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How do skeletal muscle contractions produce body heat?

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Diagram showing the structure of human striated or skeletal muscle, with labeled cross-sections from whole muscle to fascicle, muscle fiber, myofibril, and sarcomere, highlighting actin and myosin filaments. — muscle The structure of striated, or skeletal, muscle. Striated muscle tissue, such as the tissue of the human biceps muscle, consists of long fine fibers, each of which is in effect a bundle of finer myofibrils. Within each myofibril are filaments of the proteins myosin and actin; these filaments slide past one another as the muscle contracts and expands. On each myofibril, regularly occurring dark bands, called Z lines, can be seen where actin and myosin filaments overlap. The region between two Z lines is called a sarcomere; sarcomeres can be considered the primary structural and functional unit of muscle tissue.

Skeletal muscle contractions produce body heat as a byproduct of the chemical reactions that power muscle activity. When muscles contract, they utilize chemical energy derived from the metabolism of food, specifically through the breakdown of adenosine triphosphate (ATP).1234

According to the first law of thermodynamics, the energy released during these chemical reactions must be accounted for as either mechanical work or heat. While some of this energy is converted into work to facilitate movement, a significant portion is degraded into heat. The efficiency of this process is limited by the second law of thermodynamics, which dictates that not all energy can be transformed into mechanical work.12

In warm-blooded animals, this heat production is a vital function for maintaining a constant internal body temperature. For example, when the body needs to warm up, it may trigger shivering—a process where muscles rapidly and alternately contract and relax, generating substantial heat to help the body reach its optimal temperature.125