chemoreceptor or hypoxia-induced increase in [H+] at the central chemoreceptor. In some highlanders, the decrease in VRT was accompanied by an increase in sensitivity to CO2, while in others VRT remained unchanged and their sub-threshold ventilations increased, although these were not statistically significant. [source]

Motor unit recruitment in human biceps brachii during sustained voluntary contractions

THE JOURNAL OF PHYSIOLOGY, Issue 8 2008
Zachary A. Riley
The purpose of the study was to examine the influence of the difference between the recruitment threshold of a motor unit and the target force of the sustained contraction on the discharge of the motor unit at recruitment. The discharge characteristics of 53 motor units in biceps brachii were recorded after being recruited during a sustained contraction. Some motor units (n= 22) discharged action potentials tonically after being recruited, whereas others (n= 31) discharged intermittent trains of action potentials. The two groups of motor units were distinguished by the difference between the recruitment threshold of the motor unit and the target force for the sustained contraction: tonic, 5.9 ± 2.5%; intermittent, 10.7 ± 2.9%. Discharge rate for the tonic units decreased progressively (13.9 ± 2.7 to 11.7 ± 2.6 pulses s,1; P= 0.04) during the 99 ± 111 s contraction. Train rate, train duration and average discharge rate for the intermittent motor units did not change across 211 ± 153 s of intermittent discharge. The initial discharge rate at recruitment during the sustained contraction was lower for the intermittent motor units (11.0 ± 3.3 pulses s,1) than the tonic motor units (13.7 ± 3.3 pulses s,1; P= 0.005), and the coefficient of variation for interspike interval was higher for the intermittent motor units (34.6 ± 12.3%) than the tonic motor units (21.2 ± 9.4%) at recruitment (P= 0.001) and remained elevated for discharge duration (34.6 ± 9.2%versus 19.1 ± 11.7%, P < 0.001). In an additional experiment, 12 motor units were recorded at two different target forces below recruitment threshold (5.7 ± 1.9% and 10.5 ± 2.4%). Each motor unit exhibited the two discharge patterns (tonic and intermittent) as observed for the 53 motor units. The results suggest that newly recruited motor units with recruitment thresholds closer to the target force experienced less synaptic noise at the time of recruitment that resulted in them discharging action potentials at more regular and greater rates than motor units with recruitment thresholds further from the target force. [source]

Motor unit recruitment and derecruitment induced by brief increase in contraction amplitude of the human trapezius muscle

THE JOURNAL OF PHYSIOLOGY, Issue 2 2003
C. Westad
The activity pattern of low-threshold human trapezius motor units was examined in response to brief, voluntary increases in contraction amplitude (,EMG pulse') superimposed on a constant contraction at 4,7% of the surface electromyographic (EMG) response at maximal voluntary contraction (4,7% EMGmax). EMG pulses at 15,20% EMGmax were superimposed every minute on contractions of 5, 10, or 30 min duration. A quadrifilar fine-wire electrode recorded single motor unit activity and a surface electrode recorded simultaneously the surface EMG signal. Low-threshold motor units recruited at the start of the contraction were observed to stop firing while motor units of higher recruitment threshold stayed active. Derecruitment of a motor unit coincided with the end of an EMG pulse. The lowest-threshold motor units showed only brief silent periods. Some motor units with recruitment threshold up to 5% EMGmax higher than the constant contraction level were recruited during an EMG pulse and kept firing throughout the contraction. Following an EMG pulse, there was a marked reduction in motor unit firing rates upon return of the surface EMG signal to the constant contraction level, outlasting the EMG pulse by 4 s on average. The reduction in firing rates may serve as a trigger to induce derecruitment. We speculate that the silent periods following derecruitment may be due to deactivation of non-inactivating inward current (,plateau potentials'). The firing behaviour of trapezius motor units in these experiments may thus illustrate a mechanism and a control strategy to reduce fatigue of motor units with sustained activity patterns. [source]

Evidence from proprioception of fusimotor coactivation during voluntary contractions in humans

EXPERIMENTAL PHYSIOLOGY, Issue 3 2008
Trevor J. Allen
In experiments on position sense at the elbow joint in the horizontal plane, blindfolded subjects were required to match the position of one forearm (reference) by placement of their other arm (indicator). Position errors were measured after conditioning elbow muscles of the reference arm with an isometric contraction while the arm was held either flexed or extended. The difference in errors after the two forms of conditioning was large when the conditioned muscles remained relaxed during the matching process and it became less when elbow muscles were required to lift a load during the match (10 and 25% of maximal voluntary contraction, respectively). Errors from muscle conditioning were attributed to signals arising in muscle spindles and were hypothesized to result from the thixotropic property of passive intrafusal fibres. Active muscle does not exhibit thixotropy. It is proposed that during a voluntary contraction the errors after conditioning are less, because the spindles become coactivated through the fusimotor system. The distribution of errors is therefore seen to be a reflection of fusimotor recruitment thresholds. For elbow flexors most, but not all, fusimotor fibres appear to be recruited by 10% of a maximal contraction. [source]