No force can be produced contrary to the direction of shortening. At rest, myosin retains ADP and Pi. When the crossbridge is activated, force is produced before the first ATP hydrolysis. The power stroke utilizes free energy provided by ATP hydrolysis which occurred during the previous crossbridge cycle. Exposure of actin active sites to myosin heads 12.
Oxidative metabolism provides the fuel for maximum work output over short intervals.
During contraction, muscle energy requirement is proportional to force production. During contraction, muscle energy requirement is proportional to shortening velocity. During contraction, muscle energy requirement is determined by muscle type and is independent of force production or velocity of shortening. When muscle is activated, myosin is able to interact with active sites on troponin. Rearrangements on the actin filament during the work stroke of the crossbridge cycle are responsible for muscle shortening.