Alexander V. Ng

Age and gender influence muscle sympathetic nerve activity at rest in healthy humans.

Muscle sympathetic nerve activity at rest increases with age in humans. The respective influences of the aging process per se and gender on this increase and whether age and gender effects on muscle sympathetic nerve activity can be identified with plasma norepinephrine concentrations, however, have not been established. To examine these issues, nine young women (aged 24 +/- 1 years; mean +/- SEM), eight young men (aged 26 +/- 1 years), seven older women (aged 63 +/- 1 years), and eight older men (aged 66 +/- 1 years) were studied. All were healthy, normotensive (blood pressure < 140/90 mm Hg), nonobese (< 20% above ideal weight), unmedicated, nonsmokers engaged in minimal to recreational levels of chronic physical activity. Arterial blood pressure (manual sphygmomanometry, brachial artery), heart rate, muscle sympathetic nerve activity (peroneal microneurography), and antecubital venous plasma norepinephrine concentrations (radioenzymatic assay) were determined during quiet supine resting conditions. Body weight was higher in men, but there were no age-related differences, whereas estimated body fat (sum of skinfolds) was higher in women and in the older groups (p < 0.05). Estimated daily energy expenditure, arterial blood pressure, and heart rate were not different among the groups. Both muscle sympathetic nerve activity burst frequency and burst incidence at rest were progressively higher in the young women, young men, older women, and older men (10 +/- 1 versus 18 +/- 2 versus 25 +/- 3 versus 39 +/- 5 bursts/min and 16 +/- 1 versus 30 +/- 4 versus 40 +/- 3 versus 61 +/- 6 bursts/100 heartbeats, respectively; all p < 0.05 versus each other).(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitation of lower physical activity in persons with multiple sclerosis

Multiple sclerosis (MS) is a demyelinating disease of the central nervous system that often affects the motor system. We tested the hypothesis that physical activity was lower in a group of 17 MS patients (mean +/- SD; age = 46 +/- 6 yr, 11 females, 6 males) compared with 15 healthy sedentary control subjects (age = 44 +/- 7 yr, 9 females, 6 males). Physical activity was measured with a three-dimensional accelerometer and with an activity questionnaire for 7 d. Vector magnitude values from the accelerometer for the MS and sedentary control subjects were 121,027 +/- 59,336 and 185,892 +/- 60,566 arbitrary units/day, respectively (P = 0.01). Estimated energy expenditure values derived from the questionnaire were 35.9 +/- 3.0 and 36.2 +/- 4.1 Kcal.kg-1.d-1 (NS), respectively. Thus, when measured directly with an accelerometer, activity was lower in MS compared with sedentary control subjects. The data also suggest that the accelerometer was more sensitive than the questionnaire for detecting differences in activity between two relatively sedentary populations, including one with neurologic disease.

Functional relationships of central and peripheral muscle alterations in multiple sclerosis

The functional implications of central motor impairment and peripheral muscle alterations in multiple sclerosis are unclear. Muscle strength, central and peripheral activation, and symptomatic fatigue were investigated in 16 patients with multiple sclerosis (MS) and 18 control subjects. Voluntary and electrically stimulated isometric contractions were obtained from the ankle dorsiflexor muscles. Maximal voluntary contraction (MVC) was 27% lower in MS patients than controls, although electrically stimulated force was similar. Muscle fat‐free cross‐sectional area (CSA) was similar in both groups. These data indicate central activation impairment in MS. Such impairment in MS was further demonstrated by decreased foot‐tap speed, rate of voluntary force development, and central activation ratio. Peripheral activation changes in MS patients were modest. Although stimulated tetanic force was similar, force relaxation was slower in MS patients compared to controls, resulting in a left‐shifted force–frequency relationship in MS. Motor function changes were not associated with fatigue but were associated with impaired ambulation. Thus, weakness and walking impairment, but not fatigue, were related to impaired central activation in MS. These findings may help optimize rehabilitation strategies designed to improve function in persons with MS. Muscle Nerve 29: 843–852, 2004

Exercise and aging: autonomic control of the circulation.

This review describes age-related changes in autonomic control of the circulation during exercise and the associated effects on exercise capacity. The increase in heart rate during exercise becomes smaller with aging probably due to both less withdrawal of cardiac vagal tone and diminished beta-adrenergic responsiveness. The latter also appears to contribute to an attenuation in the left ventricular contractile response to exercise despite greater beta-adrenergic stimulation. At rest, muscle sympathetic nerve activity and arterial plasma norepinephrine spillover rates are elevated in older humans. With aging, sympathetically mediated vasoconstriction in nonactive muscle is augmented during brief dynamic exercise. Paradoxically, during more prolonged exercise increases in plasma norepinephrine concentrations/spillover rates are not greater with age. These age-related changes do not adversely affect submaximal exercise performance at a particular % maximal oxygen consumption. However, the lower peak heart rate and attenuated left ventricular contractile response reduce maximal cardiac output, oxygen consumption, and exercise capacity. In older humans, aerobic exercise training lowers heart rate at rest, reduces levels of heart rate and plasma catecholamines at the same absolute submaximal workload, and, at least in men, improves left ventricular performance during peak exercise, but does not reduce, and may even increase, basal sympathetic nerve activity.

Time to task failure and muscle activation vary with load type for a submaximal fatiguing contraction with the lower leg

The purpose was to compare the time to failure and muscle activation patterns for a sustained isometric submaximal contraction with the dorsiflexor muscles when the foot was restrained to a force transducer (force task) compared with supporting an equivalent inertial load and unrestrained (position task). Fifteen men and women (mean+/-SD; 21.1+/-1.4 yr) performed the force and position tasks at 20% maximal voluntary contraction force until task failure. Maximal voluntary contraction force performed before the force and position tasks was similar (333+/-71 vs. 334+/-65 N), but the time to task failure was briefer for the position task (10.0+/-6.2 vs. 21.3+/-17.8 min, P<0.05). The rate of increase in agonist root-mean-square electromyogram (EMG), EMG bursting activity, rating of perceived exertion, fluctuations in motor output, mean arterial pressure, and heart rate during the fatiguing contraction was greater for the position task. EMG activity of the vastus lateralis (lower leg stabilizer) and medial gastrocnemius (antagonist) increased more rapidly during the position task, but coactivation ratios (agonist vs. antagonist) were similar during the two tasks. Thus the difference in time to failure for the two tasks with the dorsiflexor muscles involved a greater level of neural activity and rate of motor unit recruitment during the position task, but did not involve a difference in coactivation. These findings have implications for rehabilitation and ergonomics in minimizing fatigue during prolonged activation of the dorsiflexor muscles.

The Effect of High-Load vs High-Repetition Training on Endurance Performance

&NA; Ebben, W.P., A.G. Kindler, K.A. Chirdon, N.C. Jenkins, A.J. Polichnowski, and A.V. Ng. The effect of high‐load vs. high‐repetition training on endurance performance. J. Strength Cond. Res. 18(3):513–517. 2004.—The purpose of this study was to compare the effects of high‐load (H‐load) periodized resistance training and high‐repetition (H‐rep) reverse step loading periodized resistance training on endurance performance. Twenty‐six female university rowers (age = 20 ± 1 year) were randomly assigned to H‐load (5 novice, 8 varsity) or H‐rep (7 novice, 6 varsity) groups. Subjects were pre‐ and posttested using a 2,000‐m rowing ergometer test. Outcome variables included SymbolO2 peak, time to test completion, total power, average power per stroke, total number of strokes, stroke rate, and body mass. Subjects trained for 8 weeks using identical exercises. Varsity rowers who performed H‐load training demonstrated greater improvement compared with those who performed H‐rep training. Novice rowers who performed H‐rep training demonstrated greater improvement compared with those who performed H‐load training. High‐load periodized training appears to be more effective for athletes with advanced training status, and H‐rep reverse step loading periodized training is more effective for those who are relatively untrained. Symbol. No caption available.

Central motor drive is increased during voluntary muscle contractions in multiple sclerosis

To test the hypothesis that central motor drive is increased during voluntary contractions in multiple sclerosis (MS) patients, we recorded the surface electromyogram (EMG) and force from the tibialis anterior muscle during isometric dorsiflexion in 14 MS and 18 control subjects. Measurements were obtained during contractions at 10–100% of maximal voluntary contraction (MVC), incremented by 10% MVC. Integrated EMG (% maximum) was elevated in MS compared to controls from 10 to 70% MVC (pairwise, P < 0.05; main effect, P = 0.001). MS severity (Expanded Disability Status Scale) was highly correlated to the individual slopes of the EMG/force relationship (r = −0.87, P < 0.001). We conclude that central motor drive in MS is increased throughout a large range of submaximal contractions and that this increase is associated with overall disease severity.

Ratings of Perceived Exertion During Aerobic Exercise in Multiple Sclerosis

OBJECTIVE To compare ratings of perceived exertion (RPEs) during aerobic exercise in people with multiple sclerosis (MS) and control participants. DESIGN Prospective experimental study. SETTING An exercise testing laboratory. PARTICIPANTS Sedentary adults (n=12) with mild MS (Expanded Disability Status Scale score < or = 3) aged 30 to 45 years and sedentary age-matched and sex-matched controls (n=12). INTERVENTIONS All participants underwent a graded aerobic exercise test on a cycle ergometer with breath-by-breath gas measurements and continuous heart rate monitoring. MAIN OUTCOME MEASURES After completing the Modified Fatigue Impact Scale, participants rated their effort sense every 30 seconds during exercise using the modified Borg 10-point scale. RESULTS The 2 study groups showed similar baseline characteristics except for higher fatigue scores in the MS group. There were no significant differences for any fitness measure, including oxygen cost slope (in VO(2) x min(-1) x W(-1)), VO(2), or work rate during exercise. Neither heart rate nor RPE--measured at 25%, 50%, 75%, and 100% of VO(2)peak--differed between groups. CONCLUSIONS Despite greater reported fatigue levels, participants with MS showed similar RPE and physiologic responses to submaximal and maximal exercise compared with controls. In MS, the Borg 10-point scale may help improve evidence-based exercise prescriptions, which otherwise may be limited by fatigue, motor impairment, heat sensitivity, or autonomic dysfunction.

Muscle sympathetic nerve activity during postural change in healthy young and older adults

Recent evidence suggests that during orthostatic stress the reflex increase in muscle sympathetic nerve activity may be diminished in older adults. To test this hypothesis, we measured muscle sympathetic nerve activity, plasma noradrenaline concentrations, heart rate, and arterial blood pressure in twelve young (mean, 25 years; range, 19–29 years) adults and 14 older (mean 64 years; range, 60–74 years) healthy adults, while supine and during upright sitting. Supine control levels of muscle sympathetic nerve activity were higher in the older subjects (35 ± 1 vs. 25 ± 1 bursts/min,p < 0.05), but there were no differences in plasma noradrenaline concentrations, heart rate or arterial pressure. Despite higher supine control levels in the older group, the absolute unit increases in muscle sympathetic nerve activity in response to upright sitting (p < 0.05 vs. control) were not different in the two groups (7 ± 1 vs. 7 ± 1 bursts/min), nor were the increases in plasma noradrenaline concentrations. Heart rate did not increase above supine control in response to sitting in either group. Arterial pressure increased slightly (p < 0.05, supine vs. control), but there were no age-related differences. These results indicate that, contrary to recent findings, the reflex increases in muscle sympathetic nerve activity and plasma noradrenaline concentrations and regulation of arterial pressure during this natural orthostatic stress are well preserved in older healthy men and women.

Self-efficacy and health status improve after a wellness program in persons with multiple sclerosis

Purpose: To determine if an intensive wellness program for persons with MS results in improved self-efficacy, quality of life (QOL), or physical activity outcomes. Methods: 129 subjects participated in one of seven 4-day interdisciplinary educational wellness programs throughout the United States. This intervention was based on the philosophy that health management is important to disease management. The program consisted of psychological and physiological evaluations, lectures and workshops. Before the intervention and after at 1, 3 and 6 months, self-efficacy (MS Self-Efficacy Scale, MSSE, control), health related QOL (SF-36) and physical activity (Physical Activity Scale for Persons with Physical Disabilities, PASAID) was assessed. Results: Improvements were noted at 1, 3 and 6 months post-intervention. Those present at 6 months included, MSSE, role physical, vitality and mental health scales of the SF-36. PASAID did not change. Improvements were independent of disability (EDSS). Conclusion: A 4-day multidisciplinary educational wellness program can result in improvement in self-efficacy and health-related QOL in persons with MS and can be stable up to at least 6 months. Improvements do not depend on degree of disability. Implications for Rehabilitation Multiple sclerosis is a neurologic disease that can have a significant negative impact on self-efficacy and quality of life. Some wellness based programs have been shown to be effective in improving self-efficacy and quality of life (QOL) in persons with MS. Distance or time could be barriers to access effective wellness programs. An intensive “traveling” 4-day interdisciplinary educational wellness program can result in improvements in self-efficacy and health-related quality of life in persons with MS. Improvements were not dependent on a person’s level of disability (i.e. EDSS).