Final answer:
Locomotive muscles are generally tougher than support muscles due to increased actomyosin contractility and the hypertrophy that occurs with regular mechanical stress and exercise, which increases the density of sarcomeres and myofibrils.
Step-by-step explanation:
The toughness of locomotive muscles compared to support muscles can be attributed to several physiological factors related to muscle structure and activity. One critical aspect of muscle toughness is the actomyosin contractility. In locomotive muscles, or skeletal muscles, the repeated cycles of contraction and relaxation under varying loads lead to increased density and crosslinking of actin and myosin proteins, resulting in muscles that are generally tougher. The interaction between actin and myosin, particularly the myosin heads pulling on actin filaments to create muscle contraction, is more intense in these muscles due to their role in movement and posture. Additionally, hypertrophy can contribute to muscle toughness. With increased use, skeletal muscles experience hypertrophy, where sarcomeres and myofibrils increase in number, leading to a more robust and bulkier muscle. Conversely, atrophy occurs with disuse, where these components decrease, leading to less robust muscles. Support muscles might not endure the same level of mechanical stress as locomotive muscles, thus may be less tough.