Age-related changes in leg proprioception: implications for postural control

Abstract

In addition to being a prerequisite for many activities of daily living, the ability to maintain steady upright standing is a relevant model to study sensorimotor integrative function. Upright standing requires managing multimodal sensory inputs to produce finely tuned motor output that can be adjusted to accommodate changes in standing conditions and environment. The sensory information used for postural control mainly arises from the vestibular system of the inner ear, vision, and proprioception. Proprioception (sense of body position and movement) encompasses signals from mechanoreceptors (proprioceptors) located in muscles, tendons, and joint capsules. There is general agreement that proprioception signals from leg muscles provide the primary source of information for postural control. This is because of their exquisite sensitivity to detect body sway during unperturbed upright standing that mainly results from variations in leg muscle length induced by rotations around the ankle joint. However, aging is associated with alterations of muscle spindles and their neural pathways, which induce a decrease in the sensitivity, acuity, and integration of the proprioceptive signal. These alterations promote changes in postural control that reduce its efficiency and thereby may have deleterious consequences for the functional independence of an individual. This narrative review provides an overview of how aging alters the proprioceptive signal from the legs and presents compelling evidence that these changes modify the neural control of upright standing.

Keywords: Ia afferents; balance; muscle spindles; proprioception; upright standing.

Conflict of interest statement

No conflicts of interest, financial or otherwise, are declared by the authors.

Figures

Fig. 1.

Schematic representation of sensory and fusimotor innervation of the intrafusal fibers of a muscle spindle, with the exception of α fibers that only innervate extrafusal fibers, in contrast to β fibers that innervate both extrafusal and intrafusal (bag 1) fibers. Arrows indicate the direction of the impulse conduction. Sensory afferent axons Ia and II convey information to sensory neurons located in the dorsal root ganglia.

Fig. 2.

A: schematic illustration of the spinal and ascending pathways conveying proprioceptive information. Arrows indicate the direction of flow of the neural information. B: effects of aging on the proprioceptive system. Morphological changes in muscle spindles and parent afferent fibers decrease the acuity of the proprioceptive signal. The combination of changes in afferent fibers and spinal network alters the efficacy of synaptic transmission of proprioceptive volleys from muscle spindles. Spinal and supraspinal changes alter the neural integration of the inputs from muscle spindles. EPSPs, excitatory postsynaptic potentials; MNs, motoneurons; ↑, increase; ↓, decrease or alteration.

Fig. 3.

Framework of how age-related alteration of leg proprioception changes postural control and its adaptability. ↑, increase; ↓, decrease or alteration.