Major muscles and muscle health
Knowing the major muscles helps in sports, exercise, and medicine. Muscles also adapt to how we use them.
Muscles of the head and neck
The muscles of facial expression, such as orbicularis oculi (closing the eye) and zygomaticus (smiling), insert into skin. The masseter and temporalis close the jaw for chewing; the masseter is one of the strongest muscles for its size. The sternocleidomastoid flexes and turns the head.
Muscles of the trunk
The diaphragm is the main breathing muscle. The abdominal wall includes the rectus abdominis (the 'six-pack'), the external and internal obliques, and the transversus abdominis, which support organs and flex and twist the trunk. The erector spinae muscles along the spine keep you upright.
Muscles of the upper limb
The pectoralis major (chest) and latissimus dorsi (back) move the arm at the shoulder. The deltoid abducts the arm. The rotator cuff, four small muscles, holds the head of the humerus in its shallow socket and is often injured. The biceps brachii flexes the elbow; the triceps brachii extends it.
Muscles of the lower limb
The gluteus maximus extends the hip, as in climbing stairs. The quadriceps femoris, four muscles on the front of the thigh, extend the knee; the hamstrings at the back flex it and extend the hip. The gastrocnemius and soleus (calf) attach through the Achilles tendon and point the foot.
Exercise adaptations
Resistance training makes fibres thicker (hypertrophy) by adding myofibrils, increasing strength. Endurance training increases mitochondria, capillaries, and myoglobin, improving fatigue resistance. Disuse, such as a limb in a cast or bed rest, causes atrophy, loss of muscle mass, sometimes within days.
Fatigue and soreness
Muscle fatigue is a decline in force during sustained activity, caused by several factors including ion imbalances, reduced fuel, and accumulated metabolic by-products. Delayed-onset muscle soreness, peaking 1-2 days after unaccustomed exercise, especially eccentric work, reflects microscopic damage and inflammation, not lactate, which clears within an hour.
Muscle disorders
Muscular dystrophies are inherited diseases in which muscle fibres degenerate; Duchenne muscular dystrophy, caused by lack of the protein dystrophin, is X-linked and mostly affects boys. Myasthenia gravis is an autoimmune disease attacking acetylcholine receptors, causing weakness. Strains are tears in muscle or tendon fibres from overstretching.
Worked example
After weeks with a leg in a cast, a person's thigh is visibly thinner: the quadriceps have atrophied from disuse. Physiotherapy uses gradually increasing resistance exercise to stimulate hypertrophy and restore strength, along with stretching to restore range of motion.
Key takeaways
Facial muscles insert into skin; the masseter closes the jaw. The diaphragm drives breathing; abdominal muscles support the trunk. The deltoid, rotator cuff, biceps, and triceps move the arm; the gluteus maximus, quadriceps, hamstrings, and calf muscles move the leg. Training causes hypertrophy or endurance changes; disuse causes atrophy. Soreness is from microdamage, not lactate.
Flashcard
Closes the jaw
Check your understanding
Which muscle group extends the knee, as when kicking a ball?