Aim Decreased muscle strength has been frequently observed in individuals with Parkinson's disease (PD). However, this condition is still poorly examined in physically active patients. This study compared quadriceps (Q) maximal force and the contribution of central and peripheral components of force production during a maximal isometric task between physically active PD and healthy individuals. In addition, the correlation between force determinants and energy expenditure indices were investigated. Methods Maximal voluntary contraction (MVC), resting twitch (RT) force, pennation angle (θp), physiological cross‐sectional area (PCSA) and Q volume were assessed in 10 physically active PD and 10 healthy control (CTRL) individuals matched for age, sex and daily energy expenditure (DEE) profile. Results No significant differences were observed between PD and CTRL in MVC (142 ± 85; 142 ± 47 N m), Q volume (1469 ± 379; 1466 ± 522 cm3), PCSA (206 ± 54; 205 ± 71 cm2), θp (14 ± 7; 13 ± 3 rad) and voluntary muscle‐specific torque (MVC/PCSA [67 ± 35; 66 ± 19 N m cm−2]). Daily calories and MVC correlated (r = 0.56, P = .0099). However, PD displayed lower maximal voluntary activation (MVA) (85 ± 7; 95 ± 5%), rate of torque development (RTD) in the 0‐0.05 (110 ± 70; 447 ± 461 N m s−1) and the 0.05‐0.1 s (156 ± 135; 437 ± 371 N m s−1) epochs of MVCs, whereas RT normalized for PCSA was higher (35 ± 14; 20 ± 6 N m cm−2). Conclusion Physically active PDs show a preserved strength of the lower limb. This resulted by increasing skeletal muscle contractility, which counterbalances neuromuscular deterioration, likely due to their moderate level of physical activity.

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Fatigue is one of the most disabling symptoms of multiple sclerosis (MS); it influences patients’ quality of life. The etiology of fatigue is complex, and its pathogenesis is still unclear and debated. The objective of this review was to describe potential brain structural and functional dysfunctions underlying fatigue symptoms in patients with MS. To reach this purpose, a systematic review was conducted of published studies comparing functional brain activation and structural brain in MS patients with and without fatigue. Electronic databases were searched until 24 February 2021. The structural and functional outcomes were extracted from eligible studies and tabulated. Fifty studies were included: 32 reported structural brain differences between patients with and without fatigue; 14 studies described functional alterations in patients with fatigue compared to patients without it; and four studies showed structural and functional brain alterations in patients. The results revealed structural and functional abnormalities that could correlate to the symptom of fatigue in patients with MS. Several studies reported the differences between patients with fatigue and patients without fatigue in terms of conventional magnetic resonance imaging (MRI) outcomes and brain atrophy, specifically in the thalamus. Functional studies showed abnormal activation in the thalamus and in some regions of the sensorimotor network in patients with fatigue compared to patients without it. Patients with fatigue present more structural and functional alterations compared to patients without fatigue. Specifically, abnormal activation and atrophy of the thalamus and some regions of the sensorimotor network seem linked to fatigue.

Muscle fatigue induced by voluntary exercise, which requires central motor drive, causes central fatigue that impairs endurance performance of a different, non-fatigued muscle. This study investigated the impact of quadriceps fatigue induced by electrically-induced (no central motor drive) contractions on single-leg knee-extension (KE) performance of the subsequently exercising ipsilateral quadriceps. On two separate occasions, eight males completed constant-load (85% of maximal power-output) KE exercise to exhaustion. In a counterbalanced manner, subjects performed the KE exercise with no pre-existing quadriceps fatigue in the contralateral leg on one day (No-PreF), while on the other day, the same KE exercise was repeated following electrically-induced quadriceps fatigue in the contralateral leg (PreF). Quadriceps fatigue was assessed by evaluating pre- to post-exercise changes in potentiated twitch force (ΔQtw,pot; peripheral-fatigue), and voluntary muscle activation (ΔVA; central-fatigue). As reflected by the 57±11% reduction in electrically-evoked pulse force, the electrically-induced fatigue protocol caused significant knee-extensors fatigue. KE endurance time to exhaustion was shorter during PreF compared to No-PreF (4.6±1.2 vs 7.7±2.4 min; p<0.01). While ΔQtw,pot was significantly larger in No-PreF compared to PreF (-60% vs -52%, p<0.05), ΔVA was greater in PreF (-14% vs -10%, p<0.05). Taken together, electrically-induced quadriceps fatigue in the contralateral leg limits KE endurance performance and the development of peripheral fatigue in the ipsilateral leg. These findings support the hypothesis that the crossover-effect of central fatigue is mainly mediated by group III/IV muscle afferent feedback and suggest that impairments associated with central motor drive may only play a minor role in this phenomenon.