Single myofibril linear relaxation time. (A) Single myofibrils were attached to two glass probes, one of which was bent at a 90° angle to form a cantilever that allows for measurement of force (cf. A inset, 10 μM calibration bar; SL = 2.2 μm). Myofibrils were activated with either a maximum (pCa = 4.0) or ∼50% maximum (pCa = 5.7) Ca²⁺-containing solutions using the rapid solution switching technique. Upon the rapid switch back to the low [Ca²⁺] solution (pCa = 9.0) at time = 0 s, myofibril force relaxed in a bi-phasic manner as illustrated in A. The time of transition from the slow to the fast relaxation phase (Time_lin) in the AOB example trace is indicated by the arrow. (B) Average time of transition from the slow to the fast relaxation phase (Time_lin) in Sham control (open bars) and AOB (closed bars) from either pCa = 4.0 or pCa = 5.7. n = 5 hearts per group. 15°C. Up to 30 myofibrils were studied per cohort. Subsequently, multiple muscle data were averaged to derive a single average datum per heart. *P = 0.041 Sham vs. AOB; P = 0.116 pCa = 4.0 vs. pCa = 5.7 by two-way ANOVA. Additional single myofibril data is presented in Fig. 9.