Sarcomere

The sarcomere is the repeating segment that gives striated muscle its striped appearance. Within it, thick myosin filaments and thin actin filaments overlap; when myosin heads bind actin and pull, the sarcomere shortens. Millions of sarcomeres shortening together shorten the whole muscle.

It matters because it explains the length-tension relationship: a sarcomere produces most force at a middle length where filament overlap is optimal, and less when very short or very stretched. That is the physiological reason exercises feel strongest in the middle of their range.

In practice, muscles adapt to training partly by adding sarcomeres — in parallel, which thickens the fibre, or in series, which lengthens it. Sarcomeres added in series after heavy eccentric or lengthened-position training are one proposed mechanism behind the lasting flexibility gains such training produces.