Both standing and postural threat decrease Achilles' tendon reflex inhibition from tendon electrical stimulation.

Area of Science:

• Neuroscience
• Human Motor Control
• Biomechanics

Background:

• Golgi tendon organs (GTOs) and their Ib reflexes are vital for maintaining standing balance.
• The modulation of Ib reflexes under conditions of postural threat remains largely unexplored.

Purpose of the Study:

• To investigate how upright stance influences GTO Ib reflexes.
• To determine the effect of height-induced postural threat on GTO Ib reflexes during standing balance.
• To validate tendon electrical stimulation (TStim) as a method for probing Ib reflexes in standing individuals.

Main Methods:

• Tendon electrical stimulation (TStim) was used to elicit short-latency Ib inhibitory reflexes in the medial gastrocnemius muscle.
• Experiments compared Ib inhibition during prone lying versus upright standing (postural orientation).
• Postural threat was manipulated by having participants stand on elevated surfaces of varying heights and proximity to an edge.

Main Results:

• Ib inhibition was significantly reduced (42.2% in area, 32.9% in duration) during upright standing compared to prone lying.
• Height-induced postural threat led to further significant reductions in Ib inhibition (area: 32.4%, duration: 16.4%, amplitude: 24.8%) in the high-threat condition versus the low-threat condition.
• Tendon electrical stimulation proved to be a viable technique for assessing Ib reflexes during standing balance.

Conclusions:

• Human GTO Ib reflexes are modulated by postural orientation, being reduced during upright stance.
• Ib reflexes are context-dependent, with increased postural threat leading to decreased Ib inhibition.
• These findings suggest that threat or emotional processing centers in the central nervous system modulate human Ib reflex pathways.