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

Area of Science:

• Neuroscience
• Biomechanics
• Motor Control

Background:

• Spinal excitability, measured by soleus H-reflex, decreases shortly after drop landings.
• This decrease has been presumed to result from presynaptic inhibition, but this remains unproven.

Purpose of the Study:

• To investigate the role of presynaptic inhibition during the flight and landing phases of drop landings.
• To determine the mechanisms underlying the decrease in soleus H-reflex post-landing.

Main Methods:

• Fifteen participants performed 40 cm drop landings.
• Peripheral nerve stimulations (femoral and common fibular nerves) were used to assess presynaptic inhibition via H-reflex heteronymous facilitation (H_HF) and H-reflex D1 inhibition (H_D1).
• Soleus H-reflex amplitudes were measured during quiet stance, pre-landing (PRE), and post-landing (POST) and normalized to maximal M-waves (M_max).

Main Results:

• Soleus H-reflex (H_Test) was significantly smaller POST landing compared to PRE landing (-8.5% M_max, p=0.016).
• Presynaptic inhibition (H_HF and H_D1) was observed during quiet stance but not during PRE or POST landing phases.
• The effectiveness of both facilitation and inhibition paradigms in quiet stance, contrasted with their absence during drop landings, indicates presynaptic inhibition is not responsible for the POST landing H-reflex reduction.

Conclusions:

• The decreased soleus H-reflex observed POST drop landing is not attributable to ongoing presynaptic inhibition.
• Reduced motoneuron excitability, rather than presynaptic inhibition, likely underlies the observed changes in spinal excitability after drop landings.
• Further research is needed to elucidate the specific mechanisms contributing to altered spinal excitability during and after drop landings.