To answer your question, it is important to explain the structure of skeletal muscle and the mechanism by which muscle contracts.
Muscles are made up of units of muscle fibres and each muscle fibre is made up of sub-units called sarcomeres. These sarcomeres line up end to end in a long chain to make up the muscle fibre. When a muscle contracts, the ends of the sarcomeres pull towards each other so that the sarcomeres get shorter in length.
In a study carried out by Gordon et al (1966) to measure the tension of a single muscle fibre at different sarcomere lengths, it was established that maximum tension occurs at the rest length of the sarcomere, but when the muscle fibre was stretched so that the sarcomere length increased, the tension fell from maximum to zero.