Fast Voluntary Contractions Research Articles

Attenuated activation of knee extensor muscles during fast contractions in older men and women.

Reduced physical function and increased risk of falls in older adults are accompanied by age-related reductions in torque development of leg muscles, although the mechanisms and potential sex differences are not understood. To determine the mechanistic origins (neural vs. muscular) for the age-related reduction in torque development, we compared the peak rates of torque development (RTD) during electrically-evoked and fast voluntary contractions of the knee extensors between young and older men and women. Sets of single- and double-pulse electrical stimulations evoked contractions of the knee extensor muscles in 20 young (23.0 ± 0.8years; 10 women) and 20 older adults (78.2 ± 1.5years; 10 women), followed by voluntary isometric knee extension contractions with torque development as fast as possible that matched the torque during electrically-evoked contraction (10-40% maximal torque). Peak RTD during fast-voluntary contractions was 41% less than electrically-evoked contractions (p < 0.001), but more so for older adults (44%) than young (38%, p = 0.04), with no sex differences. Peak RTD during fast-voluntary contractions was more variable between contractions for the older than young adults (77%MVC s-1 vs. 47%MVCs-1, p < 0.001). Additionally, older women exhibited greater variability than older men (81%MVCs-1 vs. 72%MVCs-1, p = 0.04) with no sex-related differences within the young adults. Older adults had slower and more variable RTD during voluntary contractions than young adults, particularly older women. The limited age-related differences in electrically-evoked RTD suggest the primary mechanism for the slower torque development of the knee extensor muscles in older men and women involve reduced neural activation.

Electromyography of pelvic floor muscles with true differential versus faux differential electrode configuration.

In pelvic floor muscle (PFM) electromyography (EMG) two different bipolar configurations are applied: "true differential" configuration (TD) measures neuromuscular activity with two ipsilateral electrodes, whereas "faux differential" configuration (FD) has two electrodes placed on each side of the PFMs. The aim of the study was to determine possible differences and the relationship between both configurations. A secondary data analysis of 28 continent (CON) and 22 stress urinary incontinent (SUI) women was performed. Surface EMG was measured using a vaginal probe during maximal voluntary (MVC) and fast voluntary (FVC) contractions. TD and FD were explored with amplitude- and time-related EMG parameters, cross-correlation coefficients (R(0)) and statistical parametric mapping (SPM). Of a total of 62 comparisons of EMG parameters of MVC and FVC, only one comparison showed significant differences between the two configurations (CON group, FVC4peak TD versus FD, p = 0.015). R(0) were high in both groups for all MVC and FVC variables (R(0) ≥ 0.989). SPM detected 3 out of 28 comparisons with short (0.124-0.404s) significant supra-threshold clusters (p < 0.025). The findings suggest that TD and FD might measure neuromuscular activity almost the same. Very high cross-correlation coefficients and a very limited number of significant results from EMG parameters, as well as SPM, suggest that in the measured sample the choice of TD or FD might remain practically irrelevant. To gain further insight into the scientific and clinical relevance of choosing either of the electrode configurations, the comparisons should be re-evaluated on a sample with more severe incontinence symptoms.

Reliability and validity of pelvic floor muscle displacement measurements during voluntary contractions.

Understanding the functioning of pelvic floor muscles (PFM) is crucial in female PFM rehabilitation. The aim of this study was to determine the intra-session retest reliability and validity to evaluate the quantity of PFM displacement. This cross-sectional observational study examined the PFM displacement of 17 young healthy nulliparous women in the midsagittal plane. Three maximal voluntary contractions (MVCs) and five fast voluntary contractions (FVCs) were simultaneously examined with an electromagnetic tracking system (ETS) and transabdominal ultrasound (TAUS) and expressed in millimeters (mean, SD). To evaluate reliability and validity, the analysis of variance, intraclass coefficient (2,1), standard error of measurement (SEM), and minimal detectable difference (MDD) were calculated. Maximal voluntary contractions and FVCs in supine position measured by an ETS (TAUS) showed a displacement of MVC: 3.5 ± 1.9mm (7.8 ± 4.5mm), FVC: 3.5 ± 2.4mm (7.6 ± 5.3mm), and during standing of MVC: 5.2 ± 1.6mm (9.4 ± 3.8mm) and FVC: 4.8 ± 2.5mm (9.7 ± 4.1mm). Intraclass correlation for the ETS (TAUS) measurement varied between 0.79 and 0.89 (0.61 and 0.74), SEM 0.52 and 1.03mm (1.54 and 3.2mm), and MDD 1.54 and 3.2mm (6.64 and 7.53mm). The correlation between an ETS and TAUS varied between 0.53 and 0.67. For MVC and FVC, ETS measurements are highly reliable and TAUS measurements are moderately reliable for both contraction types. The correlation between the TAUS and ETS measurements is moderate. An ETS seems to be a reliable and valid measurement tool for evaluating PFM displacement during voluntary contractions. In future studies, the reproducibility and validity of ETS measurements need to be investigated in impact activities.

Pelvic floor muscle activity during fast voluntary contractions in continent and incontinent women.

Stress urinary incontinence (SUI) has also been attributed to a lower speed of contraction of the pelvic floor muscles (PFM). Therefore, PFM assessment and training implement fast voluntary contractions (FVC). The purpose of the study was to explore FVC regarding feasibility, on-/offset and rate of activity determination, as well as differences between continent (CON) and stress urinary incontinent (SUI) women. Fifty women were included and examined by means of PFM EMG during rest, maximum voluntary contractions (MVC), and five FVC. MVC-peak activity was used to normalize EMG-data. On-/offset of muscle activity was determined as mean of rectified rest activity plus 1 standard deviation. Linear regression was calculated for rate of activity from onset to peak, peak to offset, and within 200 ms after both onset and peak. Peak activity and time variables related to onset, peak, and offset were calculated. On-/offsets were evaluable for 234/222 of 250 FVC by a computer-based algorithm, 16/28 on-/offsets had to be determined manually. There was no significant difference between groups (CON/SUI) regarding FVC peak (92.1/99.3 %MVC), time to peak (514.2/525.6 ms), and increase of activity (182.8/182.1 %MVC/s). The SUI group showed a significantly slower activity decrease (-120.7/-74.4 %MVC/s). FVC analyses were shown to be feasible. The significant difference observed between the groups did not refer to activity increase but instead to a prolonged relaxation phase in the SUI group. This prompts to reconsider the interpretation of FVC in PFM testing and training.

Characteristics of fast voluntary and electrically evoked isometric knee extensions during 56 days of bed rest with and without exercise countermeasure

The contractile characteristics of fast voluntary and electrically evoked unilateral isometric knee extensions were followed in 16 healthy men during 56 days of horizontal bed rest and assessed at bed rest days 4, 7, 10, 17, 24, 38 and 56. Subjects were randomized to either an inactive control group (Ctrl, n = 8) or a resistive vibration exercise countermeasure group (RVE, n = 8). No changes were observed in neural activation, indicated by the amplitude of the surface electromyogram, or the initial rate of voluntary torque development in either group during bed rest. In contrast, for Ctrl, the force oscillation amplitude at 10 Hz stimulation increased by 48% (P < 0.01), the time to reach peak torque at 300 Hz stimulation decreased by 7% (P < 0.01), and the half relaxation time at 150 Hz stimulation tended to be slightly reduced by 3% (P = 0.056) after 56 days of bed rest. No changes were observed for RVE. Torque production at 10 Hz stimulation relative to maximal (150 Hz) stimulation was increased after bed rest for both Ctrl (15%; P < 0.05) and RVE (41%; P < 0.05). In conclusion, bed rest without exercise countermeasure resulted in intrinsic speed properties of a faster knee extensor group, which may have partly contributed to the preserved ability to perform fast voluntary contractions. The changes in intrinsic contractile properties were prevented by resistive vibration exercise, and voluntary motor performance remained unaltered for RVE subjects as well.

Age-related decline in rate of torque development is accompanied by lower maximal motor unit discharge frequency during fast contractions

The aim of this study was to investigate the association between the rate of torque development and maximal motor unit discharge frequency in young and elderly adults as they performed rapid submaximal contractions with the ankle dorsiflexors. Recordings were obtained of the torque exerted by the dorsiflexors during the isometric contractions and the surface and intramuscular electromyograms (EMGs) from the tibialis anterior. The maximal rate of torque development and integrated EMG (percentage of total EMG burst) at peak rate of torque development during fast contractions were lower in elderly than young adults by 48% (P < 0.05) and 16.5% (P < 0.05), respectively. The young adults, but not the elderly adults, exhibited a positive association (r2 = 0.33; P < 0.01) between the integrated EMG computed up to the peak rate of torque development and the maximal rate of torque development achieved during the fast contractions. These age-related changes during fast voluntary contractions were accompanied by a decline (P < 0.001) in motor unit discharge frequency (19, 28, and 34% for first 3 interspike intervals, respectively) and in the percentage of units (45%; P < 0.05) that exhibited double discharges (doublets) at brief intervals (<5 ms). Because aging decreased the maximal rate of torque development of fast voluntary contractions to a greater extent ( approximately 10%) than that of an electrically evoked twitch, collectively the results indicate that the age-related decline in maximal motor unit discharge frequency likely limit, in addition to the slowing of muscle contractile properties, the performance of fast voluntary contractions.

Antagonist inhibition during rest and precontraction

The excitability of antagonist soleus motoneurons was tested during fast voluntary contractions of tibialis anterior (TA). Contractions of TA started either from zero or higher levels of tonic contraction of ankle extensors. Results showed that H reflex depression under 3 different conditions: 25% (R25) or 50% (R50) of the maximal isometric dorsiflexion from the resting state and the sequential isometric response (SW) appeared about 20–40 msec prior to the EMG onset of TA. In particular, H reflex depression was clear in the SW response, and the stronger the prior contraction of extensors the greater the H reflex depression. There was a significant difference in the integrated EMG between R25 and SW but none between R50 and SW. The different amounts of inhibition found for dorsiflexion from the resting state and sequential movement is most probably explained by presynaptic inhibition. If presynaptic inhibition were increased throughout the sequential movement, in comparison with the resting state, change in the H reflex gain would occur.

Antagonist inhibition during rest and precontraction

The excitability of antagonist soleus motoneurons was tested during fast voluntary contractions of tibialis anterior (TA). Contractions of TA started either from zero or higher levels of tonic contraction of ankle extensors. Results showed that H reflex depression under 3 different conditions: 25% (R25) or 50% (R50) of the maximal isometric dorsiflexion from the resting state and the sequential isometric response (SW) appeared about 20-40 msec prior to the EMG onset of TA. In particular, H reflex depression was clear in the SW response, and the stronger the prior contraction of extensors the greater the H reflex depression. There was a significant difference in the integrated EMG between R25 and SW but none between R50 and SW. The different amounts of inhibition found for dorsiflexion from the resting state and sequential movement is most probably explained by presynaptic inhibition. If presynaptic inhibition were increased throughout the sequential movement, in comparison with the resting state, change in the H reflex gain would occur.

Rate of tension development in isometric contractions of a human hand muscle

The amplitude and rate of tension development during both electrical stimulation and voluntary activation of a human hand muscle were compared. The contractile responses from the first dorsal interosseous muscle of 14 normal human subjects were studied to provide the experimental data. At different forces less than 400g, under conditions of both electrical stimulation and voluntary contraction, force was directly proportional to speed of contraction. This linear relationship between the speed and force of contraction, as the proportion of active motor units varied, implies that these two parameters are similarly related in motor units of various sizes. In fast voluntary contractions, the rate of rise of tension at any given force level was greater than that observed in response to repetitive stimuli delivered to the motor nerve. Even at 200 and 500 impulses per second at maximal intensity, the speed of tension development was less than that found in fast voluntary contractions. The importance of asynchronous impulses and optimum discharge sequencing in generating a faster speed of tension development in fast voluntary contractions is underscored by these findings.

Fatigue and EMG of repeated fast voluntary contractions in man.

A fatigue test consisting of repeated fast maximal contractions of the left quadriceps muscle in an isokinetic apparatus was performed by 12 healthy male subjects (19-25 yrs). EMG signals recorded from the surface of the left vastus lateralis muscle, from which also biopsies were obtained for muscle fibre classification. Only minor changes were observed in the EMG variables despite a decrease in muscle strength performance, in terms of peak torque, work and power to about 50% of initial values after 100 contractions. The concomitantly obtained positive correlation between the increase in EMG/torque ratio and the individual percentage of fast twitch (FT) muscle fibres indicated that local factors in the muscle, primarily in FT fibres, were causing the development of fatigue during repeated dynamic contractions with high power outputs.