These results demonstrate that lifelong high-intensity physical activity could potentially mitigate the loss of MU associated with aging well into the seventh decade of life.

No abstract

Natural adult aging is associated with many functional impairments of the human neuromuscular system. One of the more observable alterations is the loss of contractile muscle mass, termed sarcopenia. The loss of muscle mass occurs primarily due to a progressive loss of viable motor units, and accompanying atrophy of remaining muscle fibers. Not only does the loss of muscle mass contribute to impaired function in old age, but alterations in fiber type and myosin heavy chain isoform expression also contribute to weaker, slower, and less powerful contracting muscles. This review will focus on motor unit loss associated with natural adult aging, age-related fatigability, and the age-related differences in strength across contractile muscle actions.

Although there were no significant differences in MUNE between both older groups in the biceps brachii muscle, with the number of subjects tested here, we cannot eliminate the possibility of some whole-body neuroprotective effect. However, when compared with the remote biceps muscle, a greater influence on age-related spinal motoneuron survival was found in a chronically activated MN pool specific to the exercised muscle.

It is unclear during human aging whether healthy older adults (>70 yr old) experience greater, lesser, or the same fatigability compared with younger adults. The reported disparate findings may be related to the task-dependent nature of fatigue and the limited number of studies exploring nonisometric contractile function and aging. The purpose here was to determine the effects of fast shortening contractions on the fatigability of the triceps surae in 10 young (~24 yr old) and 10 old (~78 yr old) men using isometric and dynamic measures. Participants performed 50 maximal velocity-dependent plantar flexions at a constant load of 20% maximal voluntary isometric contraction (MVC). Isometric twitch properties and MVCs were tested at baseline and during and following the fatigue task. Voluntary activation was similar between the old and young (~98%) and was unaltered with fatigue. The old had 26% lower (P < 0.01) isometric MVC torque and 18% slower (P < 0.01) maximal shortening velocity than the young. Hence, peak power was 38% lower in the old (P < 0.01). At task termination, MVC torque was maintained in the old (P = 0.15) but decreased by 21% in the young (P < 0.01). Twitch half-relaxation time was lengthened in the old at task termination by 26% (P < 0.01) but unchanged in the young (P = 0.10). Peak power was reduced by 24% and 17% at task termination in the old and young, respectively (P < 0.01). Despite a better maintenance in isometric MVC torque production, the weaker and slower contracting triceps surae of the old was more fatigable than the young during fast dynamic efforts with an unconstrained velocity.

The documented impact of contractile level on decomposition-enhanced spike-triggered averaging motor unit number estimates (MUNEs) in young adults demonstrates the importance of selecting an objective contraction intensity that yields the most representative MUNE for a given muscle. Whether the same contraction intensity would be ideal in an altered system (e.g., by aging or disease) has yet to be examined. Thus, the main purpose of this study was to compare the effects of contraction intensity on MUNEs from the soleus muscle in young ( approximately 27 years) and old ( approximately 75 years) men. Using decomposition-enhanced spike-triggered averaging, surface and intramuscular electromyographic signals were collected from the soleus during a range of submaximal isometric plantar-flexion contractions (threshold, 10%, 20%, and 30% of maximum voluntary contraction; MVC). Five MUNEs were calculated, one for each of the four contraction intensities and an ensemble MUNE was derived from all MUs collected. Although MUNE decreased similarly with increased effort in both groups, MUNEs were not significantly reduced in the old men compared to the young men. Consequently, the ensemble MUNE was extrapolated to an intensity of approximately 15% MVC in both young and old. The results suggest that, in the soleus, the use of the same contraction intensity across age groups is a valid comparison.

Mean maximal motor unit firing rates (MUFRs) of the human soleus are lower (5-20 Hz) than other limb muscles (20-50 Hz) during brief sustained contractions. With healthy adult aging, maximal MUFRs are 20-40% lower and twitch contractile speed of lower limb muscles are 10-40% slower compared with young adults. However, it is unknown whether the inherently low maximal MUFRs for the soleus are further reduced with aging in association with age-related slowing in contractile properties. The purpose of the present study was to compare the changes in triceps surae contractile properties and MUFRs of the soleus throughout a variety of contraction intensities in six old ( approximately 75 yr old) and six young ( approximately 24 yr old) men. Neuromuscular measures were collected from the soleus and triceps surae during repeated sessions (2-6 sessions). Populations of single MUFR trains were recorded from the soleus with tungsten microelectrodes during separate sustained 6- to 10-s isometric contractions of varying intensities [25%, 50%, 75%, and 100% maximal voluntary isometric contraction (MVC)]. The old men had weaker triceps surae strength (MVC; 35% lower) and slower contractile properties (contraction duration; 20% longer) than the young men. However, there was no difference in average MUFRs of the soleus at 75% and 100% MVC ( approximately 14.5 Hz and approximately 16.5 Hz, respectively). At 25% and 50% MVC, average rates were 10% and 20% lower in the old men compared with young, respectively. Despite a significant slowing in triceps surae contraction duration, there was no age-related change in MUFRs recorded at high contractile intensities in the soleus. Thus the relationship between the whole muscle contractile properties and MUFRs found in other muscle groups may not exist between the triceps surae and soleus and may be muscle dependent.

Thus, at least for proximal upper limb muscles, mean maximal MUDR reductions with healthy adult ageing are muscle specific and not strongly related to contractile speed.