Comparison Between Isometric and Concentric Motor Fatigability in Persons With Multiple Sclerosis and Healthy Controls – exploring central and peripheral contributions of motor fatigability

Background Motor fatigability (i.e. contraction-induced reduction in muscle strength) from a concentric task associate stronger to walking and perception of fatigue in persons with multiple sclerosis (pwMS), compared with an isometric task. However, the central and peripheral contributions of motor fatigability between these tasks have not been investigated. Objective Compare the central and peripheral contributions of motor fatigability in the knee extensors in a sustained isometric fatigability protocol versus a concentric fatigability protocol and in pwMS versus healthy controls (HCs). Methods Participants (n=31 pwMS; n=15 HCs) underwent neuromuscular testing before and immediately after two knee extensor fatigability tasks (sustained isometric and concentric) in an isokinetic dynamometer. Neuromuscular testing of fatigability consisted of maximal voluntary contraction, voluntary activation (central/neural contributor), and resting twitch (peripheral/muscular contributor) determined by the interpolated twitch technique. Results Sustained isometric and concentric fatigability protocols resulted in motor fatigability for both pwMS and HCs, with no between-protocols differences for either group. Regression analysis showed that motor fatigability variance in pwMS was mainly attributed to central fatigability in the sustained isometric protocol and to both central and peripheral fatigability in the concentric protocol. In HCs, the variance in sustained isometric and concentric fatigability were attributed to both peripheral and central fatigability. Conclusion Central and peripheral contributions of motor fatigability differed between sustained isometric and concentric protocols as well as between pwMS and HCs. These between-protocol differences in pwMS provide a neuromuscular dimension to the reported difference in the strength of associations of concentric and isometric tasks to walking and perception of fatigue in pwMS.

Reliability of Tibialis Anterior Muscle Voluntary Activation Using the Interpolated Twitch Technique and the Central Activation Ratio in People with Stroke

Voluntary activation (VA) is measured by applying supramaximal electrical stimulation to a muscle during a maximal voluntary contraction (MVC). The amplitude of the evoked muscle twitch is used to determine any VA deficit, and indicates incomplete central neural drive to the motor units. People with stroke experience VA deficits and greater levels of central fatigue, which is the decrease in VA that occurs following exercise. This study investigated the between-session reliability of VA and central fatigue of the tibialis anterior muscle (TA) in people with chronic stroke (n = 12), using the interpolated twitch technique (ITT), adjusted-ITT, and central activation ratio (CAR) methods. On two separate sessions, supramaximal electrical stimulation was applied to the TA when it was at rest and maximally activated, at the start and end of a 30-s isometric dorsiflexor MVC. The most reliable measures of VA were obtained using the CAR calculation on transformed data, which produced an ICC of 0.92, and a lower bound confidence interval in the good range (95% CI 0.77 to 0.98). Reliability was lower for the CAR calculation on non-transformed data (ICC 0.82, 95% CI 0.63 to 0.91) and the ITT and adjusted-ITT calculations on transformed data (ICCs 0.82, 95% CIs 0.51 to 0.94), which had lower bound confidence intervals in the moderate range. The two ITT calculations on non-transformed data demonstrated the poorest reliability (ICCs 0.62, 95% CI 0.25 to 0.74). Central fatigue measures demonstrated very poor reliability. Thus, the reliability for VA in people with chronic stroke ranged from good to poor, depending on the calculation method and statistical analysis method, whereas the reliability for central fatigue was very poor.

Unlike voluntary contractions, stimulated contractions of a hand muscle do not reduce voluntary activation or motoneuronal excitability

We demonstrate that reductions in voluntary activation and motoneuron excitability following 2-min isometric maximal contractions in humans occur only when fatigue is produced through voluntary contractions and not through electrically stimulated contractions. This is contrary to studies that suggest that changes in the superimposed twitch and therefore voluntary activation are explained by changes in peripheral factors alone. Thus, the interpolated twitch technique remains a viable tool to assess voluntary activation and central fatigue.