Figure 4.

Apparent rate constant of force redevelopment (k_tr) A, comparison between type I fibres from patients carrying the β-Tm mutation (open bar, n = 10) and control subjects (hatched bar, n = 10). Values are means ± s.d.*Statistically significant difference. B, typical experimental force signal recording from a type I fibre from a control subject (k_tr 17.33 s⁻¹). C, typical experimental force signal recording from a patient carrying the β-Tm mutation (k_tr 7.70 s⁻¹). The scale bar denotes 400 ms. A 20% release of original fibre length rapidly introduced at one end of the fibre reduces the proportion of available cross-bridges that are attached from approximately 80% in the isometric fibre to 20% (force drops to zero). Dissociation of the remaining cross-bridges is accomplished by rapidly re-extending overall muscle length to the initial value. Coincident with this re-stretch, force transiently increased because of positive straining of attached cross-bridges. These positively strained cross-bridges rapidly dissociated since the imposed length change is much greater than estimates of the working distance of a cross-bridge.