Understanding arm anatomy is easier with a clear mental map of where each muscle sits and what it does. While I can't show you actual diagrams here, I can paint a detailed picture of your arm's muscular geography.
Front View (Anterior)
Looking at your arm from the front, palm facing forward:
Upper Arm:
Biceps Brachii (surface level)
• Takes up most of the front upper arm
• Two visible heads when developed
• Long head on the outer side (toward your body's midline when arm is at side)
• Short head on the inner side
• Becomes prominent when you flex
Brachialis (deeper, partially visible)
• Visible on the outer edge of the biceps
• Creates the "thickness" between biceps and triceps
• Shows as a bulge on the outer arm when developed
Forearm:
Brachioradialis
• The meaty muscle on the thumb side of your forearm
• Near the elbow crease
• Becomes prominent when you make a fist
Wrist Flexors
• Group of muscles on the underside (palm side) of forearm
• Run from near the elbow to the wrist
• Create the "fullness" of the inner forearm
Pro Tip
Feel each muscle as you read about it. Place your hand on the area and contract the muscle. This builds the mind-muscle connection that makes training more effective.
Back View (Posterior)
Looking at your arm from behind:
Upper Arm:
Triceps Brachii (surface level)
• Takes up the entire back of the upper arm
• Three heads create the "horseshoe" shape
• Lateral head: Outer portion, visible from the side
• Long head: Inner portion, runs down the middle/inside
• Medial head: Deeper, partially hidden by other heads
Forearm:
Wrist Extensors
• Group of muscles on the top (back) of forearm
• Run from near the elbow to the wrist
• Include the extensor carpi muscles
Side View (Lateral)
Looking at your arm from the side:
From the outside:
• Lateral head of triceps (back)
• Long head of biceps (front)
• Brachialis (between them)
• Brachioradialis (forearm)
From the inside:
• Long head of triceps (back)
• Short head of biceps (front)
• Wrist flexors (forearm)
Key Landmarks
Elbow crease (antecubital fossa):
Where biceps tendon meets forearm. Brachioradialis visible on thumb side.
Bicep peak:
The highest point of your flexed bicep. Created primarily by the long head.
Tricep horseshoe:
The U-shaped definition between tricep heads. Visible when arm is extended and flexed.
Outer arm sweep:
Where lateral tricep meets the brachialis. Creates the "rounded" look from behind.
Training Each Visible Muscle
Biceps (front upper arm):
Supinated curls, incline curls, preacher curls
Brachialis (outer edge):
Hammer curls, reverse curls
Brachioradialis (outer forearm):
Hammer curls, reverse curls
Triceps (back upper arm):
Pushdowns, overhead extensions, dips, close-grip bench
Forearm flexors:
Wrist curls, grip work
Forearm extensors:
Reverse wrist curls, reverse curls
Building Complete Arms
A complete arm routine addresses all visible muscles from all angles. Don't just train what you see in the mirror (front). Train what others see from the side and back too.
Balance is key: biceps with triceps, forearm flexors with extensors, long head with short head. That's how you build arms that look impressive from every direction.