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Applied Sciences: Psychobiology and Social Sciences

The effect of lifelong exercise on psychomotor reaction time: a study of 38 pairs of male monozygotic twins

SIMONEN, RIITTA L.; VIDEMAN, TAPIO; BATTIÉ, MICHELE C.; GIBBONS, LAURA E.

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Medicine& Science in Sports & Exercise: September 1998 - Volume 30 - Issue 9 - p 1445-1450
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Abstract

Fast psychomotor reaction time (RT) has been considered an indicator of a healthy lifestyle and perceived health (4). It is also a method of evaluating motor skills, especially central nervous system function in rapid, controlled movement execution. In studies of the elderly, physically active subjects have been shown to have faster RTs than inactive subjects (19,25,31), but contradictory results also have been found (26). There is little information available on the relationship between RT and fitness for the middle aged, when morbidities affecting RT are less likely to emerge as possible confounding factors.

Faster choice RT in exercisers versus nonexercising controls has been found by Era et al.(9) among seniors engaging in athletics, by Lupinacci et al.(19) in physically active university professors, and by Sherwood and Selder (28) in 30-59-yr-old runners versus untrained volunteers. In some studies, simple RT has been faster in exercisers as well (1,5,9,19,28). In these retrospective studies of the relationship between physical activity and psychomotor reaction time, subjects have either already adopted and maintained regular exercise or have chosen an inactive way of life, choices that may be affected by physical abilities.

In a 3-yr follow-up study of older adult subjects (aged 57-85 yr) who had been involved in an exercise program, Rikli and Edwards (25) found a faster choice RT after 1 yr of follow-up in those involved in the program as compared with controls, although the difference had not increased at the 3-yr point. In intervention studies, neither a 6-wk walking program (26), 7 wk of aerobic exercise (24), or 10 months of endurance training leading to a 29% improvement in aerobic capacity significantly improved RT (21). The intervention studies may not have been long enough or may have had inadequate sample sizes to reveal statistically significant differences between exercisers and nonexercisers.

Inconsistencies in the association between RT and exercise may be because of the uncontrolled effects of confounding factors such as cardiovascular disease (2), smoking (13), and excessive use of alcohol (11). Other physical activities besides sports and exercise, when strenuous enough, increase oxygen uptake and therefore should be controlled in studies of exercise and RT as well. Fast RT is associated with intelligence(16), which is determined partly by education (12) and genetics (23). Also, RT has a strong genetic dependency. In an EMG study (18), it was estimated that 86% of the variance in visual RT in 11-24-yr-old twins was the result of genetics. In a study of middle-aged adults and their children, inheritance accounted for 27% of the variance in decision time and 18% in movement time in psychomotor simple RT testing (22). Simple RT correlation between parents and offspring was 0.70 in a Polish family study(35). Because genetics affect exercise participation (34), aerobic capacity (3), and RT itself (18,22,35), retrospective studies of exercisers versus inactive controls contain the bias of genotype.

Based on evidence that RT might be faster in subjects with greater aerobic capacity (8,10), one might expect that exercise with enough frequency, intensity, and duration would have a positive effect on subjects' psychomotor speed ability. Current opinion suggests that frequent aerobic exercise participation delays the effects of aging on RT, but evidence of exercise-induced fast psychomotor speed among healthy middle-aged subjects is still lacking (32).

The goal of this investigation was to study the effects of different modes of exercise on RT and its component parts of decision and movement time. Studying identical twins discordant for lifetime exercise provided the opportunity to control the effects of genome and many extraneous childhood environmental exposures. Also, studying hand and foot RTs with devices of the same level of task complexity gives confirming information about the psychomotor speed of exercise discordant twins.

METHODS

Subjects. Visual hand and foot simple and choice RTs were studied in 38 identical male twin pairs aged 35-69 yr (mean = 49 yr). Pairs discordant for physical activity were chosen among 2050 identical male twin pairs from the population-based Finnish twin cohort. The discordance was determined by physical activity data collected from postal questionnaires in 1975 and 1981, as well as interview data from 1992 to 1993. The assurance of twinship and the basis of the Finnish twin registry has been reported in more detail elsewhere(17). Subjects received written information about study procedures and aims, and informed consent was obtained before participation. The study plan was approved by the Ethical Committee of the Department of Public Health at the University of Helsinki and the Human Subjects Committee at the University of Washington.

Two groups of pairs were compared. For the mean lifetime exercise contrast in the frequent versus occasional exercise groups (29 pairs, mean age 50 yr), the frequent exercisers had more than twice the frequency, at higher intensity, than their co-twins (Table 1). In the regular versus infrequent light exercise groups (9 pairs, mean age 47 yr), the regular co-twins exercised three times as often (Table 1).

TABLE 1
TABLE 1:
Lifetime and past year exercise and physical leisure time activities and occupational physical load in frequent versus occasional and regular versus Infrequent exercise contrast groups.

Anthropometric and health behavior-related factors were measured or obtained from an interview (Table 2). Four pairs in the frequent versus occasional exercise comparison group and one pair in the regular versus infrequent group had opposite handedness; otherwise, pairs were right-handed. In the frequent exercise group, 83% had an elementary school education, their co-twins 86%. In the regular versus infrequent exercise group, 89% of co-twins in either group had completed elementary school.

TABLE 2
TABLE 2:
Subject characteristics.

Health status and health habits. A medical history was obtained from an extensive standardized interview that included past and present diseases diagnosed by a physician. The year diagnosed, current status, medication, and hospitalization data were collected for each disease. Current smoking status was recorded, as well as smoking history summarized in pack-years (number of packs per day times number of years smoking). Alcohol consumption was estimated in grams of absolute alcohol per month on the basis of consumption frequency, amount, and type of reported alcohol products (Table 2). Only subjects without severe diseases or current medications affecting RT, reports of tiredness, acute infections, sensory deficits, or musculoskeletal complaints were included in the study.

Physical activity history. Physical activity refers to sports and exercise activities, other physical leisure time activities, and physical activity at work. Lifetime physical activity history was ascertained from a structured interview lasting about 2.5 h; co-twins were always questioned by the same interviewer. Physical activity from the age of 12 to the present age was coded to an accuracy of 1 yr. Information collected was activity type, age span, possible competitive level, mean months of participation per year, mean frequency per week, mean duration of the session, and perceived intensity (1 = light, 2 = moderate, 3 = strenuous). Every event lasting at least 3 months was coded separately. Lifelong and past year physical activity history, weighted by years in an activity, are presented in Table 1 for the twin contrast groups.

Subjects' lifetime occupational physical loading was also ascertained. Every job's physical load was evaluated in the interview on the basis of walking, lifting, and bending or twisting demands of work. Jobs were coded using one of 18 descriptive categories that were later reduced to 4 (1 = sedentary work, 2-4 = progressive degrees of material handling, where 4 was heavy physical work), which were weighted by the years spent in each job. Occupational physical loading characteristics are also summarized by the mean years in every loading category, mean hours sitting per day, and time spent in twisted and bended positions per day. Mean lifting load was computed by multiplying the weight of the most commonly lifted object by the frequency of lifts per day, weighted by the number of years at that job (Table 1).

Reaction time. Psychomotor performance testing was conducted in a peaceful room, and both siblings were tested within 2 h to avoid possible effects of diurnal variations in RT. Visual reaction time was tested in the seated position, first with the index finger of the dominant hand and then with an analogous device designed for use with the foot, using the first toe of the bare ipsilateral foot and then the contralateral foot.

RTs were measured with simple choice method and after that with a 7-choice method. The targets were located in a semicircle 10 cm from the waiting button in hand testing and 20 cm in the foot testing. In each session, subjects performed three practice efforts followed by 12 actual trials. The order of the trials was the same for each subject, with a randomly assigned foreperiod of 1 to 4 s. No warning signal was present.

A device recorded the decision time when a subject released his finger or toe from the waiting button and the movement time when a subject reached with his finger or toe to the target button. RT was the sum of decision and movement time. Trials in which the subject failed to hit the target were omitted. Because subjects sometimes make a false start and receive an atypically fast time, trials with decision times less than 90 ms were omitted. Completing the RT testing took about 20 min for each subject. The fastest five trials were chosen for the analysis based on reliability data with these devises (29). There the fastest five values had higher within test session repeatability (0.99), compared with the last five (0.92-0.95) or all 12 (0.95-0.97). Also, between session repeatability of the hand and foot RTs using the fastest five values was better (0.49-0.68), compared with the last five (0.38-0.60) or all 12 values (0.51-0.61). When using means of fastest five trials, Pearson correlation coefficients were from 0.53 to 0.65 for the hand and ipsilateral foot, from 0.39 to 0.61 for the hand and contralateral foot, and from 0.76 to 0.81 for the ipsilateral and contralateral foot (all p values <0.001).

Statistical methods. The means of the five fastest decision, movement, and total reaction times were used. Because the five fastest decision times and the five fastest movement times were not necessarily in the same trials, the means of these do not sum to the means of the five fastest total reaction times. Lifelong participation in exercise and other leisure time physical activities was averaged from age 12 through the actual age, and the frequency, intensity, and duration were weighted by the number of years spent in each activity. The occupational physical loading score also was weighted by the years at each job.

For each twin pair, the RT of the twin who exercised less was subtracted from the RT of the twin who exercised more. t-tests were used to test the hypothesis that the mean paired differences were zero. The statistical model for the t-test can be written Rtmore − Rtless = β0 + error, where the subscripts more and less refer to the twins with more and less frequent exercise, and β0 is the grand mean. The null hypothesis tested is that the grand mean is zero.

Occupational loading was controlled by computing the paired differences in lifetime mean job code and using this difference in a linear regression model to predict the difference in RT. This can be written as follows: Equation 1 The hypothesis of no difference in RT is still that the grand mean is zero, and this was tested using the Wald test.

RESULTS

RT differences between frequent versus occasional exercise pairs. Psychomotor RT was typically slightly faster in frequent exercisers than in occasional exercisers(0-11%), but the results were not consistent between hand, contralateral, and ipsilateral foot and for all RT components (Table 3). Before controlling occupational loading, differences were found in hand choice decision time (7% faster,P < 0.01) and RT (5% faster, P < 0.05) and contralateral foot simple RT (6% faster, P < 0.05), choice movement time (11% faster,P < 0.01), and choice RT (7% faster, P < 0.01). In other measured values, group differences were 0-9%. Once occupational physical demands were controlled, only hand choice decision time and contralateral foot choice RT remained significant (7%, P<0.01 and 7%, P < 0.05, respectively) (Table 3).

TABLE 3
TABLE 3:
Group hand and ipsilateral and contralateral foot simple and choice RTs (in ms), with standard deviations in parentheses, and RT differences. P values indicate the adjusted predicted differences (not shown), which remained statistically significant after controlling for occupational physical loading.

A linear regression analysis revealed that age did not significantly affect the within pair RT differences(0-51 ms), except for the ipsilateral foot simple and choice RTs (both P< 0.05). For these, ipsilateral foot RTs every 10-yr increase in age predicted 45-46 ms greater within pair differences, and it was more significant for the decision time than for the movement times.

RT differences between regular versus infrequent exercise pairs. No significant RT differences were found in regular versus infrequent exercise pairs, except for faster ipsilateral foot simple decision time in the regular exercisers (8%, P < 0.05). This remained statistically significant after controlling for occupational physical loading (Table 3). Although the various RT measures tended to be faster for the hand for regular than for infrequent exercisers, these differences were not statistically significant(P = 0.06-0.73). No clear pattern of greater or lesser decision time, movement time, or RT emerged between these exercise groups (Table 3).

DISCUSSION

Results suggest that frequent (4 times/wk) lifetime involvement in exercise increases psychomotor speed slightly as compared with occasional exercise participation (<2 times/wk). Regular light exercise of approximately three times per week did not have any clear benefit on RT as compared with exercising less than one time per week. These results suggest a somewhat smaller effect (1-7%) of exercise on hand RT than results from previous studies, in which exercisers had 5-18% (19,25) or even 25% (31) faster hand RTs. The slightly greater effect of exercise on choice RT than on simple RT is in concordance with other studies (8,25), although RT differences here were not seen consistently in all extremities.

In our findings, pairwise variations were much smaller than variations within an age group, which indicates that twinship helps to control for the effects of extraneous factors that may confound results in previous RT-exercise studies. Such factors may be the individual's adaptability to exercise, genetic limits of trainability, and genetic factors affecting psychomotor speed itself. Also, subjects in this study were middle aged, when the prevalence of cardiovascular diseases that affect RT are not as pronounced as later in life. Both twin siblings had a similar educational level, which also has been associated with RT (8,14).

Although the frequent versus occasional group RT differences in ipsilateral foot were not significant, they seemed to follow the same trend as seen in other extremities; RT differences were 1-7% in the hand, 0-6% in the ipsilateral foot, and 2-11% in the contralateral foot. Closer examination of this exercise contrast group revealed that all four pairs showing the largest intrapair differences were of opposite handedness, with the left-handed twins who exercised more being faster than their co-twins.

The exercise contrast in the frequent versus occasional group was somewhat greater than the regular versus infrequent group, where the regular exercisers had slightly less intensity and duration but clearly greater frequency than their co-twins who exercised occasionally. This could be one reason why an effect of more frequent exercise was not seen with this contrast. The occupational physical demands also were somewhat greater in the infrequent versus the regular exercisers, which may have diluted differences in aerobic capacity resulting from exercise participation.

Work content will presumably have two kinds of effects on RT and motor function: mental work may enhance a subject's information processing capabilities (33), and heavy physical work may have a training effect leading to better aerobic capacity (30). Some jobs may also contain task-specific training, leading to an advantage in RT task performance. Studies have documented faster RT in subjects with a higher level of education, which was found especially among middle-aged and elderly people (8). Also, subjects doing mental work did better in a finger tapping test when compared with subjects who did physical work (33). Similarly, lower-level managers and high-level managers showed better psychomotor function than manual workers (7). Controlling for occupational physical loading history in this study did not increase the association between RT and exercise. This may be the result of confounding work-related physical, psychological, and other factors affecting RT, which were not considered and should be further explored.

There are several physiological and psychological theories explaining the structural and behavioral effects of exercise on the central nervous system's information processing, which could lead to fast motor response. Improvement in cognitive performance has been thought to be mediated by increased cerebral oxygen transport and utilization, glucose metabolism, and neurotransmitter turnover (6) or by exercise-induced increases in the secretion of neurotrophic factors (20). The psychological benefits of exercise, such as increased concentration and arousal and decreased anxiety level and depression, may enhance cognitive performance (e.g., see ref. 27). Subjects with exercise backgrounds also may be more competitive and motivated to perform (34). It has been stated that vigorous exercise, in particular, reduces state anxiety and neuroticism, decreases the level of mild to moderate depression, and reduces various stress indices(15), which may help to explain our findings that frequent and vigorous exercise enhanced RT, whereas there was no such effect on regular, but light exercise.

Physical fitness measured with direct or indirect oxygen uptake would include effects that vigorous physical leisure time activities and occupational physical loading may have on performance. Such studies have shown that high oxygen uptake capacity seems to be associated with fast psychomotor speed (10), but on the other hand, in intervention studies an increase of 20% (21) or 27% (6) in maximal oxygen uptake seemed inadequate to affect choice RT. The frequency of lifelong physical activity required to have a small effect on RT seemed fairly high in our study. Based on these results, it may be unrealistic to expect general public health exercise promotion efforts to lead to exercise habits adequate to influence reaction time.

In conclusion, the study results suggest that RT may be slightly affected only by lifelong, frequent exercise and that the effect is small. The effect is seen more clearly in choice RT, which is a form of complex perceptual motor processing. There is still a need to explore the immediate and long-term health benefits of the 5-11% faster psychomotor speed induced by frequent, vigorous physical activity.

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Keywords:

DECISION TIME; MOVEMENT TIME; MOTOR SKILLS; PHYSICAL ACTIVITY; PHYSICAL FITNESS; OCCUPATIONAL PHYSICAL LOAD; TWIN STUDY

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