Muscle contraction happens only when the energy molecule called adenosine triphosphate (ATP) is present. ATP provides the energy for muscle contraction and other reactions in the body. It has three phosphate groups that it can give away, releasing energy each time.

Myosin is the motor protein that does muscle contraction by pulling on actin rods (filaments) in muscle cells. Binding of ATP to myosin causes the motor to release its grip on the actin rod. Breaking off one phosphate group of ATP and releasing the resulting two pieces is how myosin reaches out to do another stroke.

Besides ATP, muscle cells have other molecules needed for muscle contraction including NADH, FADH₂, and creatine phosphate.

ATP has three parts. A sugar molecule called ribose is at the center, connected to a molecule called adenine on one side and a chain of three phosphate groups on the other side. The energy of ATP is found the phosphate groups. Phosphate groups are highly negatively charged, meaning they naturally repel each other.

However, in ATP the three phosphate groups are held next to each other by chemical bonds. The tension between the bond the electrostatic repulsion is the stored energy. Once the bond between two phosphate groups is broken, the two phosphates push apart, which is the energy that moves the enzyme that is hugging the ATP molecule.

ATP is broken into ADP (adenosine diphosphate) and phosphate (P), so ADP has only two phosphates left.

Myosin is a family of motor proteins that generate force for moving things inside a cell. Myosin II is the motor that does muscle contraction. Myosin II is a motor that binds to and pulls on actin filaments, which are parallel rods that stretch along the length of a muscle cell.

Myosin molecules have two separate parts: the heavy chain and the light chain. The heavy chain has three regions, like a fist, wrist, and forearm.

The heavy chain has a head domain, which is like fist that binds ATP and pulls on the actin rod. The neck region is the wrist that connects the head domain to the tail. The tail domain is the forearm, which coils around the tails of other myosin motors resulting in a bundle of motors that are attached together.

Once myosin grabs on to an actin filament and pulls, myosin cannot let go until a new ATP molecule attaches. After releasing the actin filament, myosin breaks the outermost phosphate group off of ATP, which causes the myosin to head to straighten up, ready to bind and pull actin again. In this straightened position, myosin grabs on to the actin rod again.

Then myosin releases the ADP and phosphate, which resulted from breaking ATP. Ejection of these two molecules causes the myosin head to bind at the neck, like a fist that curls towards the forearm. This curling motion pulls the actin filament, which causes the muscle cell to contract. Myosin will not let go of actin until a new ATP molecule attaches.

ATP is one of the most important molecules needed for muscle contraction. Since muscle cells use up ATP at a high rate, they have ways of making ATP quickly. Muscle cells have high amounts of molecules that help generate new ATP. NAD+ and FAD+ are molecules that carry electrons in the form of NADH and FADH2, respectively.

If ATP is like a $20 bill that is enough for most enzymes to buy a typical American meal, meaning do one reaction, then NADH and FADH2 are like $5 and $3 gift cards, respectively. NADH and FADH2 give their electrons to what is called the electron transport chain, which uses the electrons to generate new ATP molecules.

Analogously, NADH and FADH2 can be thought of as saving bonds. Another molecule in muscle cells is creatine phosphate, which is a sugar that gives its phosphate group away to ADP. In this way, ADP can quickly be recharged into ATP.