Motor unit firing variability and synchronization during short-term light-load training in older adults.

We compared motor unit synchronization and firing rate variability within and across synergistic hand muscles during a pinching task following short-term light-load training to improve force steadiness in older adults. A total of 183 motor unit pairs before training and 158 motor unit pairs after training were recorded with intramuscular fine-wire electrodes within and across the first dorsal interosseous (FDI) and adductor pollicis (AdP) muscles during a pinch task performed by ten older adults before and after a 4-week short-term light-load training program. Nine younger adults performed the same experimental sessions 4 weeks apart with no training intervention. Two-minute sustained contractions of 2, 4, 8, and 12% maximal voluntary contraction (MVC) were performed with the non-dominant hand. The coefficient of variation (CV) of force was greater in older than in younger adults and was lower at the 2 and 4% MVC levels in both the finger (0.12 +/- 0.01 vs. 0.08 +/- 0.01, and 0.08 +/- 0.01 vs. 0.05 +/- 0.01, respectively) and thumb (0.11 +/- 0.01 vs. 0.08 +/- 0.01, and 0.09 +/- 0.01 vs. 0.05 +/- 0.01, respectively) compared to higher force levels following training in the older adults. There were no changes in CIS or k'-1 values following training. Motor unit firing rate variability significantly decreased at low force levels in the FDI muscle and also tended to decrease with training in the AdP muscle (p = 0.06). No changes occurred in the younger control group. These findings are the first to show that motor unit synchronization does not change during light-load training. Thus, it is likely that force steadiness in older adults improves by reducing motor unit firing variability rather than by changing motor unit synchronization.

Quantification of manual force control and tremor.

Isometric impulse frequencies associated with active tremor and force regulation were examined in 10 patients with idiopathic Parkinson's disease (PD) and in 10 older adults (OAs) who performed an isometric tracing task. The authors decoupled and analyzed the data to determine whether PD-related tremor in the thumb and in the index finger during isometric force control are related and whether PD impairs the performance of volitional force control beyond the errors contributed by tremor. After decoupling, there were clear and robust differences in PD patients' control of isometric force that could not be attributed to action-tremor error. Those errors, which occurred in the absence of movement, suggest impairment in coordinated recruitment and derecruitment of motor units during a fine-motor task.

Effects of crank length on maximal cycling power and optimal pedaling rate of boys aged 8-11 years.

It is generally reported that cycle crank length affects maximal cycling power of adults and that optimal crank length is related to leg length. This suggests that the use of standard length cycle cranks may provide nonoptimal test conditions for children. The purpose of this study was to determine the effects of cycle-crank length on maximal cycling power and optimal pedaling rate of 17 boys aged 8-11 years. The boys performed maximal cycle ergometry with standard (170 mm) cycle cranks and with a crank length that was 20% of estimated leg length (LL20). Power produced when using the 170 mm cranks [mean (SEM)] [364 (18) W] did not differ from that produced with the LL20 cranks [366 (19)]. Optimal pedaling rate was significantly greater for the LL20 cranks [129 (4) rpm] than for the 170 mm cranks [114 (4) rpm]. These data suggest that standard 170 mm cranks do not compromise maximal power measurements in boys aged 8-11 years so that the test apparatus does not bias physiological or developmental inferences made from tests of maximal cycling power.

Determinants of maximal cycling power: crank length, pedaling rate and pedal speed.

The purpose of this investigation was to determine the effects of cycle crank length on maximum cycling power, optimal pedaling rate, and optimal pedal speed, and to determine the optimal crank length to leg length ratio for maximal power production. Trained cyclists (n = 16) performed maximal inertial load cycle ergometry using crank lengths of 120, 145, 170, 195, and 220 mm. Maximum power ranged from a low of 1149 (20) W for the 220-mm cranks to a high of 1194 (21) W for the 145-mm cranks. Power produced with the 145- and 170-mm cranks was significantly (P < 0.05) greater than that produced with the 120- and 220-mm cranks. The optimal pedaling rate decreased significantly with increasing crank length, from 136 rpm for the 120-mm cranks to 110 rpm for the 220-mm cranks. Conversely, optimal pedal speed increased significantly with increasing crank length, from 1.71 m/s for the 120-mm cranks to 2.53 m/s for the 220-mm cranks. The crank length to leg length and crank length to tibia length ratios accounted for 20.5% and 21.1% of the variability in maximum power, respectively. The optimal crank length was 20% of leg length or 41% of tibia length. These data suggest that pedal speed (which constrains muscle shortening velocity) and pedaling rate (which affects muscle excitation state) exert distinct effects that influence muscular power during cycling. Even though maximum cycling power was significantly affected by crank length, use of the standard 170-mm length cranks should not substantially compromise maximum power in most adults.

Exercise dose-response effects on quality of life and independent living in older adults.

PURPOSE: The purpose of this study was to determine if exercise operates in a dose-response fashion to influence well-being and to postpone dependency. METHODS: A computer-assisted search was made by using the following key words: resistance training, strength training, function, exercise, elderly, quality of life, frailty, physical activity, independence, performance, aerobic training, mobility, well-being, and disability. Review articles and personal files were also used, and a critical review of research studies meeting the criteria described in the methods section of the article was conducted. RESULTS: In large sample correlational studies and prospective longitudinal studies, researchers consistently report that measures of physical function in old adults are related to feelings of well-being, and that old adults who are physically active also report higher levels of well-being and physical function, but the results of randomized intervention studies of aerobic and/or resistive strength training do not always support this relationship. Even if changes in well-being and physical function were reported, no evidence was found that levels of intensity operated in a dose-response fashion to influence these changes. Research design problems included ineffective aerobic or strength training treatments, widely varying participation and effort of the research participants, and both treatment and physical function tests that were not appropriate for the physical status of the participants. CONCLUSION: The most consistent results were that long-term physical activity is related to postponed disability and independent living in the oldest-old subjects. Even in individuals with chronic disease, systematic participation in physical activities enhances physical function.

A governing relationship for repetitive muscular contraction.

During repetitive contractions, muscular work has been shown to exhibit complex relationships with muscle strain length, cycle frequency, and muscle shortening velocity. Those complex relationships make it difficult to predict muscular performance for any specific set of movement parameters. We hypothesized that the relationship of impulse with cyclic velocity (the product of shortening velocity and cycle frequency) would be independent of strain length and that impulse-cyclic velocity relationships for maximal cycling would be similar to those of in situ muscle performing repetitive contraction. Impulse and power were measured during maximal cycle ergometry with five cycle-crank lengths (120-220mm). Kinematic data were recorded to determine the relationship of pedal speed with joint angular velocity. Previously reported in situ data for rat plantaris were used to calculate values for impulse and cyclic velocity. Kinematic data indicated that pedal speed was highly correlated with joint angular velocity at the hip, knee, and ankle and was, therefore, considered a valid indicator of muscle shortening velocity. Cycling impulse-cyclic velocity relationships for each crank length were closely approximated by a rectangular hyperbola. Data for all crank lengths were also closely approximated by a single hyperbola, however, impulse produced on the 120mm cranks differed significantly from that on all other cranks. In situ impulse-cyclic velocity relationships exhibited similar characteristics to those of cycling. The convergence of the impulse-cyclic velocity relationships from most crank and strain lengths suggests that impulse-cyclic velocity represents a governing relationship for repetitive muscular contraction and thus a single equation can predict muscle performance for a wide range of functional activities. The similarity of characteristics exhibited by cycling and in situ muscle suggests that cycling can serve as a window though which to observe basic muscle function and that investigators can examine similar questions with in vivo and in situ models.

Maximal power across the lifespan.

BACKGROUND: Previous investigators have reported that maximal power increases during growth and decreases with aging. These age-related differences have been reported to persist even when power is scaled to body mass or muscle size. We hypothesized that age-related differences in maximal power were primarily related to differences in muscle size and fiber-type distribution rather than to age per se. METHODS: Maximum cycling power (Pmax) and optimal pedaling rate (Vopt, a surrogate measure for muscle fiber type) were determined for 195 boys and men, 8-70 years of age, by using inertial load cycle ergometry. Anthropometric dimensions were used to estimate lean thigh volume (LTVest) of all subjects, and magnetic resonance imagery was used to determine thigh and hip muscle volume (MRIvol) for 24 subjects. RESULTS: Pmax was highly related to the product of LTVest and Vopt (LTVest X Vopt; r2 = .83). Multiple regression revealed that Pmax was significantly related to both LTVest x Vopt and age (R2 = .84). Power scaled by LTVest X Vopt was stable during growth and exhibited a small but significant decrease with aging. MRIvol was highly correlated with LTVest, and the ratio of LTVest to MRIvol was independent of age. CONCLUSIONS: These results suggest that muscle volume and optimal pedaling rate are the main determinants of maximal power across the lifespan and that the contractile properties of muscle are developed early in childhood and remain nearly intact late into the lifespan.

Age differences in the expression of manual asymmetry.

Manual dexterity declines in older adults; however, the degree to which the decline is symmetrical in the two hands is less clear. In this investigation the differences between young (aged 18 to 24) and old (aged 62 to 72) men and women in the expression of manual asymmetry across several motor tasks were examined. Five motor tasks provided a broad measurement of several motor characteristics necessary for instrumental activities of daily living. The main effect of sex was not significant; therefore data from men and women were pooled. Little support was found for generalized asymmetrical performance in old adults compared to young adults across motor tasks. An age-by-hand group analysis indicated that age was related to preferred hand performance on two tasks, but when a within-subject analysis of hand differences was performed, and the speed accuracy trade-off considered, differential asymmetry was seen only in the most complex, speeded task.

Gender differences in rowing performance and power with aging.

PURPOSE AND METHODS: Competitive indoor rowing performance times of 2487 men ages 24 to 93, and 1615 women ages 24-84 collected from a composite ranking of regional, national, and international indoor rowing competitions were analyzed to determine the impact of age and gender on ergometer rowing performance. RESULTS: For all subjects age was only modestly correlated with performance in men or women (r = 0.58 and 0.46, respectively). When regression analysis was restricted to only the 95th percentile of each 2-yr age increment (119 men, 79 women), age was a powerful predictor of performance variance in men and women (approximately 90%). In the top men, the pattern of performance decline was curvilinear. Between ages 24 and 50, performance decline was only 3% per decade, compared to 7% from ages 50 to 74. The pattern of performance decline in women was essentially linear across the same 50-yr age span. CONCLUSION: Performance time to power output conversion revealed that men and women lose absolute power at a similar rate across the age span analyzed. However, their different starting positions on the exponential power-velocity curve create distinct differences in the pattern of performance decline and the maintenance of relative power. These data suggest that differences in the effect of aging on performance across different endurance sports are caused more by physics than physiology.

Selective D1 and D2 dopamine receptor antagonists produce differential effects on reaction time in the rat.

The purpose of this investigation was to determine whether selectively blocking D1 and D2 dopamine receptors produces a differential effect on the characteristics (speed and success) of the reaction time response in rats. Animals were shaped to release a lever in response to an auditory/visual stimulus to avoid mild foot shock. The selective D1 antagonist SCH 23390 (0, 70, and 100 micrograms/kg, IP) and the selective D2 antagonists spiperone (0, 1, and 10 micrograms/kg, IP) and haloperidol (0, 10, and 100 micrograms/kg, IP) were studied for their effects on successful avoidance and response latency. SCH 23390 impaired successful avoidance and increased response latencies in a dose-dependent manner. Spiperone and haloperidol also produced dose-related decreases in successful avoidance. In contrast to the dose-related increase in response latencies produced by SCH 23990, 1 microgram/kg spiperone and 10 micrograms/kg haloperidol significantly decreased the latencies of successful responses. Spiperone (10 micrograms/kg) had little effect on response latencies, while 100 micrograms/kg haloperidol increased them. The results of these experiments demonstrate that reaction time is differentially affected by selective dopamine receptor blockade and that the speed and success of reaction time responses can be independently modulated by D1 vs. D2 receptor activity.

Apomorphine and amphetamine produce differential effects on the speed and success of reaction time responding in the rat.

Apomorphine, a nonselective, direct-acting dopamine agonist, and amphetamine, a nonselective indirect-acting dopamine agonist, were compared for their effects on the reaction time response in rats. Animals were shaped to release a lever in response to an auditory/visual stimulus to avoid mild foot shock. The characteristics of the reaction time response of primary interest were percent successful avoidance and response latency. Apomorphine (0, 1, and 5 mg/kg, IP) significantly decreased successful avoidance, but had no effect on response latencies. Thus, the decrease in successful avoidance was not a direct result of longer latencies. Amphetamine (0, 0.5, and 1 mg/kg, IP) produced a different pattern of effects on the reaction time response. Successful avoidance was not affected by amphetamine treatment. However, response latencies were dose-dependently decreased in response to amphetamine. These results demonstrate that dopamine receptor stimulation by different dopamine agonists produces a different pattern of effects on the characteristics of the reaction time response. In addition, these results demonstrate that successful avoidance can be modulated independently of response latencies.

Tolerance to the effects of ethanol on the speed and success of reaction time responding in the rat: effects of age and intoxicated practice.

An animal model of human reaction time (RT) was used to investigate the effects of age and intoxicated practice on the development of tolerance to the motor impairing effects of ethanol (EtOH). Young (8-9 month) and old (24-26 month) Fischer 344 rats were trained to release a lever in response to an auditory and visual stimulus in order to avoid mild footshock. The animals were divided into groups to receive either intoxicated (EtOH-before) or unintoxicated (EtOH-after) RT testing. Successful avoidance and response latencies were impaired in young and old rats after the initial exposure to EtOH (EtOH-before group). Tolerance developed to EtOH's effects on successful avoidance and on response latencies whether or not the rats received intoxicated RT practice; however, intoxicated practice facilitated tolerance development to EtOH's effects on successful avoidance but not on response latencies. While the initial sensitivity and the level of tolerance that developed to EtOH's effects were similar in young and old rats, the old rats were generally more sensitive to EtOH and developed tolerance at a slower rate. These results suggest that tolerance develops to the effects of EtOH on RT and that intoxicated practice can have different effects on the parameters of the behavioral response.

Age-related differences in response programming.

Age-related differential effects on reaction time (RT) performance for movement complexity and response-response (R-R) compatibility were examined in children, adolescents, and young adults. A two-choice RT paradigm involved three different finger responses, and each finger movement response was paired with every other movement response. Movement complexity was manipulated by varying the digits activated and was measured as the mean RT for a particular movement across all choice pairs. R-R compatibility was manipulated by altering the pairing of choice alternatives and was determined by the mean RT comparison for each of the movements according to the paired choice alternative. Simple RTs were also obtained for all finger movement responses for comparison with the RTs achieved in choice situations. Age-related differences were found for both movement complexity and R-R compatibility. Mean RT and response consistency improved with age. Although higher overall speeds were found with age, adolescents were not significantly slower than young adults. Adolescents did, however, make significantly fewer response errors on movements differing in complexity. Bilateral versus unilateral control and number of fingers involved in the task were found to affect both movement complexity and the compatibility between response pairs. The relationship between the alternative and choice response was found to be a robust factor affecting R-R compatibility. Choice responses were significantly slower than simple responses, and the rank ordering of movement responses was identical within the two paradigms.

Effects of adult aging on the movement complexity factor of response programming.

The interaction effects between adult aging and incremental levels of movement complexity were studied in young, middle-aged, and older healthy females. Utilizing a two-choice reaction time (RT) paradigm, movement complexity as a factor of response programming was varied in a microswitch pressing task by altering the number of sides of the body and the number of fingers controlled. The speed of response programming was found to be age dependent and to interact with movement complexity across age groups. The results confirmed that as movement complexity increases, the effects of adult aging on RT increase. This study further emphasizes the robustness of the movement complexity x age effect, in that older individuals were found to be much more sensitive to small changes in movement complexity than younger subjects. The comparison results of three adult age groups suggested this sensitivity to movement complexity is progressive over the adult life span.

Effects of route of administration of ethanol on high-speed reaction time in young and old rats.

The blood ethanol concentrations (BECs) and reactive capacity of young (8 months) and old (24 months) male Fischer 344 rats were compared at 5, 10, 20, 45, 65, and 90 min following the administration of ethanol (EtOH). The time-dependent effects of intragastric intubation (IG; 3 g/kg) and intraperitoneal injection (IP; 1.5 g/kg) of EtOH (20% w/v) were determined. Subsequent to IG delivery, BECs rose most rapidly within the first 20 min, but did not reach peak levels until 90 min for both young (240 mg/dl) and old rats (250 mg/dl). Following IP injections, BECs escalated within 5 min to 250 mg/dl in the young, to 175 mg/dl in the old, and declined gradually to a stabilized value of 150 mg/dl (young) and 130 mg/dl (old). The old rats never achieved the high BECs seen in the young. Reactive capacity, a measure of auditory/visual reaction time, was inversely related to BECs. As BECs (IP) declined, performance improved at a similar rate in both age groups, although the old rats' performance was more impaired than that of the young. However, BEC per se was not an adequate predictor of reactive capacity. When EtOH was delivered by IG so that BECs remained high for long periods of time, reactive capacity was far less impaired compared with IP delivery in which comparable BECs were present for only a few minutes. The possibility was noted that behavioral tolerance may have developed during the 90-min post-EtOH period, and that the IP delivery method may disrupt behavior in ways independent of brain ethanol levels.

Effects of ethanol on human fractionated response times.

The effects of ethanol (EtOH) on response components varying along a central vs. peripheral dimension were studied in five subjects. Reaction times (RTs) were fractionated by electromyographical recordings into premotor (central) and motor (peripheral, contractile) components. Highly practiced subjects performed a simple and discrimination RT task and related movement without significant impairment at the moderate blood ethanol concentration (BEC) (0.10%). At the higher BEC (0.17%), all components involving central processing (response time, RT and premotor time) were impaired in both simple and discrimination RT. More peripheral components (contractile time and movement time) were little affected. Contractile time was slowed slightly but significantly, but only in the combination of EtOH and the discrimination task which suggests that the stimulus discrimination stage of information processing can influence the activation of motor units involved in carrying out the movement.

Systematic aerobic exercise and components of reaction time in older women.

Reaction time and its fractionated components were studied in two groups of older women who differ in their level of regular aerobic exercise. Significant group differences were found in all dependent variables indicating that in older women regular aerobic exercise is an important factor influencing the speed of their reactions to simple and discriminatory stimuli, and in the accompanying premotor time, contractile time, and speed of movement following the reaction responses.

Reaction time and nigrostriatal dopamine function: the effects of age and practice.

Normal aged and Parkinsonian individuals lose the ability to initiate movements rapidly (increased reaction time) in parallel with changes in the nigrostriatal dopamine (DA) system. However, the ability of these individuals to improve their reaction time with practice has not been adequately assessed. We have developed a rodent model of human reaction time in which reaction time performance correlates highly with neurochemical measures of nigrostriatal DA integrity. In the present report, 15 young and 10 old male Sprague-Dawley rats were conditioned in a reaction time task to release a lever quickly in response to external stimuli in order to avoid a mild footshock. In order to examine the effects of practice on this reaction time task, the young animals were tested for 5 days at 3, 6 and 9 months of age and the old animals were tested for 5 days at 18, 21, and 24 months of age. From this well-practiced task, reaction time response latencies were measured and compared to measures of nigrostriatal DA function (steady-state levels of DA and its metabolites, D2DA receptor affinity and density). The old animals were slower in response latencies than the young animals. These age differences in response latencies, however, disappeared after several days of testing at each of the 3 test sessions, so that the old animals were not significantly slower than the young animals on days 4 and 5 of each session. As expected, the old animals showed reduced striatal D2DA receptor density with no age differences in DA receptor affinity.(ABSTRACT TRUNCATED AT 250 WORDS)

Movement initiation characteristics in young adult rats in relation to the high- and low-affinity agonist states of the striatal D2 dopamine receptor.

Changes in the speed of movement initiation as a function of age, brain damage, or rat strain are associated with altered characteristics of nigrostriatal dopamine (DA) neurons and of striatal D2 DA receptors. In the present study we investigated the relationship between movement initiation (response parameters: percent of successful responses and response latency) and the agonist binding states of the D2 DA receptor in corpus striatum in 3-month-old Sprague-Dawley rats (n = 51). In contrast to the typical experimental procedure, the variances of the behavioral and receptor binding data were intentionally made as small as possible to provide the most stringent test of putative relationships among variables. Rats were trained to release a lever as rapidly as possible in response to a light/buzzer (CS) combination in order to avoid a mild footshock (UCS). Percent avoidance scores, latencies of the fastest successful trials (successful latencies) and mean latencies for all responses (mean latencies) were collected for 1000-, 500-, 300- and 200-ms CS-UCS intervals. Twenty-four hours following the last behavioral test, animals were euthanized for measurements of the high- and low-affinity binding of DA to D2 receptors in corpus striatum. The standard errors of the mean for both behavioral and receptor binding parameters were, generally, less than 10%. The tightness of the receptor binding data appeared to be related to a lack of biological variance in the animals rather than to an artifact associated with the behavioral testing procedure, since a parallel experiment indicated that different numbers of behavioral shaping sessions had no effects on striatal D2 binding characteristics.(ABSTRACT TRUNCATED AT 250 WORDS)

Apomorphine doses impair the reaction time of fast reacting but not slow reacting rats.

Previously, several studies in our laboratories have linked the ability to initiate movement rapidly (reactive capacity) in several rat strains to markers of nigrostriatal dopamine function. In the present investigation we wished to determine the extent to which fast-reacting (FR) and slow-reacting (SR) rats responded differentially to the effects of dopamine agonist (apomorphine) administration. Fisher 344 rats were operantly conditioned on a shock-avoidance, reactive capacity task which requires extremely fast, ballistic, lever release responses. In FR, but not in SR rats, significant dose-dependent decreases in the per cent of successful avoidance trials were observed in response to apomorphine doses. Moreover, apomorphine brought responding in the FR animals to levels below that of the SR rats. Performance of the SR rats was unaffected by drug treatments. A number of interpretations are consistent with these preliminary data, including the idea that basal differences in speed of reaction and differential responses to a dopamine agonist, antiparkinsonian agent may both be associated with a similar mechanism: differential activation of compartmentalized striatal efferent systems.