Impact of strength training on the aging neuromuscular system
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Maximal muscle strength and rate of force development (RFD) decline with age. While this reduction has typically been attributed to muscle wasting, alterations in the nervous system are increasingly argued to be a quintessential contributor. The objective of this thesis was to investigate the role of the nervous system in the age-related decline in muscle strength, and study how strength training may maintain, or even reverse this debilitating outcome. In paper I evoked reflex potentials in the calf muscles revealed that efferent neural drive during maximal voluntary contraction (MVC) was markedly lower in a group of moderately active old compared to a group of moderately active young controls (Vsup/Msup: 0.12 ± 0.08 (old) 0.47 ± 0.20 (young)). The lower efferent neural drive was accompanied by a 34% reduction in plantar flexion MVC in the old group. Following 8 weeks of high intensity (75-80% of one repetition maximum (1RM)) strength training the old group increased plantar flexion MVC (19 ± 15%), and exhibited enhanced efferent neural drive, evident as a 72 ± 73% increase Vsup/Msup-ratio. However, despite these improvements, a marked neural drive deficit was still apparent compared to young; the old group exhibited ~60% lower Vsup/Msup compared to the young after training (Vsup/Msup: 0.18 ± 0.09 (old) 0.45 ± 0.19 (young)). Extending the population to include exceptionally strength trained individuals, paper II examined a group of master athletes (MA), to explore whether lifelong strength training could mitigate the age-related decline in efferent neural drive observed in paper I. Contrasting the MA to a group of recreationally active old (AO) and a group of sedentary old (SO), evoked reflex potentials in the calves revealed that strength training, but not recreational activity, could counteract the age-related decline in efferent neural drive (Vsup/Msup: 0.28 ± 0.15 (MA), 0.13 ± 0.06 (AO), 0.11 ± 0.05 (SO), 0.45 ± 0.12 (young)). Looking closer into the neural components that may contribute to efferent neural drive reduction with age, paper III investigated corticospinal excitability following strength training in a group of old. Motor potentials evoked by transcranial magnetic stimulation (TMS) showed that corticospinal excitability was lower in old than young during quadriceps contractions at 50% of MVC. Four weeks of maximal strength training (85-90% of 1RM) fully rejuvenated corticospinal excitability in the old, hence demonstrating adaptive changes of corticospinal pathways. In contrast, evoked reflex potentials in paper I and II indicated that strength-training induced adaptations of the peripheral Ia afferent reflex arc may be limited. Paper IV sought to investigate how various levels of muscle strength may translate into functional performance during daily tasks in old. The study compared the same three groups of old as in paper II; MA, AO and SO, revealing that high muscle strength is crucial to maintain performance during force-demanding functional tasks, such as stair climbing and chair rising. Importantly, participation in recreational physical activity did not induce such advantages. Strikingly, the 71 ± 4 years old MA in paper II and IV displayed 26% higher leg press 1 RM than the healthy young controls. These athletes exemplify that very high levels of muscle strength may be achieved at old age, if continuous high intensity strength training is sufficiently emphasized. In conclusion the papers of this thesis show that efferent neural drive is attenuated with age. This impairment is also reflected at higher levels of the nervous system, evident as reduced corticospinal excitability. As a countermeasure, strength training, but not habitual recreational activity, can mitigate the age-related neural decline. In turn, this may contribute to muscular strength and force-demanding physical function maintenance, or even improvement, with age. Ultimately, yielding a potential to improve the quality of life for the aging individual.