How many motor units is enough? An assessment of the influence of the number of motor units on firing rate calculations.

The number of motor units included in calculations of mean firing rates varies widely in the literature. It is unknown how the number of decomposed motor units included in the calculation of firing rate per participant compares to the total number of active motor units in the muscle, and if this is different for males and females. Bootstrapped distributions and confidence intervals (CI) of mean motor unit firing rates decomposed from the tibialis anterior were used to represent the total number of active motor units for individual participants in trials from 20 to 100 % of maximal voluntary contraction. Bootstrapped distributions of mean firing rates were constructed using different numbers of motor units, from one to the maximum number for each participant, and compared to the CIs. A probability measure for each number of motor units involved in firing rate was calculated and then averaged across all individuals. Motor unit numbers required for similar levels of probability increased as contraction intensity increased (p < 0.001). Increased levels of probability also required higher numbers of motor units (p < 0.001). There was no effect of sex (p ≥ 0.97) for any comparison. This methodology should be repeated in other muscles, and aged populations.

Different discrete motor-unit activation patterns in the flexor carpi radialis in boys and men.

BACKGROUND: Lower activation of higher threshold (type-II) motor units (MUs) has been suggested in children compared with adults. We examined child-adult differences in discrete MU activation of the flexor carpi radialis (FCR). METHODS: Fifteen boys (10.2 ± 1.4 years), and 17 men (25.0 ± 2.7 years) completed 2 laboratory sessions. Following a habituation session, maximal voluntary isometric wrist flexion torque (MVIC) was determined before completing trapezoidal isometric contractions at 70%MVIC. Surface electromyography was captured by Delsys Trigno Galileo sensors and decomposed into individual MU action potential trains. Recruitment threshold (RT), and MU firing rates (MUFR) were calculated. RESULTS: MVIC was significantly greater in men (10.19 ± 1.92 Nm) than in boys (4.33 ± 1.47 Nm) (p < 0.05), but not statistically different after accounting for differences in body size. Mean MUFR was not different between boys (17.41 ± 7.83 pps) and men (17.47 ± 7.64 pps). However, the MUFR-RT slope was significantly (p < 0.05) steeper (more negative) in boys, reflecting a progressively greater decrease in MUFR with increasing RT. Additionally, boys recruited more of their MUs early in the ramped contraction. CONCLUSION: Compared with men, boys tended to recruit their MUs earlier and at a lower percentage of MVIC. This difference in MU recruitment may explain the greater decrease in MUFR with increasing RT in boys compared with men. Overall, these findings suggest an age-related difference in the neural strategy used to develop moderate-high torque in wrist flexors, where boys recruit more of their MUs earlier in the force gradation process, possibly resulting in a narrower recruitment range.

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.

Teaching Essential EMG Theory to Kinesiologists and Physical Therapists Using Analogies Visual Descriptions, and Qualitative Analysis of Biophysical Concepts.

Electromyography (EMG) is a multidisciplinary field that brings together allied health (kinesiology and physical therapy) and the engineering sciences (biomedical and electrical). Since the physical sciences are used in the measurement of a biological process, the presentation of the theoretical foundations of EMG is most conveniently conducted using math and physics. However, given the multidisciplinary nature of EMG, a course will most likely include students from diverse backgrounds, with varying levels of math and physics. This is a pedagogical paper that outlines an approach for teaching foundational concepts in EMG to kinesiologists and physical therapists that uses a combination of analogies, visual descriptions, and qualitative analysis of biophysical concepts to develop an intuitive understanding for those who are new to surface EMG. The approach focuses on muscle fiber action potentials (MFAPs), motor unit action potentials (MUAPs), and compound muscle action potentials (CMAPs) because changes in these waveforms are much easier to identify and describe in comparison to the surface EMG interference pattern (IP).

Influence of muscle fatigue on motor task performance of the hand and wrist: A systematic review.

Muscle fatigue is represented as a reduction in force production capability; however, fatigue does not necessarily result in performance impairments. As the distal upper limb serves as the end effector when interacting or manipulating objects, it is important to understand how muscle fatigue may impact motor functionality. The aim of this study was to systematically review the literature to identify how various aspects of motor performance of the distal upper limb are impaired following muscle fatigue. Four databases were searched using 23 search terms describing the distal upper limb, muscle fatigue, and various performance metrics. A total of 4561 articles were screened with a total of 28 articles extracted and critically appraised. Evidence extracted indicates that muscle fatigue results in unique impairments based on the type of motor performance being evaluated. Furthermore, much data suggests that muscle fatigue does not result in consistent, predictable performance impairments, particularly while performing submaximal tasks. Additionally, magnitude of fatigue does not directly correlate with reductions in performance outcomes at the hand and wrist. Fatiguing protocols used highlighted the importance of fatigue specificity. When fatiguing and performance tasks are similar, performance impairment is likely to be observed. The numerous muscles found in the hand and wrist, often considered redundant, play a critical role in maintaining task performance in the presence of muscle fatigue. The presence of motor abundance (e.g. multiple muscles with similar function) is shown to reduce the impairment in multiple performance metrics by compensating for reduced function of fatigued muscles. Continued exploration into various fatiguing protocols (i.e. maximal or submaximal) will provide greater insights into performance impairments in the distal upper limb.

Effects of the Proprioceptive Neuromuscular Facilitation Contraction Sequence on Motor Skill Learning-Related Increases in the Maximal Rate of Wrist Flexion Torque Development.

Background: The proprioceptive neuromuscular facilitation (PNF) reciprocal contraction pattern has the potential to increase the maximum rate of torque development. However, it is a more complex resistive exercise task and may interfere with improvements in the maximum rate of torque development due to motor skill learning, as observed for unidirectional contractions. The purpose of this study was to examine the cost-benefit of using the PNF exercise technique to increase the maximum rate of torque development. Methods: Twenty-six participants completed isometric maximal extension-to-flexion (experimental PNF group) or flexion-only (control group) contractions at the wrist. Ten of the assigned contractions were performed on each of three sessions separated by 48-h for skill acquisition. Retention was assessed with 5 contractions performed 2-weeks after acquisition. Torque and surface electromyographic (sEMG) activity were analyzed for evidence of facilitated contractions between groups, as well as alterations in muscle coordination assessed across test sessions. The criterion measures were: mean maximal isometric wrist flexion toque; the maximal rate of torque development (dτ/d t m a x ); root-mean-square error (RMSE) variability of the rate of torque versus torque phase-plane; the rate of wrist flexion muscle activation (Q 30); a coactivation ratio for wrist flexor and extensor sEMG activity; and wrist flexor electromechanical delay (EMD). Results: There were no significant differences between groups with respect to maximal wrist flexion torque, dτ/d t m a x or RMSE variability of torque trajectories. Both groups exhibited a progressive increase in maximal strength (+23.35% p < 0.01, η 2 = 0.655) and in dτ/d t m a x (+19.84% p = 0.08, η 2 = 0.150) from the start of acquisition to retention. RMSE was lowest after a 2-week rest interval (-18.2% p = 0.04, η 2 = 0.198). There were no significant differences between groups in the rate of muscle activation or the coactivation ratio. There was a reduction in coactivation that was retained after a 2-week rest interval (-32.60%, p = 0.02, η 2 = 0.266). Alternatively, EMD was significantly greater in the experimental group (Δ 77.43%, p < 0.01, η 2 = 0.809) across all sessions. However, both groups had a similar pattern of improvement to the third consecutive day of testing (-16.82%, p = 0.049, η 2 = 0.189), but returned close to baseline value after the 2-week rest interval. Discussion: The wrist extension-to-flexion contraction pattern did not result in a greater maximal rate of torque development than simple contractions of the wrist flexors. There was no difference between groups with respect to motor skill learning. The main adaptation in neuromotor control was a decrease in coactivation, not the maximal rate of muscle activation.

Anthropometrics and electromyography as predictors for maximal voluntary isometric wrist torque: Considerations for ergonomists.

The purpose of this study was to evaluate anthropometry and forearm muscle activity as predictors of maximal isometric wrist torque. Thirteen anthropometric measures, forearm electromyography from flexor carpi radialis (FCR) and extensor carpi radialis (ECR), and maximal isometric wrist flexion/extension torque were obtained from 25 male participants. Pearson correlation coefficients assessed relationships between peak isometric torque and: (1) anthropometrics, (2) FCR and ECR activation, (3) FCR/ECR antagonist/agonist coactivation ratios. Based on significant correlations, linear regression equations were developed (SPSS v.25; p < 0.05). Hand thickness, forearm circumference and ECR activation or hand thickness, elbow circumference, FCR activation and body weight were most highly correlated with extension or flexion torque, respectively. Hand thickness, forearm circumference, and ECR activation (R2 = 54.5%; p = 0.001) and hand thickness, elbow circumference, FCR activation (R2 = 68.3%; p < 0.001) explained similar variance in torque regressions as did the addition of body weight to extension (R2 = 58.0%; p = 0.001) and flexion (R2 = 69.9%; p < 0.001) torque regression equations, respectively. Circumference measurements, a pseudo for muscle size, and activation amplitude influenced wrist force output more than limb length or coactivation.

The effect of acute low-load resistance exercise with the addition of blood flow occlusion on muscle function in boys and men.

PURPOSE: In adults, low-load resistance training with blood flow occlusion (BFO) mimics strength increases that occur from high-load training, without the need to experience high mechanical stress. In view of child-adult differences in exercise responses, this study examined whether BFO during exercise elicits differential changes in maximal voluntary contraction (MVC) and electromyographical (EMG) activity in children and adults. METHODS: Sixteen men (24.4 ± 2.5 years) and 14 boys (10.7 ± 2.0 years) performed low-load resistance exercise (25 repetitions at 35% MVC) of the wrist flexors with and without BFO. MVC wrist flexor force and EMG activity of the flexor carpi radialis (FCR) were obtained at the beginning and end of the exercise. RESULTS: Both groups demonstrated a larger decrease in MVC force following BFO (- 18.6 ± 12.5%) than the control (without BFO) condition (- 6.2 ± 15.0%; p < 0.001). Whereas the men's EMG amplitude increased 16.3 ± 20.5% (p = 0.005) during BFO, the boys' EMG amplitude did not change over time or between conditions. In both groups, the mean power frequency (MPF) of the EMG signal decreased more during BFO (- 20.1 ± 9.6%; p < 0.001) than the control condition (- 5.6 ± 9.7%; p = 0.002). CONCLUSIONS: Low-load exercise with BFO resulted in similar neuromuscular responses between boys and men, except for an observed increase in the EMG amplitude in men but not boys. While this result might suggest that men relied on a greater activation of higher-threshold motor units during BFO, it does not explain why there were similar decreases in MPF between groups. Therefore, it remains unclear whether the effectiveness of BFO training is similar for children and adults.

Sex differences in the modulation of the motor unit discharge rate leads to reduced force steadiness.

The purpose of this study was to evaluate the relationship between the variability in the motor unit inter-pulse interval and force steadiness at submaximal and maximal force outputs between the sexes. Twenty-four male and 24 female participants were recruited to perform isometric dorsiflexion contractions at 20, 40, 60, 80, and 100% maximum voluntary contraction. Tibialis anterior myoelectric signal was recorded by an intramuscular electrode. Females had lower force steadiness (coefficient of variation of force (CoV-Force), 27.3%, p < 0.01) and a greater coefficient of variation of motor unit action potential inter-pulse interval (CoV-IPI), compared with males (9.6%, p < 0.01). There was no significant correlation between the normalized CoV-IPI and CoV-Force (r = 0.19, p > 0.01), but there was a significant repeated measures correlation between the raw scores for root-mean-square force error and the standard deviation of motor unit discharge rate (r = 0.65, p < 0.01). Females also had a greater incidence of doublet discharges on average across force levels (p < 0.01). The sex differences may result from motor unit behaviours (i.e., doublet and rapid discharges, synchronization, rate coding or recruitment), leading to lower force steadiness and greater CoV-IPI in females. Novelty: Sex differences in force steadiness may be due to neural strategies. Females have lower force steadiness compared with males. Greater incidence of doublet discharges in females may result in lesser force steadiness.

The effects of local forearm heating and cooling on motor unit properties during submaximal contractions.

NEW FINDINGS: What is the central question of this study? How do temperature manipulations affect motor unit (MU) properties during submaximal contractions to the same relative percentage of maximal force? What is the main finding and its importance? MU recruitment patterns are affected by temperature manipulations at the forearm. However, the relationship between MU potential amplitude and recruitment threshold indicates no change to the order or recruitment. Additionally, the MU potential amplitude and firing rate relationship was affected by temperature, suggesting that smaller MUs are more affected by temperature changes than larger MUs. ABSTRACT: Temperature impacts muscle contractile properties, such that experiments with workloads based on thermoneutral values will produce different relative intensities if maximal force changes due to muscle temperature. We investigated how temperature affected motor unit (MU) properties with contractions performed at the same normalized percentage of maximal force. Twenty participants (10 females) completed evoked, maximal, and trapezoidal voluntary contractions during thermoneutral-, hot-, and cold-temperature conditions. Forearm temperature was established using 25 min of neutral (∼32°C), hot (∼44°C) or cold (∼13°C) water circulated through a tube-lined sleeve. Flexor carpi radialis MU properties were assessed with contractions at 30% and 60% MVC relative to each temperature using surface electromyography decomposition. Changes to contractile properties and electromechanical delay from the evoked twitch suggest that muscle contractility was changed from the thermal manipulations (effect size (d) ≥ 0.42, P < 0.05). Maximal force was not different between neutral and hot conditions (d = 0.16, P > 0.05) but decreased in the cold (d ≥ 0.34, P < 0.05). For both contraction intensities, MU potential (MUP) amplitude was larger and duration was longer in the cold compared to neutral and hot conditions (d ≥ 1.24, P < 0.05). Cumulative probability density for the number of MUs recruited revealed differences in MU recruitment patterns among temperature conditions. The relationship between MU recruitment threshold and firing rate or MUP amplitude was not different among temperature conditions (P > 0.05); however, the relationship between MUP amplitude and firing rate was (P < 0.05). Local temperature manipulations appear to affect MU recruitment patterns, which may act as compensatory mechanisms to the changes in muscle viscosity and contractile properties due to local temperature changes.

Is the 'reverse onion skin' phenomenon more prevalent than we thought during intramuscular myoelectric recordings from low to maximal force outputs?

There are isolated instances in the literature that suggest the 'onion skin' phenomenon is not always present. That is, newly recruited high threshold motor units (MU) have higher discharge rates than previously recruited low threshold MUs. Therefore, the purpose of this paper was to investigate the presence of the 'onion skin' phenomenon in a large sample of intramuscular myoelectric recordings from low to maximal force outputs. Forty-eight participants performed rapid isometric dorsiflexion contractions at 20, 40, 60, 80 and 100 % MVC while intramuscular electrical activity was recorded. A bivariate frequency-distribution of the motor unit discharge rate and motor unit action potential peak-to-peak (P-P) amplitude was assessed. There was a significant difference in bivariate frequency-distribution across force levels (D's = 0.1083-0.3094, p's < 0.001). Newly recruited high threshold MUs did have lower discharge rates, but there was an increase in the presence of high threshold, large P-P amplitude MUs with higher discharge rates than lower threshold MUs (reverse onion skin) during the stable portion of the force output. The recruitment of high threshold MUs with higher discharge rates decreased the level of common drive from the cross-correlation (Rxy) = 0.79 at 20 % MVC to Rxy = 0.68 at 100 % MVC (p < 0.01), but it remained high. As the interference pattern becomes more complex with the recruitment of more motor units at higher force outputs, intramuscular electrodes may be more discriminating while recording motor unit activity leading to the identification of both the 'reverse onion skin' and 'onion skin' phenomenon being present.

The reliability of surface EMG derived motor unit variables.

Motor unit (MU) recordings obtained from surface electromyography (sEMG) decomposition are used to investigate the neural control of muscle in response to interventions, but our understanding of the longer-term reliability of MU variables is limited. This study examined the reliability of MU variables in the flexor carpi radialis (FCR) and tibialis anterior (TA) over a three-month period. Forty college-aged participants completed isometric wrist flexion (n = 20) and dorsiflexion (n = 20). There were 3 maximal isometric voluntary contractions (MVC) and 3 ramp contractions to 60% of MVC on four separate sessions separated by a total of 13 weeks. Intraclass correlation coefficients (ICC) were calculated from a fully nested ANOVA model. Maximal force was highly reliable (ICC = 0.94-0.99). The ICC values ranged from 0.49 to 0.92 for the FCR MU variables and from 0.58 to 0.96 for the TA MU variables. All MU variables exhibited a high degree of stability of means across test session and consistency within subjects, with the exception of the number of MUs detected in the TA. Poor ICC values did not reflect poor reliability but rather, convergence towards a narrow range of physiologically normal values. Surface EMG decomposition of a large population of MUs showed no differences in common drive between FCR (0.273) and for the TA (0.267) across test sessions. Forty percent of the sampled MUs in both muscles had a common drive of 0.30 or greater, which provides indirect support for the validity of the decompositions. MU variables may be used to monitor adaptations to a longer-term intervention study.

Sex differences in motor unit discharge rates at maximal and submaximal levels of force output.

This study evaluated potential sex differences in motor unit (MU) behaviour at maximal and submaximal force outputs. Forty-eight participants, 24 females and 24 males, performed isometric dorsiflexion contractions at 20%, 40%, 60%, 80%, and 100% of a maximum voluntary contraction (MVC). Tibialis anterior electromyography was recorded both by surface and intramuscular electrodes. Compared with males, females had a greater MU discharge rate (MUDR) averaged across all submaximal intensities (Δ 0.45 pps, 2.56%). Males exhibited greater increases in MUDR above 40% MVC, surpassing females at 100% MVC (p's < 0.01). Averaged across all force outputs, females had a greater incidence of doublet and rapid discharges and a greater percentage of MU trains with doublet and rapid (5-10 ms) discharges (Δ 75.55% and 61.48%, respectively; p's < 0.01). A subset of males (n = 8) and females (n = 8), matched for maximum force output, revealed that females had even greater MUDR (Δ 1.38 pps, 7.47%) and percentage of MU trains with doublet and rapid discharges (Δ 51.62%, 56.68%, respectively; p's < 0.01) compared with males at each force output, including 100% MVC. Analysis of the subset of strength-matched males and females suggest that sex differences in MU behaviour may be a result of females needing to generate greater neural drive to achieve fused tetanus. Novelty Females had higher MUDRs and greater percentage of MU trains with doublets across submaximal force outputs (20%-80% MVC). Differences were even greater for a strength matched subset. Differences in motor unit behaviour may arise from musculoskeletal differences, requiring greater neural drive in females.

A method for editing motor unit potential trains obtained by decomposition of surface electromyographic signals.

Rather than discarding motor unit potential trains (MUPTs) because they do not meet 100% validity criteria, we describe and evaluate a novel editing routine that preserves valid discharge times, based on decreasing shape variability (variance ratio, VR) within a MUPT. The error filtered estimation (EFE) algorithm is then applied to the remaining 'high confidence' discharge times to estimate inter-discharge interval (IDI) statistics. Decomposed surface EMG data from the flexor carpi radialis recorded from 20 participants during 60% MVC wrist flexion was used. There were two levels of denoising criteria (relaxed and strict) criteria for removing MUPs to decrease the VR and increase the signal-to-noise ratio (SNR) of a MUPT. In total, VR decreased 24.88% and SNR increased 6.0% (p's < 0.05). The MUP template peak-to-peak (P-P) amplitude and P-P duration were dependent on the level of denoising (p's < 0.05). The standard error of the estimate (SEE) of the mean IDI before and after editing using the relaxed criteria (3.2% versus 3.69%), was very similar (p > 0.05). The same was true for the SEE between denoising criteria, which increased only to 5.14% for the strict criteria (p > 0.05). Editing the MUPTs resulted in a significant decrease in MUP shape variability and in the measures extracted from the MUP templates, with trivial differences between the SEE of the mean IDI between the edited and unedited MUPTs.

The effects of local muscle temperature on force variability.

PURPOSE: Force variability is affected by environmental temperature, but whether the changes are from altered muscle temperature or proprioception are unclear. We tested how forearm muscle warming and cooling affected a force tracking task. METHODS: Twelve males and four females completed evoked, maximal, and isometric wrist flexion contractions (0-30% maximal) during thermoneutral-, warm-, and cold-muscle conditions. Forearm muscle temperature was manipulated using neutral (~ 33 °C), hot (~ 44 °C), or cold (~ 13 °C) water circulated through a tube-lined sleeve. Evoked and voluntary contractions were performed before and after thermal manipulations. RESULTS: Thermal manipulations altered contractile properties as evident in the twitch half-relaxation time, rate of force development, and duration (all P < 0.05), suggesting that muscle temperature was successfully altered. Changes in surface electromyography of the flexor carpi radialis root-mean-square amplitude and mean power frequency between temperature conditions (all P < 0.05) also indicate muscle temperature changes. No changes to root-mean-square error or variance ratio of the force trace were observed with muscle temperature changes (both P > 0.05). Muscle temperature changes did not have a consistent effect on coefficient of variation during each plateau of the staircase contraction. CONCLUSIONS: Our results suggest that the ability to perform a multi-plateaued isometric force task is not affected by changes to forearm muscle temperature. As the thermal manipulation was limited to the forearm, changes to hand temperature would be minimal, thus, proprioception in the wrist and hand was preserved allowing performance to be maintained. Therefore, modest changes to forearm muscle temperature are not likely to affect force variability if proprioception is maintained.

The cross education of strength and skill following unilateral strength training in the upper and lower limbs.

Cross education is the strength gain or skill improvement transferred to the contralateral limb following unilateral training or practice. The present study examined the transfer of both strength and skill following a strength training program. Forty participants (20M, 20F) completed a 6-wk unilateral training program of dominant wrist flexion or dorsiflexion. Strength, force variability, and muscle activity were assessed pretraining, posttraining, and following 6 wk of detraining (retention). Analyses of covariance compared the experimental limb (trained or untrained) to the control (dominant or nondominant). There were no sex differences in the training response. Cross education of strength at posttraining was 6% ( P < 0.01) in the untrained arm and 13% ( P < 0.01) in the untrained leg. Contralateral strength continued to increase following detraining to 15% in the arm ( P < 0.01) and 14% in the leg ( P < 0.01). There was no difference in strength gains between upper and lower limbs ( P > 0.05). Cross education of skill (force variability) demonstrated greater improvements in the untrained limbs compared with the control limbs during contractions performed without concurrent feedback. Significant increases in V-wave amplitude ( P = 0.02) and central activation ( P < 0.01) were highly correlated with contralateral strength gains. There was no change in agonist amplitude or motor unit firing rates in the untrained limbs ( P > 0.05). The neuromuscular mechanisms mirrored the force increases at posttraining and retention supporting central drive adaptations of cross education. The continued strength increases at retention identified the presence of motor learning in cross education, as confirmed by force variability. NEW & NOTEWORTHY: We examined cross education of strength and skill following 6 wk of unilateral training and 6 wk of detraining. A novel finding was the continued increase in contralateral strength following both training and detraining. Neuromuscular adaptations were highly correlated with strength gains in the trained and contralateral limbs. Motor learning was evident in the trained and contralateral limbs during contractions performed without concurrent feedback.

Core and skin temperature influences on the surface electromyographic responses to an isometric force and position task.

The large body of work demonstrating hyperthermic impairment of neuromuscular function has utilized maximal isometric contractions, but extrapolating these findings to whole-body exercise and submaximal, dynamic contractions may be problematic. We isolated and compared core and skin temperature influences on an isometric force task versus a position task requiring dynamic maintenance of joint angle. Surface electromyography (sEMG) was measured on the flexor carpi radialis at 60% of baseline maximal voluntary contraction while either pushing against a rigid restraint (force task) or while maintaining a constant wrist angle and supporting an equivalent inertial load (position task). Twenty participants performed each task at 0.5°C rectal temperature (Tre) intervals while being passively heated from 37.1±0.3°C to ≥1.5°C Tre and then cooled to 37.8±0.3°C, permitting separate analyses of core versus skin temperature influences. During a 3-s contraction, trend analysis revealed a quadratic trend that peaked during hyperthermia for root-mean-square (RMS) amplitude during the force task. In contrast, RMS amplitude during the position task remained stable with passive heating, then rapidly increased with the initial decrease in skin temperature at the onset of passive cooling (p = 0.010). Combined hot core and hot skin elicited shifts toward higher frequencies in the sEMG signal during the force task (p = 0.003), whereas inconsistent changes in the frequency spectra occurred for the position task. Based on the patterns of RMS amplitude in response to thermal stress, we conclude that core temperature was the primary thermal afferent influencing neuromuscular response during a submaximal force task, with minimal input from skin temperature. However, skin temperature was the primary thermal afferent during a position task with minimal core temperature influence. Therefore, temperature has a task-dependent impact on neuromuscular responses.

Effects of Plyometric and Resistance Training on Muscle Strength, Explosiveness, and Neuromuscular Function in Young Adolescent Soccer Players.

McKinlay, BJ, Wallace, P, Dotan, R, Long, D, Tokuno, C, Gabriel, D, and Falk, B. Effects of plyometric and resistance training on muscle strength, explosiveness, and neuromuscular function in young adolescent soccer players. J Strength Cond Res 32(11): 3039-3050, 2018-This study examined the effect of 8 weeks of free-weight resistance training (RT) and plyometric (PLYO) training on maximal strength, explosiveness, and jump performance compared with no added training (CON), in young male soccer players. Forty-one 11- to 13-year-old soccer players were divided into 3 groups (RT, PLYO, and CON). All participants completed isometric and dynamic (240°·s) knee extensions before and after training. Peak torque (pT), peak rate of torque development (pRTD), electromechanical delay (EMD), rate of muscle activation (Q50), m. vastus lateralis thickness (VLT), and jump performance were examined. Peak torque, pRTD, and jump performance significantly improved in both training groups. Training resulted in significant (p ≤ 0.05) increases in isometric pT (23.4 vs. 15.8%) and pRTD (15.0 vs. 17.6%), in RT and PLYO, respectively. During dynamic contractions, training resulted in significant increases in pT (12.4 and 10.8% in RT and PLYO, respectively), but not in pRTD. Jump performance increased in both training groups (RT = 10.0% and PLYO = 16.2%), with only PLYO significantly different from CON. Training resulted in significant increases in VLT (RT = 6.7% and PLYO = 8.1%). There were no significant EMD changes. In conclusion, 8-week free-weight resistance and plyometric training resulted in significant improvements in muscle strength and jump performance. Training resulted in similar muscle hypertrophy in the 2 training modes, with no clear differences in muscle performance. Plyometric training was more effective in improving jump performance, whereas free-weight RT was more advantageous in improving peak torque, where the stretch reflex was not involved.

The effects of local forearm muscle cooling on motor unit properties.

PURPOSE: Muscle cooling impairs maximal force. Using needle electromyography (EMG) to assess motor unit properties during muscle cooling, is limited and equivocal. Therefore, we aimed to determine the impact of local muscle cooling on motor unit firing properties using surface EMG decomposition. METHODS: Twenty participants (12 M, 8 F) completed maximal, evoked, and trapezoidal contractions during thermoneutral and cold muscle conditions. Forearm muscle temperature was manipulated using 10-min neutral (~ 32 °C) or 20-min cold (~ 3 °C) water baths. Twitches and maximal voluntary contractions were performed prior to, and after, forearm immersion in neutral or cold water. Motor unit properties were assessed during trapezoidal contractions to 50% baseline force using surface EMG decomposition. RESULTS: Impaired contractile properties from muscle cooling were evident in the twitch amplitude, duration, and rate of force development indicating that the muscle was successfully cooled from the cold water bath (all d ≥ 0.5, P < 0.05). Surface EMG decomposition showed muscle cooling increased the number of motor units (d = 0.7, P = 0.01) and motor unit action potential (MUAP) duration (d = 0.6, P < 0.001), but decreased MUAP amplitude (d = 0.2, P = 0.012). Individually, neither motor unit firing rates (d = 0.1, P = 0.843) nor recruitment threshold (d = 0.1, P = 0.746) changed; however, the relationship between the recruitment threshold and motor unit firing rate was steeper (d = 1.0, P < 0.001) and had an increased y-intercept (d = 0.9, P = 0.007) with muscle cooling. CONCLUSIONS: Since muscle contractility is impaired with muscle cooling, these findings suggest a compensatory increase in the number of active motor units, and small but coupled changes in motor unit firing rates and recruitment threshold to produce the same force.

Isometric and dynamic strength and neuromuscular attributes as predictors of vertical jump performance in 11- to 13-year-old male athletes.

In explosive contractions, neural activation is a major factor in determining the rate of torque development, while the latter is an important determinant of jump performance. However, the contribution of neuromuscular activation and rate of torque development to jump performance in children and youth is unclear. The purpose of this study was to examine the relationships between the rate of neuromuscular activation, peak torque, rate of torque development, and jump performance in young male athletes. Forty-one 12.5 ± 0.5-year-old male soccer players completed explosive, unilateral isometric and dynamic (240°/s) knee extensions (Biodex System III), as well as countermovement-, squat-, and drop-jumps. Peak torque (pT), peak rate of torque development (pRTD), and rate of vastus lateralis activation (Q30) during the isometric and dynamic contractions were examined in relation to attained jump heights. Isometric pT and pRTD were strongly correlated (r = 0.71) but not related to jump performance. Dynamic pT and pRTD, normalized to body mass, were significantly related to jump height in all 3 jumps (r = 0.38-0.66, p < 0.05). Dynamic normalized, but not absolute pRTD, was significantly related to Q30 (r = 0.35, p < 0.05). In young soccer players, neuromuscular activation and rate of torque development in dynamic contractions are related to jump performance, while isometric contractions are not. These findings have implications in the choice of training and assessment methods for young athletes.