Muscles of the Hand

The muscles of the hand are a group of intrinsic muscles responsible for the fine motor control and intricate movements of the fingers and thumb. These muscles are located entirely within the hand, unlike the extrinsic muscles of the forearm that control gross movements. The hand muscles are essential for grasping, manipulating objects, and performing skilled actions such as writing, typing, and playing instruments.

Classification

The intrinsic hand muscles are grouped into functional compartments based on their location and action:

• Thenar muscles: Control the thumb
• Hypothenar muscles: Control the little finger
• Lumbricals: Flex the metacarpophalangeal (MCP) joints and extend interphalangeal (IP) joints
• Interossei: Perform finger abduction and adduction
• Palmar muscles: Palmaris brevis and other small muscles contribute to skin movement and grip stabilization

Location

The hand muscles are located in the palmar aspect of the hand and arranged in five compartments:

Thenar Muscles

• Abductor pollicis brevis: Abducts the thumb at the carpometacarpal joint.
• Flexor pollicis brevis: Flexes the thumb at the MCP joint.
• Opponens pollicis: Allows opposition of the thumb—critical for grasping.

Hypothenar Muscles

• Abductor digiti minimi: Abducts the fifth digit (little finger).
• Flexor digiti minimi brevis: Flexes the MCP joint of the fifth digit.
• Opponens digiti minimi: Enables opposition of the fifth digit with the thumb.

Lumbricals

• Origin: Tendons of flexor digitorum profundus
• Insertion: Extensor expansions on dorsal surfaces of fingers 2–5
• Function: Flex the MCP joints and extend both the PIP and DIP joints

Interossei

Dorsal Interossei (4 muscles)

• Action: Abduct fingers (DAB – Dorsal ABduct)
• Insertion: Base of proximal phalanges and extensor expansion

Palmar Interossei (3 muscles)

• Action: Adduct fingers (PAD – Palmar ADduct)
• Insertion: Base of proximal phalanges and extensor expansion

Other Palmar Muscles

• Palmaris brevis: Small subcutaneous muscle; wrinkles skin on the hypothenar eminence to aid grip.
• Adductor pollicis: Adducts the thumb; has two heads (transverse and oblique) and is innervated by the deep branch of the ulnar nerve.

Function

• Thumb movement: Thenar muscles enable opposition, abduction, adduction, and flexion of the thumb.
• Little finger control: Hypothenar muscles allow opposition and flexion of the fifth digit.
• Finger abduction and adduction: Interossei control fine spreading and closing movements.
• Grip modulation: Intrinsic muscles help maintain precision and power grip.
• Coordination: Lumbricals coordinate flexion of MCPs and extension of IPs—critical for smooth writing or playing instruments.

Physiological Role(s)

• Allow intricate digital control and manipulation of small objects.
• Support dynamic tension and precision in actions like drawing or sewing.
• Stabilize fingers during static holding (e.g., pinching, gripping tools).
• Assist extrinsic forearm muscles in distributing movement efficiently across joints.

Development

The hand muscles develop from the paraxial mesoderm and migrate into the limb buds by the 5th week of gestation. The thenar and lateral lumbricals are innervated by the median nerve, reflecting their origin from the lateral plate mesoderm. The interossei and hypothenar muscles derive from the medial side and are innervated by the ulnar nerve. Muscle differentiation is complete by birth, but fine motor skill refinement continues through childhood.

Clinical Significance

• Carpal tunnel syndrome: Median nerve compression affects thenar muscles; may lead to atrophy and loss of thumb opposition.
• Ulnar nerve palsy: Leads to claw hand deformity due to loss of interossei and medial lumbrical function.
• Dupuytren’s contracture: Fibrosis of palmar fascia affects underlying muscles and limits finger extension.
• Intrinsic minus hand: Loss of intrinsic muscle function causes hyperextension at MCPs and flexion at IPs.
• Lumbrical tightness test: Used clinically to assess intrinsic tightness in flexion contractures.

Surface Anatomy

• Thenar and hypothenar eminences are visible muscular bulges on the palm.
• Contraction of interossei may be palpated between the metacarpals on the dorsal hand during finger spread or pinch.
• Loss of thenar bulk is often the earliest visible sign in median nerve neuropathies.

Imaging

• MRI: Useful for assessing muscle integrity in trauma, tumors, or nerve injuries.
• Ultrasound: Helpful in evaluating masses (e.g., ganglions) and guiding injections in cases of entrapment or overuse injuries.

Anatomical Variations

• Palmaris brevis may be hypertrophied or absent.
• Accessory lumbricals and interossei are occasionally present.
• The first dorsal interosseous muscle may be fused with adductor pollicis in rare cases.