The cross-bridge of skeletal muscle is not synchronized either by length or force step.

Enrico Grazi

doi:10.3390/ijms160612064

Abstract

Force and length steps, applied to a muscle fiber in the isometric state, are believed to synchronize attached cross-bridges. This alleged synchronization facilitates the interpretation of the experiments. A rapid force step elicits an elastic response of the attached cross-bridges, followed by an isotonic phase. The decay of this second isotonic phase is of the first order. This excludes that the attached cross-bridges may decay all at the same time. The change of the X-ray interference distance during the second phase measures the stroke size only in the unrealistic case that the cross-bridges are and remain all attached. A rapid force step does not synchronize attached cross-bridges. The change of X-ray interference during the second phase does not measure the stroke size. These conclusions significantly change the picture of the mechanism of skeletal muscle contraction.

Highlights

The concept that either force or length steps, applied to a muscle fiber in the isometric state, synchronize attached cross-bridges was introduced by Huxley and Simmons [1]
By applying a rapid force step to the muscle fiber, they expected to generate a synchronous motion of the attached myosin heads and to alter the axial position of the center of scattering mass by a calculable amount
A reaction of the first order implies that the decay occurs randomly and not at all as a single event, as it should be in the case of the simultaneous power stroke of the synchronized cross-bridges

Summary

Introduction

The concept that either force or length steps, applied to a muscle fiber in the isometric state, synchronize attached cross-bridges was introduced by Huxley and Simmons [1]. This view, not really supported by experimental evidence, is still very popular, probably because it helps in the interpretation of the experiments. By applying a rapid force step to the muscle fiber, they expected to generate a synchronous motion of the attached myosin heads and to alter the axial position of the center of scattering mass by a calculable amount. From the shift of the position of the center of the scattering mass Reconditi et al [3] presumed to calculate the stroke size

Results

The First Model

The Second Model

Discussion

The Center of Mass of an Array of Cross-Bridges

Conclusions