Analyzed data from the article "Effects of aging on transmission from Ia afferents to α-motoneurons and presynaptic mechanisms across muscle lengths and contraction types"

Presynaptic inhibition of Ia afferents, particularly via primary afferent depolarization (PAD), plays a crucial role in modulating transmission from Ia afferents to α-motoneurons according to task demands. In young adults, PAD is enhanced during eccentric contractions and at longer muscle lengths, supporting adaptable motor control. However, aging is associated with reduced muscle spindle sensitivity, which may impair the regulation of presynaptic inhibition via PAD. This study investigated whether aging alters the modulation of PAD according to contraction type and muscle length. Nineteen older adults (69 ± 4 years) and sixteen young adults (23 ± 2 years) participated in three experimental sessions. The maximal H-reflex (Hmax) and maximal M-wave (Mmax) were recorded in the soleus muscle to assess Ia-alpha motoneuron transmission (Hmax/Mmax). PAD was evaluated using the D1 inhibition method (HD1/Htest), while ongoing presynaptic inhibition was assessed via heteronymous facilitation (Hfac/Htest). Measurements were performed during isometric and eccentric contractions at intermediate (0°) and long (-15° dorsiflexion) muscle lengths. Young adults showed significant modulation of both HD1/Htest and Hfac/Htest across conditions, while older adults did not (p > 0.05). Similarly, Hmax/Mmax was reduced during eccentric contractions at intermediate muscle length in young but not in older adults. These results indicate that aging impairs regulation of PAD according to muscle mechanics, potentially contributing to deficits in sensorimotor control.