What part of a muscle twitch begins as a repetitive power stroke, then pulls the thin filaments past the thick filaments, shortening the sarcomeres

contraction period

Explanation:

A muscle twitch consists of three phases: 1-latent period, 2-contraction phase, and 3-relaxation phase. The contraction period is the time required during muscle contraction. Moreover, the latent period is the time when the action potential reaches the muscle, while the relaxation phase is the time when calcium ions (Ca 2+) return to the sarcoplasmic reticulum. During the contraction period, myosin and actin bind to each other, forming cross-bridges that generate the filament movement.

that scientist would be robert hooke

igneous rocks which formed by the crystallization of magma at the surface of the earth called extrusive rocks. they are characterized by fine-grained textures because the rapid cooling at or near the surface did not provide enough time for large crystals to grow.