RESUMEN
Low-intensity contractions with blood flow occlusion (BFO) result in neuromuscular adaptations comparable with high-intensity (>70% maximal voluntary contraction, MVC) exercise. Because BFO exercise can only be applied to limb muscles, it is of interest to explore whether muscles proximal to the occlusion site are affected. Therefore, the purpose of this study was to assess neural activation of the tibialis anterior (TA) when flow is occluded proximal and distal to the active muscle. Five males completed three protocols to observe the effect of BFO on motor unit firing rates (MUFR) of the TA at a fatiguing contraction intensity of â¼15% MVC. Two occlusion protocols, one proximal (BFOprox) to and one distal (BFOdis) to the TA, were compared with a control (free-flow) protocol time-matched to BFOdis. MVC was significantly reduced following the BFOprox (â¼41%; P < 0.001) and BFOdis (â¼27%, P < 0.001), but not following the control protocol (â¼15%; P = 0.13). Surface electromyography (EMG) during BFOdis and BFOprox increased â¼14% and â¼28%, respectively, but was not different among protocols. MUFRs for BFOdis and BFOprox were significantly reduced (by â¼33% and â¼23%, respectively; P < 0.01) at task failure. Results indicate that although BFOprox results in the largest reductions of MUFRs, BFOdis shows greater impairments compared with the free-flow control condition. Novelty Effects on motor unit firing rates of proximal versus distal blood flow occlusion were compared during low-intensity fatiguing task. Proximal occlusion results in greatest fatigue and reduction in motor unit rates, but distal occlusion elicits more fatigue and rate reduction than a control task.
Asunto(s)
Contracción Isométrica/fisiología , Neuronas Motoras/fisiología , Músculo Esquelético/fisiología , Resistencia Física/fisiología , Adulto , Velocidad del Flujo Sanguíneo , Electromiografía , Humanos , Masculino , Fatiga Muscular/fisiologíaRESUMEN
INTRODUCTION: Blood flow-restricted or occlusion exercise enhances muscle hypertrophy and strength during resistance training. The acute effects on voluntary and electrically evoked muscle contractile characteristics with impaired blood flow at low- and high-contraction forces have not been explored. METHODS: On separate days, nine males completed two different protocols of concentric elbow flexor contractions. A repetitive low-force (~25% of isometric maximum voluntary contraction [MVC]) with blood flow occlusion (BFO) (300 mm Hg) protocol was compared with a high-force (~80% MVC) free blood flow protocol (HF), until range of motion (0°-90°) was impaired. Torque, velocity, and power were compared with baseline and between protocols. Maximum voluntary contraction and voluntary activation were assessed during and after each protocol. Muscle twitch, low (20 Hz) and high (50 Hz) tetanus, and compound muscle action potential (Mmax) area were measured at 0, 2, 5, 10, and 20 min of recovery. RESULTS: Repetitions to failure (FP) were lower for HF (~16) versus BFO (~21), and MVC at FP was reduced more during BFO (~77%) compared with HF (~23%), with no difference in voluntary activation (~10% loss) between protocols. At FP, velocities for BFO and HF were similarly reduced by approximately 63% and 56%, respectively; however, peak power decreased more during BFO (~90%) compared with HF (~67%). Total work for BFO was approximately 40% lower than for HF. Peak power for HF was recovered by 2 min, whereas BFO required 20 min. Low-frequency fatigue (20 Hz/50 Hz) was greater after BFO (~70% decrease vs ~29% decrease after HF), whereas Mmax area was unaffected. CONCLUSION: Concentric elbow flexions at low-force with BFO cause greater impairments in strength and power than HF and, therefore, may be a replacement for high-force exercise used in chronic training.
Asunto(s)
Brazo/fisiología , Contracción Isométrica/fisiología , Músculo Esquelético/irrigación sanguínea , Entrenamiento de Fuerza/métodos , Adulto , Estimulación Eléctrica , Humanos , Masculino , Fuerza Muscular/fisiología , Músculo Esquelético/anatomía & histología , Flujo Sanguíneo RegionalRESUMEN
PURPOSE: Blood flow restriction (BFR) with low-intensity (< 30% of 1 repetition maximum strength) muscle contraction has been used chronically (> 4 weeks) to enhance resistance training. However, mechanisms underlying muscle adaptations following BFR are not well understood. To explore changes related to chronic BFR adaptations, the current study used blood flow occlusion (BFO) during an acute bout of low-intensity isometric fatiguing contractions to assess peripheral (muscle) factors affected. METHODS: Ten males completed separate fatiguing elbow flexor protocols to failure; one with BFO and one with un-restricted blood flow (FF). Baseline, post-task failure, and 30 min of recovery measures of voluntary and involuntary contractile properties were compared. RESULTS: BFO had greater impairment of intrinsic measures compared with FF, despite FF lasting 80% longer. Following task failure, maximal voluntary contraction and 50 Hz torque decreased in both protocols (~ 60% from baseline). Voluntary activation decreased ~ 11% from baseline at failure following both protocols, but recovered at a faster rate following BFO, whereas MVC recovered to ~ 90% of baseline in both protocols. The 10/50 Hz torque ratio was decreased by ~ 68% and ~ 21% from baseline, for BFO and FF, respectively (P < 0.01). 50 Hz half-relaxation-time (HRT) was significantly longer immediately following BFO (~ 107% greater than baseline), with no change following FF. CONCLUSIONS: Thus, greater peripheral fatigue that recovers at a similar rate compared to conventional exercise is likely driving muscle adaptations observed with chronic BFR exercise. Likely BFO alters energy demand and supply of working muscle similar to chronic BFR, but is exaggerated in this paradigm.
Asunto(s)
Articulación del Codo/fisiología , Codo/fisiología , Ejercicio Físico/fisiología , Rango del Movimiento Articular/fisiología , Humanos , Contracción Isométrica/fisiología , Masculino , Músculo Esquelético/fisiologíaRESUMEN
In ramp-incremental cycling exercise, some individuals are capable of producing power output (PO) in excess of that produced at their limit of tolerance (LoT) whereas others cannot. This study sought to describe the 1) prevalence of a "power reserve" within a group of young men ( n = 21; mean ± SD: age 25 ± 4 yr; VÌo2max 45 ± 8 ml·kg-1·min-1); and 2) muscle fatigue characteristics of those with and without a power reserve. "Power reserve" (ΔPReserve) was determined as the difference between peak PO achieved during a ramp-incremental test to exhaustion and maximal, single-leg isokinetic dynamometer power determined within 45 s of completing the ramp-incremental test. Between-group differences in pre- vs. postexercise changes in voluntary and electrically stimulated single-leg muscle force production measures (maximal voluntary contraction torque, voluntary activation, maximal isotonic velocity and isokinetic power; 1-, 10-, 50-Hz torque; and 10/50-Hz ratio), VÌo2max, and constant-PO cycling time-to-exhaustion also were assessed. Frequency distribution analysis revealed a dichotomy in the prevalence of a power reserve within the sample resulting in two groups: 1) "No Reserve" (NRES: power reserve <5%; n = 10) and 2) "Reserve" (RES: power reserve >15%; n = 11). At the LoT, all participants had achieved VÌo2max. Muscle fatigue was evident in both groups, although the NRES group had greater reductions ( P < 0.05) in 10-Hz peak torque (PT), 10/50 Hz ratio, and maximal velocity. Time to the LoT during the constant PO test was 22 ± 16% greater ( P < 0.05) in RES (116 ± 19 s; PO = 317 ± 52 W) than in NRES (90 ± 23 s; PO = 337 ± 71 W), despite similar ramp-incremental exercise durations and VÌo2max between groups. Compared with the RES group, the NRES group accrued greater peripheral muscle fatigue at the LoT, suggesting that the mechanisms contributing to exhaustion in a ramp-incremental protocol are not uniform. NEW & NOTEWORTHY This study demonstrates that the mechanisms associated with the limit of tolerance during ramp-incremental cycling exercise differ between those who are capable of generating power output in excess of that at exercise termination vs. those who are not. Those without a "power reserve" exhibit greater peripheral muscle fatigue and reduced muscle endurance, supporting the hypothesis that exhaustion occurs at a specific level of neuromuscular fatigue. In contrast, those with a power reserve likely are limited by other mechanisms.
Asunto(s)
Ciclismo/fisiología , Ejercicio Físico/fisiología , Fatiga Muscular/fisiología , Músculo Esquelético/fisiología , Adulto , Prueba de Esfuerzo/métodos , Humanos , Masculino , Contracción Muscular/fisiología , Consumo de Oxígeno/fisiología , TorqueRESUMEN
Investigations of high-intensity isometric fatiguing protocols report decreases in motor unit firing rates (MUFRs), but little is known regarding changes in MUFRs following fatigue induced by high-intensity dynamic contractions. Our purpose was to evaluate MUFRs of the anconeus (an accessory elbow extensor) and elbow extension power production as a function of time to task failure (TTF) during high-velocity fatiguing concentric contractions against a moderately heavy resistance. Fine-wire intramuscular electrode pairs were inserted into the anconeus to record MUs in 12 male participants (25 ± 3 yr), over repeated sessions on separate days. MUs were tracked throughout a three-stage, varying load dynamic elbow extension protocol designed to extend the task duration for >1 min thereby inducing substantial fatigue. Mean MUFRs and peak power were calculated for three relative time ranges: 0-15% TTF (beginning), 45-60% TTF (middle) and 85-100% TTF (end). Mean duration of the overall fatigue protocol was â¼80 s. Following the protocol, isometric maximum voluntary contraction (MVC), highest velocity at 35% MVC load, and peak power decreased 37, 60, and 64% compared with baseline, respectively. Data from 20 anconeus MUs tracked successfully throughout the protocol indicated a reduction in MUFRs in relation to power loss from 36 Hz/160 W (0-15% TTF) to 28 Hz/97 W (45-60% TTF) to 23 Hz/43 W (85-100% TTF). During these high-intensity maximal effort concentric contractions, anconeus MUFRs decreased substantially (>35%). Although the absolute MUFRs were higher in the present study than those reported previously for other muscles during sustained high-intensity isometric tasks, the relative decrease in MUFRs was similar between the two tasks.
Asunto(s)
Neuronas Motoras/fisiología , Movimiento/fisiología , Contracción Muscular/fisiología , Músculo Esquelético/fisiología , Unión Neuromuscular/fisiopatología , Resistencia Física/fisiología , Rango del Movimiento Articular/fisiología , Electromiografía , Entrenamiento de Intervalos de Alta Intensidad/métodos , Humanos , Masculino , Esfuerzo Físico/fisiología , Reclutamiento Neurofisiológico/fisiología , Transmisión Sináptica/fisiología , Adulto JovenRESUMEN
During constant-power output (PO) exercise above lactate threshold (LT), pulmonary O2 uptake (VÌo2 p) features a developing slow component (VÌo2 pSC). This progressive increase in O2 cost of exercise is suggested to be related to the effects of muscle fatigue development. We hypothesized that peripheral muscle fatigue as assessed by contractile impairment would be associated with the VÌo2 pSC Eleven healthy men were recruited to perform four constant-PO tests at an intensity corresponding to â¼Δ60 (very heavy, VH) where Δ is 60% of the difference between LT and peak VÌo2 p The VH exercise was completed for each of 3, 8, 13, and 18 min (i.e., VH3, VH8, VH13, VH18) with each preceded by 3 min of cycling at 20 W. Peripheral muscle fatigue was assessed via pre- vs. postexercise measurements of quadriceps torque in response to brief trains of electrical stimulation delivered at low (10 Hz) and high (50 Hz) frequencies. During exercise, breath-by-breath VÌo2 p was measured by mass spectrometry and volume turbine. The magnitude of VÌo2 pSC increased (P < 0.05) from 224 ± 81 ml/min at VH3 to 520 ± 119, 625 ± 134, and 678 ± 156 ml/min at VH8, VH13, and VH18, respectively. The ratio of the low-to-high frequency (10/50 Hz) response was reduced (P < 0.05) at VH3 (-12 ± 9%) and further reduced (P < 0.05) at VH8 (-25 ± 11%), VH13 (-42 ± 19%), and VH18 (-46 ± 16%), mirroring the temporal pattern of VÌo2 pSC development. The reduction in 10/50 Hz ratio was correlated (P < 0.001, r(2) = 0.69) with VÌo2 pSC amplitude. The temporal and quantitative association of decrements in muscle torque production and VÌo2 pSC suggest a common physiological mechanism between skeletal muscle fatigue and loss of muscle efficiency.
Asunto(s)
Pulmón/fisiología , Fatiga Muscular/fisiología , Fuerza Muscular/fisiología , Músculo Esquelético/fisiología , Consumo de Oxígeno/fisiología , Resistencia Física/fisiología , Adulto , Entrenamiento de Intervalos de Alta Intensidad/métodos , Humanos , Masculino , Tasa de Depuración Metabólica , Modelos Biológicos , Oxígeno/metabolismo , Intercambio Gaseoso Pulmonar/fisiologíaRESUMEN
The purpose of the current study was to assess the effects of 5 brief (2s), intermittent, submaximal elbow flexors voluntary contractions at 50% of maximal voluntary contraction (MVC) on measures of central (i.e. supraspinal and spinal) excitability. Supraspinal and spinal excitability of the biceps brachii were assessed via transcranial magnetic stimulation (TMS) of the motor cortex and transmastoid electrical stimulation (TMES) of the corticospinal tract, respectively. TMS-induced motor-evoked potentials (MEPs), TMES-induced cervicomedullary-evoked potentials (CMEPs), Erb's point peripheral nerve stimulation and MVC were assessed prior to and following submaximal voluntary contractions at 50% of MVC. The MEP to CMEP ratio increased (584±77.2%; p=0.011) and CMEP amplitudes decreased (62±3.0%; p=0.02) immediately post-exercise. MVC force output did not change immediately post-exercise. The results suggest that brief, non-fatiguing intermittent submaximal voluntary contractions transiently enhance supraspinal excitability while decreasing spinal excitability. The impact of these changes on one's ability to generate or maintain force production remains unknown.
Asunto(s)
Contracción Muscular , Músculo Esquelético/fisiología , Entrenamiento de Fuerza , Médula Espinal/fisiología , Adulto , Codo , Estimulación Eléctrica , Humanos , Masculino , Músculo Esquelético/inervación , Tractos Piramidales/fisiología , Estimulación Magnética Transcraneal , Adulto JovenRESUMEN
The present study investigated the effects of exercise-induced elbow flexor fatigue on voluntary force output, electromyographic (EMG) activity and motoneurone excitability of the nonexercised knee extensor muscles. Eleven participants attended 3 testing sessions: (i) control, (ii) unilateral fatiguing elbow flexion and (iii) bilateral fatiguing elbow flexion (BiFlex). The nonfatigued knee extensor muscles were assessed with thoracic motor evoked potentials (TMEPs), maximal compound muscle action potential (Mmax), knee extensor maximal voluntary contractions (MVCs), and normalized EMG activity before and at 30 s, 3 min, and 5 min postexercise. BiFlex showed significantly lower (Δ = -18%, p = 0.03) vastus lateralis (VL) normalized EMG activity compared with the control session whereas knee extension MVC force did not show any statistical difference between the 3 conditions (p = 0.12). The TMEP·Mmax(-1) ratio measured at the VL showed a significantly higher value (Δ = +46%, p = 0.003) following BiFlex compared with the control condition at 30 s postexercise. The results suggest that the lower VL normalized EMG following BiFlex might have been due to a reduction in supraspinal motor output because spinal motoneuronal responses demonstrated substantially higher value (30 s postexercise) and peripheral excitability (compound muscle action potential) showed no change following BiFelex than control condition.
Asunto(s)
Encéfalo/fisiología , Ejercicio Físico/fisiología , Neuronas Motoras/fisiología , Contracción Muscular , Fatiga Muscular , Músculo Cuádriceps/inervación , Adulto , Codo , Estimulación Eléctrica , Electromiografía , Potenciales Evocados Motores , Femenino , Humanos , Rodilla , Masculino , Fuerza Muscular , Vías Nerviosas/fisiología , Factores de Tiempo , Adulto JovenRESUMEN
The purpose of this study was to determine if supraspinal and/or spinal motoneuron excitability of the biceps brachii were differentially modulated before: 1) arm cycling and 2) an intensity-matched tonic contraction. Surface EMG recordings of motor evoked potentials (MEPs) and cervicomedullary motor evoked potentials (CMEPs) were used to assess supraspinal and spinal motoneuron excitability, respectively. MEP amplitudes were larger and onset latencies shorter, before arm cycling and tonic contraction when compared with rest with no intent to move, but with no difference between motor outputs. CMEP amplitudes and onset latencies remained unchanged before cycling and tonic contraction compared with rest. Premovement enhancement of corticospinal excitability was due to an increase in supraspinal excitability that was not task-dependent. This suggests that a common neural drive is used to initiate both motor outputs with task-dependent changes in neural excitability only being evident once the motor outputs have begun.
Asunto(s)
Brazo/fisiología , Neuronas Motoras/fisiología , Tractos Piramidales/fisiología , Estimulación Magnética Transcraneal/métodos , Adulto , Potenciales Evocados Motores/fisiología , Humanos , Músculo Esquelético/fisiologíaRESUMEN
Human studies have not assessed supraspinal or spinal motoneurone excitability in the quiescent state prior to a rhythmic and alternating cyclical motor output. The purpose of the current study was to determine whether supraspinal and (or) spinal motoneurone excitability was modulated in humans prior to arm cycling when compared with rest with no intention to move. We hypothesized that corticospinal excitability would be enhanced prior to arm cycling due, in part, to increased spinal motoneurone excitability. Supraspinal and spinal motoneurone excitability were assessed via transcranial magnetic stimulation (TMS) of the motor cortex and transmastoid stimulation of the corticospinal tract, respectively. Surface electromyography recordings of TMS motor evoked potentials (MEPs) and cervicomedullary MEPs (CMEPs) were made from the relaxed biceps brachii muscle prior to rhythmic arm cycling and at rest with no intention to move. The amplitude of the MEPs was greater (mean increase: +9.8% of maximal M wave; p = 0.006) and their onset latencies were shorter (mean decrease: -1.5 ms; p < 0.05) prior to cycling when compared with rest. The amplitudes of the CMEPs at any of 3 stimulation intensities were not different between conditions. We conclude that premovement enhancement of corticospinal excitability is greater prior to arm cycling than at rest because of increases in supraspinal but not spinal motoneurone excitability.