Reduction In Voluntary Activation Research Articles

Reproducibility of two electrical stimulation techniques to assess neuromuscular fatigue

The main aim of this study was to compare the reliability of percutaneous electrical stimulation over the muscle (MES) and electrical stimulation over the nerve (NES) to assess quadriceps muscle voluntary activation and muscle contractile properties. A secondary aim was to determine whether MES detects the fatigue induced by prolonged exercise in the heat. For the first purpose, eight participants performed on three non-consecutive days: quadriceps maximal voluntary contractions (MVC) with superimposed electrical stimulation to assess voluntary activation, and electrically evoked contractions to assess peak force (PEVO), using NES (ball-shaped cathode) or MES (adhesive electrodes). For the second purpose, the same participants pedalled for 2 h at 60% VO2max in a 36°C environment. Quadriceps maximal voluntary contractions, voluntary activation, and PEVO were measured before and just after the completion of exercise using MES. Voluntary activation assessed with MES presented lower intra-day (1.3±0.2 vs. 2.5±0.5%) and inter-day (1.7±0.3 vs. 2.9 ±0.4%) coefficients of variation than with NES. PEVO had lower intra-day and inter-day coefficients of variation (5.6±1.0 vs. 17.0±3.3%) and higher intraclass correlation values (ICC 0.95 vs. 0.85) when using MES than with NES either when stimulating at 20 or 80 Hz. Prolonged exercise in the heat reduced MVC (11±2%) and quadriceps muscle voluntary activation (5.2±0.5%) from pre-exercise values (P<0.05), as previously found. PEVO was unaffected by exercise in the heat. In conclusion, MES is more reliable than NES to assess quadriceps muscle voluntary activation and PEVO. In addition, MES readily detects the reduction in voluntary activation induced by prolonged exercise in the heat.

Age And Sex Differences In Muscle Fatigability Maintaining The Same Absolute Force

Old adults are able to sustain a submaximal isometric contraction for a longer duration than young adults when maintained at the same relative intensity. Many daily tasks, however, require maintenance of an absolute force or load that does not vary between young and old adults. PURPOSE: The purpose was to compare the time to failure, maximal voluntary contraction (MVC), and supraspinal fatigue of young and old men and women for an isometric fatiguing contraction performed at the same absolute force with the elbow flexor muscles. METHODS: Ten young adults (5 men and 5 women; mean ± ∑D; 21± 1 years) and 10 old adults (5 men and 5 women; 71 ± 4 years) sustained an isometric fatiguing contraction at a force equivalent to a 6 kg load until the target force could no longer be achieved. Supraspinal fatigue was quantified as the reduction in voluntary activation using cortical stimulation. RESULTS: Young men were stronger than old men (99.6 ± 20.7 Nm vs 69.6 ± 6.3 Nm) but there was no significant difference between young and old women (41.8 ± 8.1 Nm vs 35.8 ± 18.6 Nm). Men were stronger than women for both age groups. Young adults sustained a relatively lower contraction force than old adults (24.8 ± 10.7 vs 31.2 ± 10.8 %MVC) and men sustained relatively less than women (18.8 ± 4.3 vs 37.2 ± 6.7 %MVC). There was no difference in time to task failure between young and old adults but there was a sex difference such that men had a longer time to failure than the women. The time to task failure of the young men and women were 20.7 ± 11.8 and min 5.2 ± 2.5 min respectively, and the old men and women were 16.4 ± 6.8 min and 5.5 ± 2.2 min respectively. Voluntary activation was similar for men and women before and after the fatiguing contraction (P < 0.05) but the age difference was P = 0.06 indicating old adults showed less voluntary activation than the young. The age and sex difference for time to failure was minimized when co-varied for the initial MVC values. Furthermore, MVC torque was the main predictor of time to task failure (r2 = 0.54, P < 0.001). CONCLUSION: The age difference in muscle fatigability often observed when the relative force is similar was minimized when sustaining a similar absolute force. In contrast, the sex difference in muscle fatigability when sustaining relative forces was reversed for a similar absolute force.

Manipulation of Rest Period Length Induces Different Causes of Fatigue in Vertical Jumping

The aim of this study was to directly compare the causes of fatigue after a short- and a long-rest interval between consecutive stretch-shortening cycle exercises. Eleven healthy males jumped with different resting period lengths (short=6.1+/-1 s, long=8.6+/-0.9 s), performing countermovement jumps at 95% of their maximal jump height until they were unable to sustain the target height. After short- and long-rest, the maximal voluntary isometric contraction knee extension torque decreased (-7%; p=0.04), comparing to values obtained before exercise protocols. No change was seen from pre- to post-exercise, for either short- or long-rest, in biceps femoris coactivation (-1%; p=0.95), peak-to-peak amplitude (1%; p=0.95) and duration (-8%; p=0.92) of the compound muscle action potential of the vastus lateralis. Evoked peak twitch torque reduced after both exercise protocols (short=-26%, long=-32%; p=0.003) indicating peripheral fatigue. However, central fatigue occurred only after short-rest evidenced by a reduction in voluntary activation of the quadriceps muscle (-14%; p=0.013) measured using the interpolated twitch technique. In conclusion, after stretch-shortening cycle exercise using short rest period length, the cause of fatigue was central and peripheral, while after using long rest period length, the cause of fatigue was peripheral.

Age‐related muscle fatigue after a low‐force fatiguing contraction is explained by central fatigue

The contribution of central fatigue during and after low- and high-force isometric contractions sustained until failure with age is not established. We compared the time to failure and changes in voluntary activation measured using motor point stimulation of 15 young and 15 old adults for an isometric contraction sustained with the elbow flexor muscles at 20% and 80% of maximal voluntary contraction (MVC) force. Young adults had a briefer time to task failure than old adults for the 20% MVC fatiguing contraction, but a similar duration for the 80% task. Voluntary activation was reduced at the end of the 20% MVC task, but by greater magnitudes for old than young adults. The reduction in MVC torque after the low-force task was associated with the reduction in voluntary activation. After the 80% task, voluntary activation declined to similar levels for the young and old adults. Electromyographic activity levels (% MVC) of the biceps brachii and brachioradialis muscles during the fatiguing contraction were greater for the old than young for the 20% MVC task, but similar with age for the 80% MVC task. Our findings indicate that intensity and duration of contraction can be manipulated in young and old adults to induce varying magnitudes of fatigue within the central nervous system. Aging increases: (1) fatigue within the central nervous system immediately after a low-force fatiguing contraction, and (2) the potential for large neural adaptations during neuromuscular rehabilitation in old adults.

Time Course of Central Fatigue Transfer ability

Fatigue is defined as any contraction-induced reduction in the ability to generate maximal muscle force or power output. Central and peripheral components contribute to the development of muscle fatigue. The possibility of central fatigue transferability from the fatigued to the unfatigued knee extensors has been established previously in our laboratory; however the time course of this event was not evident. PURPOSE: Establish the time course of central fatigue transferability and assess the contribution of spinal excitability to central activation failure. METHODS: Twelve untrained male volunteers participated in the experiment. Central fatigue was induced with a series of sub maximal, intermittent isometric voluntary contractions (30% MVC, 0.6 duty cycle) of the knee extensors of one limb. The superimposed twitch technique was used to measure changes in voluntary activation. Alterations in force-generating capacity were monitored with brief MVC performed periodically on both limbs before, during, and after the fatiguing protocol. Surface electromyography (EMG and M wave) from vastus lateralis (VL) as well as twitch contractile properties from the knee extensors were obtained throughout the fatigue protocol from both limbs. The fatigue-related changes in spinal excitability were assessed by measuring the duration of the mixed nerve silent period (SP). Two-way repeated measures ANOVA were used to analyze the pooled data. RESULTS: The average length of the fatigue protocol was 58.2±15.5 min. Central and peripheral components contributed to the 40% (p<0.01) reduction in MVC in the fatigued limb. Central fatigue was partially transferred to the unfatigued side. This was represented by 10% (p<0.03) decline in MVC together with 4.5% (p<0.01) reduction in voluntary activation. The transferability of central fatigue occurred within the first 15 min of the fatigue protocol. The SP duration was not altered in either limb throughout the experiment. CONCLUSION: The time course of central fatigue transferability was during the first 15 min of the fatigue protocol. Since spinal excitability remained unchanged, it leads us to conclude that some supraspinal mechanism(s) were responsible for the transfer of central fatigue.

EVIDENCE FOR A SUPRASPINAL CONTRIBUTION TO HUMAN MUSCLE FATIGUE

1. Muscle fatigue can be defined as any exercise-induced loss of ability to produce force with a muscle or muscle group. It involves processes at all levels of the motor pathway between the brain and the muscle. Central fatigue represents the failure of the nervous system to drive the muscle maximally. It is defined as a progressive exercise-induced reduction in voluntary activation or neural drive to the muscle. Supraspinal fatigue is a component of central fatigue. It can be defined as an exercise-induced decline in force caused by suboptimal output from the motor cortex. 2. When stimulus intensity is set appropriately, transcranial magnetic stimulation (TMS) over the motor cortex during an isometric maximal voluntary contraction (MVC) of the elbow flexors commonly evokes a small twitch-like increment in flexion force. This increment indicates that, despite the subject's maximal effort, motor cortical output at the moment of stimulation was not maximal and was not sufficient to drive the motoneurons to produce maximal force from the muscle. An exercise-induced increase in this increment demonstrates supraspinal fatigue. 3. Supraspinal fatigue has been demonstrated during fatiguing sustained and intermittent maximal and submaximal contractions of the elbow flexors where it accounts for about one-quarter of the loss of force of fatigue. It is linked to activity and the development of fatigue in the tested muscles and is little influenced by exercise performed by other muscles. 4. The mechanisms of supraspinal fatigue are unclear. Although changes in the behaviour of cortical neurons and spinal motoneurons occur during fatigue, they can be dissociated from supraspinal fatigue. One factor that may contribute to supraspinal fatigue is the firing of fatigue-sensitive muscle afferents that may act to impair voluntary descending drive.

Reduced voluntary activation of human skeletal muscle during shortening and lengthening contractions in whole body hyperthermia

This study examined the effect of whole body hyperthermia on the voluntary activation of exercised and non-exercised skeletal muscle performing a series of lengthening and shortening contractions. Thirteen subjects exercised on a cycle ergometer at 60% of maximal oxygen consumption until voluntary exhaustion in ambient conditions of approximately 40 degrees C and 60% relative humidity. Before and immediately following the cycle protocol, subjects performed a series of 25 continuous isokinetic shortening and lengthening maximal voluntary contractions (MVCs) of the leg extensors and forearm flexors. Voluntary activation for shortening and lengthening contractions for the forearm and leg was assessed prior to and following the 25 MVCs by superimposing a paired electrical stimulus to the femoral nerve and the biceps brachii during additional MVCs. Exercise to exhaustion increased rectal temperature to 39.35+/-0.50 degrees C. Voluntary activation remained unchanged following the prehyperthermia endurance set of shortening and lengthening maximal contractions in both the forearm flexors and leg extensors. Similarly, voluntary activation remained at prehyperthermic levels for the single MVCs immediately following the cycle trial. However, by the time of completion of the posthyperthermia endurance contractions, voluntary activation had declined significantly by 5.87+/-7.56 and 8.46+/-9.26% in the shortening and lengthening phases, respectively, for the leg extensors but not for the forearm flexors. These results indicate that the central nervous system (CNS) reduces voluntary drive to skeletal muscle performing both shortening and lengthening contractions following exercise-induced hyperthermia. The reductions in voluntary activation were only observed following a series of dynamic movements, indicating that the CNS allows for initial and brief 're-activation' of skeletal muscle following exercise-induced hyperthermia.

Electrophysiological observations in cases of partial and total rupture of the achilles tendon

Patients with recurrent, generally protracted, disturbances in one or both heels were subjected to electrophysiological examination consisting of recording of the electrical activity in soleus and gastrocnemius muscles during maximal voluntary contraction, recording of the H reflex response to electrical stimulation of the popliteal nerve and recording of the plantar flexion of the foot during supramaximal electrical stimulation of the popliteal nerve. In patients with partial rupture the EMG activity was reduced in that part of the muscle with the ruptured tendon. In most of the cases of partial rupture the H reflex was normal. However, in all cases of rupture of the soleus tendon and in some cases of total rupture the amplitude of the H reflex was larger in the affected leg than in the normal leg. This indicates that in these cases the reduction in voluntary activity was not due to decreased motoneurone excitability. Recording of the plantar flexion of the foot gave an isotonic contraction curve of the calf muscles. In the majority of the cases of partial rupture the shape of the contraction curve deviated from the normal. The results also suggest that the contraction time of the soleus muscle is longer than that of the gastrocnemius muscle. The investigations show that partial ruptures of the Achilles tendon give rise to characteristic electrophysiological changes which permit the demonstration not only of the presence but also the site of a rupture.