This paper examines the importance of relative versus absolute intensity of physical activity in the context of population health. The two variables are plainly interrelated and may be very similar to each other in a population that is relatively homogeneous with respect to age, gender, and fitness. In a more heterogeneous sample, the question becomes whether outcome is affected by relative intensity when people perform a given volume (absolute intensity · time) of physical activity. Most of the papers to be discussed refer to large muscle activities, but if the active muscle mass is small, relative intensities must be expressed in relation to the peak response to exercise involving this particular volume of muscle (221). Debate on the optimal pattern of health-enhancing physical activity has a long history (50,115,244). One early review of this question (211) concluded:
“Some workers, basing their views on personal experience of illness or on the one experimental study of bed rest (52), have contended that the ordinary routine of daily life produces considerable training. Others (14,32) have stated categorically that mild exercise, such as golf and bowling is not enough; a pulse rate of 140/min (60% of the maximum possible increase over the resting value (31,32)) or 150/min (34) is essential for training.” (p. 533)
An experimental study from the same era (212) exposed 39 young sedentary men (predicted maximal oxygen intake 43.4 ± 5.8 mL·kg-1·min-1) to training that varied in intensity (39, 75, or 96% of aerobic power), frequency (1, 3, or 5 sessions per week), and duration (5, 10, or 20 min per session). Step-wise multiple regression analysis suggested that increments in maximal oxygen intake were significantly related to initial aerobic power (mL·kg-1·min-1), intensity of training (mL·kg-1·min-1), and the number of training sessions per week. This study found some response even at the lowest dose of training, but the magnitude of response was influenced most strongly by the intensity of training relative to the individual’s personal level of aerobic fitness.
Early observers focused on enhancing maximal aerobic power and cardiovascular function, assuming that the needs of health would be satisfied thereby (2,12,25,37,60,64,70,76,81,83,109,138,168,183,210,213,237). An increase of physical fitness is indeed associated with a reduced risk of cardiovascular and all-cause mortality (22,23,62,199), and is important to maintaining functional independence and thus quality of life in the elderly (71,85,181,214,225,243). However, debate has shifted more recently to the minimal level of physical activity needed to achieve specific health-related outcomes, including a reduction in overall and cardiac mortality, the control of hypertension and the prevention of strokes, improvement of metabolic health (the control of obesity, blood glucose regulation, the prevention of diabetes mellitus, and optimization of lipid profile), control of osteoporosis, enhancement of immune function, reduction of the risk of various tumors, and enhancement of mental health. It seems unlikely that a single relative or absolute intensity of physical activity will meet all of these objectives (21,95), and the achievement of some outcomes is unlikely to require an increase of aerobic fitness (184). Where possible, semantic descriptions of intensity have been matched to those proposed by Howley (105). One cross-sectional study of some 350 people found that whereas gains in cardiovascular variables were correlated with perceptions of hard (“vigorous”) physical activity (219), the control of body fat was associated with frequent, moderate physical activity.
The public policy issue of whether to advocate a moderate or a hard relative intensity of physical activity is far from resolved (92,130,161,217,218,255). Discussion has gathered new impetus over recent recommendations to incorporate necessary daily physical activity into “active living” rather than formal exercise classes (15,52,57,58,102,128,161,180,198,216,218,255,258). To date, those benefiting from everyday activities have tended to be sedentary, obese, and elderly people, those in whom a given absolute intensity of effort such as brisk walking develops a substantial relative intensity of effort (186,218). A table (p. 79) in one recent consensus document emphasized the major impact of an age-related decline in maximal oxygen intake on the relationship between absolute and relative intensity of effort (28), and an accompanying paper highlighted the need to distinguish absolute from relative intensity of physical activity (95). Subsequent modification of the table expressed aerobic intensities as a fraction of the oxygen consumption reserve (8,105,255). This change overcame the problem that resting metabolism accounts for a larger fraction of total energy expenditure in women, older men, and unfit individuals. It also made percentages of the individual’s maximal aerobic potential more comparable with percentages of heart rate reserve or peak aerobic power output (140,178,239).
Many papers have continued to focus on absolute intensities, asking whether health outcomes are enhanced by some specific energy expenditure, measured in METs, METs·wk-1, or kJ·min-1(130,161). Assessments of both relative and absolute intensities of effort have been handicapped by limitations of field methodology. Field observers commonly lack the information needed to estimate relative intensities of effort. Moreover, the claim that cardiovascular health is enhanced above a threshold absolute energy expenditure of 2 MJ·wk-1(171) is challenged by (i) potential errors of up to 2 MJ·wk-1 in estimates of gross weekly energy expenditures (217), and (ii) a failure to distinguish between gross costs (where a variable 1–2 MJ·wk-1 of imputed energy expenditure is due to normal resting expenditures) and net energy costs. In some cases, apparent differences in health outcome as large as 20% have remained statistically insignificant because of problems in measuring either physical activity or health (11,256). Analyses are further complicated because hard activity is usually recalled more readily and more precisely than light or moderate effort (3,135). If a given activity such as walking is randomly assigned, it may provoke reductions in unmeasured components of active leisure (190). Further, the reporting of certain patterns of physical activity reflects the attitudes and overall lifestyle of the individual (201,220), and if data are adjusted statistically for differences of lifestyle, previously observed associations between physical activity and health outcomes may disappear (110). Finally, the association of benefit with repeat reports of a given pattern of physical activity may indicate the importance of long-sustained active behavior (for example, in the prevention of neoplasia), but it may also reflect the greater accuracy of duplicate observations.
Published articles on the relationship between intensity of physical activity and health benefits have commonly included as key-words exercise prescription, training load, exercise, prevention, energy expenditure, physical fitness and health, intensity, exercise and health (education, policy, promotion and public), absolute intensity (exercise-physiology), and relative intensity (oxygen consumption-physiology). The majority of relevant papers before 1995 were identified in an earlier search (216). The current search of MEDLINE spanned the period from 1991 through December of 1999. Over this period, 1666 papers discussed exercise + intensity and 770 exercise intensity, but a combination of intensity + exercise + health yielded only 24 citations. Likewise, absolute exercise intensity yielded 7528 citations, but when this was combined with health, the total dropped to 52. Relative intensity yielded 4441 papers, but this was reduced to 5 citations on adding the term health. The Sport Discus was reviewed for the period from January of 1995 to September 1999. Given its exercise orientation, the search was limited to exercise prescription, intensity and health; this yielded 130 hits. The papers thus identified were supplemented by those identified in the earlier search, items garnered from major reviews (8,20,27,95,96,132,161,166,174,179,180,255,265) and material in extensive personal files. Because of geographic constraints, articles were reviewed only by the present author.
A large proportion of the papers thus identified merely asserted that a certain relative or absolute energy expenditure had a beneficial influence on health (217,218). The present review was limited to the 176 articles that sought evidence of a threshold of relative or absolute intensity of effort. Unfortunately, the importance of relative versus absolute energy expenditure was often obscured even in these articles, because differing relative intensities of effort were not matched for total weekly energy expenditures.
Heavy physical activity can have potential negative effects (musculoskeletal injuries (185), cardiac incidents (227,257), immune suppression (215), and overtraining (123)). Such outcomes reduce the net health benefits of physical activity. Nevertheless, the relative intensities inducing such outcomes are large and, thus, have only limited importance for the average sedentary adult who is initiating a program of moderate physical activity. These risks are not given detailed consideration in the present review.
CURRENT STATUS OF KNOWLEDGE
At least 23 national and international consensus reports have been published over the past 10 yr (Table 1). Some of these documents have looked at specific outcomes such as hypertension (63), obesity (26), or the secondary or tertiary prevention of coronary disease (232), but most have taken a global approach to health (29,30). The emphasis has been on survival rather than quality-adjusted life expectancy, and there has been little attempt to evaluate the relative importance of competing outcomes. Often, a minimum duration of activity has been recommended for any given relative intensity of effort. Thus, de facto a minimum-condition–related volume of physical activity has also been specified. With a few exceptions (27,107,154), consensus documents generally have not discussed the importance of relative versus absolute intensity of physical activity.
Almost all expert groups (Category D Evidence) except those concerned with adolescents (9,197) and obesity (26) have concluded that a light to moderate intensity of aerobic activity (40–60% of V̇O2max; 40–50% of V̇O2R) is an appropriate minimal recommendation for population health (Table 1). However, the power of this apparent unanimity of opinion is weakened by a large overlap in membership among various expert groups. Some reports (8,98) have noted additional health benefits when people have progressed to a higher intensity of effort (up to 85% of V̇O2max or V̇O2maxR), but many recent reports have argued that “active living” offers an adequate minimum of physical activity. Meta-analyses further indicate that, at least initially, a light-to-moderate intensity of physical activity is most effective in motivating sedentary individuals (55,61,103,207). Unfortunately, only a few consensus groups (27,107,154) have examined the issue of relative versus absolute intensity of effort.
PHYSICAL ACTIVITY VERSUS ALL-CAUSE OR CARDIOVASCULAR MORTALITY OR CARDIAC RISK FACTORS
Much early research on all-cause or cardiovascular mortality was based on cross-sectional occupational comparisons, where at best it was possible to distinguish light from hard physical work (255). Occupational studies are not considered in the present report, in part because of a substantial fitness-related selection into and out of heavy employment (162,267), and in part because the categorization of intensity of effort is very different for 30 min of leisure activity and an 8-h working day. Thirty-three reports covering 29 studies of leisure behavior have been examined (Table 2). All are large-scale longitudinal observational surveys, where correlations have been sought between simple questionnaire estimates of physical activity and health outcomes. In 16 of these studies, the reported type, frequency, intensity, and duration of physical activity have been combined to yield semiquantitative estimates of weekly energy expenditures, sometimes described erroneously as “intensity” of effort. Analyses of the Harvard Alumni data suggest that both total and cardiovascular mortality are reduced progressively over the weekly energy expenditure range 2.1–8.4 MJ·wk-1(172,173).
Fifteen reports (13 studies) have looked specifically at cross-sectional differences in outcome between individuals who reported hard physical activity versus those who did not (18,67,77,126,133,135,143,152,159,160,172,173,195,208, 209,229). Although the intent seems to have been to compare relative intensities, the threshold for benefit has commonly been reported as an absolute power output, and with a few exceptions (18,135,159,160,175), the influence of relative or absolute intensity of activity has not been distinguished from associated changes in the total volume of physical activity. The majority of studies have pointed to a threshold intensity of 6 METs (126,135,159,160,172,229) or more (152). Nevertheless, some comparisons between moderate and hard physical activity have found health benefits from light to moderate intensities of effort (18,143,195). Furthermore, as discussed below, hypertensive subjects seem at increased risk during hard intensities of effort (208,209), perhaps because there is a disproportionate increase in catecholamine secretion during hard physical activity (254). Unfortunately, there have been few studies in the absolute intensity range recently recommended by many expert groups (4–6 METs).
Some analyses of one study suggested additive effects from the intensity and the absolute volume of physical activity (133,135,172); other investigators noted an effect of intensity independent of volume (159,160) or an effect of duration independent of intensity (18). Further, a cross-sectional comparison found that occasional activity >6 METs (<once per week) was associated with a 107-fold increase in the immediate risk of a myocardial infarction in the first hour postexercise, as compared with a 2.4-fold increase of risk in those pursuing the same intensity of effort five or more times per week (155).
A total weekly energy expenditure of 8.4 MJ seems necessary to regression of atherosclerotic lesions (91). Several small crossover laboratory trials (Category B Evidence) have looked at acute changes in clotting mechanisms (156,226,240,262). With one exception (259), these have suggested that benefit is much greater with hard than with moderate intensity physical activity.
A substantial proportion of available reports have noted that a threshold absolute intensity of physical activity of around 6 METs is needed for all-cause or cardiovascular benefit (Category C Evidence). Unfortunately, this threshold has not been related to maximal aerobic power, so the relative intensity of physical activity remains unclear. Moreover, most studies have not distinguished effects of relative or absolute intensity independently of associated changes in the volume of physical activity.
PHYSICAL ACTIVITY, CONTROL OF HYPERTENSION AND STROKE
Three small-scale crossover trials (69,141,146) and one large cross-sectional analysis (265) have explored the acute impact of various intensities of physical activity upon blood pressures (Table 3). The crossover studies agree that the immediate reduction of blood pressure is independent of the relative intensity of effort, but in the cross-sectional analysis, the velocity of running (and thus intensity) had a much greater impact than the volume of activity as indicated by the distance covered.
Eleven of 15 studies of physical activity and chronic reductions in blood pressure involve small samples of randomly assigned subjects; three of the remaining four are large-scale nonrandomized longitudinal studies (Table 3). Five investigators have found a U-shaped relationship, with less benefit at high than at lower relative intensities of effort (47,88,121,148,192). Possibly, adoption of too high an intensity of effort reduced the impact of physical activity upon associated risk factors such as obesity and blood lipid profile. In contrast, five reports noted no effect of intensity (31,59,157,193,241). Three other reports observed an increase of response with intensity of effort (56,68,86); in one of these (56), walking distance was controlled. One of the cross-sectional studies (177) observed a threshold volume of activity for benefit (8.4 MJ·wk-1), but it did not examine the importance of relative vs absolute intensity of effort in reaching this threshold.
Five large-scale nonrandomized longitudinal data sets and two case-control studies have looked at the influence of physical activity upon stroke. Two reports (139,151) support suggestions of a U-shaped relationship to the intensity of effort (41), with little long-term benefit beyond an intensity of 70% of V̇O2max(90). One of the remaining reports found no effect of intensity (118), and the other four (101,173,196,260) observed an increased response with greater amounts of effort, but in none of these four studies was volume controlled.
One major weakness in research to date has been a failure to differentiate between hemorrhagic and ischemic strokes. Physical inactivity is apparently a risk factor in all cases of hemorrhagic stroke, but in ischemic stroke, this is true only of smokers (1). In some studies, there has also been a failure to distinguish clearly between acute and chronic reductions in blood pressure.
The acute, exercise-induced reduction of blood pressure seems independent of the relative intensity of exercise (Category B Evidence), but the chronic response may be greater for moderate than for hard effort (Category B Evidence). The optimal intensity of effort for the prevention of stroke remains unclear.
PHYSICAL ACTIVITY AND METABOLIC HEALTH
The impact of physical activity on metabolic health occurs through a combination of the acute and chronic effects of exercise. The acute metabolic effects of a given bout of exercise may be enhanced if physical condition is improved (93).
Acute stimulation of metabolism.
Ten small trials (nine with a crossover design, one randomized) have examined the acute effect of exercise upon resting metabolism (Table 4). Five of the 10 trials (33,182,203,224,248) controlled relative intensity of effort for the total volume of activity. Six trials found an intensity-related increase in the amount (33,182,224,248) or the duration (13,231) of the excess postexercise oxygen consumption for both leg- and arm-cranking exercise, with intensity being more important than the duration of activity (224). The timing of observations postexercise may influence the observed response (182); three other reports (one with control of total volume (203)) found no effect of intensity (80,203,204), and one noted that 3 h postexercise the response to activity at 50% of V̇O2max was greater than that at 70% V̇O2max(39).
Traditionally, the total volume of physical activity has been held to be more important than the intensity of effort in achieving a significant decrease in body fat content. However, hard exercise could facilitate the process by increasing resting energy expenditure (127,247) or lean body mass (and thus the ability to undertake endurance exercise (106)). Hard effort might also be helpful in reaching the requisite total weekly energy expenditure (53,108).
Eight small-scale randomized trials and four larger cross-sectional studies have examined the impact of differing programs on changes in body composition; the total volume of physical activity was controlled in only two studies (56,84). Two reports compared structured with lifestyle activities, hard intensity activity being less in the lifestyle program (10,59). Fat loss was similar for the two groups, but in one of these comparisons, the lifestyle group lost more lean body mass (10), a point confirmed in another trial (84). Two further trials (56,200) found no significant changes in body mass relative to control at any of two or three intensities of effort. One study found reductions in skin-fold thicknesses with both resistance and aerobic exercise (42). The remaining reports disagreed as to whether the response was unchanged (72,84) or increased (35,54,104,108,246) by an increase in the relative intensity of exercise; however, total volume of physical activity was controlled in one of the two negative reports (84).
One small randomized trial and six small-scale crossover trials examined acute changes in lipid metabolism; five (49,51,82,187,249) controlled for total volume of physical activity. Postprandial lipemia did not differ with relative exercise intensity (249). In men (but not in women), the effect on plasma free fatty acid kinetics was greater with a harder relative intensity of activity (73,74), uncontrolled for total volume. One trial found no acute changes in the lipid profile (51), but in the remaining two studies of plasma lipids, changes showed a significant effect of relative intensity (82,187).
Chronic changes in lipid profile have been examined in eight small-scale randomized longitudinal trials, and seven nonrandomized studies. Five studies controlled for the distance walked (48,56,200,235,265). Four of the randomized trials found little or no change in lipid profile relative to control at any of two or three relative intensities of effort (56,59,119,200). Two other reports found greater benefit from moderate than from hard intensity programs (120,235), and in the remaining two studies, there was evidence of benefit but no effect of relative intensity (10,48). The majority of the cross-sectional studies demonstrated an effect of exercise volume rather than intensity. The report of Williams (265) specifically noted that the effect on lipid profile was 6 times larger for running distance than for running velocity.
Blood glucose regulation.
Eight small crossover trials, one randomized trial, and two cross-sectional studies have examined the influence of exercise intensity on glucose utilization and/or insulin sensitivity. Trials have differed in their choice of subjects (trained, untrained, insulin-dependent, or nondependent diabetics). Three studies (14,32,112) controlled for the total volume of activity. Three of four trials (44,74,194) found a greater increase of glucose uptake at the higher intensity of effort. Two negative studies (32,233) involved patients with noninsulin dependent diabetes mellitus, and one of these equated energy expenditures across intensities. Two of three trials (14,112) but not (268) found an improvement of insulin sensitivity only at the harder intensity of effort.
Prevention of maturity-onset diabetes mellitus.
Five large-scale studies have explored cross-sectional associations between physical activity and the incidence of maturity-onset diabetes mellitus. Two studies have pointed to the importance of activity sufficient to induce a sweat (144,145), and two others have pointed to progressive protection from an increased total volume of energy expenditure (86,100), but no one appears to have explored the absolute versus relative intensity issue systematically.
As with blood pressure, there has sometimes been a failure to distinguish between the acute and chronic metabolic effects of physical activity. Many of the metabolic benefits of physical activity such as a normalization of lipid profile seem to depend on the total volume rather than the intensity of exercise (Category B and Category C Evidence). However, the excess postexercise oxygen consumption seems to reflect the relative intensity of effort (Category B Evidence), and programs that enhance lean mass may increase the ability of overweight individuals to exercise.
The extent of weight-bearing and the application of resistive force are probably more important than either the relative or the absolute intensity of physical activity when seeking to prevent osteoporosis. Available information (Table 5) includes two small randomized controlled trials covering periods of 7 months (97) and 1 yr (188), and two larger cross-sectional studies (46,122), all performed on postmenopausal women. Hatori et al. (97) noted that lumbar vertebral bone mineral density was increased by 1.0% if the intensity of physical activity exceeded the individual’s anaerobic threshold, whereas a lighter intensity was associated with a 1.0% loss, not significantly different from the 1.7% loss seen in control subjects (97). However, in the one study where the total work performed was equalized (188), benefit was similar from contractions at 40% and 80% of 1 RM. The cross-sectional observations suggest effects from the volume of activity and from the intensity (uncontrolled for volume).
There is some suggestion (Category B Evidence) that harder relative intensities of effort are more effective in augmenting bone density, but this may be due in part to an associated increase in the volume of physical activity.
PHYSICAL ACTIVITY AND OPTIMIZATION OF IMMUNE FUNCTION
A lymphocytosis develops at the immediate end of a bout of exercise, and a suppression of the immune response then persists for some hours. Six small-scale studies have looked at the effects of various intensities of effort on immune function (Table 6); three of these studies (142,150,191) controlled for the total volume of physical activity. One of these three reports noted no change in serum IgA at any intensity of effort (150), but four of the five remaining studies (142,165,238,253) found that a hard relative intensity of effort (75–80% of V̇O2max) was needed to induce even a transient immunosuppression. The negative report considered primarily the in vitro neutrophil degranulation response to bacterial lipopolysaccharide (191).
There is good evidence (Category B) that a hard relative intensity of exercise is needed to cause even a transient suppression of circulating immune function.
Physical activity and cancer.
In the case of cancer prevention, it is particularly important to maintain the specified pattern of physical activity for many years. Seven large-scale nonrandomized longitudinal studies (4,11,40,111,136,170,261) and a meta-analysis (222) have explored the effects of the dose of physical activity upon all-cancer death rates (Table 7); only one report commented on relative intensity (261), and this failed to standardize for total volume of energy expended. Individual reports show rather inconsistent effects. A meta-analysis found a greater benefit from a large rather than a moderate dose of physical activity (222), although the effects of intensity were not clearly distinguished from volume.
Of individual site cancers, research has focused particularly on the colon. Here, data are drawn from six nonrandomized longitudinal studies (79,131,134,206,245), three case-control studies (78,228,230), and the meta-analysis (222). Again, the issue of relative intensity is not resolved. The most convincing single study (230) used the case-control technique to show a much greater protection from hard than from light intensity activity, but a rather similar effect from small, moderate, and moderately large volume relative to a large volume of physical activity. These observations would be consistent with postulated benefit from an increased local synthesis of prostaglandin F2, and thus a speeding of colon transit time (45). In contrast, a 16-yr follow-up of 53,242 men and 28,274 women found an adverse effect of regular training relative to a sedentary lifestyle in men (but not in women) (245). Further, in the meta-analysis (222), benefit was greater for light than for moderate activity.
One large study of female breast tumors found the greatest benefit from the largest volume of physical activity, although this was true only of postmenopausal women (205). The meta-analysis (222) showed little difference between light and moderate activity in the case of breast and female reproductive tract tumors, although in the case of prostate tumors the largest benefit was seen with light rather than moderate activity. Again, volume was not clearly distinguished from intensity in most of these studies.
Despite a substantial number of epidemiological studies (Category C Evidence), the issue of the relative intensity of effort needed for protection against various types of cancer remains unresolved and largely unexplored.
ENHANCEMENT OF MENTAL HEALTH
Fourteen studies (Table 8) have looked at the effects of exercise intensity upon mental health, eight considering acute and six chronic responses. Most are based on relatively small samples of subjects, and with two exceptions (147,207), the subjects have had only minor (nonclinical) disturbances of affect. Four of the acute trials found a larger effect at harder relative intensities of effort, but with one exception (65), the effect of a harder relative intensity was not distinguished from that of an increased volume of physical activity. With the exception of one study (167), the evidence on long-term decreases in anxiety suggests that a moderate relative intensity of exercise may be more effective than hard intensity effort. Indeed, one trial found an increase of anxiety with hard intensity training (116).
Further examination of the acute effects of relative exercise intensity on mental health is needed, controlling for the volume of physical activity that is undertaken. The chronic benefits of physical activity seem associated with a moderate rather than a hard relative intensity of effort (Category B Evidence).
FUTURE RESEARCH PRIORITIES
1) More precise methods are needed to assess both relative and absolute energy expenditures in epidemiological surveys.
2) Future studies comparing several relative intensities of effort need to be designed to assure consistent absolute energy expenditures between conditions.
3) There is a need to explore whether there are multiple mechanisms of benefit, with one mechanism being induced by a specified absolute intensity or volume of effort, and another by a specific relative intensity.
4) Given that differing patterns of effort may favor different health outcomes, there is a need to prioritize outcomes. Specifically, there is a need to assess which outcomes contribute most to quality-adjusted life expectancy.
5) Greater attention should be focused on age-, gender-, and fitness-related differences in physical activity requirements.
6) There is a need to distinguish possible differences in relative and absolute intensity requirements between the prevention and the treatment of disease.
7) Optimal as well as minimal requirements need to be considered, with a view to defining not only thresholds but also ceilings of relative and absolute intensity of effort.
Address for correspondence: Professor Roy J. Shephard, M.D. (Lond.)., Ph.D., DPE, P.O. Box 521, Brackendale, BC V0N 1H0 Canada; E-mail: [email protected]
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