E5 Range of Motion

Topic

Joint range of motion depends on three physical factors: the viscoelastic properties of the soft tissues surrounding the joint, the bony geometry of the articulating surfaces, and the passive muscle tension of the muscles crossing the joint.

The first factor is the viscoelastic properties of the soft tissues: the joint capsule, ligaments, and skin surrounding the joint stretch to a certain point and then offer increasing resistance; thus, the stiffer or less elastic these soft tissues are, the sooner that resistance is reached and the smaller the available joint range of motion.

The second factor is bony geometry: the shape of the bony surfaces within the joint determines—purely mechanically—how far they can move relative to one another before colliding, and this collision limits the joint range of motion regardless of the elasticity of the surrounding soft tissues.

The third factor is passive muscle tension: a muscle crossing the joint, even when not actively contracting, offers resistance to stretching that increases as it lengthens; the shorter or less extensible the muscle is at rest, the sooner this passive muscle tension limits the joint range of motion.

Together, the viscoelastic properties of soft tissues, bony geometry, and passive muscle tension determine passive range of motion—the joint range of motion achieved when an external force moves the joint, without the muscle itself contracting. Active range of motion, in contrast, is further limited by the muscle's capacity to produce force: even if the tissues and bony geometry would allow for a greater angle, the active muscle only moves as far as its own force-generating capacity permits. When distinguishing between passive and active ranges of motion, joint range of motion serves as the model linking tissue mechanics to joint function: the gap between these two values ​​reveals whether a movement deficit stems from the tissues and bony geometry or from the muscle's ability to generate the force required to reach that same angle.

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