Voluntary activation of knee extensor muscles with transcranial magnetic stimulation

Voluntary muscle activation in people with multiple sclerosis is reduced across a wide range of forces following maximal effort-fatiguing contractions

Although multiple sclerosis (MS) is frequently associated with motor impairment, little is known about how muscle activation is affected with MS. The aim of this study was to use transcranial magnetic stimulation (TMS) and motor nerve stimulation to investigate voluntary muscle activation in people with MS across a range of contraction forces. Ten people with MS (39 ± 7 yr) and 10 healthy controls (40 ± 5 yr) performed elbow flexions at target contraction forces of 25%, 50%, 75%, 90%, and 100% maximal voluntary contraction (MVC) while electromyography (EMG) of the biceps brachii was recorded. Sustained elbow flexion MVCs were then performed until force declined to 60% of baseline MVC, where the target contraction forces were again examined but after the sustained MVC. Following the sustained MVC, there was a reduction in biceps EMG amplitude (P < 0.01) and motor cortical voluntary activation (P < 0.01) for the MS group across all contraction intensities. There was also an increase in the rate of torque development for motor nerve-resting twitches in the MS group following the sustained MVC (P = 0.03). Despite the MS group reporting higher fatigue severity scale scores (P < 0.01), disease duration was a better predictor of muscle activation for the MS group (r = -0.757, P = 0.01). These findings indicate that voluntary muscle activation is compromised in people with MS following maximal effort contractions, which may be associated with disease duration rather than self-reports of fatigue.NEW & NOTEWORTHY We use transcranial magnetic stimulation to demonstrate that people with relapsing-remitting multiple sclerosis (MS) have a reduced ability to activate muscles following maximal effort-fatiguing contractions. A reduced ability to activate the elbow flexor muscles after a fatiguing contraction was associated with disease duration and not self-reported levels of fatigue.

Neural and muscular contributions to the age-related loss in power of the knee extensors in men and women

The mechanisms for the loss in limb muscle power in old (60-79 years) and very old (≥80 years) adults and whether the mechanisms differ between men and women are not well-understood. We compared maximal power of the knee extensor muscles between young, old, and very old men and women and identified the neural and muscular factors contributing to the age-related loss of power. 31 young (22.9±3.0 years, 15 women), 83 old (70.4±4.9 years, 39 women), and 16 very old adults (85.8±4.2 years, 9 women) performed maximal isokinetic contractions at 14 different velocities (30-450°/s) to identify peak power. Voluntary activation (VA) and contractile properties were assessed with transcranial magnetic stimulation to the motor cortex and electrical stimulation of the femoral nerve. The age-related loss in power was ~6.5 W·year-1 for men (R2=0.62, p<0.001), which was a greater rate of decline (p=0.002) than the ~4.2 W·year-1 for women (R2=0.77, p<0.001). Contractile properties were the most closely associated variables with power output for both sexes, such as the rate of torque development of the potentiated twitch (men: R2=0.69, p<0.001; women: R2=0.57, p<0.001). VA was weakly associated with power in women (R2=0.13, p=0.012) but not men (p=0.191), whereas neuromuscular activation (EMG amplitude) during the maximal power contraction was not associated with power in men (p=0.347) or women (p=0.106). These data suggest that the age-related loss in power of the knee extensor muscles is due primarily to factors within the muscle for both sexes, although neural factors may play a minor role in older women.

Voluntary activation does not differ when using two different methods to determine transcranial magnetic stimulator output

According to current guidelines, when measuring voluntary activation (VA) using transcranial magnetic stimulation (TMS), stimulator output (SO) should not exceed the intensity that, during a maximal voluntary contraction (MVC), elicits a motor evoked potential (MEP) from the antagonist muscle >15%-20% of its maximal M-wave amplitude. However, VA is based on agonist evoked-torque responses [i.e., superimposed twitch (SIT) and estimated resting twitch (ERT)], which means limiting SO based on electromyographic (EMG) responses will often lead to a submaximal SIT and ERT, possibly underestimating VA. Therefore, the purpose of this study was to compare elbow flexor VA calculated using the original method (i.e., intensity based on MEP size; SOMEP) and a method based solely on eliciting the largest SIT at 50% MVC torque (SOSIT), regardless of triceps brachii MEP size. Fifteen healthy, young participants performed 10 sets of brief contractions at 100%, 75%, and 50% MVC torque, with TMS delivered at SOMEP (73.0 ± 13.5%) or SOSIT (92.0 ± 10.8%) for five sets each. Although the mean ERT torque was greater using SOSIT (15.2 ± 4.8 Nm) compared with SOMEP (13.0 ± 3.7 Nm; P = 0.031), the SIT amplitude at 100% MVC torque was not different (SOMEP: 0.69 ± 0.49 Nm vs. SOSIT: 0.74 ± 0.52 Nm; P = 0.604). Despite the ERT disparity, VA scores were not different between SOMEP (94.6 ± 3.5%) and SOSIT (95.0 ± 3.3%; P = 0.572). Even though SOSIT did not lead to a higher VA score than the SOMEP method, it has the benefit of yielding the same result without the need to record antagonist EMG or perform MVCs when determining SO, which can induce fatigue before measuring VA.NEW & NOTEWORTHY When using transcranial magnetic stimulation (TMS) to determine voluntary activation (VA) of the elbow flexors, we hypothesized that a stimulator output designed to limit antagonist muscle activity would evoke submaximal agonist superimposed twitch amplitudes, thus underestimating VA. Contrary to our hypothesis, VA was not greater with an output based on maximal superimposed twitch amplitude. Nevertheless, our findings advance methodological practices by simplifying the equipment and minimizing the time required to determine VA using TMS.

People with multiple sclerosis have reduced TMS-evoked motor cortical output compared with healthy individuals during fatiguing submaximal contractions

People with multiple sclerosis (PwMS) typically experience greater levels of exercise-induced fatigue compared with healthy individuals. Therefore, this study examined performance fatigability in PwMS when executing a prolonged submaximal contraction. Nine PwMS (38 ± 7 yr, 6 females) and nine healthy controls (35 ± 6 yr, 4 females) performed an elbow flexion at 15% maximal voluntary contraction (MVC) for 26 min. MVCs were performed every 2 min during, and following, the contraction to determine if maximal force was impaired by the low-intensity contraction. Single-pulse transcranial magnetic stimulation (TMS) was delivered to the primary motor cortex with a circular coil during each MVC and during the submaximal contraction. Superimposed and resting twitches were calculated from elbow flexion torque, whereas motor-evoked potentials were calculated from biceps brachii electromyography. Ratings of perceived exertion (RPE) were obtained before each MVC. During the fatiguing contraction protocol, the MS group exhibited a reduced MVC torque compared with the healthy control group (P = 0.044), which aligned with group differences in biceps brachii EMG activity (P = 0.022) and superimposed twitch amplitude (P = 0.016). Fatigue-related decrements in MVC torque (P = 0.044) and biceps brachii EMG activity (P = 0.043) demonstrated in the MS group persisted throughout recovery. However, MS did not affect the RPE during the fatigue task. These findings suggest that PwMS may have greater levels of performance fatigability due to decreased voluntary drive from the motor cortex, which is not associated with greater ratings of perceived exertion.NEW & NOTEWORTHY By combining TMS and motor nerve stimulation during a low-intensity exercise task, we were able to uncover the contribution that different levels of the CNS have during fatiguing exercise in PwMS. Our findings are novel and revealed that PwMS experienced decreased voluntary drive from the motor cortex during a low-intensity sustained fatiguing task that was associated with heightened levels of performance fatigability.

Early Detection of Prolonged Decreases in Maximal Voluntary Contraction Force after Eccentric Exercise of the Knee Extensors

PURPOSE

We examined whether the magnitude of muscle damage indicated by changes in maximal voluntary isometric contraction (MVIC) strength 1 to 3 d after unaccustomed eccentric exercise (ECC) was correlated with changes in central and peripheral neuromuscular parameters immediately post-ECC.

METHODS

Twenty participants (19-36 yr) performed six sets of eight eccentric contractions of the knee extensors. Rate of force development (RFD) during knee extensor MVIC, twitch force, rate of force development (RFDRT) and rate of relaxation (RRRT) of the resting twitch, maximal M-wave (MMAX), voluntary activation, silent period duration, motor-evoked potentials (MEP) and short-interval intracortical inhibition were assessed before, immediately after, and 1 to 3 d post-ECC. Relationships between changes in these variables immediately post-ECC and changes in MVIC strength at 1 to 3 d post-ECC were examined by Pearson product-moment (r) or Spearman correlations.

RESULTS

Maximal voluntary isometric contraction strength decreased (-22.2% ± 18.4%) immediately postexercise, and remained below baseline at 1 (-16.3% ± 15.2%), 2 (-14.7% ± 13.2%) and 3 d post-ECC (-8.6% ± 15.7%). Immediately post-ECC, RFD (0-30-ms: -38.3% ± 31.4%), twitch force (-45.9% ± 22.4%), RFDRT (-32.5% ± 40.7%), RRRT (-38.0% ± 39.7%), voluntary activation (-21.4% ± 16.5%) and MEP/MMAX at rest (-42.5% ± 23.3%) also decreased, whereas the silent period duration at 10%-MVIC increased by 26.0% ± 12.2% (P < 0.05). Decreases in RFD at 0 to 30 ms, 0 to 50 ms, and 0 to 100 ms immediately post-ECC were correlated (P < 0.05) with changes in MVIC strength at 1 d (r = 0.56-0.60) and 2 d post-ECC (r = 0.53-0.63). Changes in MEP/MMAX at 10%-MVIC immediately post-ECC were correlated with changes in MVIC strength at 1 d (r = -0.53) and 2 d (r = -0.54) post-ECC (P < 0.05).

CONCLUSIONS

The magnitude of decrease in MVIC strength at 1 to 3 d after ECC was associated with the magnitude of changes in RFD and MEP/MMAX immediately post-ECC. However, based on individual data, these markers were not sensitive for the practical detection of muscle damage.

On the reliability and validity of central fatigue determination

Physical performance fatigue can be ascribed to both peripheral and central components. Central fatigue, however, is an elusive entity, consisting of cognitive/sensory component and presumably also a neuro-physiological component that are difficult to tease apart and assess independently of each other. The most widely accepted method for the assessment of central fatigue is based on the premise that decreasing volitional muscle activation (VA), as determined by the interpolated twitch technique (ITT) in fatiguing muscles, reflects increasing central fatigue. Suffering its own shortcomings, the validity of VA determination under fatigued conditions has never been proven and is only assumed. This review presents evidence that questions ITT's reliability and validity in reflecting VA in the fatiguing muscle and, consequently, VA's validity for central fatigue assessment. Specifically highlighted is the paradox of children and endurance athletes, who share striking endurance characteristics, being claimed as more centrally fatigable than untrained adults. Further research and new directions are needed for confirming and quantifying central fatigue and teasing apart its psychologic and neuromotor components.