Presynaptic Inhibition Does not Mediate Reduced Soleus H-Reflex Amplitudes During Drop Landings

During drop landings, shortly after ground contact, spinal excitability is decreased. This decrease, as measured by soleus H-reflex, has been presumed, but not proven, to originate from presynaptic inhibition, facilitated by the descending drive from supraspinal centers. Therefore, the aim of this study was to examine presynaptic inhibition during the flight and landing phases of drop landings. Fifteen participants received peripheral nerve stimulations during quiet stance and pre (PRE) and post (POST) ground contact of 40 cm drop landings. Stimulations during drop landings were timed to elicit soleus H-reflexes 30-0 ms PRE and 30-60 ms POST landings, respectively. Presynaptic inhibition was assessed by conditioning the soleus H-reflex with femoral nerve stimulations, eliciting H-reflex heteronymous facilitation (HHF) and common fibular nerve stimulations, eliciting H-reflex D1 inhibition (HD1). Conditioned soleus H-reflex amplitudes were normalized to maximal M-waves (Mmax) and compared with the unconditioned H-reflexes (HTest) during quiet stance, PRE, and POST. EMG of soleus, medial gastrocnemius, tibialis anterior, and vastus medialis as well as hip, knee, and ankle joint angles were measured throughout drop landings and quiet stance. HTest POST was significantly smaller than PRE (-8.5% Mmax, p = 0.016). Facilitation and inhibition were observed in quiet stance (HHF-HTest: +7.8% Mmax, p < 0.001; HD1-HTest: -9.5%Mmax, p = 0.003), but not during PRE or POST (all p = 1.000). Both paradigms were effective in quiet stance, but not during drop landings, suggesting that the decreased soleus H-reflex POST is not due to ongoing presynaptic inhibition. Instead, reduced motoneuron excitability may indicate other underlying mechanisms.