The myosin projections/cross-bridges on electron micrographs of skeletal muscle are observed to extend perpendicularly from the thick filaments in a resting state. However, in the contracted state, these myosin cross-bridges slant towards the center of the sarcomere, indicating the ratchet action of the myosin head. The mechanism responsible for this cross-bridge cycling is known as the sliding filament theory because it involves the myosin cross-bridge pulling the actin thin filament towards the center of the sarcomere, resulting in the apparent "sliding" of the thin filament past the thick filament. It is important to note that the myosin cross-bridge cycle stops in relaxed living muscle due to the actions of regulatory mechanisms.

During the cross-bridge cycle, ATP is partially hydrolyzed in the relaxed state (state a). In the presence of elevated myoplasmic Ca++ (state b), myosin binds to actin. The hydrolysis of ATP is then completed (state c), leading to a conformational change in the myosin molecule that pulls the actin filament towards the center of the sarcomere. Subsequently, a new ATP molecule binds to myosin, causing the release of the cross-bridge (state d). The newly bound ATP undergoes partial hydrolysis, precocking the myosin head for further binding. If myoplasmic Ca++ levels remain elevated, the cycle repeats. However, if myoplasmic Ca++ levels are low, relaxation occurs.

The number of myosin molecules contributing to force generation and the involvement of both myosin heads within a myosin molecule in this process remain uncertain. It has been estimated that there could be around 600 myosin heads per thick filament, with a stoichiometry of 1 myosin head per 1.8 actin molecules. However, due to steric considerations, it is unlikely that all myosin heads can interact with actin. Calculations suggest that even during maximal force generation, only 20% to 40% of the myosin heads bind to actin.

The conversion of chemical energy (ATP) into mechanical energy by muscle is highly efficient. In isolated muscle preparations, maximum mechanical efficiency (approximately 65% efficiency) is achieved at a submaximal force of 30% of maximal tension. In humans performing steady-state ergometer exercise, mechanical efficiencies range from 40% to 57%.

Skeletal muscle fibers can be classified into two main groups based on their contraction speed: fast-twitch and slow-twitch muscle fibers. The speed of shortening is correlated with myosin ATPase activity.
12.3 ). These myosin projections/cross-bridges can be seen on electron micrographs of skeletal muscle and appear to extend perpendicular from the thick filaments at rest. In the contracted state, the myosin cross-bridges slant toward the center of the sarcomere, which is consistent with the ratchet action of the myosin head.
The cross-bridge cycling mechanism just described is called the sliding filament theory because the myosin
Cycle stops here in relaxed living muscle (because of actions of regulatory mechanisms)
Tension Time0 G. 40 S. 90 msec L.R. 7.5 0 0.5 1.0 •Fig. 12.13. Cross-Bridge Cycle. Intherelaxedstate(statea),ATPispartiallyhydrolyzed(M•ADP•Pi).InthepresenceofelevatedmyoplasmicCa++ (stateb),myosin(M)bindstoactin(A).HydrolysisofATPiscompleted(statec)andcausesaconformationalchangeinthemyosinmoleculethatpullstheactinfilamenttowardthecenterofthesarcomere.AnewATPmoleculebindstomyosinandcausesreleaseofthecross-bridge(stated).PartialhydrolysisofthenewlyboundATPrecocksthemyosinhead,whichisnowreadytobindagainandagain.Ifmyoplasmic[Ca++]isstillelevated,thecyclerepeats.Ifmyoplasmic[Ca++]islow,relaxationresults.
cross-bridge is pulling the actin thin filament toward the center of the sarcomere, which results in an apparent “sliding” of the thin filament past the thick filament. There is, however, uncertainty about how many myosin molecules contribute to the generation of force and whether both myosin heads in a given myosin molecule are involved. It has been calculated that there may be 600 myosin heads per thick filament, with a stoichiometry of 1 myosin head per 1.8 actin molecules. As a result of steric considerations, it is unlikely that all myosin heads can interact with actin, and calculations suggest that even during maximal force generation, only 20% to 40% of the myosin heads bind to actin.
The conversion of chemical energy (i.e., ATP) to mechanical energy by muscle is highly efficient. In isolated muscle preparations, maximum mechanical efficiency (≈65% efficiency) is obtained at a submaximal force of 30% maximal tension. In humans performing steady-state ergometer exercise, mechanical efficiencies range from 40% to 57%.
Skeletal muscle fibers can be classified into two main groups according to the speed of contraction: fast-twitch •Fig. 12.14. A, Musclesvaryintermsofthespeedofcontraction.G,gastrocnemiusmuscleoftheleg;LR,lateralrectusmuscleoftheeye;S,soleusmuscleoftheleg.B, ThespeedofshorteningiscorrelatedwithmyosinATPaseactivity.N-SOL,normalsoleusmuscle(slowtwitch);N-EDL,normalextensordigitorumlongusmuscle(fasttwitch);S-EDL,self-innervatedextensordigitorumlongusmuscle(EDLmotornervetransectedandresutured);S-SOL,self-innervatedsoleusmuscle(soleusmotornervetransectedandresutured);X-EDL,cross-innervatedextensordigitorumlongusmuscle(EDLinnervatedbysoleusmotornerve);X-SOL,cross-innervatedSOLmuscle(soleusinnervatedbyEDLmotornerve).(A, FromMontcastleV[ed].Medical Physiology. 12thed.St.Louis:Mosby;1974.B, FromBárányM,CloseRI.J Physiol. 1971;213:455.) and slow-twitch muscle fibers. As shown in