# The Cross-Bridge of Skeletal Muscle Is Not Synchronized Either by Length or Force Step

## TL;DR

This paper challenges the idea that force or length steps synchronize cross-bridges in muscle fibers, changing our understanding of muscle contraction.

## Contribution

It shows that cross-bridge synchronization by force or length steps is not valid, altering interpretations of muscle mechanics.

## Key findings

- A rapid force step does not synchronize attached cross-bridges.
- The decay of the isotonic phase is first-order, indicating asynchronous cross-bridge detachment.
- X-ray interference during the second phase does not accurately measure stroke size.

## 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.