Diurnal changes in the amplitude of the Hoffmann reflex in the human soleus but not in the flexor carpi radialis muscle

Changes in the reflex amplitude throughout the day have been observed in non-human mammals. The present experiment tested whether diurnal fluctuations also occur in humans. Hoffmann reflex (H-reflex) amplitude was measured in soleus and flexor carpi radialis (FCR) muscles from the data collected over a 12-h period between 7:00-9:00 a.m. arid 7:00-9:00 p.m. At 4-h intervals, M/H recruitment curves were obtained, and two measures of H-reflex excitability were calculated. The maximal H-reflex(H-max) was calculated as the average of the three largest H-reflexes. H-reflexes were also sampled from the ascending limb of the M/H recruitment curve (H-A, n = 10), with a corresponding M-wave of 5% M-max. All values were normalized to the maximal M-wave (M-max). Soleus H-reflex amplitude and plantar flexion maximal voluntary isometric contraction force (MVIC) were significantly smaller (p < 0.05) in the morning (H-max=57.2% M-max, H-A=42.3%, M-max, MVIC = 162.1 Nm) than in the evening (H-max = 69.1 % M-max, a 20.1 % increase, H-A = 54.1 % M-max, a 27.4% increase and MVIC = 195.8 Nm, a 20.8% increase). In contrast, FCR H-reflex amplitude and FCR MVIC were unchanged across all testing sessions. The data show that diurnal fluctuations are present in the amplitude of the human soleus but not in the FCR H-reflex.. Diurnal fluctuation in the human soleus H-reflex amplitude must be considered when interpreting H-reflex data, especially when a repeated measures design spanning several days is utilized.