Artwork showing a muscle fibre damaged by over-exertion (left), and an intact muscle fibre (right). Extended exertion results in breakdown of glycogen stores (blue) into glucose within muscle fibres. Glucose fuels the continued contraction of the muscle. Continued strenuous exertion can result in depletion of glycogen and damage to myofibrils (the contractile elements within the fibres). This may result in delayed onset muscle soreness (DOMS) after exercise. The body repairs the damaged fibres with additional tissue, which leads to increase in muscle size and strength. However excessive strain on a muscle may cause enough damage to produce a muscle injury.