Trunk Stiffness

Trunk stiffness is the mechanical resistance of the trunk to deformation or movement when an external force or moment is applied. In biomechanics, it can be estimated from the relationship between an applied perturbation and the resulting displacement or angular movement of the trunk. Under comparable measurement conditions, greater stiffness means that a given disturbance produces less movement .^1,2

Trunk stiffness arises from several sources. Passive structures such as ligaments, fascia, intervertebral discs, and other tissues provide intrinsic resistance, while active contraction of trunk muscles can substantially increase stiffness. Simultaneous activation of opposing muscle groups, known as co-contraction, can increase trunk stiffness even when little visible movement occurs .^1,2

Stiffness can be adjusted according to the task. Muscle activation may increase before or during lifting, balance challenges, or unexpected perturbations, helping the trunk resist unwanted motion. Reflex responses to disturbances can also contribute to trunk stabilization after a perturbation begins .^2,3

Trunk stiffness is related to, but not synonymous with, trunk or spinal stability. Stiffness is a mechanical property that can contribute to stability, whereas stability refers more broadly to maintaining control of trunk posture and movement despite disturbances. Effective trunk control therefore depends on coordinated, task-specific contributions from muscles, passive tissues, and sensory-motor control rather than stiffness alone .^2,3