Rhythmic arm cycling modulates Hoffmann reflex excitability differentially in the ankle flexor and extensor muscles

Area of Science:

• Neuroscience
• Human Motor Control
• Exercise Physiology

Background:

• Rhythmic arm movements are known to influence leg muscle reflex excitability, potentially via descending pathways from central pattern generators (CPGs).
• Previous research has primarily focused on the effects on ankle extensors, specifically the soleus (SOL) muscle, leaving the generalizability to other lower leg muscles unconfirmed.

Purpose of the Study:

• To investigate whether rhythmic arm cycling differentially modulates H-reflex amplitude in both ankle extensors (SOL) and flexors (tibialis anterior, TA).
• To explore the neural mechanisms underlying the observed modulations, considering CPG output and cortical drive.

Main Methods:

• H-reflex recruitment curves were measured bilaterally in the SOL and TA muscles during rest and 1Hz rhythmic arm cycling.
• Participants underwent both control and arm cycling conditions to compare reflex responses.

Main Results:

• Significant suppression of SOL H-reflex amplitude (Hmax) was observed in both legs during arm cycling.
• In contrast, TA H-reflex amplitude exhibited bidirectional modulation (either suppression or facilitation), unlike the consistent suppression in SOL.

Conclusions:

• Rhythmic arm movement differentially regulates H-reflex responses in ankle flexors and extensors.
• These differences may stem from distinct CPG outputs to SOL and TA, and potentially differential cortical influences during arm cycling.
• Findings offer insights for rehabilitative strategies targeting ankle motor control.