Figure 16.26: The process of actin and myosin sliding past one another is called crossbridge
cycling, and it occurs in all muscle types. Myosin is a molecular motor that moves along
the passive actin. Each thick myosin filament has little extensions or “heads,” that “walk”
along the thin actin filaments during contraction. In this way the thick filament slides over
thin filament. The actin filaments transmit the force generated by myosin to the ends of the
muscle, which causes the muscle to shorten. ( 22 )
Muscles and Exercise
Your muscles are important for carrying out everyday activities. The ability of your body
to carry out your daily activities without getting out of breath, sore, or overly tired is called
physical fitness. Physicalfitnessalsodescribesthebody’sabilitytorespondtoemergencies
and to avoid getting sick. A person can have a good level of physical fitness or a poor level
of fitness. For example, a person who becomes breathless and tired after climbing a flight of
stairs is not physically fit.
Physical exerciseis any activity that maintains or improves physical fitness and overall
health. Regular physical exercise is important in preventing lifestyle diseases such as heart
disease, cardiovascular disease, Type 2 diabetes, and obesity.
Regular exercise improves the health of the muscular system. Muscles that are exercised are
bigger and stronger than muscles that are not exercised. Exercise improves both muscular
strength and muscular endurance. Muscular strengthis the ability of a muscle to exert
force during a contraction. Muscular enduranceis the ability of a muscle to continue to
contract over a long time without getting tired. Two types of exercises help improve the
fitness of muscles,anaerobic exerciseandaerobic exercise.
Exercises are grouped into three types depending on the effect they have on the body:
- Aerobic exercises such as cycling, walking, and running, increase muscular en-
durance. - Anaerobic exercisessuch as weight training, or sprinting increase muscle strength.
- Flexibility exercisessuch as stretching, improve the range of motion of muscles and
joints. Regular stretching helps avoid activity-related injuries.