Golgi tendon organs (GTOs) sit in series with collagen fascicles at the muscle–tendon junction. As the muscle develops tension, collagen strands straighten and compress the sensory (Ib) nerve terminals woven between them, producing a firing rate that rises steeply at low force and saturates at high force — a saturating, roughly sigmoidal tension–response curve:
FR(F) = FR_max · F^p / (F^p + F_h^p)
F = muscle–tendon force (% of maximum)
F_h = half-saturation force
p = curve steepness (Hill coefficient)
In the spinal cord, Ib afferents synapse on an inhibitory interneuron that produces autogenic inhibition of the same (homonymous) motor pool — the classic "inverse myotatic reflex" that protects a muscle from tearing under quiet, static loads. But during locomotion the very same Ib pathway is task-dependently gated: interneurons activated by hip-extension and other afferent signals during the stance phase suppress the inhibitory relay and open an alternate route, so the identical tension signal now excites the extensor motoneurons instead. This produces positive force feedback — rising tendon force reflexively recruits more extensor drive — which helps the limb support body weight through stance without the nervous system having to compute load in advance. During swing, when the limb carries no load, the pathway is largely gated off. This phase-dependent reflex reversal is documented in cat locomotion (Pearson, Duysens, McCrea) and in human treadmill studies using nerve or tendon-force feedback (Sinkjaer, Stephens & Yang).
- Quiet standing — gate = −1 (inhibitory): as force rises, the model's motoneuron drive is reflexively cut, so force settles at a lower, self-limited equilibrium — overload protection.
- Stance phase — gate = +1 (excitatory): rising force reflexively adds drive, closing a positive-feedback loop that pushes force toward a higher equilibrium set by GTO saturation, assisting weight support.
- Swing phase — gate = 0: the Ib pathway contributes nothing; force simply tracks the postural load demand with a short lag.
- Reflex gain slider — scales how strongly the gated Ib signal adds to or subtracts from motoneuron drive, modelling descending (supraspinal) control over the strength of the reflex.
The force never runs away because the GTO's own firing-rate curve saturates: as force approaches maximum, additional force produces almost no additional firing, capping the positive-feedback gain below 1 and keeping the stance-phase loop stable.