Much of our understanding of the mechanism of muscle contraction has come from excellent biochemical studies performed from the 1950s to the mid-1970s (Webb and Trentham, 83). It was during this period that methods for isolating specific muscle proteins were developed as well as the methods for measuring their physicochemical and biochemical properties.
In its simplest form, biochemical experiments on muscle contractile proteins have shown that, during the cross-bridge cycle, actin (A) combines with myosin (M) and ATP to produce force, adenosine diphosphate (ADP) and inorganic phosphate, Pi This can be represented as a chemical reaction in the form
A + M + ATP -> A + M + ADP + Pi + Force
However, we also know that upon the death of a muscle, a rigor state is entered whereby actin and myosin interact to form a very stiff connection. This can be represented as
A + M -> A.M "rigor" complex
If actin and myosin can interact by themselves, where does ATP come into the picture during contraction? Experiments have demonstrated that the myosin molecule can hydrolyze ATP into ADP and Pi. In other words,
M + ATP -> M + ADP + Pi
Scientists now agree that ATP serves at least two functions in skeletal muscle systems: First, ATP disconnects actin from myosin, and second, ATP is hydrolyzed by the myosin molecule to produce the energy required for muscle contraction. This description of the different biochemical steps involved in muscle contraction is referred to as the Lymn-Taylor actomyosin ATPase hydrolysis mechanism. (Webb and Trentham, 83)
The relationship between the Lymn-Taylor kinetic scheme and the mechanical cross-bridge cycle is not fully known. However, Lymn and Taylor proposed that their biochemical data could be incorporated into a four-step cross-bridge cycle that could be envisioned thus:
