The effect of rate of torque development on motor unit recruitment and firing rates during isometric voluntary trapezoidal contractions

Blood flow restriction increases necessary muscle excitation of the elbow flexors during a single high-load contraction

PURPOSE

To investigate the effects of blood flow restriction (BFR) on electromyographic amplitude (EMGRMS)-force relationships of the biceps brachii (BB) during a single high-load muscle action.

METHODS

Twelve recreationally active males and eleven recreationally active females performed maximal voluntary contractions (MVCs), followed by an isometric trapezoidal muscle action of the elbow flexors at 70% MVC. Surface EMG was recorded from the BB during BFR and control (CON) visits. For BFR, cuff pressure was 60% of the pressure required to completely occlude blood at rest. Individual b (slope) and a terms (gain) were calculated from the log-transformed EMGRMS-force relationships during the linearly increasing and decreasing segments of the trapezoid. EMGRMS during the steady force segment was normalized to MVC EMGRMS.

RESULTS

For BFR, the b terms were greater during the linearly increasing segment than the linearly decreasing segment (p < 0.001), and compared to the linearly increasing segment for CON (p < 0.001). The a terms for BFR were greater during the linearly decreasing than linearly increasing segment (p = 0.028). Steady force N-EMGRMS was greater for BFR than CON collapsed across sex (p = 0.041).

CONCLUSION

BFR likely elicited additional recruitment of higher threshold motor units during the linearly increasing- and steady force-segment. The differences between activation and deactivation strategies were only observed with BFR, such as the b terms decreased and the a terms increased for the linearly decreasing segment in comparison to the increasing segment. However, EMGRMS-force relationships during the linearly increasing- and decreasing-segments were not different between sexes during BFR and CON.

Motor unit firing rates during slow and fast contractions in boys and men

BACKGROUND

Motor unit (MU) activation during maximal contractions is lower in children compared with adults. Among adults, discrete MU activation differs, depending on the rate of contraction. We investigated the effect of contraction rate on discrete MU activation in boys and men.

METHODS

Following a habituation session, 14 boys and 20 men completed two experimental sessions for knee extension and wrist flexion, in random order. Maximal voluntary isometric torque (MVIC) was determined before completing trapezoidal isometric contractions (70%MVIC) at low (10%MVIC/s) and high (35%MVIC/s) contraction rates. Surface electromyography was captured from the vastus lateralis (VL) and flexor carpi radialis (FCR) and decomposed into individual MU action potential (MUAP) trains.

RESULTS

In both groups and muscles, the initial MU firing rate (MUFR) was greater (p < 0.05) at high compared with low contraction rates. The increase in initial MUFR at the fast contraction in the VL was greater in men than boys (p < 0.05). Mean MUFR was significantly lower during fast contractions only in the FCR (p < 0.05). In both groups and muscles, the rate of decay of MUFR with increasing MUAP amplitude was less steep (p < 0.05) during fast compared with slow contractions.

CONCLUSION

In both groups and muscles, initial MUFRs, as well as MUFRs of large MUs were higher during fast compared with slow contractions. However, in the VL, the increase in initial MUFR was greater in men compared with boys. This suggests that in large muscles, men may rely more on increasing MUFR to generate torque at faster rates compared with boys.

Post-activation potentiation and potentiated motor unit firing patterns in boys and men

BACKGROUND

Post-activation potentiation (PAP) describes the enhancement of twitch torque following a conditioning contraction (CC) in skeletal muscle. In adults, PAP may be related to muscle fibre composition and is accompanied by a decrease in motor unit (MU) firing rates (MUFRs). Muscle fibre composition and/or activation is different between children and adults. This study examined PAP and MU firing patterns of the potentiated knee extensors in boys and men.

METHODS

Twenty-three boys (10.5 ± 1.3 years) and 20 men (23.1 ± 3.3 years) completed familiarization and experimental sessions. Maximal isometric evoked-twitch torque and MU firing patterns during submaximal contractions (20% and 70% maximal voluntary isometric contraction, MVIC) were recorded before and after a CC (5 s MVIC). PAP was calculated as the percent-increase in evoked-twitch torque after the CC. MU firing patterns were examined during submaximal contractions before and after the CC using Trigno Galileo surface electrodes (Delsys Inc) and decomposition algorithms (NeuroMap, Delsys Inc). MU action potential amplitudes (MUAPamp) and MUFRs were calculated for each MU and exponential MUFR-MUAPamp relationships were calculated for each participant and trial.

RESULTS

PAP was higher in men than in boys (98.3 ± 37.1% vs. 68.8 ± 18.3%, respectively; p = 0.002). Following potentiation, the rate of decay of the MUFR-MUAPamps relationship decreased in both contractions, with a greater decrease among boys during the high-intensity contractions.

CONCLUSION

Lower PAP in the boys did not coincide with smaller changes in potentiated MU firing patterns, as boys had greater reductions in MUFRs with potentiation compared with men in high-intensity contractions.

An exploration of the motor unit behaviour during the concentric and eccentric phases of a squat task performed at different speeds

Despite squatting being important in strength training and rehabilitation, few studies have investigated motor unit (MU) behaviour. This study explored the MU behaviour of vastus medialis (VM) and vastus lateralis (VL) during the concentric and eccentric phases of a squat exercise performed at two speeds. Twenty-two participants had surface dEMG sensors attached over VM and VL, and IMUs recorded thigh and shank angular velocities. Participants performed squats at 15 and 25 repetitions per minute in a randomised order, and EMG signals were decomposed into their MU action potential trains. A four factor (muscle × speed × contraction phase × sexes) mixed methods ANOVA revealed significant main effects for MU firing rates between speeds, between muscles and between sexes, but not contraction phases. Post hoc analysis showed significantly greater MU firing rates and amplitudes in VM. A significant interaction was seen between speed and the contraction phases. Further analysis revealed significantly greater firing rates during the concentric compared to the eccentric phases, and between speeds during the eccentric phase only. VM and VL respond differently during squatting depending on speed and contraction phase. These new insights in VM and VL MU behvaviour may be useful when designing training and rehabilitation protocols.

Influence of Pennation Angle and Muscle Thickness on Mechanomyographic Amplitude-Torque Relationships and Sex-Related Differences in the Vastus Lateralis

This study examined potential sex-related differences and correlations among the pennation angle (PA), muscle thickness (MT), and mechanomyographic amplitude (MMG_RMS)-torque relationships of the vastus lateralis (VL) in 11 healthy males and 12 healthy females. The PA and MT of the VL were quantified with ultrasound. Participants performed an isometric muscle action of the knee extensors that linearly increased to 70% of maximal strength followed by a 12 s plateau. MMG was recorded from the VL. Linear regression models were fit to the log-transformed MMG_RMS-torque relationships to calculate b terms (slopes) for the linearly increasing segment. MMG_RMS was averaged during the plateau. Males exhibited greater PA (p < 0.001), MT (p = 0.027), b terms (p = 0.005), and MMG_RMS (p = 0.016). The b terms were strongly (p < 0.001, r = 0.772) and moderately correlated (p = 0.004, r = 0.571) with PA and MT, respectively, while MMG_RMS was moderately correlated with PA (p = 0.018, r = 0.500) and MT (p = 0.014, r = 0.515). The greater mechanical behavior of individuals possessing a larger PA and MT of the VL may reflect increased cross-bridge activity within the muscle fibers. Additionally, PA may help explain sex-related differences in MMG_RMS between sexes.

An examination of motor unit firing rates during steady torque of maximal efforts with either an explosive or slower rate of torque development

NEW FINDINGS

What is the central question of this study? The aim was to explore agonist and antagonist motor unit firing rates during maximal efforts performed with either an explosive or a slower rate of torque development. What is the main finding and its importance? The antagonist muscle presented a motor unit firing rate relationship similar to the agonist muscle. Additionally, the motor units of both muscles exhibited higher firing rates during explosive maximal contractions than during maximal contractions performed at a slower rate of torque development. These results could prove useful to future research analysing the effects of age, disease, resistance training and/or fatigue-related alterations to motor unit firing rates.

ABSTRACT

The primary purpose of the present study was to examine motor unit (MU) firing rates in agonist and antagonist muscles during periods of steady, maximal efforts using explosive and slower rates of torque development. A secondary purpose was to analyse the MU firing rate versus action potential amplitude relationships of the agonist and antagonist muscles during maximal efforts. Thirteen subjects (mean ± SD; age, 21.2 ± 3.6 years; mass 81.1 ± 21.3 kg; and stature, 177.1±9.9 cm) performed two maximal isometric trapezoid muscle actions of the elbow flexors that included either an explosive or a slower, linearly increasing rate (ramp) of torque development. Surface EMG signals of the biceps brachii (BB) and triceps brachii (TB) muscles were collected and decomposed into their constituent MU action potential trains. The MU firing rate versus action potential amplitude relationships of the BB (agonist) and TB (antagonist) muscles were analysed. Moderate to strong relationships (|r| ≥ 0.65) were present for the explosive and ramp contractions in the agonist and antagonist muscles. Firing rates of smaller and larger MUs were higher during the explosive [mean ± SD; agonist = 18.1 ± 6.9 pulses per second (pps), antagonist = 22.0±3.9 pps] than the ramp (agonist = 14.0 ± 5.1 pps, antagonist = 18.3 ± 4.4 pps) contractions for the agonist (P = 0.013) and antagonist muscles (P = 0.007). The antagonist muscle exhibits a similar MU firing rate versus action potential amplitude relationship to the agonist muscle at maximal efforts. Future research should investigate the effects of short-term resistance training on antagonist firing rates and the involvement of peripheral feedback on firing rates during maximal efforts performed at various rates of torque development.

Firing rate trajectories of human motor units during isometric ramp contractions to 10, 25 and 50% of maximal voluntary contraction

During low torque graded isometric contractions, motor units (MU) exhibit initial firing rate acceleration followed by saturation demonstrating a non-linear response attributed to persistent inward currents (PICs) which contribute to the net excitatory input. Firing rate saturation studies have been done exclusively at recruitment thresholds of low firing threshold MUs below 10% of isometric maximal voluntary contraction(MVC). It remains unclear whether later recruited (i.e. higher-threshold) MUs follow a similar firing rate trajectory as low-threshold units. Thus, MU firing rate trajectories were explored in relation to MU recruitment threshold (RT) at contraction levels between 10 and 50% of MVC. During graded isometric contractions to 10, 25 and 50% of MVC, single MU potentials were recorded from the tibialis anterior from 5 participants using tungsten microelectrodes. To characterize the firing rate trajectory, each MU train was fit by competing functions of torque as an exponential (i.e. saturated) and simple linear regression, using previous analysis methods (Fuglevand et al. 2015). Throughout a RT range of 0.02-41% of MVC, 261 MUs were compared. In 87% of MUs the better fit was by a linear function, whereas the remaining MUs (13%) were fit better with an exponential (saturated) firing rate trajectory. There was no statistical difference in the number of MUs better fit by the exponential function between low (<10% MVC) and relatively higher threshold MUs (>10% MVC; both p < 0.05). Increasing RT and rate of torque development (RTD) of the ramps were correlated with increased firing rate variability (larger error) in both fits (r = 0.3 and r = 0.4, both p < 0.01). Additionally, there was a 4-fold increase in peak antagonist surface electromyography (EMG) from 10 to 50% MVC contraction ramps. When all MUs were plotted with a normalized firing onset (i.e. 0% MVC) the data visually displayed an initial firing rate acceleration followed by a linear response (biphasic trajectory). Increased synaptic drive and greater antagonist surface EMG during moderate torque outputs may dampen PIC activity as compared with MUs during lower torque (<10% MVC) recruitment levels.

An examination of a potential organized motor unit firing rate and recruitment scheme of an antagonist muscle during isometric contractions

The primary purpose of the present study is to determine if an organized control scheme exists for the antagonist muscle during steady isometric torque. A secondary focus is to better understand how firing rates of the antagonist muscle change from a moderate- to higher-contraction intensity. Fourteen subjects performed two submaximal isometric trapezoid muscle actions of the forearm flexors that included a linearly increasing, steady force at both 40% and 70% maximum voluntary contraction, and linearly decreasing segments. Surface electromyographic signals of the biceps and triceps brachii were collected and decomposed into constituent motor unit action potential trains. Motor unit firing rate versus recruitment threshold, motor unit action potential amplitude versus recruitment threshold, and motor unit firing rate versus action potential amplitude relationships of the biceps brachii (agonist) and triceps brachii (antagonist) muscles were analyzed. Moderate- to-strong relationships (|r| ≥ 0.69) were present for the agonist and antagonist muscles for each relationship with no differences between muscles (P = 0.716, 0.428, 0.182). The y-intercepts of the motor unit firing rate versus recruitment threshold relationship of the antagonist did not increase from 40% to 70% maximal voluntary contractions (P = 0.96), unlike for the agonist (P = 0.009). The antagonist muscle exhibits a similar motor unit control scheme to the agonist. Unlike the agonist, however, the firing rates of the antagonist did not increase with increasing intensity. Future research should investigate how antagonist firing rates adapt to resistance training and changes in antagonist firing rates in the absence of peripheral feedback.NEW & NOTEWORTHY This is the first study to explore a potential motor unit control scheme and quantify changes in firing rates with increasing intensity of an antagonist muscle during isometric contractions. We demonstrate that the antagonist muscle possesses an organized motor unit firing rate and recruitment scheme similar to the agonist muscle during isometric forearm flexion, but unlike the agonist muscle, there was no significant increase in firing rates from a moderate- to higher-intensity isometric contraction.

Kinetic, muscle structure, and neuromuscular determinants of weight transfer phase prior to a lateral choice reaction step in older adults

The present study aimed to investigate the association between rate of torque development (RTD), rate of activation (RoA), and muscle structure [muscle cross-sectional area (CSA), intramuscular fat (IMAT) and high density lean muscle (HDL)] with the weight transfer phase (WTP) during a choice reaction step test (CST) in older adults. Fifteen healthy older adults (7 females) participated in this study. Stance leg hip adductors RTD at 100, 150, and 200 ms, showed a significant inverse correlation with WTP (r ≥ 0.658, P ≤ 0.010). There was a significant inverse relationship between WTP and adductor magnus and tensor fascia latae RoA at all time points (RoA0-50-RoA0-200; r ≥ 0.707, P ≤ 0.033). In contrast, the WTP was not significantly associated with the hip abductor RTD, gluteus medius RoA, or muscle structure (CSA, IMAT, and HDL). Swing leg showed no significant relationship between WTP and RTD, RoA or muscle structure of the hip abductor or adductor muscles. In conclusion, the present study showed that hip adductor torque-time capacity, as well as neuromuscular activation of the adductor magnus and tensor fascia latae of the stance leg during a maximal isometric test, is associated with the ability to transfer body weight before a step to the side occurs.

Application of Modern Multi-Sensor Holter in Diagnosis and Treatment

Modern Holter devices are very trendy tools used in medicine, research, or sport. They monitor a variety of human physiological or pathophysiological signals. Nowadays, Holter devices have been developing very fast. New innovative products come to the market every day. They have become smaller, smarter, cheaper, have ultra-low power consumption, do not limit everyday life, and allow comfortable measurements of humans to be accomplished in a familiar and natural environment, without extreme fear from doctors. People can be informed about their health and 24/7 monitoring can sometimes easily detect specific diseases, which are normally passed during routine ambulance operation. However, there is a problem with the reliability, quality, and quantity of the collected data. In normal life, there may be a loss of signal recording, abnormal growth of artifacts, etc. At this point, there is a need for multiple sensors capturing single variables in parallel by different sensing methods to complement these methods and diminish the level of artifacts. We can also sense multiple different signals that are complementary and give us a coherent picture. In this article, we describe actual interesting multi-sensor principles on the grounds of our own long-year experiences and many experiments.