Muscle length and joint angle influence spinal but not corticospinal excitability to the biceps brachii across forearm postures

Transcranial Magnetic Stimulation Inter-Pulse Interval Does Not Influence Corticospinal Excitability to the Biceps Brachii During Submaximal Isometric Elbow Flexion

Previous research on resting muscles has shown that inter-pulse interval (IPI) duration influences transcranial magnetic stimulation (TMS) responses, which can introduce serious confounding variables into investigations if not accounted for. However, it is far less clear how IPI influences TMS responses in active muscles. Thus, the purpose of this study was to examine the relationship between IPI and corticospinal excitability during submaximal isometric elbow flexion. Corticospinal excitability to the biceps and triceps brachii was measured using motor evoked potentials (MEPs) elicited via TMS. Stimulation intensity was set to 120% of the biceps brachii's active motor threshold while participants produced 10% of their biceps' maximal muscle activity. TMS was delivered as separate trains of five stimulations, with experimental conditions differing between IPIs of 4, 6, 8, 10, 12 or 14 s. Results demonstrated that IPI had no influence on MEP amplitudes for either the biceps or triceps. However, when MEP amplitudes were expressed as a unitless ratio to pre-stimulus muscle activity, a main effect of time was found for the biceps; MEP amplitudes progressively decreased with successive stimulations (MEP 1:32.8 ± 5.9; MEP 5:27.7 ± 4.3, p < 0.05). These results suggest that IPI is unlikely to represent a confounding variable in TMS studies utilizing active contractions. However, studies looking to compare the amplitudes of single MEPs over time should be aware of the possibility that amplitudes may decrease with continuous stimulation. Future research should seek to examine even longer IPIs and explore the influence of higher stimulation intensities.

Inducing ipsilateral motor-evoked potentials in the biceps brachii muscle in healthy humans

To assess reticulospinal tract excitability, high-intensity transcranial magnetic stimulation (TMS) has been used to elicit ipsilateral motor-evoked potentials (iMEPs). However, there is no consensus on robust and valid methods for use in human studies. The present study proposes a standardized method for eliciting and analysing iMEPs in the biceps brachii. Twenty-four healthy young adults participated in this study. Electromyography (EMG) electrodes recorded contralateral MEPs (cMEPs) from the right and iMEPs from the left biceps brachii. A dynamic preacher curl task was used with ~15% of the subject's one-repetition maximum load. The protocol included maximal compound action potential (M-max) determination of the right biceps brachii muscle, TMS hotspot determination, and four sets of five repetitions where 100% stimulator output was delivered at an elbow angle of 110° of flexion. We normalized cMEP amplitude by M-max (% M-max) and iMEP by cMEP amplitude ratio (ICAR). Clear iMEPs above background EMG were observed in 21 subjects (88%, ICAR = .31 ± .19). Good-to-excellent agreement (intraclass correlation coefficient [ICC] = .795-1.000) and low bias (.01-.08 mV and .60-1.11 ms) were demonstrated when comparing two different analysis methods (i.e. fixed time-window vs. manual onset detection) to determine the cMEP and iMEP amplitude and latency, respectively. Most subjects demonstrated clear iMEPs above background EMG triggered at a pre-determined joint angle during a light-load dynamic preacher curl exercise. Similar results were obtained when comparing a single-trial manual identification of iMEP and a semi-automated time-window data analysis approach.

Forearm Posture Affects the Corticospinal Excitability of Intrinsic and Extrinsic Hand Muscles in Dominant and Nondominant Sides

Different forearm postures can modulate corticospinal excitability. However, there is no consensus on whether handedness plays a role in such a mechanism. This study investigated the effects of 3 forearm postures (pronation, neutral, and supination) on the corticospinal excitability of muscles from the dominant and nondominant upper limbs. Surface electromyography was recorded from the abductor digiti minimi, flexor pollicis brevis, and flexor carpi radialis from both sides of 12 right-handed volunteers. Transcranial magnetic stimulation pulses were applied to each muscle's hotspot in both cerebral hemispheres. Motor-evoked potential peak-to-peak amplitude and latency and resting motor threshold were measured. The data were evaluated by analysis of variance. The level of significance was set at 5%. The resting motor threshold was similar for the 3 muscles and both sides. Motor-evoked potential peak-to-peak amplitude from flexor pollicis brevis was lower during supination, and the dominant upper limb latency was longer. The flexor carpi radialis presented lower motor-evoked potential peak-to-peak amplitudes for neutral and shorter latencies during supination. Abductor digiti minimi seemed not to be affected by posture or side. Different muscles from dominant and nondominant sides may undergo corticospinal modulation, even distally localized from a particular joint and under rest.

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.

Effect of Forearm Postures and Elbow Joint Angles on Elbow Flexion Torque and Mechanomyography in Neuromuscular Electrical Stimulation of the Biceps Brachii

Neuromuscular electrical stimulation plays a pivotal role in rehabilitating muscle function among individuals with neurological impairment. However, there remains uncertainty regarding whether the muscle's response to electrical excitation is affected by forearm posture, joint angle, or a combination of both factors. This study aimed to investigate the effects of forearm postures and elbow joint angles on the muscle torque and MMG signals. Measurements of the torque around the elbow and MMG of the biceps brachii (BB) muscle were conducted in 36 healthy subjects (age, 22.24 ± 2.94 years; height, 172 ± 0.5 cm; and weight, 67.01 ± 7.22 kg) using an in-house elbow flexion testbed and neuromuscular electrical stimulation (NMES) of the BB muscle. The BB muscle was stimulated while the forearm was positioned in the neutral, pronation, or supination positions. The elbow was flexed at angles of 10°, 30°, 60°, and 90°. The study analyzed the impact of the forearm posture(s) and elbow joint angle(s) on the root-mean-square value of the torque (TQRMS). Subsequently, various MMG parameters, such as the root-mean-square value (MMGRMS), the mean power frequency (MMGMPF), and the median frequency (MMGMDF), were analyzed along the longitudinal, lateral, and transverse axes of the BB muscle fibers. The test-retest interclass correlation coefficient (ICC21) for the torque and MMG ranged from 0.522 to 0.828. Repeated-measure ANOVAs showed that the forearm posture and elbow flexion angle significantly influenced the TQRMS (p < 0.05). Similarly, the MMGRMS, MMGMPF, and MMGMDF showed significant differences among all the postures and angles (p < 0.05). However, the combined main effect of the forearm posture and elbow joint angle was insignificant along the longitudinal axis (p > 0.05). The study also found that the MMGRMS and TQRMS increased with increases in the joint angle from 10° to 60° and decreased at greater angles. However, during this investigation, the MMGMPF and MMGMDF exhibited a consistent decrease in response to increases in the joint angle for the lateral and transverse axes of the BB muscle. These findings suggest that the muscle contraction evoked by NMES may be influenced by the interplay between actin and myosin filaments, which are responsible for muscle contraction and are, in turn, influenced by the muscle length. Because restoring the function of limbs is a common goal in rehabilitation services, the use of MMG in the development of methods that may enable the real-time tracking of exact muscle dimensional changes and activation levels is imperative.

Can pelvic floor muscle training positions be selected according to the functional status of pelvic floor muscles?

Background

Pelvic floor muscle (PFM) training varies according to the functional status of PFM. It is used to strengthen underactive PFM and relax overactive PFM.

Aim

This study aimed to determine the appropriate PFM training positions according to the functional status of the PFM in women with pelvic floor dysfunction.

Materials and Methods

Seventy-six women diagnosed with pelvic floor dysfunction were included. After the digital palpation, participants were divided into four groups according to the functional status of PFM: normal, overactive, underactive, and nonfunctional. Participants' PFM and abdominal muscle functions were assessed with superficial electromyography in three positions (modified butterfly pose-P1, modified child pose-P2, and modified deep squat with block pose-P3). Friedman's analysis of variance and the Kruskal-Wallis test were used to assess whether the function of the muscles differed according to the functional status of the PFM and training positions.

Results

Normal PFM maximally contracted and relaxed in P1, whereas nonfunctional PFM was in P3 (P > 0.05). Overactive and underactive PFM was most contracted in P2 (P > 0.05) and relaxed in P1 (P < 0.001). In each functional state of the PFM, all abdominal muscles were most relaxed in P1, while their most contracted positions varied (P < 0.05).

Conclusion

This study showed that the positions in which the PFM relaxes and contracts the most may vary according to the functional status of the PFM. Therefore, different PFM training positions may be preferred according to the functional status of the PFM in women with pelvic floor dysfunction. However, more study needs to be done in this subject.

Enhanced corticospinal excitability in the tibialis anterior during static stretching of the soleus in young healthy individuals

Corticospinal excitability is known to be affected by afferent inflow arising from the proprioceptors during active or passive muscle movements. Also during static stretching (SS) afferent activity is enhanced, but its effect on corticospinal excitability received limited attention and has only been investigated as a single average value spread over the entire stretching period. Using transcranial magnetic stimulation (TMS) the present study was conducted to explore the time course of corticospinal excitability during 30 seconds SS. Motor evoked potentials (MEPs) after TMS were recorded from soleus (SOL) and tibialis anterior (TA) muscles in 14 participants during: a passive dynamic ankle dorsiflexion (DF), at six different time points during maximal individual SS (3, 6, 9, 18, 21 and 25 seconds into stretching), during a passive dynamic ankle plantar flexion (PF) and following SS. To explore the time course of corticospinal excitability during the static lengthened phase of a muscle stretch, the stretching protocol was repeated several times so that it was possible to collect a sufficient number of stimulations at each specific time point into SS, as well as during DF and PF. During passive DF, MEPs amplitude was greater than baseline in both TA and SOL (p = .001 and p = .005 respectively). During SS, MEPs amplitude was greater than baseline in TA (p = .006), but not in SOL. No differences between the investigated time points were found and no trend was detected throughout the stretching time. No effect in either muscle was observed during passive PF and after SS. These results could suggest that an increased activity of secondary afferents from SOL muscle spindles exert a corticomotor facilitation on TA. The muscle-nonspecific response observed during passive DF could instead be attributed to an increased activation within the sensorimotor cortical areas as a result of the awareness of the foot passive displacements.

Does forearm position matter in subpectoral biceps tenodesis? A randomised controlled trial

INTRODUCTION

The optimal position of the elbow and forearm during biceps tenodesis is a debated topic. The aim of our study was to compare two different forearm positions, pronation-extension (PE) or neutral, for fixation of the long head of the biceps tendon (LHB) in biceps tenodesis.

MATERIALS AND METHODS

Fifty patients who underwent shoulder arthroscopy between February 2016 and January 2019 were included in our study. After diagnostic arthroscopy, the LHB was cut from its origin with a thermal ablator. The LHB was then tenodesed beneath the inferior border of the pectoralis major tendon for 25 patients in the PE position and for 25 patients in the neutral position. Patients were evaluated preoperatively and 3rd, 6th and 12th months postoperatively according to the visual analog scale (VAS), American Shoulder and Elbow Surgeons (ASES) shoulder and Constant scores. Flexion and supination force measurements were made with a digital dynamometer device, compared to the healthy side for both groups.

RESULTS

ASES and VAS scores were statistically better in the PE group compared with the neutral group (p < 0.05), but there was no statistically significant difference between Constant scores at 3 and 6 months (p > 0.05). No significant difference was found in both groups for 3 scores at 12 months. Comparison of the PE group with the contralateral extremity and comparing the neutral group with the contralateral extremity in terms of flexion strength showed no statistically significant difference. No statistically significant difference was found between the supination powers of both comparative groups.

CONCLUSION

Functional scoring in the PE position is better at 3 and 6 months because patients experience less pain at 3 and 6 months. The simple change of the fixation position causes patients to feel less pain in the early period.

LEVEL OF EVIDENCE: 1

Sustained Isometric Wrist Flexion and Extension Maximal Voluntary Contractions on Corticospinal Excitability to Forearm Muscles during Low-Intensity Hand-Gripping

The wrist extensors demonstrate an earlier fatigue onset than the wrist flexors. However, it is currently unclear whether fatigue induces unique changes in muscle activity or corticospinal excitability between these muscle groups. The purpose of this study was to examine how sustained isometric wrist extension/flexion maximal voluntary contractions (MVCs) influence muscle activity and corticospinal excitability of the forearm. Corticospinal excitability to three wrist flexors and three wrist extensors were measured using motor evoked potentials (MEPs) elicited via transcranial magnetic stimulation. Responses were elicited while participants exerted 10% of their maximal handgrip force, before and after a sustained wrist flexion or extension MVC (performed on separate sessions). Post-fatigue measures were collected up to 10-min post-fatigue. Immediately post-fatigue, extensor muscle activity was significantly greater following the wrist flexion fatigue session, although corticospinal excitability (normalized to muscle activity) was greater on the wrist extension day. Responses were largely unchanged in the wrist flexors. However, for the flexor carpi ulnaris, normalized MEP amplitudes were significantly larger following wrist extension fatigue. These findings demonstrate that sustained isometric flexion/extension MVCs result in a complex reorganization of forearm muscle recruitment strategies during hand-gripping. Based on these findings, previously observed corticospinal behaviour following fatigue may not apply when the fatiguing task and measurement task are different.

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.