Age-related Changes in Motor Function (I). Mechanical and Neuromuscular Factors

Abstract

This two-part narrative review aims to provide an insight into the age-related mechanical and neuromuscular factors contributing to: (1) decreased maximal muscle strength and power; (2) decreased force control; and (3) increased fatigability. Structural and functional changes from the macro-level of the muscle-tendon unit to the micro-level of the single muscle fibre have been reviewed and are described. At the muscle-tendon unit level, muscle volume, thickness and cross-sectional area, as well as pennation angle and fascicle length all decrease as part of the natural ageing process. These changes negatively affect muscle quality, muscle and tendon stiffness and Young’s modulus and account for impairment in motor performance. A progressive age-related alteration in neuromuscular function is also well-established, with reduction in number and firing rate of the motor unit, contractile velocity and specific tension of muscle fibres, and stability of neuromuscular junction. These could be the result of structural alterations in the: (i) motor neuron, with number reduced, size and collateral sprouting increased; (ii) neuromuscular junction, with decreased post-synaptic junctional fold and density of active zones and increased pre-synaptic branching and post-synaptic area; and (iii) muscle fibre, with decreased number and size and increased type I and co-expression of myosin heavy chain.

Age Related Changes in Motor Function (II). Decline in Motor Performance Outcomes

“…Many studies have shown that reduced muscle strength measured in elderly individuals is associated with substantial alterations in muscle architecture [32,33]. As mentioned in the first part of our review [1], age-related reductions in muscle thickness and pennation angle result in less contractile tissues being attached to a given tendon or aponeurosis area; resulting in a smaller muscle CSA, which suggests a reduction in the number of sarcomeres placed in parallel and a resultant lower relative force output [34,35]. In turn, shortened fascicle lengths imply a reduction in the number of sarcomeres placed in series (i. e. contractile tissues), but not of the sarcomere's length [36], which reduces the relative shortening velocities and the length range of force development, as well as the power generating capacity [37,38].…”

“…Although, the force exerted by a muscle results from the activation of various MUs, each generating a force of varying amplitude, the last recruited MU has the largest influence on force variability [97,115]. In older individuals, due to the age-associated MU remodeling described in the first part of our review [1], fewer but larger activated MUs with greater firing variability are preferentially recruited, thus resulting in a more variable force output [116,117]. This effect is particularly noticeable at lower intensities of contraction, where each recruited MU provides a large contribution to the net force [115,118].…”

“…A selective shift toward the slow-twitch MUs and the altered myocyte contractile machinery could be responsible for most of the slowing of the isometric twitch in advanced age [17]. Another potential factor, discussed in the first part of our review [1], could be linked to the reduction in the rate of Ca 2 + release and reuptake by the sarcoplasmic reticulum (SR), as well as to alterations in function and expression of contractile protein [51][52][53]. Indeed, a relationship between the duration of the isometric twitch and SR Ca 2 + uptake activity has been demonstrated previously [54].…”

“…As illustrated in ▶Fig. 1 in the first part of the review [1], the mechanism for the age-associated difference in fatigability under different types of muscle contraction could be attributable to a number of factors including: the slowing of muscle contractile properties; the age-related MU remodeling with a selective shift in myosin heavy chain isoforms to a predominately slow type expression; a greater type I:II muscle fiber composition ratio; a higher innervation rate (i. e. each MU innervating a greater number of muscle fibers) and a lower reliance on glycolytic metabolism observed in older compared with young individuals [17,51,70,128,136,137]. For instance, during isometric fatiguing contractions, the aged muscle may experience a lower ATP cost due to a reduced reliance on anaerobic glycolysis and therefore a lower level of accumulation of metabolic by-products and a reduced intramuscular acidification [H + ] [128,138].…”

“…Aging of the musculoskeletal system is of particular interest in our society, as it is a major contributing factor to progressively impaired mobility and loss of functional independence among older individuals. In the first part of this narrative review, we discussed the mechanical and neuromuscular factors responsible for age-related impairments in motor performance [1]. We provided a comprehensive overview of the structural and functional alterations affecting both the muscle-tendon unit and the motor unit (MU).…”