Remote Muscle Research Articles

Effects of pre-induced fatigue vs. concurrent pain on exercise tolerance, neuromuscular performance and corticospinal responses of locomotor muscles.

Fatigue and muscle pain induced in a remote muscle group has been shown to alter neuromuscular performance in exercising muscles. Inhibitory neural feedback associated with activation of mechano- and metabo-sensitive muscle afferents has been implicated in this phenomenon. The present study aimed to quantify and compare the effects of pre-induced fatigue and concurrent rising pain (evoked by muscle ischaemia) on the contralateral leg exercise capacity, neuromuscular performance, and corticomotor excitability and inhibition of knee extensor muscles. Pre-induced fatigue in one leg had a greater detrimental effect than the concurrent rising pain on the contralateral limb cycling capacity. Furthermore, pre-induced fatigue, but not concurrent rising pain, reduced corticospinal inhibition recorded from tested contralateral muscles. Regardless of the origin or mechanisms modulating sensory afferents during single-leg cycling exercise (i.e. pre-induced fatigue vs. concurrent rising pain), the limit of exercise tolerance remained the same and exercise was terminated upon achievement of a sensory tolerance limit. Individuals often need to maintain voluntary contractions during high intensity exercise in the presence of fatigue and pain. This investigation examined the effects of pre-induced fatigue and concurrent rising pain (evoked by muscle ischaemia) in one leg on motor fatigability and corticospinal excitability/inhibition of the contralateral limb. Twelve healthy males undertook four experimental protocols including unilateral cycling to task failure at 80% of peak power output with: (i) the right-leg (RL); (ii) the left-leg (LL); (iii) RL immediately preceded by LL protocol (FAT-RL); and (iv) RL when blood flow was occluded in the contralateral (left) leg (PAIN-RL). Participants performed maximal and submaximal 5s right-leg knee extensions during which transcranial magnetic and femoral nerve electrical stimuli were delivered to elicit motor-evoked and compound muscle action potentials, respectively. The pre-induced fatigue reduced the right leg cycling time-to-task failure (mean±SD; 332±137s) to a greater extent than concurrent pain (460±158s), compared to RL (580±226s) (P< 0.001). The maximum voluntary contraction force declined less following FAT-RL (P<0.019) and PAIN-RL (P<0.032) compared to RL. Voluntary activation declined and the corticospinal excitability recorded from knee extensors increased similarly after the three conditions (P<0.05). However, the pre-induced fatigue, but not concurrent pain, reduced corticospinal inhibition compared to RL (P<0.05). These findings suggest that regardless of the origin and/or mechanisms modulating sensory afferent feedback during single-leg cycling (e.g. pre-induced fatigue vs. concurrent rising pain), the limit of exercise tolerance remains the same, suggesting that exercise will be terminated upon achievement of sensory tolerance limit.

Pain Sensitivity And Psychological Variables Affect Delayed Onset Muscle Soreness (DOMS)

Delayed onset muscle soreness (DOMS) is an acute muscle pain condition occurring after eccentric muscular activity in some but not all people. Physiological and psychological factors may affect DOMS. PURPOSE: To investigate whether individual pain sensitivity and psychological variables levels predict DOMS. METHODS: Thirty two participants completed pain-related psychological questionnaires and quantitative sensory testing (QST) and thereafter the DOMS protocol was carried out for the upper extremity. The second session was held 24 h later, participants completed the DOMS-related interference questionnaire and QST was then reapplied. To compare QST parameters and psychological variables between those developing DOMS and those who did not, independent sample t-tests were conducted. Multiple regression analyses were used to determine the predictive role of QST and psychological variables on DOMS intensity. RESULTS: Out of the 32 participants, 17 showed a DOMS response. Participants who developed DOMS had higher trait anxiety (p=0.010), depression (p=0.025), and stress (p=0.034) scores, compared to those who did not develop DOMS. Trait anxiety predicted the intensity of DOMS (r=0.63, P<0.000). Additionally, those who developed DOMS demonstrated a higher systemic pain sensitivity at baseline, expressed by a lower pressure pain threshold in the muscle that was exercised and in a remote muscle, and by a lower pain inhibition efficiency (P<0.02). No correlation was found between the level of pain sensitivity at baseline and the intensity of DOMS. CONCLUSIONS: The endogenous ability to regulate pain has a significant impact on pain development in DOMS. Development of DOMS is affected by baseline systemic pain sensitivity as well as psychological and personality traits. Our findings highlight the contribution of physiological and psychological factors to the development of DOMS.

Heart and musculoskeletal hemodynamic responses to repetitive bouts of quadriceps static stretching.

The role of sympathetic and parasympathetic activity in relation to the repetitive exposure to static stretching (SS) on heart and musculoskeletal hemodynamics in stretched and resting muscles is still a matter of debate. The aim of the study was to determine cardiac and musculoskeletal hemodynamics to repetitive bouts of unilateral SS. Sympathetic and parasympathetic activity contribution to the central hemodynamics and local difference in circulation of stretched and resting muscles were also investigated. In eight participants, heart rate (HR), cardiac output (CO), mean arterial pressure (MAP), HR variability (HRV), blood pressure variability (BPV), and blood flow in passively stretched limb (SL) and control (CL, resting limb) were measured during five bouts of unilateral SS (45 s of knee flexion and 15 s of knee extension). SS increased sympathetic (~20%) and decreased parasympathetic activity (~30%) with a prevalence of parasympathetic withdrawal. During SS, HR, CO, and MAP increased by ~18 beats/min, ~0.29 l/min, ~12 mmHg, respectively. Peak blood flow in response to the first stretching maneuver increased significantly (+377 ± 95 ml/min) in the SL and reduced significantly (-57 ± 48 ml/min) in the CL. This between-limb difference in local circulation response to SS disappeared after the second SS bout. These results indicate that heart hemodynamic responses to SS are primarily influenced by the parasympathetic withdrawal rather than by the increase in sympathetic activity. The balance between neural and local factors contributing to blood flow regulation was affected by the level of SS exposure, likely associated with differences in the bioavailability of local vasoactive factors throughout the stretching bouts.NEW & NOTEWORTHY Repetitive exposure to static stretching (SS) on heart and musculoskeletal hemodynamics in stretched and remote muscles may be influenced by neural and local factors. We documented that SS-induced heart hemodynamic responses are primarily influenced by parasympathetic withdrawal. The balance between neural and local factors contributing to the regulation of musculoskeletal hemodynamics is dependent on SS exposure possibly because of different local vasoactive factor bioavailability during the subsequent stretching bouts.

Baropodometric quantification and implications of muscle coactivation in the lower limbs caused by head movement: A prospective study

BackgroundIn healthy young adults, muscle coactivation can sometimes be induced by remote voluntary contractions when the motor task is forceful, maximal, tiring, or cyclic and brief. ObjectivesTo show that a change in plantar pressure is an unequivocal response to backward movement of the head, and to contribute to a better understanding of physiotherapy methods that involve remote muscle activation. MethodsInvoluntary coactivation was quantified as a percentage of the anteroposterior plantar pressure distribution, using a baropodometric platform in a population of young adults. The baropodometric data were collected from a 1s recording after 30 s in the reference condition, and from 1s recordings during the first second and then during the 120th second in the test condition. The results were analyzed with Bayesian statistics (Markov chains and Monte Carlo integration techniques). Results90 adults participated in the study (age range: 19–26; 38 males and 52 females). The forefoot plantar pressure increased in all cases, by a mean multiplicative factor (on a logit scale) of 1.12 (from 72.24% to 74.45%) when the head was aligned over the trunk. ConclusionsThis 90-participant trial confirmed our initial hypothesis: a increase in forefoot plantar pressure is a systematic response to the motor task (head movement), and suggests greater recruitment of the plantar flexor muscles. A spinal reflex and/or a previously unknown form of motor overflow might be involved in this phenomenon. These results support the development of inductive physiotherapy techniques based on remote muscle activation in the treatment of musculoskeletal disorders. Clinicalstrial.gov identifierNCT02320097.

Remote muscle contraction enhances spinal reflexes in multiple lower-limb muscles elicited by transcutaneous spinal cord stimulation.

Transcutaneous spinal cord stimulation (tSCS) is a useful technique for the clinical assessment of neurological disorders. However, the characteristics of the spinal cord circuits activated by tSCS are not yet fully understood. In this study, we examined whether remote muscle contraction enhances the spinal reflexes evoked by tSCS in multiple lower-limb muscles. Eight healthy men participated in the current experiment, which required them to grip a dynamometer as fast as possible after the presentation of an auditory cue. Spinal reflexes were evoked in multiple lower-limb muscles with different time intervals (50-400ms) after the auditory signals. The amplitudes of the spinal reflexes in all the recorded leg muscles significantly increased at 50-250ms after remote muscle activation onset. This suggests that remote muscle contraction simultaneously facilitates the spinal reflexes in multiple lower-limb muscles. In addition, eight healthy men performed five different tasks (i.e., rest, hand grip, pinch grip, elbow flexion, and shoulder flexion). Compared to control values recorded just before each task, the spinal reflexes evoked at 250ms after the auditory signals were significantly enhanced by the above tasks except for the rest task. This indicates that such facilitatory effects are also induced by remote muscle contractions in different upper-limb areas. The present results demonstrate the existence of a neural interaction between remote upper-limb muscles and spinal reflex circuits activated by tSCS in multiple lower-limb muscles. The combination of tSCS and remote muscle contraction may be useful for the neurological examination of spinal cord circuits.

CORP: Measurement of upper and lower limb muscle strength and voluntary activation.

Muscle strength, the maximal force-generating capacity of a muscle or group of muscles, is regularly assessed in physiological experiments and clinical trials. An understanding of the expected variation in strength and the factors that contribute to this variation is important when designing experiments, describing methodologies, interpreting results, and attempting to replicate methods of others and reproduce their findings. In this review (Cores of Reproducibility in Physiology), we report on the intra- and inter-rater reliability of tests of upper and lower limb muscle strength and voluntary activation in humans. Isometric, isokinetic, and isoinertial strength exhibit good intra-rater reliability in most samples (correlation coefficients ≥0.90). However, some tests of isoinertial strength exhibit systematic bias that is not resolved by familiarization. With the exception of grip strength, few attempts have been made to examine inter-rater reliability of tests of muscle strength. The acute factors most likely to affect muscle strength and serve as a source of its variation from trial-to-trial or day-to-day include attentional focus, breathing technique, remote muscle contractions, rest periods, temperature (core, muscle), time of day, visual feedback, body and limb posture, body stabilization, acute caffeine consumption, dehydration, pain, fatigue from preceding exercise, and static stretching >60 s. Voluntary activation, the nervous system's ability to drive a muscle to create its maximal force, exhibits good intra-rater reliability when examined with twitch interpolation (correlation coefficients >0.80). However, inter-rater reliability has not been formally examined. The methodological factors most likely to influence voluntary activation are myograph compliance and sensitivity; stimulation location, intensity, and inadvertent stimulation of antagonists; joint angle (muscle length); and the resting twitch.

Effects of opposite directional resistive static contraction of the muscles around the scapulae on the unexercised contralateral soleus H-reflex

An indirect approach is useful in clinical practice when a direct approach is difficult to use due to insufficient muscle contraction caused by muscle weakness. The purpose of this study was to compare the neurophysiological remote effects and after-effects of resistive static contraction of the muscles around the scapulae, with consideration of the resistant direction. The participants were 12 healthy subjects with a mean (SD) age of 23.7 (3.4) years. A static contraction of the muscles around the scapula was performed for 10 s, utilizing the resistive static contraction of scapular anterior elevation (SCAE) and scapular posterior depression (SCPD) using proprioceptive neuromuscular facilitation. The subjects were asked to maintain the scapular position against the traction force at a level of resistance that was 50% of the maximum voluntary contraction. The left soleus H-reflexes were elicited sequentially without interruption for a period of 210 s, which was divided into a rest phase (20 s), task phase (10 s); and a rest phase after each task (180 s). For comparison, each H-reflex amplitude during and after each resistive exercise was normalized to the corresponding maximal M wave (Mmax), as expressed by the H/Mmax ratio. Data were obtained from a randomized block experiment with three tasks (SCAE, SCPD, rest) for each subject (individual factor) performed over a period of 190 s (19 conditions [time course]: condition C1–C19). Three-way analysis of variance for the H/Mmax ratio with Bonferroni post-hoc analysis revealed that the SCAE task had significantly smaller effects than the SCPD and rest tasks, and that the SCPD task had significantly larger effects than the rest task ( P < 0.05). SCAE may induce neurophysiological inhibitory effects as remote after-effects, which may induce relaxation of remote muscles.

Lower body blood flow restriction training may induce remote muscle strength adaptations in an active unrestricted arm.

We examined the concurrent characteristics of the remote development of strength and cross-sectional area (CSA) of upper body skeletal muscle in response to lower body resistance training performed with an applied blood flow restriction (BFR). Males allocated to an experimental BFR group (EXP; n = 12) or a non-BFR control group (CON; n = 12) completed 7-weeks of resistance training comprising three sets of unilateral bicep curls [50% 1-repetition maximum (1-RM)], then four sets of bilateral knee extension and flexion exercises (30% 1-RM). EXP performed leg exercises with an applied BFR (60% limb occlusion pressure). 1-RM strength was measured using bilateral leg exercises and unilateral bicep curls in both trained and untrained arms. Muscle CSA was measured via peripheral quantitative computed tomography in the dominant leg and both arms. 1-RM in the trained arm increased more in EXP (2.5 ± 0.4kg; mean ± SEM) than the contralateral untrained arm (0.8 ± 0.4kg), and the trained arm of CON (0.6 ± 0.3kg, P < 0.05). The increase in knee extension 1-RM was twofold that of CON (P < 0.01). Knee flexion 1-RM, leg CSA, and trained arm CSA increased similarly between groups (P > 0.05), while untrained arm CSA did not change (P > 0.05). Lower limb BFR training increased trained arm strength more than the contralateral untrained arm, and the trained arm of controls. However, there was no additional effect on muscle CSA. These findings support evidence for a BFR training-derived remote strength transfer that may be relevant to populations with localised movement disorders.

Localized and systemic variations in central motor drive at different local skin and muscle temperatures.

This study investigated the ability to sustain quadriceps central motor drive while subjected to localized heat and metaboreceptive feedback from the contralateral leg. Eight active males each completed two counter-balanced trials, in which muscle temperature (Tm) of a single-leg (TEMP-LEG) was altered to 29.4°C (COOL) or 37.6°C (WARM), while the contralateral leg (CL-LEG) remained thermoneutral: 35.3°C and 35.2°C Tm in COOL and WARM, respectively. To activate metaboreceptive feedback, participants first performed one 120-s isometric maximal voluntary contraction (MVC) of the knee extensors in the TEMP-LEG, immediately followed by postexercise muscle ischemia (PEMI) via femoral blood flow occlusion. To assess central motor drive of a remote muscle group immediately following PEMI, another 120-s MVC was subsequently performed in the CL-LEG. Voluntary muscle activation (VA) was assessed using the twitch interpolation method. Perceived mental effort and limb discomfort were also recorded. In a cooled muscle, a significant increase in mean force output and mean VA (force, P < 0.001; VA, P < 0.05), as well as a significant decrease in limb discomfort (P < 0.05) occurred during the sustained MVC in the TEMP-LEG. However, no differences between Tm were observed in mean force output, mean VA, or limb discomfort during the sustained MVC in the CL-LEG (force, P = 0.33; VA, P > 0.68; and limb discomfort, P = 0.73). The present findings suggest that elevated local skin temperature and Tm can increase limb discomfort and decrease central motor drive, but this does not limit systemic motor activation of a thermoneutral muscle group.

P238 Homonymous Ib inhibition during remote muscle contraction

Objective Conflicting theoretical models exist regarding the mechanism related to the ability of the Jendrassek maneuvre to reinforce reflex parameters. Our objective was to investigate if eventually inhibition of the spinal Ib motoneurone during remote muscle contraction would contribute to H-reflex enhancement. Method Soleus H-reflex from 26 healthy subjects was evoked on both sides by stimulating the tibial nerve at rest and during bilateral vigorous hadgrip. This procedure was repeated using paired stimulation at interstimulus intervals ranging from 2 ms to 15 ms. Mean conditioned H-wave amplitude expressed as a percentage of its unconditioned value (H-ratio) evoked during handgrip was compared with the H-ratio obtained at the same interstimulus interval at rest. Results The size of H-reflex evoked during handgrip was significantly higher then of that evoked at rest. The H-ratio value plotted against the interstimulus interval showed both at rest and during handgrip a similar decrease that was maximal at 5 ms and lasted less than 10 ms. Discussion The size of the conditioned H-reflex is determined by the balance between test Ia excitation and Ib inhibition elicited by the conditioning stimulus. According to our results the interneurones intercalated in pathways from Ib afferents retain their inhibitory effect on the alpha motoneurone during remote muscle contraction. Conclusion H-reflex enhancement during Jendrassek maneuvre is not due to inhibition of Ib interneurones. Significance Further studies are needed in order to test the role of other spinal circuits or descending drives that produce subthreshold excitation of alpha motoneurones by remote muscle contraction.

Interplay between body stabilisation and quadriceps muscle activation capacity

The study aimed to distinguish the effect of stabilisation and muscle activation on quadriceps maximal isometric voluntary contraction (MVC) torque generation. Nine subjects performed (a) an MVC with restrained leg and pelvis (Typical MVC), (b) a Typical MVC with handgrip (Handgrip MVC), (c) an MVC focusing on contracting the knee extensors only (Isolated knee extension MVC), and (d) an MVC with unrestrained leg and pelvis (Unrestrained MVC). Torque and activation capacity between conditions were compared with repeated measures ANOVA and dependent t-tests. EMG (from eleven remote muscles) was compared using Friedman’s and Wilcoxon. Typical MVC (277.2±49.6Nm) and Handgrip MVC (261.0±55.4Nm) were higher than Isolated knee extension MVC (210.2±48.3Nm, p<0.05) and Unrestrained MVC (195.2±49.7Nm, p<0.05) torque. Typical MVC (83.1±15.9%) activation was higher than Isolated knee extension MVC (68.9±24.3%, p<0.05), and both Typical MVC and Handgrip MVC (81.8±17.4%) were higher than Unrestrained MVC (64.9±16.2%, p<0.05). Only flexor carpi radialis, biceps brachii, triceps brachii and external oblique muscles showed EMG differences, with Isolated knee extension MVC consistently lower than Typical MVC or Handgrip MVC. Stabilisation of the involved segments is the prime concern allowing fuller activation of the muscle, reinforcing the need for close attention to stabilisation during dynamometry-based knee joint functional assessment.

Intra- and Interorgan Communication in the Cardiovascular System: A Special View on Redox Regulation.

Intraorgan communication in the cardiovascular system is exerted not only by direct cell-cell contacts but also by locally released factors, which modulate neighboring cells by paracrine signals (e.g., NO, vascular endothelial growth factor, adenosine, reactive oxygen species). Moreover, cells in close proximity to the typical cardiovascular cells such as fibroblasts, red blood cells, as well as resident and invading immune cells must be considered in attempts to understand cardiovascular function in physiology and pathology. The second level of communication is the interorgan communication, which may be distinguished from intraorgan communication, since it involves signaling from remote organs to the heart and circulation. Therefore, mediators released by, for example, the kidney or skeletal muscle reach the heart and modulate its function. This is not only the case under physiological conditions, because there is increasing evidence that the organ-specific response to a primary insult may affect also the function of remote organs by the release of factors. This Forum will summarize novel mechanisms involved in intraorgan and interorgan communication of the cardiovascular system, with a special view on the remote organs, skeletal muscle and kidney. Antioxid. Redox Signal. 26, 613-615.

Masseter Muscle Activity in Track and Field Athletes: A Pilot Study.

Teeth clenching has been shown to improve remote muscle activity (by augmentation of the Hoffmann reflex), and joint fixation (by decreased reciprocal inhibition) in the entire body. Clenching could help maintain balance, improve systemic function, and enhance safety. Teeth clenching from a sports dentistry viewpoint was thought to be important and challenging. Therefore, it is quite important to investigate mastication muscles’ activity and function during sports events for clarifying a physiological role of the mastication muscle itself and involvement of mastication muscle function in whole body movement. Running is a basic motion of a lot of sports; however, a mastication muscles activity during this motion was not clarified. Throwing and jumping operation were in a same situation. The purpose of this study was to investigate the presence or absence of masseter muscle activity during track and field events. In total, 28 track and field athletes took part in the study. The Multichannel Telemetry system was used to monitor muscle activity, and the electromyograms obtained were synchronized with digital video imaging. The masseter muscle activity threshold was set 15% of maximum voluntary clenching. As results, with few exceptions, masseter muscle activity were observed during all analyzed phases of the 5 activities, and that phases in which most participants showed masseter muscle activity were characterized by initial acceleration, such as in the short sprint, from the commencement of throwing to release in both the javelin throw and shot put, and at the take-off and landing phases in both jumps.

Microfluidic device for the formation of optically excitable, three-dimensional, compartmentalized motor units.

Motor units are the fundamental elements responsible for muscle movement. They are formed by lower motor neurons and their muscle targets, synapsed via neuromuscular junctions (NMJs). The loss of NMJs in neurodegenerative disorders (such as amyotrophic lateral sclerosis or spinal muscle atrophy) or as a result of traumatic injuries affects millions of lives each year. Developing in vitro assays that closely recapitulate the physiology of neuromuscular tissues is crucial to understand the formation and maturation of NMJs, as well as to help unravel the mechanisms leading to their degeneration and repair. We present a microfluidic platform designed to coculture myoblast-derived muscle strips and motor neurons differentiated from mouse embryonic stem cells (ESCs) within a three-dimensional (3D) hydrogel. The device geometry mimics the spinal cord-limb physical separation by compartmentalizing the two cell types, which also facilitates the observation of 3D neurite outgrowth and remote muscle innervation. Moreover, the use of compliant pillars as anchors for muscle strips provides a quantitative functional readout of force generation. Finally, photosensitizing the ESC provides a pool of source cells that can be differentiated into optically excitable motor neurons, allowing for spatiodynamic, versatile, and noninvasive in vitro control of the motor units.

Differential pain response at local and remote muscle sites following aerobic cycling exercise at mild and moderate intensity.

Physical exercise has been shown to inhibit experimental pain response in the post-exercise period. Modulation of the pain system may be differentiated between muscle sites engaging in contractile activity. The purpose of this study was to assess the pain response at remote and local muscle sites following aerobic exercise at different work intensities. Participants included 10 healthy and physically active males (mean age ± SD, 21.2 ± 3.4). Somatic pressure pain threshold (PPT) at the rectus femoris (local) and brachioradialis (remote) muscle site was measured at before (Pre), 5 min after (Post1), and 15 min after (Post2) aerobic cycling exercise at 70 and 30 % of peak oxygen uptake (VO2peak) performed on different occasions in a counterbalanced order, separated by minimum of 3 days interval. Repeated measures ANOVA for PPT reveals significant main effect for time (f = 3.581, p = 0.049, observed power = 0.588) and muscle site (f = 17.931, p = 0.002, observed power = 0.963). There was a significant interaction shown for exercise intensity by time (f = 11.390, p = 0.012, observed power = 0.790). PPT at rectus femoris following cycling exercise at 70 % of VO2peak reveals a significant increase between Pre-Post1 (p = 0.040). PPT for rectus femoris following cycling exercise at 30 % of VO2peak revealed a significant decrease between Pre-Post1 (p = 0.026) and Pre-Post2 (p = 0.008). The PPT for brachioradialis following cycling exercise at 30 % of VO2peak revealed a significant decrease between Pre-Post1 (p = 0.011) and Pre-Post2 (p = 0.005). These results show that aerobic exercise increases PPT locally at the exercise muscle site following exercise at 70 % of VO2peak but reduces PPT following exercise at 30 % of VO2peak.

Effects of muscle relaxation on sustained contraction of ipsilateral remote muscle.

The objective of this study was to clarify the temporal change of muscle activity during relaxation of ipsilateral remote muscles. While participants maintained a constant right wrist extensor isometric force, they dorsiflexed the ipsilateral ankle from resting position or relaxed from dorsiflexed position in response to an audio signal. The wrist extensor force magnitude increased in the 0–400 msec period after the onset of foot contraction compared to that of the resting condition (P < 0.05). On the other hand, wrist extensor force magnitude and electromyographic (EMG) activity decreased in the 0–400 msec period after the onset of ankle dorsiflexion compared to that of the resting condition (P < 0.05). Our findings suggest that foot muscle relaxation induces temporal reduction in hand muscle EMG activity and force magnitude.

Spinal μ-opioid receptor-sensitive lower limb muscle afferents determine corticospinal responsiveness and promote central fatigue in upper limb muscle.

We investigated the influence of group III/IV lower limb muscle afferents on the development of supraspinal fatigue and the responsiveness of corticospinal projections to an arm muscle. Eight males performed constant-load leg cycling exercise (80% peak power output) for 30 s (non-fatiguing) and to exhaustion (∼9 min; fatiguing) both under control conditions and with lumbar intrathecal fentanyl impairing feedback from μ-opioid receptor-sensitive lower limb muscle afferents. Voluntary activation (VA) of elbow flexors was assessed via transcranial magnetic stimulation (TMS) during maximum voluntary contraction (MVC) and corticospinal responsiveness was monitored via TMS-evoked potentials (MEPs) during a 25% MVC. Accompanied by a significant 5 ± 1% reduction in VA from pre- to post-exercise, elbow flexor MVC progressively decreased during the fatiguing trial (P < 0.05). By contrast, with attenuated feedback from locomotor muscle afferents, MVC and VA remained unchanged during fatiguing exercise (P > 0.3). MEPs decreased by 36 ± 6% (P < 0.05) from the start of exercise to exhaustion under control conditions, but this reduction was prevented with fentanyl blockade. Furthermore, fentanyl blockade prevented the significant increase in elbow flexor MEP observed from rest to non-fatiguing exercise under control conditions and resulted in a 14% lower corticospinal responsiveness during this short bout (P < 0.05). Taken together, in the absence of locomotor muscle fatigue, group III/IV-mediated leg muscle afferents facilitate responsiveness of the motor pathway to upper limb flexor muscles. By contrast, in the presence of cycling-induced leg fatigue, group III/IV locomotor muscle afferents facilitate supraspinal fatigue in remote muscle not involved in the exercise and disfacilitate, or inhibit, the responsiveness of corticospinal projections to upper limb muscles.

Concurrent activation potentiation enhances performance of swimming race start

Concurrent activation potentiation (CAP) is recently developed method for enhancing athletic performance in speed-strength exercises. This phenomenon is based on the stimulatory effect of the voluntary contraction of remote muscles, which facilitates activation of prime movers and synergists for targeted exercise. Physiological prerequisites for this ergogenic effect are associated with the integrative function of the cerebral motor cortex and increased excitability of spinal motor neurons. The present study was aimed at evaluating the ergogenic effect producing by CAP treatment on the performance of the race start in competitive swimming. The CAP modified start technique presupposed the teeth clenching and voluntary contraction of abdominal muscles after the preliminary starting command. Eight elite and sub-elite swimmers executed four crawl stroke starts: two CAP modified and two conventional trials with objective registration of start reaction and time to complete a 15-m segment. Statistical analysis included a repeated measures two-way ANOVA test and size effect. Both the p-value and d-value between experimental and control sets were calculated. The average benefit of the CAP modified technique was equal to 0.08 s, which includes an advantage in start reaction equal to 0.05 s. As a result, application of the CAP modified technique to swimming race start resulted in a significant ergogenic effect, which was evaluated by start reaction and performance time on the 15-m segment of distance.

Influence of injection of Chinese botulinum toxin type A on the histomorphology and myosin heavy chain composition of rat gastrocnemius muscles

Botulinum toxin type A (BoNT/A) is a metalloprotease that blocks synaptic transmission via the cleavage of a synaptosomal-associated protein of 25 kDa (SNAP-25). It has gained widespread use as a treatment for cerebral palsy and skeletal muscle hypertrophy. In China, Chinese botulinum toxin type A (CBTX-A), a type of BoNT/A, is in widespread clinical use. However, the changes in the morphological and biochemical properties of treated muscles and in remote muscles from the CBTX-A injection site are relatively unknown. Therefore, we investigated the changes in histomorphology and myosin heavy chain (MyHC) isoform composition and distribution in rat gastrocnemius muscles after intramuscular injection of CBTX-A. The weakness of the injected muscles was assessed periodically to identify their functional deficiency. Muscle slices were stained with hematoxylin-eosin (HE) and adenosine triphosphatase (ATPase). MyHC isoform composition was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) to uncover changes in morphological and biochemical properties. Our findings demonstrate that following injection of CBTX-A 5 U into rat gastrocnemius muscles, shifts in MyHC isoform composition emerged on the third day after injection and peaked in the fourth week. The composition remained distinctly different from that of the control group after the twelfth week. More specifically, there was a decrease in the proportion of the type IIb isoform and an increase in the proportions of type IIx, type IIa, and type I isoforms. Data revealed that CBTX-A led to a shift in MyHC composition towards slower isoforms and that the MyHC composition remained far from normal six months after a single injection. However, no noticeable remote muscle weakness was induced.

Tissue oxygen saturation changes during intramedullary nailing of lower-limb fractures

The systemic complications of acute intramedullary nailing (IMN) in trauma patients are well known. There are no reliable methods available to predict these adverse outcomes. Noninvasive near-infrared spectroscopy (NIRS) allows measurement of oxygen saturation within muscle tissue (StO2) and quantification of the potential metabolic and microcirculatory effects of IMN in real time. The aim of this study was to characterize tissue oxygenation changes occurring during reamed IMN. Patients undergoing reamed IMN for fixation of a tibia or femur fracture and patients having an open reduction and internal fixation of the ankle (to control for potential effects of anesthesia) had a noninvasive NIRS probe attached to the thenar eminence of the hand. Tissue oxygenation was monitored continuously throughout the operation and digitally recorded for later analysis. Vascular occlusion tests, an established technique with the NIRS device, were performed before canal opening and after nail insertion (at equivalent times in the control group), to establish the presence and nature of changes in systemic microcirculation occurring during the duration of the operation. Tissue oxygenation data were collected on 23 patients undergoing 26 IMN. (mean [SD] age, 36 [19] years; median Injury Severity Score [ISS], 9; interquartile range, 9-12). The control group consisted of 19 patients (mean [SD] age, 41 [18] years; ISS, 4). Remote muscle tissue desaturated significantly faster after IMN compared with the control operation (mean [SD] difference in IMN desaturation rate, 1.8% per minute [2.6% per minute]; mean [SD] difference in control group desaturation rate, -0.6% per minute [1.5% per minute]; p = 0.014). Near infrared-derived muscle oxygen consumption (NIR VO(2)) was significantly increased during the course of IMN compared with the control (mean [SD] difference in IMN NIR VO(2), 19.9 [32.1]; mean [SD] difference in control NIR VO(2), -4.2 [17.9]; p = 0.041). IMN causes significant remote microcirculatory changes. The responsiveness of the microcirculation could be a predictor of secondary organ dysfunction. Epidemiologic study, level III.