John milked cows by hand each morning before school. One morning, he overslept and had to hurry to get to school on time. While he was milking the cows as fast as he could, his hands became very tired, and then for a short time he could neither release his grip nor squeeze harder. Explain what happened.

John’s muscles have run out of ATP as they used ATP faster then they were producing it. There is not enough ATP to allow the myosin to release the actin. The myosin heads are locked in place, so the muscle continues to contract. Since there is not enough ATP, the calcium is not pumped back into the sarcoplasmic reticulum. This constant state of contraction is called physiological contracture. John will fully recover after a couple of minutes, after the cells are able to make more ATP.