The effect of shoulder position on motor evoked and maximal muscle compound action potentials of the biceps brachii

Effect of muscle length in a handgrip task on corticomotor excitability of extrinsic and intrinsic hand muscles under resting and submaximal contraction conditions

The neurophysiological mechanisms underlying muscle force control for different wrist postures still need to be better understood. To further elucidate these mechanisms, the present study aimed to investigate the effects of wrist posture on the corticospinal excitability by transcranial magnetic stimulation (TMS) of extrinsic (flexor [FCR] and extensor carpi radialis [ECR]) and intrinsic (flexor pollicis brevis (FPB)) muscles at rest and during a submaximal handgrip strength task. Fourteen subjects (24.06 ± 2.28 years) without neurological or motor disorders were included. We assessed how the wrist posture (neutral: 0°; flexed: +45°; extended: -45°) affects maximal handgrip strength (HGSmax ) and the motor evoked potentials (MEP) amplitudes during rest and active muscle contractions. HGSmax was higher at 0° (133%) than at -45° (93.6%; p < 0.001) and +45° (73.9%; p < 0.001). MEP amplitudes were higher for the FCR at +45° (83.6%) than at -45° (45.2%; p = 0.019) and at +45° (156%; p < 0.001) and 0° (146%; p = 0.014) than at -45° (106%) at rest and active condition, respectively. Regarding the ECR, the MEP amplitudes were higher at -45° (113%) than at +45° (60.8%; p < 0.001) and 0° (72.6%; p = 0.008), and at -45° (138%) than +45° (96.7%; p = 0.007) also at rest and active conditions, respectively. In contrast, the FPB did not reveal any difference among wrist postures and conditions. Although extrinsic and intrinsic hand muscles exhibit overlapping cortical representations and partially share the same innervation, they can be modulated differently depending on the biomechanical constraints.

Pairing Transcranial Magnetic Stimulation and Loud Sounds Produces Plastic Changes in Motor Output

Most current methods for neuromodulation target the cortex. Approaches for inducing plasticity in subcortical motor pathways, such as the reticulospinal tract, could help to boost recovery after damage (e.g., stroke). In this study, we paired loud acoustic stimulation (LAS) with transcranial magnetic stimulation (TMS) over the motor cortex in male and female healthy humans. LAS activates the reticular formation; TMS activates descending systems, including corticoreticular fibers. Two hundred paired stimuli were used, with 50 ms interstimulus interval at which LAS suppresses TMS responses. Before and after stimulus pairing, responses in the contralateral biceps muscle to TMS alone were measured. Ten, 20, and 30 min after stimulus pairing ended, TMS responses were enhanced, indicating the induction of LTP. No long-term changes were seen in control experiments which used 200 unpaired TMS or LAS, indicating the importance of associative stimulation. Following paired stimulation, no changes were seen in responses to direct corticospinal stimulation at the level of the medulla, or in the extent of reaction time shortening by a loud sound (StartReact effect), suggesting that plasticity did not occur in corticospinal or reticulospinal synapses. Direct measurements in female monkeys undergoing a similar paired protocol revealed no enhancement of corticospinal volleys after paired stimulation, suggesting no changes occurred in intracortical connections. The most likely substrate for the plastic changes, consistent with all our measurements, is an increase in the efficacy of corticoreticular connections. This new protocol may find utility, as it seems to target different motor circuits compared with other available paradigms.SIGNIFICANCE STATEMENT Induction of plasticity by neurostimulation protocols may be promising to enhance functional recovery after damage such as following stroke, but current protocols mainly target cortical circuits. In this study, we developed a novel paradigm which may generate long-term changes in connections between cortex and brainstem. This could provide an additional tool to modulate and improve recovery.

Slouched and erect sitting postures affect upper limb maximum voluntary force levels and fatiguability: a randomized experimental study

Occupational ApplicationsModifying the spinal curvature is an empirical approach to treating upper limb musculoskeletal disorders, often attributed to the balance between physical stress and individual functional capacities. We completed an experimental biomechanical study to quantify the effect of seated spinal posture on upper limb functional capacities. Isometric maximum muscle voluntary forces (MVFs) were measured at participants' shoulder, elbow, and wrist. Fatiguability was also assessed during a repetitive painting task. Participants were asked to assume both slouched and erect spinal postures, in a random order. In the erect posture, participants achieved higher shoulder and elbow isometric MVF levels and took longer to reach a fatigue threshold. Thus, spinal posture tends to remotely influence upper limb functional capacities, especially at the shoulder and elbow. Ergonomists should consider spinal posture even when focusing on musculoskeletal disorders of the upper limb.Technical AbstractBackground: Musculoskeletal disorders are a major public health issue, and current treatments often remain unsatisfactory. Treatments based on spinal curvature modifications are empirically used for upper limb musculoskeletal disorders.

PURPOSE

To determine whether a slouched or erect sitting posture has an effect on upper limb functional capacities, with tests and outcomes focused on the risk of upper limb musculoskeletal disorders.

METHODS

Randomized experimental study, crossover design. Twenty-two right-handed healthy participants from the local area were assessed in a research laboratory. Participants' spinal curvatures were increased or decreased, through verbal instructions and light touch, to place them in a slouched or an erect posture that was stable and easily maintained, in a random order. Isometric maximum muscle voluntary forces (MVFs) were measured. Participants also performed a repetitive task that simulated painting, with fatigue level assessed using the CR10 Borg scale. Upper limb positioning, task setting, and instructions to participants were standardized., and the investigator was blind to the results of MVF measurements. The main outcomes were normalized differences in MVF values and time-to-reach "7" on the CR10 scale.

RESULTS

There were significantly higher MVF values in the erect posture for the shoulder and elbow, with respective mean (SD) normalized differences of 11.4 (18.2) and 11.8 (19.2)%; differences approached significance at the wrist [7.7 (18.5)%]. The normalized difference in time-to-reach "7" on the CR10 scale was significantly higher in the erect posture (by 11.4%).

CONCLUSIONS

Spinal posture modified individual upper limb functional capacities and could thus influence the risk of upper limb musculoskeletal disorders.

Endurance-exercise training adaptations in spinal motoneurones: potential functional relevance to locomotor output and assessment in humans

It is clear from non-human animal work that spinal motoneurones undergo endurance training (chronic) and locomotor (acute) related changes in their electrical properties and thus their ability to fire action potentials in response to synaptic input. The functional implications of these changes, however, are speculative. In humans, data suggests that similar chronic and acute changes in motoneurone excitability may occur, though the work is limited due to technical constraints. To examine the potential influence of chronic changes in human motoneurone excitability on the acute changes that occur during locomotor output, we must develop more sophisticated recording techniques or adapt our current methods. In this review, we briefly discuss chronic and acute changes in motoneurone excitability arising from non-human and human work. We then discuss the potential interaction effects of chronic and acute changes in motoneurone excitability and the potential impact on locomotor output. Finally, we discuss the use of high-density surface electromyogram recordings to examine human motor unit firing patterns and thus, indirectly, motoneurone excitability. The assessment of single motor units from high-density recording is mainly limited to tonic motor outputs and minimally dynamic motor output such as postural sway. Adapting this technology for use during locomotor outputs would allow us to gain a better understanding of the potential functional implications of endurance training-induced changes in human motoneurone excitability on motor output.

Longing for a Longitudinal Proxy: Acutely Measured Surface EMG Amplitude is not a Validated Predictor of Muscle Hypertrophy

Surface electromyography amplitudes are commonly measured in acute sports and exercise science studies to make inferences about muscular strength, performance, and hypertrophic adaptations that may result from different exercises or exercise-related variables. Here, we discuss the presumptive logic and assumptions underlying these inferences, focusing on hypertrophic adaptations for simplicity's sake. We present counter-evidence for each of its premises and discuss evidence both for and against the logical conclusion. Given the limited evidence validating the amplitude of surface electromyograms as a predictor of longitudinal hypertrophic adaptations, coupled with its weak mechanistic foundation, we suggest that acute comparative studies that wish to assess stimulus potency be met with scrutiny.

Moving forward: methodological considerations for assessing corticospinal excitability during rhythmic motor output in humans

The use of transcranial magnetic stimulation to assess the excitability of the central nervous system to further understand the neural control of human movement is expansive. The majority of the work performed to-date has assessed corticospinal excitability either at rest or during relatively simple isometric contractions. The results from this work are not easily extrapolated to rhythmic, dynamic motor outputs, given that corticospinal excitability is task-, phase-, intensity-, direction-, and muscle-dependent (Power KE, Lockyer EJ, Forman DA, Button DC. Appl Physiol Nutr Metab 43: 1176-1185, 2018). Assessing corticospinal excitability during rhythmic motor output, however, involves technical challenges that are to be overcome, or at the minimum considered, when attempting to design experiments and interpret the physiological relevance of the results. The purpose of this narrative review is to highlight the research examining corticospinal excitability during a rhythmic motor output and, importantly, to provide recommendations regarding the many factors that must be considered when designing and interpreting findings from studies that involve limb movement. To do so, the majority of work described herein refers to work performed using arm cycling (arm pedaling or arm cranking) as a model of a rhythmic motor output used to examine the neural control of human locomotion.

Effects of Vertical Lifting Distance on Upper-Body Muscle Fatigue

Manual material handling (MMH) is commonly demanded in the manufacturing industry. Occupational muscle fatigue of the arm, shoulder, and back, which arise from MMH tasks, can cause work absences and low efficiency. The available literature presents the lack of the fatigue comparison between targeted muscles, on the same part or on different parts. The main aim of the present study was to evaluate and compare the fatigue of upper-body muscles during repetitive bending tasks, an experiment involving 12 male subjects has been conducted to simulate material handling during furniture board drilling. The vertical lifting distance was chosen to be the single independent variable, and the three levels were 0, 250, and 500 mm. Surface electromyography (sEMG) was used to measure the muscle fatigue of the biceps brachii, upper trapezius, and multifidus, while the sEMG parameters, including the normalized electromyographic amplitude (Normalized EA) and mean power frequency (MPF), of the target muscles were analyzed. The experimental results reveal that during the manual handling tasks, the biceps brachii was the most relaxed muscle, contributing the least muscle tension, while the multifidus was the most easily fatigued muscle. Furthermore, the EMG MPF fatigue threshold (MPFFT) of multifidus muscle tension was tested to estimate its maximum workload in the long-term muscle contraction. In conclusion, bending angle should be maintained to a small range or bending should even be avoided during material-handling tasks.

Corticospinal excitability and motor representation after long-term resistance training

It is poorly understood how the central nervous system adapts to resistance training, especially after years of exposure. We compared corticospinal excitability and motor representation assessed with transcranial magnetic stimulation (TMS) between long-term resistance trained (LRT, ≥3 years) versus untrained (UNT) males (n = 15/group). Motor-evoked potentials (MEPs) were obtained from the biceps brachii during isometric elbow flexion. Stimulus-response curves were created at the hotspot during 10% maximum voluntary torque (MVT) contractions. Maximum peak-to-peak MEP amplitude (MEPmax) was acquired with 100% stimulator output intensity, whilst 25%-100% MVT was produced. Maps were created during 10% MVT contractions, with an individualised TMS intensity eliciting 20% MEPmax at the hotspot. LRT had a 48% lower stimulus-response curve slope than UNT (p < .05). LRT also had a 66% larger absolute map size, although TMS intensity used for mapping was greater in LRT versus UNT (48% vs. 26% above active motor threshold) to achieve a target 20% MEPmax at the hotspot, due to the lower slope of LRT. Map size was strongly correlated with the TMS intensity used for mapping (r = 0.776, p < .001). Once map size was normalised to TMS intensity, there was no difference between the groups (p = .683). We conclude that LRT had a lower stimulus-response curve slope/excitability, suggesting higher neural efficiency. TMS map size was overwhelmingly determined by TMS intensity, even when the MEP response at the hotspot was matched among individuals, likely due to larger current spread with higher intensities. Motor representation appears similar between LRT and UNT given no difference in the normalised map size.

Posture-dependent neuromuscular contributions to three-dimensional isometric shoulder torque generation

This study is the first to identify the muscle synergies underlying three-dimensional isometric shoulder torque generation. Although the overall structure of these synergies was unaffected by arm posture, the weighted contributions of several muscles composing two synergy patterns changed as a function of the elevation or plane of elevation of the shoulder. Our findings provide valuable insight for the development of targeted interventions for the restoration of shoulder function after neuromuscular or orthopedic pathologies.

Elbow angle modulates corticospinal excitability to the resting biceps brachii at both spinal and supraspinal levels

NEW FINDINGS

What is the central question of this study? Corticospinal excitability to biceps brachii is known to modulate according to upper-limb posture. Here, cervicomedullary stimulation was used to investigate potential spinal contributions to elbow angle-dependent changes in corticospinal excitability at rest. What is the main finding and its importance? At more extended elbow angles, biceps responses to cervicomedullary stimulation were decreased, whereas cortically evoked responses (normalized to cervicomedullary-evoked responses) were increased. Results suggest decreased spinal excitability but increased cortical excitability as the elbow is placed in a more extended position, an effect that is unlikely to be attributable to cutaneous stretch receptor activation.

ABSTRACT

Corticospinal excitability to biceps brachii is known to modulate according to upper-limb posture. In study 1, our aim was to investigate potential spinal contributions to this modulation and the independent effect of elbow angle. Biceps responses to transcranial magnetic stimulation (motor evoked potentials; MEPs) and electrical cervicomedullary stimulation (cervicomedullary motor evoked potentials; CMEPs) were measured at five elbow angles ranging from full extension to 130 deg of flexion. In study 2, possible contributions of cutaneous stretch receptors to elbow angle-dependent excitability changes were investigated by eliciting MEPs and CMEPs in three conditions of skin stretch about the elbow (stretch to mimic full extension, no stretch or stretch to mimic flexion). Each study had 12 participants. Evoked potentials were acquired at rest, with participants seated, the shoulder flexed 90 deg and forearm supinated. The MEPs and CMEPs were normalized to maximal compound muscle action potentials. In study 1, as the elbow was moved to more extended positions, there were no changes in MEPs (P = 0.963), progressive decreases in CMEPs (P < 0.0001; CMEPs at 130 deg flexion ∼220% of full extension) and increases in the MEP/CMEP ratio (P = 0.019; MEP/CMEP at 130 deg flexion ∼20% of full extension). In study 2, there were no changes in MEPs (P = 0.830) or CMEPs (P = 0.209) between skin stretch conditions. Therefore, although results suggest a decrease in spinal and an increase in supraspinal excitability at more extended angles, the mechanism for these changes in corticospinal excitability to biceps is not cutaneous stretch receptor feedback.

Neuromuscular fatigue during repeated sprint exercise: underlying physiology and methodological considerations

Neuromuscular fatigue occurs when an individual’s capacity to produce force or power is impaired. Repeated sprint exercise requires an individual to physically exert themselves at near-maximal to maximal capacity for multiple short-duration bouts, is extremely taxing on the neuromuscular system, and consequently leads to the rapid development of neuromuscular fatigue. During repeated sprint exercise the development of neuromuscular fatigue is underlined by a combination of central and peripheral fatigue. However, there are a number of methodological considerations that complicate the quantification of the development of neuromuscular fatigue. The main goal of this review is to synthesize the results from recent investigations on the development of neuromuscular fatigue during repeated sprint exercise. Hence, we summarize the overall development of neuromuscular fatigue, explain how recovery time may alter the development of neuromuscular fatigue, outline the contributions of peripheral and central fatigue to neuromuscular fatigue, and provide some methodological considerations for quantifying neuromuscular fatigue during repeated sprint exercise.