Fig. 2. Recordings of force output (lower trace) and muscle length change (Length) (upper trace) during an isotonic contraction illustrating the method used to determine the stiffness of the series elastic component. The example shown is from a muscle shortening against a load of 0.6P₀, where P₀ is the maximum isometric twitch force. The dashed vertical line indicates the time at which shortening started. The solid straight lines were fitted to the muscle length change trace (upper trace) at the start of shortening and to the force trace (lower trace) immediately before shortening started. The slopes of these lines give the initial velocity of shortening (dL/dt) and rate of force development just prior to shortening (dP/dt), respectively. The ratio of dP/dt to dL/dt gives the stiffness of the elastic component in series with the contractile element at that load. The sampling rate for these experiments was 1000 Hz.

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