Sustained Isometric Contraction Research Articles

Effect of blood flow occlusion on corticospinal excitability during sustained low-intensity isometric elbow flexion.

Blood flow occlusion (BFO) has been used to study the influence of group III/IV muscle afferents after fatiguing exercise, but it is unknown how BFO-induced activity of these afferents affects motor cortical and motoneuronal excitability during low-intensity exercise. Therefore, the purpose of this study was to assess the acute effect of BFO on peripheral [maximal M wave (Mmax)], spinal [cervicomedullary motor evoked potential (CMEP) normalized to Mmax], and motor cortical [motor evoked potential (MEP) normalized to CMEP] excitability. Nine healthy men completed a sustained isometric contraction of the elbow flexors at 20% of maximal force under three conditions: 1) contractile failure with BFO, 2) a time-matched trial without restriction [free flow (FFiso)], and 3) contractile failure with free flow (FFfail). Time to failure for BFO (and FFiso) were ~80% shorter than that for FFfail (P < 0.05). For FFfail and FFiso, Mmax area decreased ~17% and ~7%, respectively (P < 0.05), with no change during BFO. CMEP/Mmax area increased ~226% and ~80% during BFO and FFfail, respectively (P < 0.05), with no change during FFiso (P > 0.05). The increase in normalized CMEP area was greater for BFO and FFfail compared with FFiso and for BFO compared with FFfail. MEP/CMEP area was not different among the protocols (P > 0.05) and increased ~64% with time (P < 0.05). It is likely that group III/IV muscle afferent feedback to the spinal cord modulates the large increase in motoneuronal excitability for the BFO compared with FFfail and FFiso protocols.NEW & NOTEWORTHY We have observed how blood flow occlusion modulates motor cortical, spinal, and peripheral excitability during and immediately after a sustained low-intensity isometric elbow flexion contraction to failure. We conclude that blood flow occlusion causes a greater and more rapid increase in motoneuronal excitability.

Effect of blood flow occlusion on neuromuscular fatigue following sustained maximal isometric contraction.

Sustained isometric maximal voluntary contractions (IMVCs) have blood flow occlusive effects on the microvasculature. However, it is unknown if this effect would be magnified with additional blood flow restriction via a cuff and what the influence on fatigue development would be. Twelve healthy male participants performed a 1-min IMVC of the knee extensors with and without additional blood flow occlusion induced by pneumatic cuff in counterbalanced order on separate days. Vastus lateralis muscle deoxygenation was estimated via near-infrared spectroscopy-derived tissue oxygen saturation (SmO2) throughout the fatiguing contraction. Central and peripheral measures of neuromuscular fatigue (NMF) were assessed via surface electromyography (EMG) and force responses to voluntary contractions and peripheral nerve/transcranial magnetic stimulations before, immediately after, and throughout an 8-min recovery period. SmO2, force, and EMG amplitude decreased during the 1-min IMVC, but there were no between-condition differences. Similarly, no significant (p > 0.05) between-condition differences were detected for any dependent variable immediately after the fatiguing contraction. Transcranial magnetic stimulation (TMS)-derived voluntary activation was lower (p < 0.05) in the no-cuff condition during the recovery period. Sustained IMVC results in a similar degree of muscle deoxygenation and NMF as IMVCs with additional occlusion, providing further evidence that a sustained IMVC induces full ischemia. Novelty NMF etiology, muscle oxygenation, and corticospinal factors during an IMVC are similar with or without an occlusion cuff. Contrary to all other measures, TMS-evaluated voluntary activation returned to baseline faster following the occluded condition.

Fatigue Related Changes in Electromyographic Coherence between Homologous Vastus Lateralis Muscles

Background. The aim of this study was to examine changes in the coherence between EMG signals of homologous quadriceps (vastus lateralis, VL) muscles during in the beginning and the end of multiple sustained isometric bilateral contraction at maximal effort. Methods. Twelve young healthy males participated in the study. The increase in coherence between EMG signals of homologous VL muscles in alpha band, beta band and gamma band was observed during the performance of sustained maximal bilateral isometric contraction with simultaneous decreases in bilateral force, CAR. EMG frequency and EMG amplitude.Results. The increase in beta band coherence during sustained maximal isometric contraction was negatively correlated with the decrease in bilateral force, CAR, EMG amplitude and EMG frequency indicating the possible role of beta coherence to improve VL muscle activation efficiency during maximal bilateral isometric contraction and overcome the effects of VL muscle fatigue in young healthy males. Keywords: muscle fatigue, EMG coherence, bilateral contraction, common drive.

Correlations Between Myoelectric and Hemodynamic Parameters Changes in Biceps Brachii During Sustained Isometric Contraction in Healthy Elderly

The purpose of the present study was to evaluate the Pearson correlations between hemodynamic parameters obtained with near infrared spectroscopy (NIRS), namely total hemoglobin (tHb) and tissue oxygen index (TOI %), as indicators of blood flow and oxygen extraction, and myoelectric parameters obtained with surface multichannel electromyography (sEMG), namely fractal dimension (FD) and conduction velocity (CV) slopes, as descriptors of central and peripheral fatigue, during sustained high level isometric contractions at 60% maximal voluntary contraction (MVC), in elderly subjects. Ten recreationally healthy elderly subjects (age 67.7 ± 4.6 years), 5 males and 5 females, performed one isometric contraction of the elbow flexors at 120° joint angle at 60% of MVC until exhaustion in two subsequent trials 1 week apart, one for the sEMG and the other one for NIRS recordings from the biceps brachii. A negative strong Pearson’s correlation between TOI % slope in the functional hyperemic phase (HP) of contraction and a positive correlation between the slope of TOI % recorded during the ischemic phase (IP) and the CV slope were found (r = − 0.76, P < 0.05; r = 0.91, P < 0.01, respectively). A negative strong correlation between tHb IP slope and tHb HP slope and TOI % IP slope and TOI % HP slope (r = − 0.78, P < 0.01; r = − 0.78, P < 0.01, respectively) were also found. No correlation was found between FD slope and hemodynamic parameters (FD slope vs. HP tHb slope, r = − 0.19; FD slope vs. IP tHb slope, r = 0.16; FD slope vs. HP TOI % slope, r = 0.19; FD slope vs. IP TOI % slope, r = 0.27). In aged individuals, CV slope correlated with muscle O2 desaturation (TOI %) rather than reduced muscle blood flow (tHb) in the ischemic phase of isometric contraction to exhaustion. No correlations were found between myoelectric manifestations of central fatigue (FD slope) and data obtained by NIRS.

An assessment of fatigability following nerve transfer to reinnervate elbow flexor muscles.

Improvements in the evaluation of outcomes following peripheral nerve injury are needed. Recent studies have identified muscle fatigue as an inevitable consequence of muscle reinnervation. This study aimed to quantify and characterize muscle fatigue within a standardized surgical model of muscle reinnervation. This retrospective cohort study included 12 patients who underwent Oberlin nerve transfer in an attempt to restore flexion of the elbow following brachial plexus injury. There were ten men and two women with a mean age of 45.5 years (27 to 69). The mean follow-up was 58 months (28 to 100). Repeated and sustained isometric contractions of the elbow flexors were used to assess fatigability of reinnervated muscle. The strength of elbow flexion was measured using a static dynamometer (KgF) and surface electromyography (sEMG). Recordings were used to quantify and characterize fatigability of the reinnervated elbow flexor muscles compared with the uninjured contralateral side. The mean peak force of elbow flexion was 7.88 KgF (sd 3.80) compared with 20.65 KgF (sd 6.88) on the contralateral side (p < 0.001). Reinnervated elbow flexor muscles (biceps brachialis) showed sEMG evidence of fatigue earlier than normal controls with sustained (60-second) isometric contraction. Reinnervated elbow flexor muscles also showed a trend towards a faster twitch muscle fibre type. The assessment of motor outcomes must involve more than peak force alone. Reinnervated muscle shows a shift towards fast twitch fibres following reinnervation with an earlier onset of fatigue. Our findings suggest that fatigue is a clinically relevant characteristic of reinnervated muscle. Adoption of these metrics into clinical practice and the assessment of outcome could allow a more meaningful comparison to be made between differing forms of treatment and encourage advances in the management of motor recovery following nerve transfer. Cite this article: Bone Joint J 2019;101-B:867-871.

The influence of transcranial direct current stimulation on pain affect and endurance exercise

IntroductionTranscranial direct current stimulation (tDCS) is a non-invasive form of electrical brain stimulation that has been offered as a novel method to enhance endurance exercise performance. While the underlying mechanisms for these performance enhancing effects remain unclear, one explanation relates to a reduction in pain experienced during exercise. Research examining this explanation however, has failed to consider the widely recognised motivational-affective component of the pain experience. ObjectivesThe present study aimed to determine whether pain experienced during exercise involves an affective component, and whether a reduction in pain affect may account for the performance-enhancing effects of tDCS. MethodsHealthy, pain-free individuals (n = 23), including 11 males and 12 females aged 18–39 years, were recruited for participation in a randomised, placebo-controlled, participant blinded, repeated measures design study. Participants attended two testing sessions separated by at least five days. In each session, participants received either active (2 mA, 20 min) or sham (placebo control) tDCS delivered to the left dorsolateral prefrontal cortex (DLPFC). Participants then completed an endurance exercise task comprised of a sustained isometric contraction of the leg extensors performed to exhaustion at an intensity corresponding to 25% of their maximal voluntary contraction. During this task, participants provided ratings of pain intensity and pain unpleasantness at 20 s intervals. ResultsRatings of pain intensity (p < .001) and pain affect (p < .001) increased throughout the endurance exercise task. However, the tDCS intervention did not enhance endurance exercise performance (p > .05), nor manipulate perceptions of pain affect (p > .05). ConclusionsWhile the endurance exercise task induced pain affect, tDCS over the left DLPFC was not effective in reducing this component of the pain experience, nor enhancing exercise performance.

Fatigue reduces the complexity of knee extensor torque during fatiguing sustained isometric contractions

The temporal structure, or complexity, of muscle torque output reflects the adaptability of motor control to changes in task demands. This complexity is reduced by neuromuscular fatigue during intermittent isometric contractions. We tested the hypothesis that sustained fatiguing isometric contractions would result in a similar loss of complexity. To that end, nine healthy participants performed, on separate days, sustained isometric contractions of the knee extensors at 20% MVC to task failure and at 100% MVC for 60 s. Torque and surface EMG signals were sampled continuously. Complexity and fractal scaling were quantified by calculating approximate entropy (ApEn) and the detrended fluctuation analysis (DFA) α scaling exponent. Global, central and peripheral fatigue were quantified using maximal voluntary contractions (MVCs) with femoral nerve stimulation. Fatigue reduced the complexity of both submaximal (ApEn from 1.02 ± 0.06 to 0.41 ± 0.04, P < 0.05) and maximal contractions (ApEn from 0.34 ± 0.05 to 0.26 ± 0.04, P < 0.05; DFA α from 1.41 ± 0.04 to 1.52 ± 0.03, P < 0.05). The losses of complexity were accompanied by significant global, central and peripheral fatigue (all P < 0.05). These results demonstrate that a fatigue-induced loss of torque complexity is evident not only during fatiguing intermittent isometric contractions, but also during sustained fatiguing contractions.

Central Command, but not the Muscle Metaboreflex, is Responsible for Intensity‐Dependent Increases in Muscle Sympathetic Nerve Activity to Contracting Muscle during High‐Intensity Static Isometric Exercise in Humans

Post‐exercise ischaemia has been used to demonstrate muscle metaboreflex‐mediated increases in muscle sympathetic nerve activity (MSNA) to an inactive limb during static isometric exercise. In contrast, we have previously shown a rapid centrally‐mediated increase in MSNA to a contracting leg during sustained isometric exercise, which promptly returned to resting levels at the cessation of exercise despite the presence of post‐exercise muscle ischaemia of the ipsilateral limb. However, these observations have been made during relatively low‐intensity exercise. The present study tested the hypothesis that central command, but not the muscle metaboreflex, increases MSNA to the contracting muscle during sustained isometric contractions even at higher intensities. MSNA was recorded continuously (microneurography) from the right common peroneal nerve. Twelve subjects (seven males, five females) performed two‐minute ankle dorsiflexions at 10, 20, 30, 40 and 50% of maximum voluntary contraction (MVC) ipsilateral to the MSNA recording, which was immediately followed by six minutes of post‐exercise ischaemia. There was at least six minutes rest between contractions. MSNA was analysed by measuring negative‐going cardiac‐locked sympathetic spikes from the raw neurogram. Compared with MSNA at rest (82 ± 5 spikes/min), MSNA to contracting muscle increased in an intensity‐dependent manner within one‐minute of contraction at 10 %MVC (145 ± 27 spikes/min, P=0.04), 20 %MVC (219 ± 28 spikes/min, P=0.002), 30 %MVC (480 ± 92 spikes/min, P=0.002), 40 %MVC (716 ± 185 spikes/min, P=0.01) and 50 %MVC (934 ± 275 spikes/min, P=0.02). Blood pressure increased in an intensity‐dependent manner during contractions up to 50 %MVC (76 ± 5 vs 93 ± 7 mmHg, P=0.02). Within one‐minute after each contraction MSNA returned close to resting levels even in the presence of post‐exercise ischaemia. These findings indicate that central command, and not the metaboreflex, is responsible for the increase in MSNA to contracting muscle during sustained isometric contractions, even at intensities up to 50% of MVC.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

(276) Acute Exercise Decreases Situational Pain Catastrophizing of a Noxious Stimulus

There is limited evidence regarding the variability in pain response following exercise, including the impact of pain catastrophizing. The purpose of this study was to investigate the relation of the pain response after exercise with pain catastrophizing levels in young healthy individuals. Fifty-seven participants (20.75±1.81 years, 29 women) completed two randomized sessions (exercise and quiet rest). Exercise consisted of sustained submaximal isometric contraction of the knee extensors, performed until task failure. Temporal summation (TS) was assessed before and after exercise and quiet rest, by applying a constant mechanical stimulus to the index finger for one minute after reported pain threshold. During the one minute, participants were asked to rate their pain every 15 seconds (0-10 Numerical Pain Rating Scale). TS was calculated as the last pain rating minus the first pain rating. Pain catastrophizing scale (PCS) was completed at the beginning of the first session. Situational catastrophizing (S-PCS) was completed after each TS protocol. Based on the change in pain following exercise, participants were divided into two groups, responders (hypoalgesia) and non-responders (no change or hyperalgesia). There was no difference in baseline PCS between groups (p=0.974). For S-PCS, there was a session x trial x group interaction (p

Reduced blood oxygen levels induce changes in low-threshold motor unit firing that align with the individual's tolerance to hypoxia.

This study aimed to quantify how acute hypoxia impacts firing characteristics of biceps brachii motor units (MUs) during sustained isometric elbow flexions. MU data were extracted from surface electromyography (EMG) during 25% maximal voluntary contractions (MVC) in 10 healthy subjects (age 22 ± 1 yr). Blood oxygen saturation (SpO2) was then stabilized at 80% by reducing 1% of the fraction of inspired oxygen every 3 min for 35 min. MU data were once again collected 1 h and 2 h following the 35-min desaturation phase. Although MVC remained unaffected during 2 h of 80% SpO2, subject-specific changes in MU firing rate were observed. Four of 10 subjects exhibited a decrease in firing rate 1 h postdesaturation (12 ± 11%) and 2 h postdesaturation (16 ± 12%), whereas 6 of 10 subjects exhibited an increase in firing rate 1 h (9 ± 6%) and 2 h (9 ± 4%) postdesaturation. These bidirectional changes in firing rate were strongly correlated to the desaturation phase and the subjects' SpO2 sensitivity to oxygen availability, where subjects who had decreased firing rates reached the target SpO2 20 min into the desaturation phase (R2 = 0.90-0.98) and those who had increased firing rates reached the target SpO2 35 min into the desaturation phase (R2 = 0.87-0.98). It is unlikely that a single mechanism accounted for these subject-specific changes in firing rate. Instead, differences in intrinsic properties of the neurons, afferent input to the motoneurons, neuromodulators, and sympathetic nerve activity may exist between groups. NEW & NOTEWORTHY The mechanisms of compromised motor control when exposed to hypoxia are largely unknown. The current study examined how severe acute hypoxia affects motor unit firing rate during sustained isometric contractions of the bicep brachii. The response to hypoxia was different across subjects, where motor unit firing rate increased for some individuals and decreased for others. This bidirectional change in firing rate was associated with how fast subjects desaturated during hypoxic exposure.

Pre-Exercise Rehydration Attenuates Central Fatigability during 2-Min Maximum Voluntary Contraction in Hyperthermia.

Background and objectives: Hyperthermia with dehydration alters several brain structure volumes, mainly by changing plasma osmolality, thus strongly affecting neural functions (cognitive and motor). Here, we aimed to examine whether the prevention of significant dehydration caused by passively induced whole-body hyperthermia attenuates peripheral and/or central fatigability during a sustained 2-min isometric maximal voluntary contraction (MVC). Materials and Methods: Ten healthy and physically active adult men (21 ± 1 years of age) performed an isometric MVC of the knee extensors for 2 min (2-min MVC) under control (CON) conditions, after passive lower-body heating that induced severe whole-body hyperthermia (HT, Tre > 39 °C) with dehydration (HT-D) and after HT with rehydration (HT-RH). Results: In the HT-D trial, the subjects lost 0.94 ± 0.15 kg (1.33% ± 0.13%) of their body weight; in the HT-RH trial, their body weight increased by 0.1 ± 0.42 kg (0.1% ± 0.58%). After lower-body heating, the HT-RH trial (vs. HT-D trial) was accompanied by a significantly lower physiological stress index (6.77 ± 0.98 vs. 7.40 ± 1.46, respectively), heart rate (47.8 ± 9.8 vs. 60.8 ± 13.2 b min−1, respectively), and systolic blood pressure (−12.52 ± 5.1 vs. +2.3 ± 6.4, respectively). During 2-min MVC, hyperthermia (HT-D; HT-RH) resulted in greater central fatigability compared with the CON trial. The voluntary activation of exercising muscles was less depressed in the HT-RH trial compared with the HT-D trial. Over the exercise period, electrically (involuntary) induced torque decreased less in the HT-D trial than in the CON and HT-RH trials. Conclusions: Our results suggest that pre-exercise rehydration might have the immediate positive effect of reducing physiological thermal strain, thus attenuating central fatigability even when exercise is performed during severe (Tre > 39 °C) HT, induced by passive warming of the lower body.

Sensitivity of the human ventilatory response to muscle metaboreflex activation during concurrent mild hypercapnia.

What is the central question of this study? What is the relationship between the level of systemic hypercapnia and the magnitude of the additional hyperpnoea produced in response to a standardized level of muscle metaboreflex activation? What is the main finding and its importance? When a standardized activation of the muscle metaboreflex was combined with exposure to increasing levels of hypercapnia, the hyperpnoea this caused increased linearly. The concept of a synergistic interaction between the muscle metaboreflex and the central chemoreflex in humans is supported by this finding. Ventilation increases during muscle metaboreflex activation when postexercise circulatory occlusion (PECO) traps metabolites in resting human muscle, but only in conditions of concurrent systemic hypercapnia. We hypothesize that a linear relationship exists between the level of hypercapnia and the magnitude of the additional hyperpnoea produced in response to a standardized level of muscle metaboreflex activation. Fifteen male subjects performed four trials, in which the end-tidal partial pressure of carbon dioxide ( ) was elevated by 1, 3, 7 or 10mmHg above resting values using a dynamic end-tidal forcing system. In each trial, subjects were seated in an isometric dynamometer designed to measure ankle plantar flexor force. Rest for 2 min in room air was followed by 15min of exposure to one of the four levels ofhypercapnia, at which 5min further rest was followed by 2 min of sustained isometric calf muscle contraction at 50% of predetermined maximal voluntary strength. Immediately before cessation of exercise, a cuff around the upper leg was inflated to a suprasystolic pressure to cause PECO for 3min, before its deflation and a further 5min of rest, concluding exposure to hypercapnia. The PECO consistently elevated mean arterial blood pressure by ∼10mmHg in all trials, indicating similar levels of metaboreflex activation. Increased ventilation during PECO was related to as described by the following linear regression equation: Change in minute ventilation (lmin-1 )=0.85× (mmHg)+0.80 (lmin-1 ). This finding supports our hypothesis and furthers the idea of a synergistic interaction between muscle metaboreflex activation and central chemoreflex stimulation.

Accelerated muscle contractility and decreased muscle steadiness following sauna recovery do not induce greater neuromuscular fatigability during sustained submaximal contractions

Acute whole-body hyperthermia (WBH) increases blood markers concentration of stress, impairs motor drive to exercising muscles, and decreases resistance to neuromuscular fatigability. The functional natural residual consequences of WBH on neuromuscular functions remain unclear. We aimed to investigate the effects of residual WBH on voluntary and electrically induced ankle plantar flexor contractility properties, motor drive transmission (reflexes), muscle torque steadiness, resistance to neuromuscular fatigability, and markers of stress as the body temperature recovers naturally to normothermia. WBH was induced by Finnish sauna bathing in 16 apparently healthy young (24 ± 4 years) adult men. Motor performance was monitored before and 2 h after the sauna, and immediately after submaximal exercise (120 s at 50% of maximal voluntary contraction). Markers of stress were monitored before and 2 h after the sauna. Finnish sauna exposure induced moderate to severe WBH (rectal temperature, 38.5–39.6 °C). At 2 h after the sauna, rectal temperature had recovered to the preheating level (preheating 37.11 ± 0.33 °C versus postheating 37.00 ± 0.29 °C, p > .05). Post-sauna recovery was accompanied by slowed salivary free cortisol diurnal kinetics, whereas noradrenaline, dopamine, and serotonin did not persist into the 2 h recovery after the sauna. Although recovery to normothermia after a sauna led to a greater acceleration of muscle contractility properties and decreased muscle steadiness, sustained isometric submaximal contraction did not provoke greater neuromuscular fatigability.

Clinical utility of portable electrophysiology to measure fatigue in treatment-naïve non-small cell lung cancer.

Cancer-related fatigue (CRF) biology remains poorly understood. Responsible mechanisms may be central or peripheral and originate anywhere from the brain to muscle fiber. Objective measurement is complex and previously limited to specialized laboratories. Portable electroencephalography (EEG) and electromyography (EMG) may enhance objective measurement. This study evaluated the feasibility and acceptability of portable EMG-EEG in CRF assessment. A prospective observational feasibility study compared ten outpatients with inoperable, treatment-naïve non-small cell lung cancer and CRF to ten healthy volunteers. All completed a sustained isometric hand-grip contraction at 30% maximal level until self-perceived exhaustion. 128-channel EEG and 2-channel EMG signals of forearm muscles were recorded. Device acceptability was evaluated by questionnaire. The task was evaluated in two stages; first and last 20s. CRF cohort perceived exhaustion earlier than volunteers (mean 137 ± 76s vs 208 ± 51s). As fatigue progressed, EMG amplitude increased significantly (CRF p =0.02; volunteers: p =0.04) in both groups as did EMG beta band power (CRF p =0.008; volunteers: p =0.006). The increase was significantly less in CRF (amplitude p =0.032; beta power: p =0.014). EEG beta band power in the contralateral motor cortex increased significantly (CRF p =0.03; volunteers: p =0.019) in both cohorts but to greater extent (p =0.024) in CRF. One hundred percent device acceptability was reported. A laboratory-based evaluation was successfully adapted to the outpatient setting during routine visits. High acceptability supports clinical utility. In CRF, a higher degree of cortical activation was required to drive a much lower level of muscle performance. This suggests impairment of both central and peripheral mechanisms in CRF.

Oxygen Saturation of Dorsiflexor Muscles during Sustained Isometric Contraction

Oxygen Saturation of Dorsiflexor Muscles during Sustained Isometric Contraction

Short-Term Preconditioning With Blood Flow Restricted Exercise Preserves Quadriceps Muscle Endurance in Patients After Anterior Cruciate Ligament Reconstruction.

Surgical ACL reconstruction performed with a tourniquet induces compression and ischemic stress of the quadriceps femoris (QF) muscle which can accelerate postoperative weakness. Given that low-load blood flow restricted (BFR) exercise is potent in enhancing muscle oxygenation and vascular function, we hypothesized that short-term preconditioning with low-load BFR exercise can attenuate QF muscle endurance deterioration in the postoperative period. Twenty subjects undergoing arthroscopic ACL reconstruction performed 5 exercise sessions in the last 8 days prior to surgery. They were assigned into either BFR group, performing low-load BFR knee-extension exercise, or SHAM-BFR group, replicating equal training volume with sham occlusion. Blood flow (near-infrared spectroscopy) and surface EMG of QF muscle during sustained isometric contraction at 30% of maximal voluntary isometric contraction (MVIC) torque performed to volitional failure were measured prior to the intervention and again 4 and 12 weeks after surgery. There was an overall decrease (p = 0.033) in MVIC torque over time, however, no significant time-group interaction was found. The time of sustained QF contraction shortened (p = 0.002) in SHAM-BFR group by 97 ± 85 s at week 4 and returned to preoperative values at week 12. No change in the time of sustained contraction was detected in BFR group at any time point after surgery. RMS EMG amplitude increased (p = 0.009) by 54 ± 58% at week 4 after surgery in BFR group only. BFm increased (p = 0.004) by 52 ± 47% in BFR group, and decreased (p = 0.023) by 32 ± 19% in SHAM-BFR group at week 4 after surgery. Multivariate regression models of postoperative changes in time of sustained QF contraction revealed its high correlation (R2 = 0.838; p < 0.001) with changes in BFm and RMS EMG in the SHAM-BFR group, whereas no such association was found in the BFR group. In conclusion, enhanced endurance of QF muscle was triggered by combination of augmented muscle fiber recruitment and enhanced muscle perfusion. The latter alludes to a preserving effect of preconditioning with BFR exercise on density and function of QF muscle microcirculation within the first 4 weeks after ACL reconstruction.

Do aerobic characteristics explain isometric exercise-induced neuromuscular fatigue and recovery in upper and lower limbs?

ABSTRACTThis study investigated the relationships between aerobic characteristics and (i) neuromuscular fatigue induced by 2-min sustained isometric maximal voluntary contractions (MVC) and (ii) subsequent recovery, in the upper and lower limbs. In a pseudo-randomized order, eleven healthy males completed four sessions on different days: maximal incremental cycling test (100 W + 40 W every 2 min); maximal arm-cranking test (50 W + 20 W every 2 min); and 2-min sustained isometric MVCs of the knee extensors (KE) and elbow flexors (EF). Neuromuscular assessment was performed with transcranial magnetic and peripheral nerve stimulation to evaluate central and peripheral neuromuscular factors of fatigue and the subsequent recovery. Peak oxygen uptake, gas exchange threshold and the corresponding power outputs were correlated with recovery of voluntary force after the 2-min KE MVC. Regression analysis showed that power output at the gas exchange threshold alone explained 72% of the variability in ∆recovery of KE voluntary force. No relationships with fatigue or recovery in EF were observed. These results suggest that participants with greater aerobic capacities experience the same amount of fatigue and faster recovery of voluntary force in KE but not EF. The potential reasons behind the relationship in KE but not EF are discussed.

Maximal strength training increases muscle force generating capacity and the anaerobic ATP synthesis flux without altering the cost of contraction in elderly.

Aging is associated with a progressive decline in skeletal muscle function, then leading to impaired exercise tolerance. Maximal strength training (MST) appears to be a practical and effective intervention to increase both exercise capacity and efficiency. However, the underlying physiological mechanisms responsible for these functional improvements are still unclear. Accordingly, the purpose of this study was to examine the intramuscular and metabolic adaptations induced by 8 weeks of knee-extension MST in the quadriceps of 10 older individuals (75 ± 9 yrs) by employing a combination of molecular, magnetic resonance 1H-imaging and 31P-spectroscopy, muscle biopsies, motor nerve stimulation, and indirect calorimetry techniques. Dynamic and isometric muscle strength were both significantly increased by MST. The greater torque-time integral during sustained isometric maximal contraction post-MST (P = 0.002) was associated with increased rates of ATP synthesis from anaerobic glycolysis (PRE: 10 ± 7 mM·min-1; POST: 14 ± 7 mM·min-1, P = 0.02) and creatine kinase reaction (PRE: 31 ± 10 mM·min-1; POST: 41 ± 10 mM·min-1, P = 0.006) such that the ATP cost of contraction was not significantly altered. Expression of fast myosin heavy chain, quadriceps muscle volume, and submaximal cycling net efficiency were also increased with MST (P = 0.005; P = 0.03 and P = 0.03, respectively). Overall, MST induced a shift toward a more glycolytic muscle phenotype allowing for greater muscle force production during sustained maximal contraction. Consequently, some of the MST-induced improvements in exercise tolerance might stem from a greater anaerobic capacity to generate ATP, while the improvement in exercise efficiency appears to be independent from an alteration in the ATP cost of contraction.

Larger strength losses and muscle activation deficits in plantar flexors induced by backward downhill in reference to distance-matched forward uphill treadmill walk

We tested the hypothesis that backward downhill walking (eccentric component) impairs both voluntary activation and muscle contractile properties in the plantar flexors and delays recovery as compared to a gradient and distance-matched uphill walk. Fourteen males performed two 30-min walking exercises (velocity: 1 m/ s; grade: 25%; load: 12% of body weight), one downhill (DW) and one uphill (UP), in a counterbalanced order, separated by 6 weeks. Neuromuscular test sessions were performed before, after, 24-, 48- and 72-h post-exercise, including motor nerve stimulations during brief (5 s) and sustained (1 min) maximal isometric voluntary contractions of the plantar flexors. DW (−18.1 ± 11.1%, P < .001), but not UP (−6.0 ± 7.7%, P =.15), decreased torque production during brief contractions for at least three days post-exercise (P < .05). Voluntary activation during brief contractions decreased after DW (P < .05), but not UP, and recovered by 24 h. Both UP (−9.3 ± 9.0%, P = .024) and DW (−25.6 ± 10.3%, P < .001) decreased torque production during sustained contractions but voluntary activation (P = .001) was lower in DW than UP. Peak twitch torque and maximum rates of torque development and relaxation were equally reduced after UP and DW (P < .05), and recovered by 24 h. DW induced an increase in muscle soreness with peak values observed 48 h post-walking (P < .001), whereas post-UP exercise changes were non-significant (all P > .05). Using a direct comparison, the capacity to drive the plantar flexors during sustained contractions remains sub-optimal during the three-day recovery period in response to non-exhaustive, downhill backward walking in reference to an uphill exercise matched for distance covered.

Characterization of knee extensor muscles fatigability during isokinetic concentric contractions in patients with multiple sclerosis

Increased muscle fatigability has been reported in patients with multiple sclerosis (MS) assessed during sustained isometric contractions however rehabilitation programs and daily life activities often involve concentric contractions. The aim of this study was to characterize knee extensor muscles fatigability in patients with MS during concentric contractions. Thirty-eight patients with MS and 14 healthy persons were included in this study. An isokinetic assessment of knee extensor muscles fatigability was carried out during a fatiguing task of 50 concentric contractions, associated with an electromyographic activity analysis of the rectus femoris, vastus lateralis, biceps femoris and semitendinosus muscles. Patients with MS experienced lower knee extensor muscles fatigability than healthy subjects. When torque was normalized to maximal isometric peak torque, patients with MS and healthy persons had the same level of relative strength at the end of the fatigue protocol (50% of the maximal isometric peak torque). During the fatiguing task neuromuscular efficiency decreased but coactivations of knee flexor muscles remained unchanged. Fatigability is a common complaint in patients with MS but they experienced lower muscle fatigability than healthy persons during maximal concentric contractions, nevertheless they finished fatigue protocol at the same level of relative strength.