Aging and Muscle Fatigability in the Upper Extremity

Aging is accompanied by reductions in strength and contraction velocity, and increased fatigability of limb muscles during high-velocity dynamic contractions. These age-related changes affect functional tasks and are well described for the lower limb, with less known about the upper limb muscles. The aims of the thesis were to compare in young and old men and women: (1) maximal torque and power of the elbow flexor muscles across a range of isokinetic velocities, and (2) the neural (supraspinal) and muscular mechanisms of fatigue induced by high-velocity dynamic contractions of the elbow flexor muscles. 28 young (23.2 ± 2.6 years) men (n = 14) and women (n = 14) and 33 (72.6 ± 5.6 years) old men (n = 18) and women (n = 15) with the elbow flexor muscles performed: (1) maximal isokinetic contractions at 15 velocities to assess strength and power (0-450°/s), and (2) a dynamic fatiguing task involving 80 fast, maximal-effort contractions with a load equivalent to 20% of maximal voluntary isometric torque (MVIC). Before and after the fatiguing task the following were assessed: voluntary activation using motor cortical stimulation as a measure of supraspinal fatigue, and contractile properties evoked with electrical stimulation as a measure of muscular mechanisms. The elbow flexor muscles of the old adults were weaker and less powerful than young adults across all the velocities assessed (P\u3c0.01), although voluntary activation was similar between the age groups (P\u3e0.05). Some young and old adults were not able attain higher contraction velocities, primarily driven by the women. Old adults were more fatigable than young adults (P\u3c0.001, 15% difference) with now sex differences (P\u3e0.05). Old adults exhibited a larger reduction in voluntary activation (P=0.036, 7.5% age difference) and greater increase in relaxation in the old adults (55%) than the young (36%) following the fatiguing task. The elbow flexor muscles of old men and women were weaker and less powerful than young, but this was not due to differences in voluntary activation. The greater fatigability of elbow flexor muscles in the old adults however, was due to both supraspinal mechanisms and slowing of the muscle that occurs with aging

Mechanisms for the Age-related Increase in Fatigability of the Knee Extensors in Old and Very Old Adults

The mechanisms for the age-related increase in fatigability during high-velocity contractions in old and very old adults ({greater than or equal to}80 yrs) are unresolved. Moreover, whether the increased fatigability with advancing age and the underlying mechanisms differ between men and women are not known. The purpose of this study was to quantify the fatigability of knee extensor muscles and identify the mechanisms of fatigue in 30 young (22.6 {plus minus} 0.4 yrs; 15 men), 62 old (70.5 {plus minus} 0.7 yrs; 33 men), and 12 very old (86.0 {plus minus} 1.3 yrs; 6 men) men and women elicited by high-velocity concentric contractions. Participants performed 80 maximal velocity contractions (1 contraction per 3 s) with a load equivalent to 20% of the maximum voluntary isometric contraction. Voluntary activation and contractile properties were quantified before and immediately following exercise (\u3c10 \u3es) using transcranial magnetic stimulation and electrical stimulation. Absolute mechanical power output was 97% and 217% higher in the young compared to old and very old adults, respectively. Fatigability (reductions in power) progressively increased across age groups, with a power loss of 17% in young, 31% in old, and 44% in very old adults. There were no sex differences in fatigability among any of the age groups. The age-related increase in power loss was strongly associated with changes in the involuntary twitch amplitude (r=0.75,

Physical Activity Modulates Corticospinal Excitability of the Lower Limb in Young and Old Adults

Aging is associated with reduced neuromuscular function, which may be due in part to altered corticospinal excitability. Regular physical activity (PA) may ameliorate these age-related declines, but the influence of PA on corticospinal excitability is unknown. The purpose of this study was to determine the influence of age, sex, and PA on corticospinal excitability by comparing the stimulus-response curves of motor evoked potentials (MEP) in 28 young (22.4 ± 2.2 yr; 14 women and 14 men) and 50 old adults (70.2 ± 6.1 yr; 22 women and 28 men) who varied in activity levels. Transcranial magnetic stimulation was used to elicit MEPs in the active vastus lateralis muscle (10% maximal voluntary contraction) with 5% increments in stimulator intensity until the maximum MEP amplitude. Stimulus-response curves of MEP amplitudes were fit with a four-parameter sigmoidal curve and the maximal slope calculated (slopemax). Habitual PA was assessed with tri-axial accelerometry and participants categorized into either those meeting the recommended PA guidelines for optimal health benefits (\u3e10,000 steps/day, high-PA; n = 21) or those not meeting the guidelines (n = 41). The MEP amplitudes and slopemax were greater in the low-PA compared with the high-PA group (P \u3c 0.05). Neither age nor sex influenced the stimulus-response curve parameters (P \u3e 0.05), suggesting that habitual PA influenced the excitability of the corticospinal tract projecting to the lower limb similarly in both young and old adults. These findings provide evidence that achieving the recommended PA guidelines for optimal health may mediate its effects on the nervous system by decreasing corticospinal excitability

Physical Activity Modulates Corticospinal Excitability of the Lower Limb in Young and Old Adults

Aging is associated with reduced neuromuscular function, which may be due in part to altered corticospinal excitability. Regular physical activity (PA) may ameliorate these age-related declines, but the influence of PA on corticospinal excitability is unknown. The purpose of this study was to determine the influence of age, sex, and PA on corticospinal excitability by comparing the stimulus-response curves of motor evoked potentials (MEP) in 28 young (22.4 ± 2.2 yr; 14 women and 14 men) and 50 old adults (70.2 ± 6.1 yr; 22 women and 28 men) who varied in activity levels. Transcranial magnetic stimulation was used to elicit MEPs in the active vastus lateralis muscle (10% maximal voluntary contraction) with 5% increments in stimulator intensity until the maximum MEP amplitude. Stimulus-response curves of MEP amplitudes were fit with a four-parameter sigmoidal curve and the maximal slope calculated (slopemax). Habitual PA was assessed with tri-axial accelerometry and participants categorized into either those meeting the recommended PA guidelines for optimal health benefits (\u3e10,000 steps/day, high-PA; n = 21) or those not meeting the guidelines (n = 41). The MEP amplitudes and slopemax were greater in the low-PA compared with the high-PA group (P \u3c 0.05). Neither age nor sex influenced the stimulus-response curve parameters (P \u3e 0.05), suggesting that habitual PA influenced the excitability of the corticospinal tract projecting to the lower limb similarly in both young and old adults. These findings provide evidence that achieving the recommended PA guidelines for optimal health may mediate its effects on the nervous system by decreasing corticospinal excitability

Is There a Sex Difference With Aging In Stimulus-Response Characteristics of The Lower Limb?

PURPOSE: The aim of this study was to compare the input-output characteristics of the motor cortex of young and old men and women. METHODS: MEPs of the vastus lateralis (VL) were recorded from 11 young (19-30 yr, 6 women) and 26 old adults (61-79 yr, 13 women) with bipolar EMG electrodes by delivering single-pulse TMS over the motor cortex. The active motor threshold (AMT) was determined and MEPs elicited at increasing intensities (5% increments of simulator output, SO) during intermittent isometric contractions at 10% maximal voluntary contraction. MEP amplitudes were expressed relative to the resting maximal compound muscle action potential of the VL (Mmax) and plotted against the SO (%AMT) to generate an input-output curve and determine the following parameters: the estimated maximal MEP amplitude (MEPmax), the stimulus intensity required to elicit a response equal to half MEPmax (S50), and the peak slope of the sigmoidal curve. RESULTS: AMT was similar between the young and old adults (44.9 ± 9 vs 47.4 ± 4.4 %SO, respectively) and between men and women (54.4 ± 9.6 vs 54.8 ± 12.2 %SO respectively). Mmax was less in young women (10.41 ± 1.72 mV) compared with young men (17.28 ± 2.8 mV, Pmax, young women and men were similar for the MEPmax (35.3 ± 15.1 vs 34.8 ± 23.7 %Mmax, P\u3e0.05), peak slope (0.08 ± 0.04 vs 0.13 ± 0.06, P\u3e0.05). However, S50 was lower in young women compared with young men (110.9 ± 5.9 vs 124.3 ± 5.5 %AMT). Mmax was less in old women compared with old men (6.5 ± 2.4 vs 10.9 ± 3.7 mV, Pmax, old women and men were similar for the MEPmax (30.7 ± 17.5 vs 31.7 ± 11.7 %Mmax, P\u3e0.05), peak slope (0.091 ± 0.02 vs 0.084 ± 0.03, P\u3e0.05) and the S50 (113.9 ± 13.2 vs 123.3 ± 17.5 %AMT, P\u3e0.005). CONCLUSION: When normalized to the compound muscle action potential, there were minimal sex differences for the young and old adults in most input-output characteristics and AMT of the knee extensor muscles