Mechanical model of muscle contraction. 5. Tension rise after phase 1 of a length step

The theoretical approaches developed in accompanying Papers 1 to 3 lead to calculations of the isometric tetanus tension (T0) and the minimum tension (T1) observed at the end of phase 1 of a length step where the fiber is shortened (see Paper 4). During the next three phases, the time rise of the tension (T), from T1 to T0, is determined for any step (see Supplement S5.K). The tension T is expressed as a master equation which is the sum of five terms: (a) T1, (b) a positive or zero contribution resulting from the relaxation induced by the disappearance of the viscosity forces present during phase 1, (c) a positive contribution of elastic origin resulting from the new myosin II heads initiating a working stroke (WS) in the blank areas, (d) a negative contribution caused by the fast detachment of the heads still strongly attached and whose orientation of the levers is beyond the up position, (e) a negative contribution caused by the slow detachment of WS heads whose orientation of the levers is close to the up position. An agreement between the model equation and the experimental results referenced in the physiological literature is proven (r2>97.5%). The kinetics of each of the theoretical curves make it possible to distinguish phases 2, 3 and 4 characteristics of the tension rise to T0. The criteria defined to describe the tension at the end of phase 2 (T2) are applied to the master equation. There is an adequate adjustment between the theoretical and experimental T2 values for shortenings less than 8 nm in modulus (r2 > 97%).