The cause of muscle damage and injury is often attributed to large strains. Studies have suggested that strain is the principal cause of muscle damage rather than force. In this paper we show that force is the principal cause of muscle damage whereas strain is a means of increasing force. The subtle difference has important implications, as many studies use strain as an indicator for injury risk. In addition, we show that the data better supports a theory of stress as the principal cause of injury, rather than force alone, and aligns with both the myofibril and observational data. The implications of a stress-based model of injury is discussed within the paper.