Figure 2. Tension-pCa relationships.

Mean (±s.d.) tension-pCa relationships for cardiac myocytes (○), fast-twitch skeletal muscle fibres (•), and slow-twitch skeletal muscle fibres (▾). Steepness of the tension-pCa relationships was quantified for Ca²⁺-activated forces of <=50 %P_4.5 (i.e. Hill coefficient, n₂) since most of the cooperative activation of the thin filament is apparent at these forces. The Hill coefficients (n₂, mean ±s.d.) were 5.45 ± 2.39 for cardiac myocytes, 9.80 ± 1.10 for fast-twitch fibres and 2.54 ± 0.30 for slow-twitch fibres, indicative of a higher cooperativity of Ca²⁺ activation in fast-twitch fibres and cardiac myocyte preparations than in slow-twitch fibres.