Describe the biomechanics of the hand.

Hand Biomechanics

Structural Framework and Organization

The hand functions as a triplex mechanical system: a central rigid framework flanked by two mobile hinged segments (the thumb and small finger rays), with multijointed digital appendages distally that enable sophisticated manipulation beyond simple claw-like grasp. 1

• The bony architecture, soft tissue arrangement, and neuromuscular organization work together to produce intricate movements and finely controlled force production 2
• The central rigid framework moves at the wrist through flexors, extensors, abductors, and adductors, while the mobile lateral segments are essential for pinch and increasing grasp strength 1
• Each digit has unique mechanical properties regarding bone and joint architecture, muscles, tendons, and functional independence 1

Thumb Biomechanics

The thumb's highly mobile carpometacarpal joint combined with the absence of a middle phalanx creates a two-phalanx system that achieves excellent coordination without requiring a lumbrical muscle. 1

• Hypermobility of the thumb specifically predisposes to carpometacarpal osteoarthritis, contrasting with its protective effect at interphalangeal joints 3
• The thumb's unique joint configuration allows opposition and precision pinch mechanics that are fundamental to hand function 1
• Thumb base pain with localized tenderness and clinical joint instability indicates CMC osteoarthritis, particularly in older adults 3

Intrinsic Muscle Function

The interosseous muscles serve as the cornerstone of hand function, providing a foundation for all intrinsic and extrinsic hand movements despite their small excursion. 4

• These ulnar nerve-innervated muscles are organized in dorsal and palmar layers and have profound impact on finger balance, grip, and pinch function 4
• Denervation or contracture of the interosseous muscles significantly impairs hand function 4
• The lumbrical muscles function mechanically as deflexors of the proximal interphalangeal joints 1
• Because lumbricals are richly endowed with muscle spindles, their passive stretch by flexor digitorum profundus contraction may inhibit finger extensors while facilitating wrist extensors 1

Tendon Mechanics and Force Transmission

The cross-over of tendons from flexors to extensors in the extensor mechanism is mechanically necessary to produce fingertip force in every direction. 5

• The anatomical routing of multiarticular muscles makes co-contraction unavoidable for many manipulation tasks 5
• The viscoelastic tendinous networks of the extensor mechanism create complex muscle-bone-ligament interactions in finger articulations 5
• Tendon injuries alter force transmission and overall hand function, requiring MRI for surgical planning with sensitivities ranging from 28% to 85% for extensor hood injuries 6

Joint Kinematics and Range of Motion

Each digit's unique joint architecture determines its specific range of motion and functional independence, requiring individualized assessment rather than generalized hand models. 1, 7

• Anthropometry describes hand dimensions and measurements that vary between individuals 7
• Kinematics encompasses hand movements and finger joint range of motion 7
• The absence of a proximal interphalangeal joint in the thumb eliminates mechanical need for a lumbrical, demonstrating how joint configuration dictates muscle requirements 1

Force Production and Control

Precision pinch ability—the capacity to produce and control fingertip forces—emerges from complex interactions between mechanical laws, task specifications, and sensorimotor signals. 5

• Kinetics analysis includes hand models for tendon and joint force calculations 7
• Electromyography evaluates hand muscle activity associated with specific functions using signal-processing technology 7
• Driving fingers to sensorimotor performance limits is instrumental for elucidating motor control strategies 5

Clinical Implications of Biomechanical Disruption

Injury to any structural component—bone, joint, tendon, ligament, or nerve—alters the overall biomechanical function and complicates therapeutic management. 2

• Stener lesions occur when the adductor aponeurosis interposes between a retracted ulnar collateral ligament and its insertion, preventing natural healing and requiring surgical repair 3, 8
• MRI demonstrates 100% sensitivity and 94% specificity for detecting displaced UCL tears 8
• Ultrasound identifies Stener lesions by showing a hypoechoic mass proximal to the metacarpal tubercle with absent UCL 8

Common Pitfalls in Biomechanical Assessment

• Assuming all digits function identically ignores their unique mechanical properties and leads to inaccurate modeling 1
• Overlooking the mechanical necessity of tendon cross-over in the extensor mechanism misrepresents force production capabilities 5
• Failing to recognize that co-contraction is anatomically unavoidable for many tasks leads to misinterpretation of muscle activity patterns 5
• Treating the hand as a simple mechanical system rather than recognizing its triplex framework oversimplifies functional analysis 1