The Excitability of Ipsilateral Motor Evoked Potentials Is Not Task-specific and Spatially Distinct From the Contralateral Motor Hotspot

ObjectiveThe role of ipsilateral descending motor pathways in voluntary movement of humans is still a matter of debate. Few studies have examined the task dependent modulation of ipsilateral motor evoked potentials (iMEPs). Here, we determined the location of upper limb biceps brachii (BB) representation within the ipsilateral primary motor cortex. MethodsMR-navigated transcranial magnetic stimulation mapping of the dominant hemisphere was undertaken with twenty healthy participants who made tonic unilateral, bilateral homologous or bilateral antagonistic elbow flexion-extension voluntary contractions. Map center of gravity (CoG) and area for each BB were obtained. ResultsThe map CoG of the ipsilateral BB was located more anterior-laterally than those of the contralateral BB within the primary motor cortex. However different tasks had no effect on either the iMEP CoG location or the size. ConclusionOur data suggests that ipsilateral and contralateral MEP might originate in distinct adjacent neural populations in the primary motor cortex, independent of task dependence.

Quadriceps Muscle Fatigue Reduces Extension and Flexion Power During Maximal Cycling

Purpose: To investigate how quadriceps muscle fatigue affects power production over the extension and flexion phases and muscle activation during maximal cycling.Methods: Ten participants performed 10-s maximal cycling efforts without fatigue and after 120 bilateral maximal concentric contractions of the quadriceps muscles. Extension power, flexion power and electromyographic (EMG) activity were compared between maximal cycling trials. We also investigated the associations between changes in quadriceps force during isometric maximal voluntary contractions (IMVC) and power output (flexion and extension) during maximal cycling, in addition to inter-individual variability in muscle activation and pedal force profiles.Results: Quadriceps IMVC (−52 ± 21%, P = 0.002), voluntary activation (−24 ± 14%, P < 0.001) and resting twitch amplitude (−45 ± 19%, P = 0.002) were reduced following the fatiguing task, whereas vastus lateralis (P = 0.58) and vastus medialis (P = 0.15) M-wave amplitudes were unchanged. The reductions in extension power (−15 ± 8%, P < 0.001) and flexion power (−24 ± 18%, P < 0.001) recorded during maximal cycling with fatigue of the quadriceps were dissociated from the decreases in quadriceps IMVC. Peak EMG decreased across all muscles while inter-individual variability in pedal force and EMG profiles increased during maximal cycling with quadriceps fatigue.Conclusion: Quadriceps fatigue induced by voluntary contractions led to reduced activation of all lower limb muscles, increased inter-individual variability and decreased power production during maximal cycling. Interestingly, power production was further reduced over the flexion phase (24%) than the extension phase (15%), likely due to larger levels of peripheral fatigue developed in RF muscle and/or a higher contribution of the quadriceps muscle to flexion power production compared to extension power during maximal cycling.

Effect of Periodic Voluntary Interventions on Trapezius Activation and Fatigue During Light Upper Limb Activity

Objective The effects of diverse periodic interventions on trapezius muscle fatigue and activity during a full day of computer work were investigated. Background Musculoskeletal disorders, including trapezius myalgia, may be associated with repeated exposure to prolonged low-level activity, even during light upper-extremity tasks including computer work. Methods Thirty healthy adults participated in a study that simulated two 6-hour workdays of computer work. One workday involved imposed periodic passive and active interventions aimed at disrupting trapezius contraction monotony (Intervention day), whereas the other workday did not (Control day). Trapezius muscle activity was quantified by the 3-dimensional acceleration of the jolt movement of the acromion produced by electrically induced muscle twitches. The spatio-temporal distribution of trapezius activity was measured through high-density surface electromyography (HD-EMG). Results The twitch acceleration magnitude in one direction was significantly different across measurement periods ( p = 0.0156) on Control day, whereas no significant differences in any direction were observed ( p > 0.05) on Intervention day. The HD-EMG from Intervention day showed that only significant voluntary muscle contractions (swing arms, Jacobson maneuver) induced a decrease in the muscle activation time and an increase in the spatial muscle activation areas ( p < 0.01). Conclusion Disruption of trapezius monotonous activity via brief voluntary contractions effectively modified the ensuing contraction pattern (twitch acceleration along one axis, active epochs reduction, and larger spatial distribution). The observed changes support an associated reduction of muscle fatigue. Application This study suggests that disruptive intervention activity is efficient in reducing the impact of trapezius muscle fatigue.

Distribution of motor unit properties across human muscles

The purpose of our review was to compare the distribution of motor unit properties across human muscles of different sizes and recruitment ranges. Although motor units can be distinguished based on several different attributes, we focused on four key parameters that have a significant influence on the force produced by muscle during voluntary contractions: the number of motor units, average innervation number, and the distributions of contractile characteristics and discharge rates within motor unit pools. Despite relatively few publications on this topic, current data indicate that the most influential factor in the distribution of these motor unit properties between muscles is innervation number. Nonetheless, despite a five-fold difference in innervation number between a hand muscle (first dorsal interosseus) and a lower leg muscle (tibialis anterior), the general organization of their motor unit pools and the range of discharge rates appears to be relatively similar. These observations provide foundational knowledge for studies on the control of movement and the changes that occur with ageing and neurological disorders.

Effect of low-cost transcranial magnetic stimulation navigation on hotspot targeting and motor evoked potential variability in the biceps brachii

Background: Transcranial magnetic stimulation (TMS) can monitor or modulate brain excitability. However, reliability of TMS outcomes depends on consistent coil placement during stimulation. Neuronavigated TMS systems can address this issue, but their cost limits their use outside of specialist research environments. Objective: The objective was to evaluate the performance of a low-cost navigated TMS approach in improving coil placement consistency and its effect on motor evoked potentials (MEPs) when targeting the biceps brachii at rest and during voluntary contractions. Methods: We implemented a navigated TMS system using a low-cost 3D camera system and open-source software environment programmed using the Unity 3D engine. MEPs were collected from the biceps brachii at rest and during voluntary contractions across 2 sessions in ten non-disabled individuals. Motor hotspots were recorded and targeted via two conditions: navigated and conventional. Results: The low-cost navigated TMS system reduced coil orientation error (pitch: 1.18°±1.2°, yaw: 1.99°±1.9°, roll: 1.18°±2.2° with navigation, versus pitch: 3.7°±5.7°, yaw: 3.11°±3.1°, roll: 3.8°±9.1° with conventional). The improvement in coil orientation had no effect on MEP amplitudes and variability. Conclusions: The low-cost system is a suitable alternative to expensive systems in tracking the motor hotspot between sessions and quantifying the error in coil placement when delivering TMS. Biceps MEP variability reflects physiological variability across a range of voluntary efforts, that can be captured equally well with navigated or conventional approaches of coil locating.

Rhabdomyolysis After Implementing Whole-Body Electromyostimulation: A Case Study

Whole-body electromyostimulation is a new approach used to achieve weight loss and build muscles by activating different muscles groups through voluntary contractions generated by electrical impulses. This alternative approach is operated by a certified personal trainer, regulating the intensity of the electrical impulses to aid the muscular system to contract. Unaccustomed use of this technique leads to muscle destruction and a rise in creatine kinase level. A 25-year-old female began this therapy and experienced right flank tenderness associated with elevated serum creatine kinase (CK) levels. An impression of Rhabdomyolysis in the Emergency Department was given, followed by intravenous (IV) fluid administration. The patient consequently improved and was discharged with instructions. The study was projected to spread awareness regarding Whole-body electromyostimulation, which is tremendously promoted in Bahrain. This therapy may aid in client monitoring and improve the prognosis. Keywords: Adult; Electric stimulation therapy; Muscle; Musculoskeletal physiological phenomena; Rhabdomyolysis; Weight loss

Should We Use Unilateral or Bilateral Tasks to Assess Maximal and Explosive Knee Extensor Strength in Patients with Knee Osteoarthritis? A Cross-Sectional Study

Deficits in maximal and explosive knee extensor strength, which are usually assessed with unilateral tasks, are substantial in patients with knee osteoarthritis (KOA). The aim of this study was to investigate the clinical relevance of unilateral vs. bilateral tasks for assessing knee extensor strength in patients with KOA. This was achieved primarily by comparing unilateral and bilateral inter-limb strength asymmetries and secondarily by examining the relationship between unilaterally and bilaterally measured strength, and performance-based and self-reported function. Twenty-four patients with unilateral KOA (mean age: 65 ± 7 years) performed isometric gradual and explosive maximal voluntary contractions to assess, respectively their maximal and explosive strength. Performance-based and self-reported function were also evaluated with standard functional tests and questionnaires, respectively. Inter-limb asymmetries of maximal and explosive strength did not differ significantly between unilateral (mean asymmetry: 26 ± 15%) and bilateral tasks (22 ± 21%). In the same way, the relationships between knee extensor strength—measured either unilaterally or bilaterally—and performance-based or self-reported function were not influenced by the type of task. In conclusion, it does not seem to make a difference in terms of clinical relevance whether maximal and explosive knee extensor strength are evaluated with unilateral or bilateral tasks in KOA patients.

Biomechanical Evaluation of Back-Support Exoskeletons during Patient Transfers

The purpose of this study was to investigate the effects of three back-support exoskeletons (FLx ErgoSkeleton, V22 ErgoSkeleton, Laevo V2.5) and patient transfer methods (Squat pivot, stand pivot, scoot) on the musculoskeletal loading and self-reported usability measures during patient transfers between a bed and a wheelchair. In a repeated-measures laboratory study, 20 experienced caregivers (17 females and 3 males) performed a series of 24 bed-to-wheelchair transfer tasks (2 directions × 4 exoskeleton conditions × 3 patient transfer methods). The trunk flexion and lateral flexion angles, bilateral hand pull forces, and muscle activities of the erector spinae were significantly different by exoskeleton conditions and patient transfer methods (p’s < 0.01). The usability measures were significantly affected by exoskeleton designs (p’s < 0.01). There were significant two-way interaction effects on the trunk flexion and lateral flexion angles and muscle activities of the erector spinae (p’s < 0.01). For the squat pivot method, three back-support exoskeletons showed the largest reduction of erector spinae muscle activities (47.4 to 83.5% reference voluntary contractions) compared to no exoskeleton. This indicated the effects of exoskeleton conditions on the trunk postures and erector spinae muscle activities depended on the patient transfer method. More research could be needed to improve the trunk postures and usability of back-support exoskeletons suitable for patient handling.

Intra-limb modulations of posterior root-muscle reflexes evoked from the lower-limb muscles during isometric voluntary contractions

Although voluntary muscle contraction modulates spinal reflex excitability of contracted muscles and other muscles located at other segments within a limb (i.e., intra-limb modulation), to what extent corticospinal pathways are involved in intra-limb modulation of spinal reflex circuits remains unknown. The purpose of the present study was to identify differences in the involvement of corticospinal pathways in intra-limb modulation of spinal reflex circuits among lower-limb muscles during voluntary contractions. Ten young males performed isometric plantar-flexion, dorsi-flexion, knee extension, and knee flexion at 10% of each maximal torque. Electromyographic activity was recorded from soleus, tibialis anterior, vastus lateralis, and biceps femoris muscles. Motor evoked potentials and posterior root-muscle reflexes during rest and isometric contractions were elicited from the lower-limb muscles using transcranial magnetic stimulation and transcutaneous spinal cord stimulation, respectively. Motor evoked potential and posterior root-muscle reflex amplitudes of soleus during knee extension were significantly increased compared to rest. The motor evoked potential amplitude of biceps femoris during dorsi-flexion was significantly increased, whereas the posterior root-muscle reflex amplitude of biceps femoris during dorsi-flexion was significantly decreased compared to rest. These results suggest that corticospinal and spinal reflex excitabilities of soleus are facilitated during knee extension, whereas intra-limb modulation of biceps femoris during dorsi-flexion appeared to be inverse between corticospinal and spinal reflex circuits.