In pairs: B squeezes A's index and middle fingers as hard as possible for 3 s, wrist straight, then wrist fully flexed. A rates each squeeze 1–10 (or use a dynamometer). Swap.
Active insufficiency: a muscle that crosses two or more joints is shortened across all of them at once. Its fibres are so short that actin and myosin overlap poorly, so it cannot produce useful active force, however hard it is driven.
Passive insufficiency: the same muscle is lengthened across all of its joints at once. Passive tension in the stretched muscle limits motion in the opposite direction (the hamstrings stopping a straight-leg raise). Some texts use the term for the muscle itself being unable to lengthen further; the page shows both views, the passive force and the point where it stops the joint.
One-joint muscles are included for contrast: changing the adjacent joint does not change their length.
L/L₀ = n₀ + Σ kⱼ·θⱼ (θ in rad)
active = exp(−(L−1)²/0.20)
passive = (e^{4(L−1)/0.6} − 1)/(e⁴ − 1), L > 1
F = activation × active + passive
Hill-type, isometric (no force–velocity effect), rigid tendon, sagittal plane only, forearm supinated. kⱼ is the change in normalised fibre length per radian at joint j. Moment arms are constant, so joint moment follows the force curves. Coefficients are teaching values tuned so that the operating ranges and the published results below are roughly reproduced; they are not taken from one musculoskeletal model.
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