The effect of age on post-activation depression of the flexor radialis H-reflex

Postactivation 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 is thought to depend on intrinsic neuronal properties of the Ia fibres, associated with a decreased probability of transmitter release from the repetitively activated afferents. 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 verify along years the efficacy of drugs and rehabilitation therapy. It is clear that this type of longitudinal assessment could be impaired by discrepancies attributable to age-related changes of PaD. To investigate the possible age effects on PaD, we examined the frequency-related depression of the flexor carpi radialis (FCR) H-reflex in two groups of older and younger healthy subjects. PaD was evaluated by comparing the H-reflex amplitude obtained with a stimulation frequency of 0.1 Hz with those obtained using higher frequencies. The results showed that the frequency-related depression of the FRC H-reflex is not an age-dependent phenomenon, stating that this parameter can be a reliable method to assess PaD in the longitudinal assessment of spasticity.