What Are Limbs?

Limbs are the jointed appendages that extend from the trunk of the body and carry out manipulation and locomotion. In humans there are four: two upper limbs, each comprising the shoulder, arm, forearm, and hand, and two lower limbs, each comprising the hip, thigh, leg, and foot. Anatomically they belong to the appendicular skeleton and are also called the extremities. In engineering terms a limb is a serial kinematic chain of rigid links connected by joints, driven by muscles that act as tendon-coupled actuators, and governed by a control system that combines spinal reflexes with descending commands from the motor cortex.

The upper and lower limbs share an evolutionary blueprint but have diverged in function. The upper limb trades stability for range of motion and terminates in a hand with a high density of sensory receptors and more than twenty degrees of freedom. The lower limb is optimized for weight bearing and cyclic locomotion, with heavier bones, larger muscle volumes, and elastic tendons that recycle energy across each stride.

Upper Limb Structure and Manipulation

The upper limb begins at the shoulder girdle, where the clavicle and scapula float on muscle rather than locking to the axial skeleton, which is what gives the shoulder joint its extraordinary workspace at the cost of stability. Below it, the elbow provides flexion and extension while the radioulnar joints supply forearm pronation and supination. The wrist adds two more rotational degrees of freedom before the hand, whose thumb opposition enables both a power grasp for force and a precision pinch for fine manipulation. Grip force, position, and object slip are regulated by cutaneous mechanoreceptors and muscle spindles operating at latencies short enough to correct a slipping object before it falls. Reproducing that combination of dexterity and feedback remains the central difficulty in prosthetic hand design.

Lower Limb Structure and Locomotion

The lower limb is anchored by the pelvis and the hip joint, a deep ball and socket articulation that trades workspace for load transfer. The knee behaves as a hinge with rolling and sliding contact between femoral condyles and the tibial plateau, and the ankle complex separates dorsiflexion from inversion and eversion across two axes. During walking, the limb alternates between a stance phase, roughly sixty percent of the gait cycle, and a swing phase, with the ankle plantarflexors supplying most of the positive mechanical work at push-off and the Achilles tendon returning stored elastic energy. Quantifying these patterns through motion capture, force plates, and inverse dynamics is the basis of clinical gait analysis and of the specification for any device meant to restore lower limb function.

Limb Loss, Prosthetics, and Neural Interfaces

Amputation, congenital limb difference, and paralysis all create demand for artificial limbs, and the engineering response differs by level and by limb. Lower limb devices must handle load and cyclic energy return; upper limb devices must handle dexterity and intent decoding. Surface electromyography is the dominant control signal for powered arms, and a review of myoelectric prosthesis control traces the progression from direct two-site control to pattern recognition over multichannel electrode arrays. Surgical techniques have widened the available signal space: targeted muscle reinnervation in upper extremity amputation reroutes severed nerves onto spare muscle so that those muscles act as biological amplifiers of motor intent, and a review of control strategies for reinnervated prostheses evaluates how much of that added information reaches the device.

Applications

Study of the limbs supports work in a range of fields, including:

• Prosthetics, orthotics, and rehabilitation robotics
• Exoskeletons for mobility assistance and industrial load handling
• Surgical planning and orthopedic implant design
• Anthropomorphic robot arms and dexterous manipulation
• Ergonomics and workplace injury prevention
• Sports biomechanics and performance analysis