Maximal discharge rate of motor units determines the maximal rate of force development during ballistic contractions in human

Investigating the impact of external load on muscle synergies during bipedal squats

PURPOSE

A broad functional movement repertoire is crucial for engaging in physical activity and reducing the risk of injury, both of which are central aspects of lifelong health. As a fundamental exercise in both recreational and rehabilitative training regimes, the bipedal squat (SQBp) incorporates many everyday movement patterns. Crucially, SQBp can only be considered functional if the practitioner can meet the coordinative demands. Many factors affect coordinative aspects of an exercise, most notably external load. Since compound movements are assumed to be organized in a synergistic manner, we employed muscle synergy analysis to examine differences in muscle synergy properties between various external load levels during SQBp.

METHODS

Ten healthy male recreational athletes were enrolled in the present study. Each participant performed three sets of ten SQBp on a smith machine at three submaximal load levels (50%, 62.5%, and 75% of 3 repetition maximum) across three non-consecutive days. Muscle activity was recorded from 12 prime movers of SQBp by way of electromyography (EMG). Muscle synergies were analyzed in terms of temporal activation patterns, i.e., waveform, as well as the relative input of each muscle into individual synergies, i.e., weight contribution.

RESULTS

Waveforms of muscle synergies did not differ between loads. Weight contributions showed significant differences between load levels, albeit only for the gastrocnemius muscle in a single synergy.

CONCLUSION

Taken together, our results imply mostly stable spatiotemporal composition of muscle activity during SQBp, underlining the importance of technical competence during compound movement performance in athletic and rehabilitative settings.

Sex differences in strength, functional capacity and mobility in patients with multiple sclerosis: An exploratory analysis

BACKGROUND

Physical exercise programs are commonly designed without consideration for sex differences. Nevertheless, disease progression exhibits sex-specific patterns, resulting in different functionality and strength performances.

OBJECTIVES

To analyze sex differences in strength, functional capacity, and mobility, and to evaluate sex-dependent differences in leg strength in multiple sclerosis (MS) patients.

METHODS

A cross-sectional study was conducted with 35 participants (female: n = 19; Expanded Disability Status Scale (EDSS)= 3.0 ± 1.2, male: n = 16; EDSS= 3.3 ± 1.2). Body composition, maximal voluntary isometric contraction (MVIC), explosive strength (rate of force development, RFD), central activation ratio (CAR), functional capacity, and mobility were assessed.

RESULTS

Differences were observed between males and females (p = 0.001) in height, lean body mass and MVIC. No differences were observed in the other variables. Regarding the leg asymmetry, men showed higher values in the stronger leg for both MVIC (p < 0.001, d=large) and RFD, whereas women showed higher values only in RFD. Men with MS demonstrated a greater capacity to produce maximal strength than women with this disease.

CONCLUSIONS

The results found suggest that maximum strength differs between men and women in our sample of patients with multiple sclerosis. Furthermore, the weaker leg, regardless of sex, exhibits poorer results in explosive strength compared to the stronger leg. However, maximum strength only shows differences in men and not in women. Therefore, these findings should serve as a basis for rehabilitation professionals when planning training programs for this population.

Why so slow? Models of parkinsonian bradykinesia

Bradykinesia, or slowness of movement, is a defining feature of Parkinson disease (PD) and a major contributor to the negative effects on quality of life associated with this disorder and related conditions. A dominant pathophysiological model of bradykinesia in PD has existed for approximately 30 years and has been the basis for the development of several therapeutic interventions, but accumulating evidence has made this model increasingly untenable. Although more recent models have been proposed, they also appear to be flawed. In this Perspective, I consider the leading prior models of bradykinesia in PD and argue that a more functionally related model is required, one that considers changes that disrupt the fundamental process of accurate information transmission. In doing so, I review emerging evidence of network level functional connectivity changes, information transfer dysfunction and potential motor code transmission error and present a novel model of bradykinesia in PD that incorporates this evidence. I hope that this model may reconcile inconsistencies in its predecessors and encourage further development of therapeutic interventions.

The impact of cerebellar transcranial direct current stimulation on isometric bench press performance in trained athletes

Athletic development centers on optimizing performance, including technical skills and fundamental motor abilities such as strength and speed. Parameters such as maximum contraction force and rate of force development, influence athletic success, although performance gains become harder to achieve as athletic abilities increase. Non-invasive transcranial direct current stimulation of the cerebellum (CB-tDCS) has been used successfully to increase force production in novices, although the potential effects in athletes remain unexplored. The present study examined the effects of CB-tDCS on maximum isometric voluntary contraction force (MVCiso) and isometric rate of force development (RFDiso) during a bench press task in well-trained athletes. 21 healthy, male, strength-trained athletes participated in a randomized, sham-controlled, double-blinded crossover design. Each participant completed the isometric bench press (iBP) task on two separate days, with at least 5 days between sessions, while receiving either CB-tDCS or sham stimulation. Electromyography (EMG) recordings of three muscles involved in iBP were acquired bilaterally to uncover differences in neuromuscular activation and agonist-antagonist co-contraction between conditions. Contrary to our hypothesis, no significant differences in MVCiso and RFDiso were observed between CB-tDCS and sham conditions. Furthermore, no tDCS-induced differences in neuromuscular activation or agonist-antagonist co-contraction were revealed. Here, we argue that the effects of CB-tDCS on force production appear to depend on the individual's training status. Future research should study individual differences in tDCS responses between athletes and novices, as well as the potential of high-definition tDCS for precise brain region targeting to potentially enhance motor performance in athletic populations.

Evaluating peripheral neuromuscular function with brief movement-evoked pain

Movement-evoked pain is an understudied manifestation of musculoskeletal conditions that contributes to disability, yet little is known about how the neuromuscular system responds to movement-evoked pain. The present study examined whether movement-evoked pain impacts force production, electromyographic (EMG) muscle activity, and the rate of force development (RFD) during submaximal muscle contractions. Fifteen healthy adults (9 males; age = 30.3 ± 10.2 yr, range = 22-59 yr) performed submaximal isometric first finger abduction contractions without pain (baseline) and with movement-evoked pain induced by laser stimulation to the dorsum of the hand. Normalized force (% maximal voluntary contraction) and RFD decreased by 11% (P < 0.001) and 15% (P = 0.003), respectively, with movement-evoked pain, without any change in normalized peak EMG (P = 0.77). Early contractile RFD, force impulse, and corresponding EMG amplitude computed within time segments of 50, 100, 150, and 200 ms relative to the onset of movement were also unaffected by movement-evoked pain (P > 0.05). Our results demonstrate that movement-evoked pain impairs peak characteristics and not early measures of submaximal force production and RFD, without affecting EMG activity (peak and early). Possible explanations for the stability in EMG with reduced force include antagonist coactivation and a reorganization of motoneuronal activation strategy, which is discussed here.NEW & NOTEWORTHY We provide neurophysiological evidence to indicate that peak force and rate of force development are reduced by movement-evoked pain despite a lack of change in EMG and early rapid force development in the first dorsal interosseous muscle. Additional evidence suggests that these findings may coexist with a reorganization in motoneuronal activation strategy.

Model-based closed-loop control of thalamic deep brain stimulation

Introduction: Closed-loop control of deep brain stimulation (DBS) is beneficial for effective and automatic treatment of various neurological disorders like Parkinson's disease (PD) and essential tremor (ET). Manual (open-loop) DBS programming solely based on clinical observations relies on neurologists' expertise and patients' experience. Continuous stimulation in open-loop DBS may decrease battery life and cause side effects. On the contrary, a closed-loop DBS system uses a feedback biomarker/signal to track worsening (or improving) of patients' symptoms and offers several advantages compared to the open-loop DBS system. Existing closed-loop DBS control systems do not incorporate physiological mechanisms underlying DBS or symptoms, e.g., how DBS modulates dynamics of synaptic plasticity. Methods: In this work, we propose a computational framework for development of a model-based DBS controller where a neural model can describe the relationship between DBS and neural activity and a polynomial-based approximation can estimate the relationship between neural and behavioral activities. A controller is used in our model in a quasi-real-time manner to find DBS patterns that significantly reduce the worsening of symptoms. By using the proposed computational framework, these DBS patterns can be tested clinically by predicting the effect of DBS before delivering it to the patient. We applied this framework to the problem of finding optimal DBS frequencies for essential tremor given electromyography (EMG) recordings solely. Building on our recent network model of ventral intermediate nuclei (Vim), the main surgical target of the tremor, in response to DBS, we developed neural model simulation in which physiological mechanisms underlying Vim-DBS are linked to symptomatic changes in EMG signals. By using a proportional-integral-derivative (PID) controller, we showed that a closed-loop system can track EMG signals and adjust the stimulation frequency of Vim-DBS so that the power of EMG reaches a desired control target. Results and discussion: We demonstrated that the model-based DBS frequency aligns well with that used in clinical studies. Our model-based closed-loop system is adaptable to different control targets and can potentially be used for different diseases and personalized systems.

Fatigue effects on the knee flexors neuromuscular parameters during repeated sprinting

BACKGROUND

To identify at which point fatigue on neuromuscular parameters occurs in the knee flexors during a repeated sprint protocol.

METHODS

Physical active males without previous hamstring strain injury were recruited. Neuromuscular parameters such as peak torque (PT) and rate of torque development (RTD) were assessed after every two sprints in a 5 × (2 x 30 m) repeated sprint protocol.

RESULTS

Twenty physical active males participated in the study. A significant effect of sprint number was found (p < 0.001; η2p = 0.643) with a decreased sprint speed by 6.9% from fastest to slowest sprint. No significant differences were observed in the time between finishing the sprint and performing the first MVIC (46.3 ± 4.7s; p = 0.423), nor in the time between finishing a set and starting the next set (121.2 ± 7.6s; p = 0.503). Regarding neuromuscular parameters, the only significant difference found was in PT between before and after two sprints (117.95 ± 5.61 N⋅m vs. 110.64 ± 5.71 N⋅m; p = 0.048, d = 0.289) and on RTD 0-50ms before and after ten sprints (465.78 ± 223.76 N⋅m/s vs. 382.30 ± 189.56 N⋅m/s; p = 0.008; η2p = 0.149).

CONCLUSIONS

A recovery time of 46s between sprints and testing neuromuscular parameters (due to experimental design) seems sufficient to restore the neuromuscular system. Therefore, it can be suggested that time recovery is the principal factor in detecting fatigue on neuromuscular parameters.

Decreased neural drive affects the early rate of force development after repeated burst-like isometric contractions

The neural drive to the muscle is the primary determinant of the rate of force development (RFD) in the first 50 ms of a rapid contraction. It is still unproven if repetitive rapid contractions specifically impair the net neural drive to the muscles. To isolate the fatiguing effect of contraction rapidity, 17 male adult volunteers performed 100 burst-like (i.e., brief force pulses) isometric contractions of the knee extensors. The response to electrically-evoked single and octet femoral nerve stimulation was measured with high-density surface electromyography (HD-sEMG) from the vastus lateralis and medialis muscles. Root mean square (RMS) of each channel of HD-sEMG was normalized to the corresponding M-wave peak-to-peak amplitude, while muscle fiber conduction velocity (MFCV) was normalized to M-wave conduction velocity to compensate for changes in sarcolemma properties. Voluntary RFD 0-50 ms decreased (d = -0.56, p < 0.001) while time to peak force (d = 0.90, p < 0.001) and time to RFDpeak increased (d = 0.56, p = 0.034). Relative RMS (d = -1.10, p = 0.006) and MFCV (d = -0.53, p = 0.007) also decreased in the first 50 ms of voluntary contractions. Evoked octet RFD 0-50 ms (d = 0.60, p = 0.020), M-wave amplitude (d = 0.77, p = 0.009) and conduction velocity (d = 1.75, p < 0.001) all increased. Neural efficacy, i.e., voluntary/octet force ratio, largely decreased (d = -1.50, p < 0.001). We isolated the fatiguing impact of contraction rapidity and found that the decrement in RFD, particularly when calculated in the first 50 ms of muscle contraction, can mainly be explained by a decrease in the net neural drive.

Unique Neural Mechanisms Underlying Speed Control of Low-Force Ballistic Contractions

According to the speed-control hypothesis, the rate of force development (RFD) during ballistic contractions is dictated by force amplitude because time to peak force (TPF) remains constant regardless of changes in force amplitude. However, this hypothesis has not been tested at force levels below 20% of an individual's maximum voluntary contraction (MVC). Here, we examined the relationship between the RFD and force amplitude from 2 to 85% MVC and the underlying structure of muscle activity in 18 young adults. Participants exerted ballistic index finger abductions for 50 trials in each of seven randomly assigned force levels (2, 5, 15, 30, 50, 70, and 85% MVC). We quantified TPF, RFD, and various EMG burst characteristics. Contrary to the speed-control hypothesis, we found that TPF was not constant, but significantly varied from 2 to 85% MVC. Specifically, the RFD slope from 2 to 15% MVC was greater than the RFD slope from 30 to 85% MVC. Longer TPF at low force levels was associated with the variability of EMG burst duration, whereas longer TPF with higher force levels was associated with the EMG burst integral. Contrary to the speed-control hypothesis, we found that the regulation of TPF for low and high force levels was different, suggesting that neuronal variability is critical for force levels below 30% MVC and neuronal amplitude for force levels above 30% MVC. These findings present compelling new evidence highlighting the limitations of the speed-control hypothesis underscoring the need for a new theoretical framework.

Predictive Contribution of the Superficial Neck Muscles to Short-Latency Rate of Force Development of the Head and Neck

PURPOSE

To evaluate the contribution of splenius capitis, sternocleidomastoid, and upper fibers of trapezius activation to the gains in rate of force development (RFD) of the head and neck during maximum voluntary ballistic contractions.

METHODS

RFD gain was facilitated by a single-session intervention for maximum voluntary ballistic contractions in the anterior direction, oriented at 45° to the midsagittal plane, which require active restraint of axial rotation. Muscle activation for the agonist (sternocleidomastoid) and 2 antagonists (splenius capitis and upper fibers of trapezius) was evaluated. The study sample included 12 physically active men (mean age, 22.6 y). RFD (N·m·s-1; 0-100 ms) and integrated muscle activity (50 ms before and 100 ms after force onset) were measured at 10 minutes, 20 minutes, and 2 days postintervention, relative to baseline. Muscle activation predictive of RFD gains was evaluated by linear regression analysis. RFD reproducibility was evaluated using the coefficient of variation of the typical error.

RESULTS

The intervention yielded a 1.95- to 2.39-fold RFD gain (P ≤ .05), with greater RFD gain for participants with a lower peak moment of force (<10.9 N·m) than those with a higher peak moment (≥10.9 N·m) at baseline (P ≤ .002). For the low group, 65% to 74% of the RFD gain was predicted by ipsilateral sternocleidomastoid activation, with ipsilateral splenius capitis activation predicting 77% to 92% of RFD gain for the high group. Absolute peak and impulse of static force were greater for the high than for the low group (P ≤ .04). RFD reproducibility was high (coefficient of variation of the typical error ≤ 14.4%).

CONCLUSIONS

The agonist- and antagonist-focused synergies might reflect different functional priorities, higher RFD gain compared with higher head-neck force.

Analysis of the rate of force development reveals high neuromuscular fatigability in elderly patients with chronic kidney disease

BACKGROUND

Chronic kidney disease (CKD) induces muscle wasting and a reduction in the maximum voluntary force (MVF). Little is known about the neuromuscular fatigability in CKD patients, defined as the reduction of muscle force capacities during exercise. Neuromuscular fatigability is a crucial physical parameter of the daily living. The quantification of explosive force has been shown to be a sensitive means to assess neuromuscular fatigability. Thus, our study used explosive force estimates to assess neuromuscular fatigability in elderly CKD patients.

METHODS

Inclusion criteria for CKD patients were age ≥ 60 years old and glomerular filtration rate (GFR) < 45 mL/min/1.73 m2 not on dialysis, and those for controls were GFR > 60 mL/min/1.73 m2 , age and diabetes matched. The fatigability protocol focused on a handgrip task coupled with surface electromyography (sEMG). Scalars were extracted from the rate of force development (RFD): absolute and normalized time periods (50, 75, 100, 150 and 200 ms, RFD50 , RFD75 , RFD100 , RFD150 and RFD200 , respectively), peak RFD (RFDpeak in absolute; NRFDpeak normalized), time-to-peak RFD (t-RFDpeak ) and the relative force at RFDpeak (MVF-RFDpeak ). A statistical parametric mapping approach was performed on the force, impulse and RFD-time curves. The integrated sEMG with time at 0-30, 0-50, 0-100 and 0-200 ms time intervals relative to onset of sEMG activity was extracted and groups were compared separately for each sex.

RESULTS

The cohort of 159 individuals had a median age of 69 (9IQR ) years and body mass index was 27.6 (6.2IQR ) kg/m2 . Propensity-score-matched groups balanced CKD patients and controls by gender with 66 males and 34 females. In scalar analysis, CKD patients manifested a higher decrement than controls in the early phase of contraction, regarding the NRFDpeak (P = 0.009; η2 p  = 0.034) and RFD75 and RFD100 (for both P < 0.001; η2 p  = 0.068 and 0.064). The one-dimensional analysis confirmed that CKD males manifest higher and delayed neuromuscular fatigability, especially before 100 ms from onset of contraction. sEMG was lower in CKD patients than controls in the 0-100 ms (at rest: P = 0.049, Cohen's d = 0.458) and 0-200 ms (at rest: P = 0.016, Cohen's d = 0.496; during exercise: P = 0.006, Cohen's d = 0.421) time windows. Controls showed greater decrease of sEMG than CKD patients in the 0-30 ms (P = 0.020, Cohen's d = 0.533) and 0-50 ms (P = 0.010, Cohen's d = 0.640) time windows. As opposite to females, males showed almost the same differences between groups.

CONCLUSIONS

Our study is the first to show that CKD patients have higher fatigability than controls, which may be associated with an impaired motor-unit recruitment, highlighting a neural drive disturbance with CKD. Further studies are needed to confirm these findings.

Muscarinic acetylcholine activity modulates cortical silent period, but not motor evoked potentials, during muscle contractions

This study used transcranial magnetic stimulation (TMS) to determine if muscarinic receptor blockade affects muscle responses during voluntary contractions. Motor evoked potentials (MEPs) were recorded from biceps brachii in 10 subjects (age: 23 ± 2) during 10%, 25%, 50%, 75%, and 100% maximal voluntary contractions (MVCs). Each contraction intensity was examined under non-fatigued and fatigued conditions. All measurements were obtained post-ingestion of 25 mg promethazine or placebo. MEP area and the duration of the TMS-evoked silent period (SP) were calculated for all contractions. No drug-related differences were detected for MEP area during non-fatigued or fatigued contractions. A main effect of drug was detected for the SP (p = 0.019) where promethazine increased SP duration by an average of 0.023 [Formula: see text] 0.015 s. This drug effect was only identified for the unfatigued contractions and not following the sustained fatiguing contractions (p = 0.105). The cholinergic system does not influence corticospinal excitability during voluntary muscle contractions, but instead affects neural circuits associated with the TMS-evoked SP. Given the prevalence of cholinergic properties in prescription and over-the-counter medications, the current study enhances our understanding of mechanisms that may contribute to motor side-effects.

Cervical Proprioception Assessed through Targeted Head Repositioning: Validation of a Clinical Test Based on Optoelectronic Measures

Neck proprioception is commonly assessed with head repositioning tests. In such a test, an operator rotates the head of a blindfolded individual to a target position. After returning to the rest position, the participant actively repositions the head to the target. Joint Position Error (JPE) is the angular difference between the target angle (however oriented in a 3D space) and the actively reached positions (the smaller the difference, the better the proprioception). This study aimed to validate a head-to-target (HTT) repositioning test using an optoelectronic system for also measuring the components of the JPE in the horizontal, frontal, and sagittal planes. The head movements requested by the operator consisted of 30° left-right rotations and 25° flexion-extension. The operators or subjects could not obtain these movements without modest rotations in other planes. Two operators were involved. Twenty-six healthy participants (13 women) were recruited (mean (SD): 33.4 (6.3) years). The subjects’ JPE in the requested (intended) plane of motion (JPEint-component) was a few degrees only and smaller for flexion-extensions than for left-right rotations (right rotation: 5.39° (5.29°); left rotation: 5.03° (4.51°), extension: 1.79° (3.94°); flexion: 0.54° (4.35°)). Participants’ average error in unintended planes was around 1° or less. Inter-operator consistency and agreement were high. The smallest detectable change, at p < 0.05, for JPEint-component ranged between 4.5° and 6.98°. This method of optoelectronic measurement in HTT repositioning tests provides results with good metric properties, fostering application to clinical studies.

Flexor hallucis brevis motor unit behavior in response to moderate increases in rate of force development

Background

Studies on motor unit behaviour with varying rates of force development have focussed predominantly on comparisons between slow and ballistic (i.e., very fast) contractions. It remains unclear how motor units respond to less extreme changes in rates of force development. Here, we studied a small intrinsic foot muscle, flexor hallucis brevis (FHB) where the aim was to compare motor unit discharge rates and recruitment thresholds at two rates of force development. We specifically chose to investigate relatively slow to moderate rates of force development, not ballistic, as the chosen rates are more akin to those that presumably occur during daily activity.

Methods

We decomposed electromyographic signals to identify motor unit action potentials obtained from indwelling fine-wire electrodes in FHB, from ten male participants. Participants performed isometric ramp-and-hold contractions from relaxed to 50% of a maximal voluntary contraction. This was done for two rates of force development; one with the ramp performed over 5 s (slow condition) and one over 2.5 s (fast condition). Recruitment thresholds and discharge rates were calculated over the ascending limb of the ramp and compared between the two ramp conditions for matched motor units. A repeated measures nested linear mixed model was used to compare these parameters statistically. A linear repeated measures correlation was used to assess any relationship between changes in recruitment threshold and mean discharge rate between the two conditions.

Results

A significant increase in the initial discharge rate (i.e., at recruitment) in the fast (mean: 8.6 ±  2.4 Hz) compared to the slow (mean: 7.8 ± 2.3 Hz) condition (P = 0.027), with no changes in recruitment threshold (P = 0.588), mean discharge rate (P = 0.549) or final discharge rate (P = 0.763) was observed. However, we found substantial variability in motor unit responses within and between conditions. A small but significant negative correlation (R² = 0.33, P = 0.003) was found between the difference in recruitment threshold and the difference in mean discharge rate between the two conditions.

Conclusion

These findings suggest that as force increases for contractions with slower force development, increasing the initial discharge rate of recruited motor units produces the increase in rate of force development, without a change in their recruitment thresholds, mean or final discharge rate. However, an important finding was that for only moderate changes in rate of force development, as studied here, not all units respond similarly. This is different from what has been described in the literature for ballistic contractions in other muscle groups, where all motor units respond similarly to the increase in neural drive. Changing the discharge behaviour of a small group of motor units may be sufficient in developing force at the required rate rather than having the discharge behaviour of the entire motor unit pool change equally.

Ergogenic Effects of Caffeine on Ballistic (Throwing) Performance: A Meta-Analytical Review

Ballistic exercise is characterized by high velocity, force, and muscle activation. Typical examples of ballistic exercise are jumping and throwing activities. While several studies explored caffeine’s effects on throwing performance, the between study findings varied. Therefore, we performed a meta-analysis exploring caffeine’s effects on throwing performance (e.g., shot put, medicine ball throw, bench press throw). Seven databases were searched for eligible research. Ten studies (n = 151) were included. In the main meta-analysis, there was a significant ergogenic effect of caffeine on throwing performance (standardized mean difference [SMD]: 0.19; 95% confidence interval [CI]: 0.05, 0.33; p = 0.007). There was a significant ergogenic effect of caffeine in the subgroup analysis for studies that evaluated throwing velocity (SMD: 0.24; 95% CI: 0.10, 0.37; p = 0.0006) and used caffeine doses ≤3 mg/kg (SMD: 0.18; 95% CI: 0.05, 0.31; p = 0.006). There was no significant difference between caffeine and placebo in the subgroup analysis for studies that evaluated throwing distance (SMD: 0.15; 95% CI: −0.09, 0.40; p = 0.22) and used caffeine doses >3 mg/kg, (SMD: 0.17; 95% CI: −0.08, 0.41; p = 0.19). However, after one outlier study was excluded as part of a sensitivity analysis, an ergogenic effect was also observed for throwing distance and caffeine doses >3 mg/kg. Based on the results of this review, we conclude that individuals interested in the acute enhancement of throwing performance may consider caffeine supplementation.

Age-related performance fatigability: a comprehensive review of dynamic tasks

Adult ageing is associated with a myriad of changes within the neuromuscular system, leading to reductions in contractile function of old adults. One of the consequences of these age-related neuromuscular adaptations is altered performance fatigability, which can limit the ability of old adults to perform activities of daily living. Whereas age-related fatigability of isometric tasks has been well characterized, considerably less is known about fatigability of old adults during dynamic tasks involving movement about a joint, which provides a more functionally relevant task compared to static contractions. This review provides a comprehensive summary of age-related fatigability in dynamic contractions, where the importance of task specificity is highlighted with a brief discussion of the potential mechanisms responsible for differences in fatigability between young and old adults. The angular velocity of the task is critical for evaluating age-related fatigability, as tasks which constrain angular velocity (i.e., isokinetic) produce equivocal age-related differences in fatigability, whereas tasks involving unconstrained velocity (i.e., isotonic-like) consistently induce greater fatigability of old compared to young adults. These unconstrained velocity tasks, that are more closely associated with natural movements, offer an excellent model to uncover the underlying age-related mechanisms of increased fatigability. Future work evaluating the mechanisms of increased age-related fatigability of dynamic tasks should be evaluated using task-specific contractions (i.e., dynamic), particularly for assessment of spinal and supra-spinal components. Advancing our understanding of age-related fatigability is likely to yield novel insights and approaches for improving mobility limitations in old adults.

Startling stimuli increase maximal motor unit discharge rate and rate of force development in humans

Maximal rate of force development in adult humans is determined by the maximal motor unit discharge rate, however the origin of the underlying synaptic inputs remains unclear. Here, we tested a hypothesis that the maximal motor unit discharge rate will increase in response to a startling cue, a stimulus that purportedly activates the pontomedullary reticular formation neurons that make mono- and disynaptic connections to motoneurons via fast-conducting axons. Twenty-two men were required to produce isometric knee extensor forces "as fast and as hard" as possible from rest to 75% of maximal voluntary force, in response to visual (VC), visual-auditory (VAC; 80 dB), or visual-startling cue (VSC; 110 dB). Motoneuron activity was estimated via decomposition of high-density surface electromyogram recordings over the vastus lateralis and medialis muscles. Reaction time was significantly shorter in response to VSC compared to VAC and VC. The VSC further elicited faster neuromechanical responses including a greater number of discharges per motor unit per second and greater maximal rate of force development, with no differences between VAC and VC. We provide evidence, for the first time, that the synaptic input to motoneurons increases in response to a startling cue, suggesting a contribution of subcortical pathways to maximal motoneuron output in humans.

Circulating testosterone and dehydroepiandrosterone are associated with individual motor unit features in untrained and highly active older men

Long-term exercise training has been considered as an effective strategy to counteract age-related hormonal declines and minimise muscle atrophy. However, human data relating circulating hormone levels with motor nerve function are scant. The aims of the study were to explore associations between circulating sex hormone levels and motor unit (MU) characteristics in older men, including masters athletes competing in endurance and power events. Forty-three older men (mean ± SD age: 69.9 ± 4.6 years) were studied based on competitive status. The serum concentrations of dehydroepiandrosterone (DHEA), total testosterone (T) and estradiol were quantified using liquid chromatography mass spectrometry. Intramuscular electromyographic signals were recorded from vastus lateralis (VL) during 25% of maximum voluntary isometric contractions and processed to extract MU firing rate (FR), and motor unit potential (MUP) features. After adjusting for athletic status, MU FR was positively associated with DHEA levels (p = 0.019). Higher testosterone and estradiol were associated with lower MUP complexity; these relationships remained significant after adjusting for athletic status (p = 0.006 and p = 0.019, respectively). Circulating DHEA was positively associated with MU firing rate in these older men. Higher testosterone levels were associated with reduced MUP complexity, indicating reduced electrophysiological temporal dispersion, which is related to decreased differences in conduction times along axonal branches and/or MU fibres. Although evident in males only, this work highlights the potential of hormone administration as a therapeutic interventional strategy specifically targeting human motor units in older age.

The effect of small changes in rate of force development on muscle fascicle velocity and motor unit discharge behaviour

When rate of force development is increased, neural drive increases. There is presently no accepted explanation for this effect. We propose and experimentally test the theory that a small increase in rate of force development increases medial gastrocnemius fascicle shortening velocity, reducing the muscle’s force-generating capacity, leading to active motor units being recruited at lower forces and with increased discharge frequencies. Participants produced plantar flexion torques at three different rates of force development (slow: 2% MVC/s, medium: 10% MVC/s, fast: 20% MVC/s). Ultrasound imaging showed that increased rate of force development was related to higher fascicle shortening velocity (0.4 ± 0.2 mm/s, 2.0 ± 0.9 mm/s, 4.1 ± 1.9 mm/s in slow, medium, fast, respectively). In separate experiments, medial gastrocnemius motor unit recruitment thresholds and discharge frequencies were measured using fine-wire electromyography (EMG), together with surface EMG. Recruitment thresholds were lower in the fast (12.8 ± 9.2% MVC) and medium (14.5 ± 9.9% MVC) conditions compared to the slow (18.2 ± 8.9% MVC) condition. The initial discharge frequency was lower in the slow (5.8 ± 3.1 Hz) than the fast (6.7 ± 1.4 Hz), but not than the medium (6.4 ± 2.4 Hz) condition. The surface EMG was greater in the fast (mean RMS: 0.029 ± 0.017 mV) compared to the slow condition (0.019 ± 0.013 mV). We propose that the increase in muscle fascicle shortening velocity reduces the force-generating capacity of the muscle, therefore requiring greater neural drive to generate the same forces.

The role of the neural stimulus in regulating skeletal muscle hypertrophy

Resistance training is frequently performed with the goal of stimulating muscle hypertrophy. Due to the key roles motor unit recruitment and mechanical tension play to induce muscle growth, when programming, the manipulation of the training variables is oriented to provoke the correct stimulus. Although it is known that the nervous system is responsible for the control of motor units and active muscle force, muscle hypertrophy researchers and trainers tend to only focus on the adaptations of the musculotendinous unit and not in the nervous system behaviour. To better guide resistance exercise prescription for muscle hypertrophy and aiming to delve into the mechanisms that maximize this goal, this review provides evidence-based considerations for possible effects of neural behaviour on muscle growth when programming resistance training, and future neurophysiological measurement that should be tested when training to increase muscle mass. Combined information from the neural and muscular structures will allow to understand the exact adaptations of the muscle in response to a given input (neural drive to the muscle). Changes at different levels of the nervous system will affect the control of motor units and mechanical forces during resistance training, thus impacting the potential hypertrophic adaptations. Additionally, this article addresses how neural adaptations and fatigue accumulation that occur when resistance training may influence the hypertrophic response and propose neurophysiological assessments that may improve our understanding of resistance training variables that impact on muscular adaptations.

Blockade of 5-HT2 receptors suppresses rate of torque development and motor unit discharge rate during rapid contractions

Serotonin (5-HT) is a neuromodulator that is critical for regulating the excitability of spinal motoneurons and the generation of muscle torque. However, the role of 5-HT in modulating human motor unit activity during rapid contractions has yet to be assessed. Nine healthy participants (23.7 ± 2.2 yr) ingested 8 mg of the competitive 5-HT₂ antagonist cyproheptadine in a double-blinded, placebo-controlled, repeated-measures experiment. Rapid dorsiflexion contractions were performed at 30%, 50% and 70% of maximal voluntary contraction (MVC), where motor unit activity was assessed by high-density surface electromyographic decomposition. A second protocol was performed where a sustained, fatigue-inducing dorsiflexion contraction was completed prior to undertaking the same 30%, 50% and 70% MVC rapid contractions and motor unit analysis. Motor unit discharge rate (p < 0.001) and rate of torque development (RTD; p = 0.019) for the unfatigued muscle were both significantly lower for the cyproheptadine condition. Following the fatigue inducing contraction, cyproheptadine reduced motor unit discharge rate (p < 0.001) and RTD (p = 0.024), where the effects of cyproheptadine on motor unit discharge rate and RTD increased with increasing contraction intensity. Overall, these results support the viewpoint that serotonergic effects in the central nervous system occur fast enough to regulate motor unit discharge rate during rapid powerful contractions.

Lack of increased rate of force development after strength training is explained by specific neural, not muscular, motor unit adaptations

While maximal force increases following short-term isometric strength training, the rate of force development (RFD) may remain relatively unaffected. The underlying neural and muscular mechanisms during rapid contractions after strength training are largely unknown. Since strength training increases the neural drive to muscles, it may be hypothesized that there are distinct neural or muscular adaptations determining the change in RFD independently of an increase in maximal force. Therefore, we examined motor unit population data acquired from surface electromyography during the rapid generation of force before and after four weeks of strength training. We observed that strength training did not change the RFD because it did not influence the number of motor units recruited per second or their initial discharge rate during rapid contractions. While strength training did not change motoneuron behaviour in the force increase phase of rapid contractions, it increased the discharge rate of motoneurons (by ~4 spikes/s) when reaching the plateau phase (~150 ms) of the rapid contractions, determining an increase in maximal force production. Computer simulations with a motor unit model that included neural and muscular properties, closely matched the experimental observations and demonstrated that the lack of change in RFD following training is primarily mediated by an unchanged maximal recruitment speed of motoneurons. These results demonstrate that maximal force and contraction speed are determined by different adaptations in motoneuron behaviour following strength training and indicate that increases in the recruitment speed of motoneurons are required to evoke training-induced increases in RFD.

Isometric knee extension test: A practical, repeatable, and suitable tool for lower-limb screening among institutionalized older adults

We aimed to analyze the isometric knee extension test (IKE) test in terms of i) intra- and inter-session repeatability, and ii) relationship with functional and body composition factors of sarcopenia among institutionalized older adults. Thirteen institutionalized older adults (age = 87 ± 10 years, body mass [BM] = 73.1 ± 10.9 kg, body mass index [BMI] = 28.5 ± 3.8 kg·m²) were recruited from a nursing home. Variability of maximal isometric force registered in three IKE trials performed on the same day was used to examine intra-session repeatability, whereas inter-session repeatability was analyzed by comparing maximal isometric force from two different days. Furthermore, functional (Handgrip, 6-m Gait Speed, Time Up and Go [TUG], and Sit-to-stand tests) and body composition (appendicular lean mass adjusted by BMI, ALM/BMI) evaluations were conducted. Statistics included the intraclass correlation coefficient (ICC) and the standard error of measurement (SEM), expressed in both absolute (N·kg^-1) and relative terms (coefficient of variation, CV = 100 × SEM / mean). High to very high intra-session repeatability was found for both the dominant and non-dominant legs (CV ≤ 6.0%, ICC ≥ 0.989). Similarly, both legs showed high inter-session repeatability (SEM ≤ 0.26 N·kg^-1, ICC ≥ 0.959). On the other hand, significant relationships were found between Dominant and Non-dominant IKE tests and 6-m Gait Speed (r = 0.77; r = 0.58), ALM/BMI (r = 0.62; r = 0.58), and Non-dominant Handgrip/BM (r = 0.60; r = 0.68). In addition, a significant association was found between Dominant IKE/BM and TUG (r = -0.74), as well as between Non-dominant IKE/BM and Dominant Handgrip/BM (r = 0.67). These findings suggest that the IKE test is a repeatable and suitable strategy for lower-limb screening in institutionalized older adults.

A cerebellar-based solution to the nondeterministic time delay problem in robotic control

[Figure: see text].

Faster intrinsic rate of torque development in elbow flexors than knee extensors: Effect of muscle architecture?

We studied the effect of pennate vs. fusiform muscle architecture on the rate of torque development (RTD) by examining the predominately fusiform elbow flexors (EF) and highly-pennate knee extensors (KE). Seventeen male volunteers (28.4 ± 6.2 years) performed explosive isometric EF and KE contractions (MVCs). Biceps brachii and vastus lateralis fascicle angles were measured to confirm their architecture, and both the rate of voluntary muscle activation (root-mean-square EMG in the 50 ms before contraction onset; EMG_-50) and electromechanical delay (EMD; depicting muscle-tendon series elasticity) were assessed as control variables to account for their influence on RTD. MVC torque, early (RTD₅₀) and late (RTD₂₀₀) RTDs were calculated and expressed as absolute and normalized values. Absolute MVC torque (+412%), RTD₅₀ (+215%), and RTD₂₀₀ (+427%) were significantly (p < 0.001) higher in KE than EF. However, EF RTD₅₀ was faster (+178%) than KE after normalization (p = 0.02). EMG_-50 and EMD did not differ between muscle groups. The results suggest that the faster absolute RTD in KE is largely associated with its higher maximal torque capacity, however in the absence of differences in rates of muscle activation, fiber type, and EMD the fusiform architecture of EF may be considered a factor allowing its faster early RTD relative to strength capacity.

Evidence of resistance training-induced neural adaptation in older adults

The deleterious effects of aging on force production are observable from the age of 40 upwards, depending on the measure. Neural mechanisms contributing to maximum force production and rate of force development have been suggested as descending drive from supraspinal centers, spinal motoneuron excitability, and corticospinal inhibition of descending drive; all of which influence motor unit recruitment and/or firing rate. Resistance-trained Master athletes offer a good source of information regarding the inevitable effects of aging despite the countermeasure of systematic resistance-training. However, most evidence of neural adaptation is derived from longitudinal intervention studies in previously untrained (i.e. resistance-training naïve) older adults. There is good evidence for the effect of resistance-training on the end-point of neural activation, i.e. motor unit behavior, but little to no data on the generation of descending drive from e.g. transcranial magnetic stimulation or cortical imaging studies in older adults. This, along with tracking master athletes over several years, would provide valuable information and could be the focus of future research.

Forearm muscles fatigue induced by repetitive braking on a motorcycle is best discriminated by specific kinetic parameters

Maneuvering a motorcycle in racing conditions or for prolonged time is sufficiently demanding that on many occasions forearm muscles reach a state of functional failure when riders cannot properly brake or operate the throttle. This study intends to discriminate which ones of the several dynamometric parameters used in the literature to characterize the Force-time (F-t) curve during voluntary contractions are more sensitive to neuromuscular fatigue in simulated motorcycle-riding conditions. Thirty-three adults performed an intermittent fatiguing protocol (IFP) that simulated the brake-pulling and throttle-twisting actions, by using a hydraulic system equipped with a pressure sensor. Sixty pressure-time (P-t) curve parameters, including the rate of pressure development (RPD) and area under the curve were measured to characterize the time course of the braking maximal voluntary contraction (MVC). Two types of variables were used to analyze the P-t curve: 1) Times interval (from 0 to 30-50-100-500-1000 and 2000 ms); 2) Percentages of MVC (10-30-60-90%MVC). Overall significant (p ≤ 0.05) fatigue-related declines were observed only at time intervals longer than 100 ms and contraction intensities higher than 30%MVC. Strong and significant linear declines (p < 0.001) were observed at 500 ms and 1 s for normalized pressures, as well as for the ratio RPD_60%MVC/MVC (p < 0.003) throughout the IFP. Our results suggest considering RPD at time windows of 0–500 ms and 0–1 s, and contraction intensities comprised between 30% and 60% of MVC, as more suitable criteria to study fatigue-related decrements in performance rather than the classical MVC force.

Changes in corticospinal excitability during the preparation phase of ballistic and ramp contractions

KEY POINTS

Changes in corticospinal excitability prior to a contraction may depend on its characteristics, including the rate of torque development. This study compared the specific modulation of cortical and spinal excitability during the preparation phase (last 500 ms before contraction) of fast (ballistic) and ramp contractions of ankle dorsiflexors, using transcranial magnetic stimulation and peripheral nerve stimulation. The results indicate earlier changes at the cortical than at the spinal level during the preparation phase of both contraction types. However, these adjustments are delayed prior to ballistic relative to ramp contractions. This study suggests that the time course of change in cortical and spinal excitability during the preparation phase of a voluntary action is specific to the intended rate of torque development of the upcoming contraction.

ABSTRACT

The present study investigated cortical and spinal excitability during the preparation phase of ballistic (BAL) and ramp (RAMP) isometric contractions. To this end, young adults performed BAL and RAMP (1500 ms torque rise time) contractions, reaching a similar torque level, with the ankle dorsiflexor muscles. Transcranial magnetic stimulation of the motor cortex was randomly applied to record motor evoked potentials (MEP) in the tibialis anterior during the last 500 ms preceding the contraction (n = 16). Short-interval intracortical inhibition (SICI; n = 10) and spinal motor neurone excitability (F-wave occurrence; n = 8) were also assessed during this period. Data were averaged over 100 ms time windows beginning 500 ms prior to the onset of contractions. An increase in MEP amplitude and a decrease in SICI were observed from the 200-100 ms and 300-200 ms time windows prior to BAL and RAMP contractions (P < 0.05), respectively, with greater changes prior to RAMP than to BAL within the 300-200 ms time window (P < 0.05). F-wave occurrence, used to assess spinal motor neurone excitability, increased prior to RAMP (200-100 ms, P < 0.05) but not BAL contractions. Data obtained in a few participants during the last 100 ms confirmed a delayed and steeper rise in corticospinal excitability prior to BAL contractions. These results indicate earlier changes at the cortical than at the spinal level, with delayed changes prior to BAL contractions. This study suggests that the time course of change in cortical and spinal excitability during the preparation phase of a voluntary action is specific to the intended rate of torque development of the upcoming contraction.

Influence of sex on the age-related adaptations of neuromuscular function and motor unit properties in elite masters athletes

KEY POINTS

Masters athletes maintain high levels of activity into older age and allow an examination of the effects of aging dissociated from the effects of increased sedentary behaviour. Evidence suggests masters athletes are more successful at motor unit remodelling, the reinnervation of denervated fibres acting to preserve muscle fibre number, but little data are available in females. Here we used intramuscular electromyography to demonstrate that motor units sampled from the tibialis anterior show indications of remodelling from middle into older age and which does not differ between males and females. The age-related trajectory of motor unit discharge characteristic differs according to sex, with female athletes progressing to a slower firing pattern that was not observed in males. Our findings indicate motor unit remodelling from middle to older age occurs to a similar extent in male and female athletes, with discharge rates progressively slowing in females only.

ABSTRACT

Motor unit (MU) remodelling acts to minimise loss of muscle fibres following denervation in older age, which may be more successful in masters athletes. Evidence suggests performance and neuromuscular function decline with age in this population, although the majority of studies have focused on males, with little available data on female athletes. Functional assessments of strength, balance and motor control were performed in 30 masters athletes (16 male) aged 44-83 years. Intramuscular needle electrodes were used to sample individual motor unit potentials (MUPs) and near-fibre MUPs in the tibialis anterior (TA) during isometric contractions at 25% maximum voluntary contraction, and used to determine discharge characteristics (firing rate, variability) and biomarkers of peripheral MU remodelling (MUP size, complexity, stability). Multilevel mixed-effects linear regression models examined effects of age and sex. All aspects of neuromuscular function deteriorated with age (P < 0.05) with no age × sex interactions, although males were stronger (P < 0.001). Indicators of MU remodelling also progressively increased with age to a similar extent in both sexes (P < 0.05), whilst MU firing rate progressively decreased with age in females (p = 0.029), with a non-significant increase in males (p = 0.092). Masters athletes exhibit age-related declines in neuromuscular function that are largely equal across males and females. Notably, they also display features of MU remodelling with advancing age, probably acting to reduce muscle fibre loss. The age trajectory of MU firing rate assessed at a single contraction level differed between sexes, which may reflect a greater tendency for females to develop a slower muscle phenotype.

Improvements in Hip Extensor Rate of Torque Development Influence Hip and Knee Extensor Feed-Forward Control

Females have been reported to utilize a feedforward control strategy during landing in which they compensate for decreased rate of torque development (RTD) of the hip extensors through earlier pre-activation of the knee extensors. The purpose of this study was to determine the influence of a 4-week hip-focused training program on hip extensor RTD and feedforward control of the hip and knee extensors. Twenty-one females underwent hip extensor RTD evaluation and electromyographic assessment of the hip and knee extensors during a drop-jump task. Post-training, there was a significant improvement in hip extensor RTD (21.68 ± 5.44 to 23.33 ± 5.45 Nm/kg s, p = 0.009), and pre-activation of the hip extensors (87.1 ± 63.6 to 56.2 ± 60.9 ms, p < 0.001) and knee extensors (272.3 ± 113.8 to 124.0 ± 67.7 ms, p < 0.001) occurred closer to ground contact. A negative association was found between the change in hip extensor RTD and the change in knee extensor onset (r = -0.48, p = 0.03). We propose that the observed change in feedforward control is reflective of the decreased need for preparatory muscle activity owing to the improved capacity of the hip extensors to rapidly generate force.

The Effect of Spinal Manipulation on the Electrophysiological and Metabolic Properties of the Tibialis Anterior Muscle

There is growing evidence showing that spinal manipulation increases muscle strength in healthy individuals as well as in people with some musculoskeletal and neurological disorders. However, the underlying mechanism by which spinal manipulation changes muscle strength is less clear. This study aimed to assess the effects of a single spinal manipulation session on the electrophysiological and metabolic properties of the tibialis anterior (TA) muscle. Maximum voluntary contractions (MVC) of the ankle dorsiflexors, high-density electromyography (HDsEMG), intramuscular EMG, and near-infrared spectroscopy (NIRS) were recorded from the TA muscle in 25 participants with low level recurring spinal dysfunction using a randomized controlled crossover design. The following outcomes: motor unit discharge rate (MUDR), strength (force at MVC), muscle conduction velocity (CV), relative changes in oxy- and deoxyhemoglobin were assessed pre and post a spinal manipulation intervention and passive movement control. Repeated measures ANOVA was used to assess within and between-group differences. Following the spinal manipulation intervention, there was a significant increase in MVC (p = 0.02; avg 18.87 ± 28.35%) and a significant increase in CV in both the isometric steady-state (10% of MVC) contractions (p < 0.01; avg 22.11 ± 11.69%) and during the isometric ramp (10% of MVC) contractions (p < 0.01; avg 4.52 ± 4.58%) compared to the control intervention. There were no other significant findings. The observed TA strength and CV increase, without changes in MUDR, suggests that the strength changes observed following spinal manipulation are, in part, due to increased recruitment of larger, higher threshold motor units. Further research needs to investigate the longer term and potential functional effects of spinal manipulation in various patients who may benefit from improved muscle function and greater motor unit recruitment.

Is impaired dopaminergic function associated with mobility capacity in older adults?

The capacity to move is essential for independence and declines with age. Slow movement speed, in particular, is strongly associated with negative health outcomes. Prior research on mobility (herein defined as movement slowness) and aging has largely focused on musculoskeletal mechanisms and processes. More recent work has provided growing evidence for a significant role of the nervous system in contributing to reduced mobility in older adults. In this article, we report four pieces of complementary evidence from behavioral, genetic, and neuroimaging experiments that, we believe, provide theoretical support for the assertion that the basal ganglia and its dopaminergic function are responsible, in part, for age-related reductions in mobility. We report four a posteriori findings from an existing dataset: (1) slower central activation of ballistic force development is associated with worse mobility among older adults; (2) older adults with the Val/Met intermediate catecholamine-O-methyl-transferase (COMT) genotype involved in dopamine degradation exhibit greater mobility than their homozygous counterparts; (3) there are moderate relationships between performance times from a series of lower and upper extremity tasks supporting the notion that movement speed in older adults is a trait-like attribute; and (4) there is a relationship of functional connectivity within the medial orbofrontal (mOFC) cortico-striatal network and measures of mobility, suggesting that a potential neural mechanism for impaired mobility with aging is the deterioration of the integrity of key regions within the mOFC cortico-striatal network. These findings align with recent basic and clinical science work suggesting that the basal ganglia and its dopaminergic function are mechanistically linked to age-related reductions in mobility capacity.

Effects of small-sided recreational team handball training on mechanical muscle function, body composition and bone mineralization in untrained young adults-A randomized controlled trial

Prolonged physical inactivity in young adults may lead to deficiencies in musculoskeletal fitness, and thus a need exists to develop physical activity and exercise programmes that are effective of increasing musculoskeletal fitness. The aim of this study, therefore, was to investigate the effects of small-sided team handball training on lower limb muscle strength, postural balance and body composition in young adults. Twenty-six men and twenty-eight women were stratified for peak oxygen uptake (VO_2peak) and body fat percentage and randomly allocated to either 12 wks of small-sided recreational team handball training (THG: 14 men and 14 women, age 24.1±2.6 yrs (mean±SD), VO_2peak 39.8±5.9 ml/kg/min and body fat percentage 32.7±8.7%) or serving as non-exercising controls (CON: 12 men and 14 women, age 24.8±3.1 yrs, VO_2peak 39.7±5.0 ml/kg/min, body fat percentage 31.7±9.7%). THG trained on average 1.8 times/week for 12 wks. At 0 and 12 wks, lower limb muscle strength, rate of force development (RFD), vertical jump height and power, postural balance, body composition and muscle biopsies were assessed. No training effects were observed for maximal isokinetic or isometric knee extensor strength, maximal vertical jump height or take-off power, fibre type distribution or capillarization. Late phase (RFD) increased (+7.4%, p<0.05) and postural sway excursion length was improved after training (-9%, p<0.05) in THG with no difference from CON (p>0.05). Further, THG demonstrated a decrease in body fat percentage (-3.7%) accompanied by increases in whole-body fat free mass (FFM) (+2.2%), leg FFM (+2.5%), total bone mineral content (BMC) (+1.1%), leg BMC (+1.2%), total hip bone mineral density (+1.6%) and hip T-score (+50%) which differed from CON (all p<0.05). In conclusion, recreational small-sided team handball training appears to effectively improve rapid force capacity, postural balance, lean and fat body mass and bone health in previously untrained young adults. The study was registered at ClinicalTrials.gov (NCT04247724).

ClinicalTrials.gov ID number: NCT04247724

Surface Electromyography: What Limits Its Use in Exercise and Sport Physiology?

The aim of the present paper is to examine to what extent the application of surface electromyography (sEMG) in the field of exercise and, more in general, of human movement, is adopted by professionals on a regular basis. For this purpose, a brief history of the recent developments of modern sEMG techniques will be assessed and evaluated for a potential use in exercise physiology and clinical biomechanics. The idea is to understand what are the limitations that impede the translation of sEMG to applied fields such as exercise physiology. A cost/benefits evaluation will be drawn in order to understand possible causes that prevents sEMG from being routinely adopted. Among the possible causative factors, educational, economic and technical issues will be considered. Possible corrective interventions will be proposed. We will also give an overview of the parameters that can be extracted from the decomposition of the sHDEMG signals and how this can be related by professionals for assessing the health and disease of the neuromuscular system. We discuss how the decomposition of surface EMG signals might be adopted as a new non-invasive tool for assessing the status of the neuromuscular system. Recent evidences show that is possible to monitor the changes in neuromuscular function after training of longitudinally tracked populations of motoneurons, predict the maximal rate of force development by an individual via motoneuron interfacing, and identify possible causal relations between aging and the decrease in motor performance. These technologies will guide our understanding of motor control and provide a new window for the investigation of the underlying physiological processes determining force control, which is essential for the sport and exercise physiologist. We will also illustrate the challenges related to extraction of neuromuscular parameters from global EMG analysis (i.e., root-mean-square, and other global EMG metrics) and when the decomposition is needed. We posit that the main limitation in the application of sEMG techniques to the applied field is associated to problems in education and teaching, and that most of the novel technologies are not open source.

Spinal motoneurons of the human newborn are highly synchronized during leg movements

Motoneurons of neonatal rodents show synchronous activity that modulates the development of the neuromuscular system. However, the characteristics of the activity of human neonatal motoneurons are largely unknown. Using a noninvasive neural interface, we identified the discharge timings of individual spinal motoneurons in human newborns. We found highly synchronized activities of motoneurons of the tibialis anterior muscle, which were associated with fast leg movements. Although neonates' motor units exhibited discharge rates similar to those of adults, their synchronization was significantly greater than in adults. Moreover, neonatal motor units showed coherent oscillations in the delta band, which is directly translated into force generation. These results suggest that motoneuron synchronization in human neonates might be an important mechanism for controlling fast limb movements, such as those of primitive reflexes. In addition to help revealing mechanisms of development, the proposed neural interface might monitor children at risk of developing motor disorders.

Familiarization and Reliability of the Isometric Knee Extension Test for Rapid Force Production Assessment

Despite the rising interest in the use of portable force sensors during isometric exercises to inform on neuromuscular performance, the design of practical field-based methods to obtain reliable measures is an ongoing challenge. We aim at identifying the intra-session and test-retest reliability of a rapid, isometric knee extension test to evaluate the maximal voluntary concentric force (MVC), rate of force development (RFD) and impulse following a field-based approach. On two occasions, 14 athletes unfamiliar with the test completed three sets of 2 s ballistic contractions (as fast and hard as possible) with 30 s rest. Raw and filtered data were collected in real time using a portable force sensor. RFD and impulse were highly reliability during “late” phases of the contraction (0–250 ms) since the first session (coefficient of variation (CV) < 9.8%). Earlier phases (0–150 ms) achieved a moderate reliability after one familiarization session (CV < 7.1%). Measures at 0–50 ms did not reach sufficient reliability (CV~14%). MVC was accurately assessed. Dominant limbs were not importantly altered by the familiarization. In opposite, non-dominant limbs showed large variations. New evidence is provided about the positive effects of a single familiarization session to improve the reliability the isometric knee extension test for rapid force production assessment. Coaches and practitioners may benefit of from these findings to conduct practical and reliable assessments of the rapid force production using a portable force sensor and a field-based approach.

Neural and muscular determinants of maximal rate of force development

The ability to produce rapid forces requires quick motor unit recruitment, high motor unit discharge rates, and fast motor unit force twitches. The relative importance of these parameters for maximum rate of force development (RFD), however, is poorly understood. In this study, we systematically investigated these relationships using a computational model of motor unit pool activity and force. Across simulations, neural and muscular properties were systematically varied in experimentally observed ranges. Motor units were recruited over an interval starting from contraction onset (range: 22–233 ms). Upon recruitment, discharge rates declined from an initial rate (range: 89–212 pulses per second), with varying likelihood of doublet (interspike interval of 3 ms; range: 0–50%). Finally, muscular adaptations were modeled by changing average twitch contraction time (range: 42–78 ms). Spectral analysis showed that the effective neural drive to the simulated muscle had smaller bandwidths than the average motor unit twitch, indicating that the bandwidth of the motor output, and thus the capacity for explosive force, was limited mainly by neural properties. The simulated RFD increased by 1,050 ± 281% maximal voluntary contraction force per second from the longest to the shortest recruitment interval. This effect was more than fourfold higher than the effect of increasing the initial discharge rate, more than fivefold higher than the effect of increasing the chance of doublets, and more than sixfold higher than the effect of decreasing twitch contraction times. The simulated results suggest that the physiological variation of the rate by which motor units are recruited during ballistic contractions is the main determinant for the variability in RFD across individuals.

NEW & NOTEWORTHY An important limitation of human performance is the ability to generate explosive movements by means of rapid development of muscle force. The physiological determinants of this ability, however, are poorly understood. In this study, we show using extensive simulations that the rate by which motor units are recruited is the main limiting factor for maximum rate of force development.

Force-stabilizing synergies can be retained by coordinating sensory-blocked and sensory-intact digits

The present study examined the effects of selective digital deafferentation on the multi-finger synergies as a function of total force requirement and the number of digits involved in isometric pressing. 12 healthy adults participated in maximal and sub-maximal isometric pressing tasks with or without digital anesthesia to selective digits from the right hand. Our results indicate that selective anesthesia paradigm induces changes in both anesthetized (local) and non-anesthetized (non-local) digits’ performance, including: (1) decreased maximal force abilities in both local and non-local digits; (2) reduced force share during multi-finger tasks from non-local but not local digits; (3) decreased force error-making; and (4) marginally increased motor synergies. These results reinforce the contribution of somatosensory feedback in the process of maximal voluntary contraction force, motor performance, and indicate that somatosensation may play a role in optimizing secondary goals during isometric force production rather than ensuring task performance.

Post-activation Potentiation Versus Post-activation Performance Enhancement in Humans: Historical Perspective, Underlying Mechanisms, and Current Issues

Post-activation potentiation (PAP) is a well-described phenomenon with a short half-life (~28 s) that enhances muscle force production at submaximal levels of calcium saturation (i.e., submaximal levels of muscle activation). It has been largely explained by an increased myosin light chain phosphorylation occurring in type II muscle fibers, and its effects have been quantified in humans by measuring muscle twitch force responses to a bout of muscular activity. However, enhancements in (sometimes maximal) voluntary force production detected several minutes after high-intensity muscle contractions are also observed, which are also most prominent in muscles with a high proportion of type II fibers. This effect has been considered to reflect PAP. Nonetheless, the time course of myosin light chain phosphorylation (underpinning “classic” PAP) rarely matches that of voluntary force enhancement and, unlike PAP, changes in muscle temperature, muscle/cellular water content, and muscle activation may at least partly underpin voluntary force enhancement; this enhancement has thus recently been called post-activation performance enhancement (PAPE) to distinguish it from “classical” PAP. In fact, since PAPE is often undetectable at time points where PAP is maximal (or substantial), some researchers have questioned whether PAP contributes to PAPE under most conditions in vivo in humans. Equally, minimal evidence has been presented that PAP is of significant practical importance in cases where multiple physiological processes have already been upregulated by a preceding, comprehensive, active muscle warm-up. Given that confusion exists with respect to the mechanisms leading to acute enhancement of both electrically evoked (twitch force; PAP) and voluntary (PAPE) muscle function in humans after acute muscle activity, the first purpose of the present narrative review is to recount the history of PAP/PAPE research to locate definitions and determine whether they are the same phenomena. To further investigate the possibility of these phenomena being distinct as well as to better understand their potential functional benefits, possible mechanisms underpinning their effects will be examined in detail. Finally, research design issues will be addressed which might contribute to confusion relating to PAP/PAPE effects, before the contexts in which these phenomena may (or may not) benefit voluntary muscle function are considered.

Passive muscle stretching impairs rapid force production and neuromuscular function in human plantar flexors

PURPOSE

We examined the effect of muscle stretching on the ability to produce rapid torque and the mechanisms underpinning the changes.

METHODS

Eighteen men performed three conditions: (1) continuous stretch (1 set of 5 min), (2) intermittent stretch (5 sets of 1 min with 15-s inter-stretch interval), and (3) control. Isometric plantar flexor rate of torque development was measured during explosive maximal voluntary contractions (MVC) in the intervals 0-100 ms (RTD_V100) and 0-200 ms (RTD_V200), and in electrically evoked 0.5-s tetanic contractions (20 Hz, 20 Hz preceded by a doublet and 80 Hz). The rate of EMG rise, electromechanical delay during MVC (EMD_V) and during a single twitch contraction (EMD_twitch) were assessed.

RESULTS

RTD_V200 was decreased (P < 0.05) immediately after continuous (- 15%) and intermittent stretch (- 30%) with no differences between protocols. The rate of torque development during tetanic stimulations was reduced (P < 0.05) immediately after continuous (- 8%) and intermittent stretch (- 10%), when averaged across stimulation frequencies. Lateral gastrocnemius rate of EMG rise was reduced after intermittent stretch (- 27%), and changes in triceps surae rate of EMG rise were correlated with changes in RTD_V200 after both continuous (r = 0.64) and intermittent stretch (r = 0.65). EMD_V increased immediately (31%) and 15 min (17%) after intermittent stretch and was correlated with changes in RTD_V200 (r = - 0.56). EMD_twitch increased immediately after continuous (4%), and immediately (5.4%), 15 min (6.3%), and 30 min after (6.4%) intermittent stretch (P < 0.05).

CONCLUSIONS

Reductions in the rate of torque development immediately after stretching were associated with both neural and mechanical mechanisms.

The relative strength of common synaptic input to motor neurons is not a determinant of the maximal rate of force development in humans

Correlation between motor unit discharge times, often referred to as motor unit synchronization, is determined by common synaptic input to motor neurons. Although it has been largely speculated that synchronization should influence the rate of force development, the association between the degree of motor unit synchronization and rapid force generation has not been determined. In this study, we examined this association with both simulations and experimental motor unit recordings. The analysis of experimental motor unit discharges from the tibialis anterior muscle of 20 healthy individuals during rapid isometric contractions revealed that the average motor unit discharge rate was associated with the rate of force development. Moreover, the extent of motor unit synchronization was entirely determined by the average motor unit discharge rate ( R > 0.7, P < 0.0001). The simulation model demonstrated that the relative proportion of common synaptic input received by motor neurons, which determines motor unit synchronization, does not influence the rate of force development ( R = 0.03, P > 0.05). Nonetheless, the estimates of correlation between motor unit spike trains were significantly correlated with the rate of force generation ( R > 0.8, P < 0.0001). These results indicate that the average motor unit discharge rate, but not the degree of motor unit synchronization, contributes to most of the variance of human contractile speed among individuals. In addition, estimates of correlation between motor unit discharge times depend strongly on the number of identified motor units and therefore are not indicative of the strength of common input.

NEW & NOTEWORTHY It is commonly assumed that motor unit synchronization has an impact on the rate of force development of a muscle. Here we present computer simulations and experimental data of human tibialis anterior motor units during rapid contractions that show that motor unit synchronization is not a determinant of the rate of force production. This conclusion clarifies the neural determinants of rapid force generation.

Differences in Maximum Voluntary Excitation Between Isometric and Dynamic Contractions are Age-Dependent

Obtaining true maximum voluntary excitation appears to be more difficult in older populations than in young populations. The aims of this study were (1) to determine whether differences in maximum voluntary excitation obtained from maximum voluntary isometric contraction (MVIC) and (sub-)maximum voluntary dynamic contraction [(s-)MVDC] are age dependent, and (2) to determine how normalizing electromyographic signals to corresponding maximum voluntary excitations affects variance between participants and the likelihood of normalized signals exceeding 100%. MVIC, s-MVDC, and MVDC were recorded in 10 young women, and MVIC and s-MVDC were recorded in 19 older women. A significant age × contraction mode interaction effect was found for vastus lateralis (P = .04). In young women, MVDC elicited the highest maximum voluntary excitation for vastus lateralis and rectus femoris (P < .05). In older women, no differences in maximum voluntary excitation were found (P > .05). Normalization to dynamic contractions resulted in lower between-participant variance of electromyography amplitudes, though not for all muscles, and decreased the number of normalized signals exceeding 100% in young women. These findings indicate that differences in maximum voluntary excitation across contraction modes are age dependent. Therefore, one should be cautious when comparing normalized signals between age groups; however, overall dynamic contractions may be preferable over isometric contractions for normalization purposes.

You are as fast as your motor neurons: speed of recruitment and maximal discharge of motor neurons determine the maximal rate of force development in humans

KEY POINTS

We propose and validate a method for accurately identifying the activity of populations of motor neurons during contractions at maximal rate of force development in humans. The behaviour of the motor neuron pool during rapid voluntary contractions in humans is presented. We show with this approach that the motor neuron recruitment speed and maximal motor unit discharge rate largely explains the individual ability in generating rapid force contractions. The results also indicate that the synaptic inputs received by the motor neurons before force is generated dictate human potential to generate force rapidly. This is the first characterization of the discharge behaviour of a representative sample of human motor neurons during rapid contractions.

ABSTRACT

During rapid contractions, motor neurons are recruited in a short burst and begin to discharge at high frequencies (up to >200 Hz). In the present study, we investigated the behaviour of relatively large populations of motor neurons during rapid (explosive) contractions in humans, applying a new approach to accurately identify motor neuron activity simultaneous to measuring the rate of force development. The activity of spinal motor neurons was assessed by high-density electromyographic decomposition from the tibialis anterior muscle of 20 men during isometric explosive contractions. The speed of motor neuron recruitment and the instantaneous motor unit discharge rate were analysed as a function of the impulse (the time-force integral) and the maximal rate of force development. The peak of motor unit discharge rate occurred before force generation and discharge rates decreased thereafter. The maximal motor unit discharge rate was associated with the explosive force variables, at the whole population level (r2  = 0.71 ± 0.12; P < 0.001). Moreover, the peak motor unit discharge and maximal rate of force variables were correlated with an estimate of the supraspinal drive, which was measured as the speed of motor unit recruitment before the generation of afferent feedback (P < 0.05). We show for the first time the full association between the effective neural drive to the muscle and human maximal rate of force development. The results obtained in the present study indicate that the variability in the maximal contractile explosive force of the human tibialis anterior muscle is determined by the neural activation preceding force generation.

Multi-joint rate of force development testing protocol affects reliability and the smallest detectible difference

Isometric tests have been used to assess rate of force development (RFD), however variation in testing methodologies are known to affect performance outcomes. The aim of this study was to assess the RFD in the isometric squat (ISqT) using two test protocols and two testing angles. Eleven participants (age: 26.8 ± 4.5 years, strength training experience: 7.1 ± 3.03 years) completed test and retest sessions one week apart, whereby two test protocols with respect to duration and instructions were compared. Isometric peak force (ISqT^peak) and isometric explosive force (ISqT^exp) tests were assessed at two joint angles (knee flexion angle 100° and 125°). Force-time traces were sampled and subsequently analysed for RFD measures. Average and instantaneous RFD variables did not meet reliability minimum criteria in ISqT^peak at 100° or 125°. The ISqT^exp test at 100° met reliability criteria in the RFD 0-200 and 0-250ms variables. The ISqT^exp test at 125° met reliability criteria in the RFD 0-150, 0-200 and 0-250ms variables. Force-time characteristics were optimized at the higher knee joint angle. Average and instantaneous RFD measures obtained using a traditional peak force test do not meet basic reliability criteria. Researchers assessing multi-joint RFD should employ the explosive RFD test protocol as opposed to the traditional isometric peak force protocol.

Kinetic analysis of push-up exercises: a systematic review with practical recommendations

Push-ups represent one of the simplest and most popular strengthening exercise. The aim of this study was to systematically review and critically appraise the literature on the kinetics-related characteristics of different types of push-ups, with the objective of optimising training prescription and exercise-related load. A systematic search was conducted in the electronic databases PubMed, Google Scholar and Science Direct up to April 2018. Studies that reported kinetic data (e.g. initial and peak-force supported by the upper-limbs, impact-force, peak-flexion-moment of the elbow-joint, rate of propulsive- and impact-, and vertebral-joint compressive-forces) related to push-ups and included trained, recreational and untrained participants, were considered. The risk of bias in the included studies was assessed using the Critical Appraisal Skills Programme scale. From 5290 articles retrieved in the initial search, only 26 studies were included in this review. Kinetic data for 46 push-up variants were assessed. A limitation of the current review is that the relationship between our findings and actual clinical or practical consequences is not statistically proven but can only be inferred from our critical descriptive approach. Overall, this review provides detailed data on specific characteristics and intensities of push-up variations, in order to optimise exercise prescription for training and rehabilitation purposes.