Neuromuscular Impairment of the Plantar Flexors In Adults With and Without Prader-Willi Syndrome

Abstract

Muscle weakness is common in individuals with Prader-Willi Syndrome (PWS), but the source of weakness is unclear. PURPOSE: The purpose of this study was to compare neuromuscular function, and muscle size and quality of the plantar flexor muscles between individuals with and without PWS. METHODS: Ten participants with PWS were matched on sex to 10 obese control and 10 lean control participants. Hoffman (H) reflex and muscle response (M-wave) were obtained from the soleus by stimulating the tibial nerve to determine the H:M ratio. Isometric plantar flexor strength was assessed using an isokinetic dynamometer to find peak torque (PT), early (RTD100) and late (RTD200) rate of torque development. Surface electromyography (EMG) was recorded from the soleus and gastrocnemii during strength assessments to determine early (RER100) and late (RER200) rise in EMG, and early (I100) and late (I200) integrated EMG. EMG data were normalized to peak EMG amplitude collected during the MVIC trial. Strength variables were normalized to lean mass. Ultrasound imaging was used to quantify gastrocnemii cross sectional area (CSA) and echointensity (EI). One-way ANOVA was used to compare dependent variables between groups. RESULTS: There were group differences in H:M ratio (p=0.03), RTD100 (p<0.01), RTD200 (p=0.01), RER100 (p=0.02), and CSA (p<0.01). Post hoc tests indicated that the PWS group had lower H:M ratio (0.29 ± 0.18 vs. 0.52 ± 0.13, p=0.03), RTD100 (1.49 ± 0.64 vs. 5.19 ± 3.26 Nm/kg, p<0.01), and RTD200 (2.20 ± 1.05 vs. 4.95± 2.48 Nm/kg p<0.01) compared to lean controls. The PWS group had lower RER100 (1.07± 1.14 vs. 2.84 ± 1.49 %MVIC/sec, p=0.02) in the soleus compared to obese controls. Obese controls had larger CSA compared to lean controls (2527.99 ± 579.02 vs. 1638.55 ± 354.52 mm2, p<0.01) and compared to the PWS group (1797.29 ± 764.77 mm2, p=0.026). There were no differences between groups in RER200, I100, I200, or EI. CONCLUSIONS: A lower H:M ratio in adults with PWS compared to controls may indicate lower α-motoneuron excitability. Similarly, lower RTD100 in adults with PWS compared to controls may indicate lower motor unit recruitment and firing rate, which is supported by lower RER100. Conversely, lower RTD200 may be attributed to smaller CSA. Muscle weakness in adults with PWS may originate from neural and morphologic factors.