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Anatomy
Anatomy

The 12 Major Muscle Groups

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Historical reference: its only source is the original reference manual, and some of it may be out of date. Not used for AI answers until re-sourced.

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DefinitionA short reference note, not a full lesson.
Definition

For balanced development and reduced injury risk, strengthen the 12 major muscle groups (plus obliques and adductors/abductors).

Detail

For balanced muscle development and to reduce the risk of injuries, it is essential to strengthen the 12 major muscle groups:
1. Trapezius 5. Erector Spinae 9. Quadriceps
2. Deltoids 6. Triceps 10. Glutes / Hip
3. Pectorals 7. Biceps 11. Calves
4. Latissimus Dorsi 8. Abdominals 12. Hamstrings

If possible, it is also advisable to train the obliques and the adductors/abductors. Training some muscles while excluding others typically produces poorer overall results and may lead to muscle injuries.

Key facts

  • 12 major muscle groups: trapezius, deltoids, pectorals, latissimus dorsi, erector spinae, triceps, biceps, abdominals, quadriceps, glutes/hip, calves, hamstrings.
  • Also recommended: obliques and adductors/abductors.

Connections

Sources

Historical reference · reference manual (page cited)
p.388p.516
Cited authorities in source: Fleck & Kraemer (1997).

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Atlas Connections

Abdominals (Midsection)
Four muscles — rectus abdominis, external oblique, internal oblique, transverse abdominis — that flex, rotate, and laterally flex the trunk and compress the abdomen.
Biceps Brachii
The strongest flexor of the elbow joint (long and short heads); flexes the elbow, supinates the forearm, and weakly flexes the shoulder; a 2-joint muscle (ROM 135°).
Calves (Gastrocnemius and Soleus)
The calf comprises the gastrocnemius (crosses knee and ankle) and the soleus (crosses only the ankle); both plantar-flex the foot.
Deltoids
The three-headed shoulder muscle — anterior, medial, and posterior deltoid — each with distinct actions.
Erector Spinae (Sacrospinalis)
Three muscle-group columns (iliocostalis, longissimus, spinalis) running skull-to-sacrum that extend, laterally flex, and rotate the vertebral column.
Gluteus Maximus
The strongest, largest muscle in the body; the primary hip extensor and a lateral rotator of the femur.
Hamstrings
Three 2-joint muscles — biceps femoris, semitendinosus, semimembranosus — that flex the knee (primary) and extend the hip (secondary); knee-flexion ROM 125°.
Latissimus Dorsi
A 2-joint back muscle; its strongest action is adduction (drawing the arm to the side from side-horizontal, rotating inward), with extension second.
Pectoralis Major
The chest muscle (clavicular and sternal portions); its primary action is horizontal adduction (flexion) of the arm — a powerful action.
Quadriceps
The four knee extensors — rectus femoris, vastus medialis, vastus lateralis, vastus intermedius (rectus femoris also flexes the hip); the leg-extension arc is ~90° (knee bent to straight).
Trapezius
A muscle that functions as three different muscles — upper, middle, and lower traps — moving the scapula.
Triceps Brachii
The three-headed elbow extensor (long, medial, lateral heads); the long head also assists shoulder extension.
Principle of Muscle Balance
Train all major muscle groups — and both sides of every joint — because a disproportionately strong muscle predisposes its weaker antagonist to injury.