What causes muscle failure during a set?
Muscle fatigue may be defined as a reversible decrease in contractile strength that occurs after repeated muscular activity. Human fatigue is a complex experience that includes failure at more than one site along the chain of events that lead to muscle fiber stimulation. Fatigue involves a central component, which puts an upper limit to the number of command signals sent to the muscles. One peripheral component is reduced force output (inability to complete another repetition).
In brief, the force of contraction relates directly to the electrical activation sent by the central nervous system and the number of motor units recruited. Therefore, as fatigue increases, the recruitment of motor units decreases.
It is important to remember, quality in movement is the most important factor in exercise! You cannot expect optimal results unless the target muscles are recruited during an exercise. Synergists can "take over" a movement pattern, allowing the motion to occur by compromising the quality of motion. For example, the traditional sit up with the feet anchored. As the trunk fatigues, the hip flexors dominate the movement. Concentration to ensure precision of motion during an exercise is critical. Once form is compromised (form fatigue), stop!