Muscles are made out of a substantial number of individual cells and when the muscle all in all is under strain these muscle cells contract. To do this they require energy and this energy is gotten from breathing which brings a supply of sustenance and oxygen to the phones. The cell separates ATP to discharge energy. It is this requirement for a constant supply of energy top our muscle cells that influences us to feel tired when we hold up a weight.
This withdrawal can't be kept up without continual addition of energy and so the muscle continually relaxes and contracts.
Energy provided by blood system to biceps to keep muscle gotten, no work done on objects as it doesn't move, Energy provided heat muscles → heat to environment