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Muscle sound. Bases for the introduction of a mechanomyographic signal in the muscle studies.
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Soundmyogram analysis during sustained maximal voluntary contraction in sprinters and long distance runners
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Motor output relative error during static linear maximal torque ramp: influence of the age
No full textAim To evaluate if the torque output relative error during linearly varying static contractions can be influenced by possible changes in motor unit properties and activation/deactivation strategies (MUAS and MUDS) due to the ageing process.
Method Eleven young (Y, 23.90 ± 3.72 years) and eleven old (O, 69.63 ± 4.34 years) subjects were recruited for the study. After the first dorsal interosseous (FDI) maximal voluntary contraction (MVC) during static abduction, the subject performed one triangular isometric contraction (0-100-0 %). The rate of the up-going (UGR) and down-going ramp (DGR) was 13.3% MVC/s. The requested output tension (% MVC target) was provided on a PC screen together with the force from the subject for the necessary visual feedback. The global, positive and negative relative errors (E%) were calculated for the whole triangle and for the up going and down-gong phases separately according to the formula E%= absolute value of (exerted - requested target force)/requested force*100. The filtered force signal (bandwidth: 0-20 Hz) was analyzed using the Spike Shape Analysis (SSA) technique. Statistical significance of the calculated parameters between Y and O was set at p<0.05.
Results As reported in the table the calculated E% were always greater in old subjects. The SSA reported lower number of longer spikes, with similar absolute amplitude, in O with respect to Y subjects. When the spike amplitude is scaled to the individual MVC the spikes of O subjects are greater than in Y.
Conclusion MUAS and MUDA adopted by Y and O subjects determine a different a more precise motor output in Y than in O. The key factors may be: I. the larger number of error correction (reflected in the spike number) and II. the larger motor unit size due to the muscle fibers reinnervation process with age (reflecting the larger relative spike amplitude in O compared to Y)
SURFACE MECHANOMYOGRAM AMPLITUDE IS NOT ATTENUATED BY INTRAMUSCULAR PRESSURE.
No full textThe electromyogram (EMG) and intramuscular pressure (IMP) increase linearly with force during voluntary static contractions, while the surface mechanomyogram (MMG) increases linearly only up to approximately 70% of the maximal voluntary contraction (MVC) and then levels off. The aim of this study was to investigate the possible influence of IMP on the non-linear MMG increase with force and hence on the signal generation process. Seven subjects performed static contractions of the elbow flexors during: (1) ramp contractions from 0 to 60% of the MVC, and (2) steps at 10, 20 and 40% of the MVC. An external pressure of 0 and 50 mmHg for the ramps or 0, 20, 40, 60, 80 and 100 mmHg for the steps was applied by means of a sphygmomanometer cuff in separate trials. The EMG and the MMG were detected in the biceps brachii by means of a pair of surface electrodes and an accelerometer. The IMP was measured using a Millar tipped pressure transducer, and the data was presented as the mean and standard deviation in each case. The IMP was strongly and linearly related to the external pressure and contraction force both during ramps and steps. The EMG(rms) and MMG(rms) were never reduced as a consequence of the IMP increments. In contrast, a steeper MMG(rms) versus %MVC relationship during ramps at 50 mmHg cuff pressure, and an influence of the cuff pressure at 40% of MVC on MMG(rms) were evident. We conclude that IMP per se does not attenuate the MMG generation process during voluntary contraction, suggesting that the previously described MMG(rms) decrease at near maximal static efforts must be attributed to other determinants, such as a fusion-like situation due to the high motor unit firing rate
Changes of the force-frequency relationship in human tibialis anterior at fatigue.
No full textThis work estimates the influence of the single twitch (ST) parameters changes on specific regions of the force-frequency relationship (FFR) in fatigued human tibialis anterior (TA). In 20 subjects (age 20-40) the TA underwent three stimulation phases: (a) five STs at 1 Hz followed by 5 s stimulation with increasing rate (1-50 Hz, to obtain FFR); (b) fatiguing stimulation (35 Hz for 40 s); (c) same as in "a". By the average STs (mean of the five responses) of a and c phases, the peak twitch (Pt) was calculated. Moreover, after ST normalization to Pt, the maximum contraction rate (MCR) and the maximum relaxation rate (MRR) were computed. By the FFR, normalized to the 50 Hz force, we first defined the threshold frequency (TF) when the force oscillation presented the same value in (a) and (c), and then the areas below the FFR in the 1 Hz-TF and in the TF-50 Hz ranges. RESULTS: In unfatigued and fatigued muscle Pt, and MRR changed from 6.12 +/- 3.08 to 3.27 +/- 1.16 N and from 0.87 +/- 0.13 to 0.65 +/- 0.09% Pt/ms, respectively. MCR did not change significantly. The 1 Hz-TF area ratio (c/a) was > 1 for muscles having fatigued Pt > 60% of its basal value. The TF-50 Hz area ratio (c/a) was mostly below 1. CONCLUSIONS: At fatigue, MRR reduction, leading to a better fusion of muscle mechanical output, is able to compensate, in the 1 Hz-TF frequency range, up to 40% Pt loss; beyond TF, the changes of FFR are related to the degree of force loss indicated by the fatigued Pt
Effect of acclimatization to high altitute (5050 m) on motor unit activation pattern and muscle performance
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