The effect of age on post-activation depression of the upper limb H-reflex

Post-activation depression (PaD) refers to the inhibition of the H-reflex induced by a preceding conditioning stimulus able to activate the afferents mediating the H-reflex itself. PaD can be investigated assessing the frequency-related depression of the H-reflex. This parameter, which is highly correlated to the severity of spasticity, has been used in the longitudinal assessment of spastic patients, in particular to assess the effect of drugs and rehabilitation over the years. However, in such longitudinal assessment, changes observed might be age related and not only disease related. The aim of this study was to investigate the possible age effects on PaD.

The frequency-related depression of the flexor carpi radialis (FCR) H-reflex was examined in two groups of young (20 subjects; 28 ± 3 years) and aged (18 subjects; 69 ± 6 years) healthy subjects. PaD was evaluated by comparing the H-reflex amplitudes obtained with a stimulation frequency of 0.1 Hz with those obtained using higher frequencies (0.33-0.5-1-2 Hz).

The results showed that frequency-related depression of the FCR H-reflex is similar in young and elderly subjects at all frequencies, with the exception of 2 Hz.

Our study shows that ageing does not affect the frequency-related depression of the FCR H-reflex at the frequencies of 1 Hz or lower, supporting the reliability of this method to assess PaD in the clinical practice, particularly for the longitudinal assessment of spasticity. A decrease of GABA-ergic presynaptic inhibition seems to be the more likely explanation for the age-related changes that we observed at the frequency of 2 Hz.