Key result
Patients with SMA types 3 and 4 had a twofold lower maximal exercise duration (p < 0.001) compared to controls during a maximal arm-cycling task, without primary limitation from motor unit recruitment.
Why the study?
Exercise intolerance is an important impairment in patients with SMA, but mechanisms underlying this symptom are poorly understood.
Does reduced motor unit and capillary recruitment capacity contribute to exercise intolerance in adolescent and adult patients with SMA types 3 and 4?
Case-Control (n=30)
Does reduced motor unit and capillary recruitment capacity contribute to exercise intolerance in adolescent and adult patients with SMA types 3 and 4?
Exercise intolerance in SMA types 3 and 4 is not primarily limited by motor unit or capillary recruitment capacity in upper arm muscles.
Answers are grounded in this paper and cite their sources.
No evidence implicates motor unit or capillary recruitment deficits as primary limits to exercise tolerance in SMA types 3-4; leaves open mitochondrial and other mechanisms for study.
BACKGROUND: Exercise intolerance is an important impairment in patients with SMA, but little is known about the mechanisms underlying this symptom. OBJECTIVE: To investigate if reduced motor unit and capillary recruitment capacity in patients with SMA contribute to exercise intolerance. METHODS: Adolescent and adult patients with SMA types 3 and 4 (n = 15) and age- and gender matched controls (n = 15) performed a maximal upper body exercise test. We applied respiratory gas analyses, non-invasive surface electromyography (sEMG) and continuous wave near-infrared spectroscopy (CW-NIRS) to study oxygen consumption, arm muscle motor unit- and capillary recruitment, respectively. RESULTS: Maximal exercise duration was twofold lower (p < 0.001) and work of breathing and ventilation was 1.6- and 1.8-fold higher (p < 0.05) in patients compared to controls, respectively. Regarding motor unit recruitment, we found higher normalized RMS amplitude onset values of sEMG signals from all muscles and the increase in normalized RMS amplitudes was similar in the m. triceps brachii, m. brachioradialis and m. flexor digitorum in SMA compared to controls. Median frequency, onset values were similar in patients and controls. We found a similar decrease in median frequencies of sEMG recordings from the m. biceps brachii, a diminished decrease from the m. brachioradialis and m. flexor digitorum, but a larger decrease from the m. triceps brachii. With respect to capillary recruitment, CW-NIRS recordings in m. biceps brachii revealed dynamics that were both qualitatively and quantitatively similar in patients and controls. CONCLUSION: We found no evidence for the contribution of motor unit and capillary recruitment capacity of the upper arm muscles in adolescent and adult patients with SMA types 3 and 4 as primary limiting factors to premature fatigue during execution of a maximal arm-cycling task.
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Habets et al. (2022) conducted a case-control in Spinal Muscular Atrophy types 3 and 4 (n=30). Spinal Muscular Atrophy types 3 and 4 vs. Age- and gender-matched controls was evaluated on Maximal exercise duration (p=<0.001). Patients with SMA types 3 and 4 had a twofold lower maximal exercise duration (p < 0.001) compared to controls during a maximal arm-cycling task, without primary limitation from motor unit recruitment.
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