Holding a stretch triggers the muscle spindle and Golgi tendon organ to gradually relax (autogenic inhibition), letting the joint rotate further without injury. Static stretching holds one end position; dynamic stretching moves through the range repeatedly; PNF alternates contraction and relaxation for the fastest gains. The tension glow around the joint fades as the hold progresses.
tension(t) = tension0 · e^(−k·t·intensity)
ROM(t) = ROM0 + (ROMmax − ROM0)·(1 − e^(−k·t))
sessions_to_gain ≈ ln(1 − Δ/ΔROMmax) / −k_session (adaptation curve)
- Stretch type — static (steady hold), dynamic (repeated swings), PNF (contract-relax cycles) — each has a different relaxation rate k.
- Hold duration — how long, in simulated seconds, the stretch is sustained per rep.
- Stretch intensity — how deep the joint is pushed toward its end range.
- Reset — returns the joint to its resting angle and full muscle tension.
Physical therapists use this static-hold relaxation principle to design rehab protocols that safely increase a patient's range of motion after injury or surgery.