J Appl Physiol 111: 1211–1217, 2011.
First published July 21, 2011; doi:10.1152/japplphysiol.00421.2011.
HIGHLIGHTED TOPIC
Review
Physiology and Pathophysiology of Physical Inactivity
Metabolic deterioration of the sedentary control group in clinical trials
Mahesh J. Patel, Cris A. Slentz, and William E. Kraus
Division of Cardiovascular Medicine, Duke University Medical Center, Durham, North Carolina
Submitted 6 April 2011; accepted in final form 20 July 2011
physical inactivity
SINCE THE LANDMARK STUDY of bus drivers and conductors by
Morris et al. in 1953 (31), numerous analyses from observational studies such as the Harvard Alumni Health Study (25,
41) and the Aerobics Center Longitudinal Study (4, 5) have
demonstrated the long-term health benefits of physical activity
and fitness. More recently, observational studies have demonstrated that specific sedentary behaviors, such as prolonged
periods of sitting, are also related to adverse long-term health
outcomes and that these relations are statistically independent
of physical activity levels (21, 32, 50). These observations help
to establish that both physical activity and inactivity play key
roles in maintaining and improving health and in disease
development over the course of a lifetime.
While observational data have provided insights into the
long-term effects of physical activity and inactivity on health,
clinical trials have provided insights into the mechanisms of
these effects in the shorter term. Here, we will review evidence
that highlights the progressive metabolic decline associated
with physically inactive lifestyles. These data are based on
observations of the sedentary control groups from clinical
exercise training trials. We will discuss the minimal amount of
physical activity needed to prevent this metabolic deterioration
and how this amount relates to current physical activity rec-
Address for reprint requests and other correspondence: M. J. Patel, Duke
Univ. Medical Center, Durham, NC 27710 (e-mail: [email protected]).
http://www.jap.org
ommendations for the general population. Finally, we will
discuss how the minimal amount of physical activity needed
for the general population may differ from what is needed for
any given individual and argue for the need to advance our
understanding of these individual differences to move toward
the next level in lifestyle medicine: personalized physical
activity recommendations.
SEDENTARY CONTROL GROUPS
Over the last decade, relatively large randomized controlled
trials of exercise training have been performed in sedentary
populations (8, 9, 22, 42). While the primary purpose of these
studies was to understand the physiological effects of different
modes, amounts, and intensities of exercise training regimens
on health parameters, analyses of the sedentary control groups
from these studies have also provided a greater understanding
of the natural history of metabolic parameters in those who are
leading sedentary lifestyles. By evaluating the change in various physiological parameters over the sedentary control period, insights have been gleaned about the physiological effects
of physical inactivity over periods of ⬃6 mo. Our group has
published extensively on this topic, based on findings from the
sedentary control group of the Studies of Targeted Risk Reduction Interventions through Defined Exercise (STRRIDE;
45– 47).
8750-7587/11 Copyright © 2011 the American Physiological Society
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Patel MJ, Slentz CA, Kraus WE. Metabolic deterioration of the sedentary
control group in clinical trials. J Appl Physiol 111: 1211–1217, 2011. First
published July 21, 2011; doi:10.1152/japplphysiol.00421.2011.—Randomized
clinical trials of exercise training regimens in sedentary individuals have provided
a mechanistic understanding of the long-term health benefits and consequences of
physical activity and inactivity. The sedentary control periods from these trials have
provided evidence of the progressive metabolic deterioration that results from as
little as 4 – 6 mo of continuing a physically inactive lifestyle. These clinical trials
have also demonstrated that only a modest amount of physical activity is required
to prevent this metabolic deterioration, and this amount of physical activity is
consistent with current physical activity recommendations (150 min/wk of moderate intensity physical activity). These recommendations have been issued to the
general population for a vast array of health benefits. While greater adherence to
these recommendations should result in substantial improvements in the health of
the population, these recommendations still remain inadequate for many individuals. An individual’s physical activity requirements are influenced by such factors as
an individual’s diet, nonexercise physical activity patterns, genetic profile, and
medications. Improving the understanding of how these factors influence an
individual’s physical activity requirements will help advance the field and help
move the field toward the development of more personalized physical activity
recommendations.
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METABOLIC DETERIORATION OF THE SEDENTARY CONTROL GROUP
Table 1. Metabolic effects of 6 mo of continued physical
inactivity in sedentary individuals
Body Mass
2Insulin Sensitivity
Waist circumference
Waist/hip ratio
Visceral fat
Total abdominal fat
Fasting insulin
Total LDL particle number
Small LDL particle number
LDL cholesterol
Fasting glucose
2Cardiorespiratory fitness (TTE)
2LDL particle size
LDL, low-density lipoprotein; TTE, time to exhaustion on a treadmill
exercise tolerance test. Findings are from analyses of STRRIDE (9, 13–19).
J Appl Physiol • VOL
Fig. 1. Comparison of the effects of continued physical inactivity (controls)
and 3 different exercise training regimens on mean changes (Chg) in visceral
abdominal fat (A), subcutaneous abdominal fat (B), and total abdominal fat (C).
Findings are from an analysis of STRRIDE (45).The mean percent changes in
the control, low amount-moderate intensity, low amount-high intensity, and
high amount-high intensity groups for visceral abdominal fat were 8.6%, 1.7%,
2.5%, and ⫺6.9% (A); subcutaneous fat 1.1%, ⫺1.2%, 3.1%, and ⫺7.1% (B);
and total abdominal fat 3.9%, 0.2%, 2.0%, and ⫺6.8% (C).
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The original study design of STRRIDE allowed for the
exploration of the optimal amount and intensity of exercise
training in overweight to obese, sedentary individuals with
mild-to-moderate dyslipidemia. The details of this design have
been described previously (23). In brief, STRRIDE included a
sedentary control group, wherein study subjects continued their
physically inactive lifestyles, and three exercise training
groups, during which subjects participated in one of the following regimens for 6 mo.
Low amount, moderate intensity physical activity—prescribed 14 kcal·kg⫺1·wk⫺1 of physical activity at 40 –55% of
peak V̇O2 (caloric equivalent of walking ⬃19 km/wk).
Low amount, vigorous intensity physical activity–prescribed
14 kcal·kg⫺1·wk⫺1 of physical activity at 65– 80% of peak
V̇O2. (caloric equivalent of walking/jogging ⬃19 km/wk).
High amount, vigorous intensity physical activity—prescribed 23
kcal·kg⫺1·wk⫺1 of physical activity at 65– 80% of peak V̇O2
(caloric equivalent of walking/jogging ⬃32 km/wk).
In addition to evaluating the effects of different amounts and
intensities of exercise training on cardiometabolic risk factors,
STRRIDE allowed for an investigation of whether cardiometabolic parameters change after only a 6-mo period of continued
physical inactivity in the control group. Subjects in the sedentary control group experienced a deterioration in several metabolic factors, including increases in measures of adiposity (23,
38) and worsening of lipoprotein profiles (22, 47), cardiorespiratory fitness (10), and parameters of glycemic regulation (16,
49). A list of the specific metabolic variables that worsened
over the 6-mo sedentary control period of STRRIDE is presented in Table 1. Due to concerns that any observed changes
in metabolic parameters over the control period might have
been due to changes in subjects’ activity patterns, control
subjects were extensively counseled on the importance of not
changing their physical activity from their prerandomization
inactive lifestyles. As a reflection of good compliance with this
protocol, no significant changes in physical activity, caloric
consumption, or cardiorespiratory fitness were observed over
the control period (10, 15, 22).
One key driver of the progression of these metabolic abnormalities over the inactive control period was an increase in
body mass. Subjects in the inactive group gained ⬃1% of body
mass and 2 cm of waist circumference over 6 mo, whereas
subjects in all three groups of exercise training regimens lost
body mass in a dose-response manner, despite a lack of change
in caloric intake (46). Also, the changes in body composition
observed in the inactive group were driven by increases in
visceral adiposity (45; Fig. 1). Clearly, the gain in body mass
and adiposity observed in the inactive comparison group was
due to an imbalance of total caloric intake over energy expenditure that accrued over the 6 mo.
Other large contemporary clinical trials of exercise training
in sedentary populations have also observed evidence of metabolic deterioration in the sedentary comparison groups (8, 9).
In the Health Benefits of Aerobic and Resistance Training in
individuals with Type 2 diabetes study (8), the sedentary
control group had an increase in hemoglobin A1c levels, which
was severe enough to prompt the data and safety monitoring
board to advise stopping randomization into this sedentary
control group (8). This deterioration in insulin sensitivity was
attributable to 9 mo of continued physical inactivity, as there
was no evidence of changes in physical activity, caloric consumption, or cardiorespiratory fitness over the control period.
In the Dose Response to Exercise in Postmenopausal Women
(DREW) study (9), metabolic deterioration was not observed
over the course of this sedentary control period; however, this
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METABOLIC DETERIORATION OF THE SEDENTARY CONTROL GROUP
Table 2. Changes in body composition after 4 mo of
continued physical inactivity in sedentary individuals
Mean change in body mass in kg (SD)
0.91 (0.14) P value ⬍0.0001
Mean change in waist circumference in cm
(SD)
0.66 (0.19) P value ⫽ 0.0008
Mean change in percent body fat mass (SD) 0.37 (0.13) P value ⫽ 0.006
SD, ⫾ SD. Findings are from an analysis of STRRIDE-AT/RT.
J Appl Physiol • VOL
ration associated with continued physical inactivity in the short
and medium terms (on the order of weeks and months, respectively) is clear. As a result, greater consideration now needs to
be made about the ethics of mandating that participants remain
physically inactive within the context of physical activity trials.
In addition, because the efficacy of physical activity over
inactivity is well established, further studies testing this question add little additional knowledge to our understanding of
physical activity and health. Instead, greater emphasis should
be placed on comparative effectiveness research, which avoids
the use of physically inactive control groups and allows for
comparisons of the effects of different types of physical activity regimens (varying by exercise modality, intensity, duration,
and frequency) on various health outcomes (17). Such studies
will help advance our understanding of the optimal physical
activity regimens needed to maintain health and treat disease.
MINIMUM AMOUNT OF PHYSICAL ACTIVITY NEEDED TO
PREVENT METABOLIC DETERIORATION
Somewhere between the amounts of physical activity related
to metabolic deterioration and metabolic improvements, there
is a minimal level of physical activity required to maintain
stable cardiometabolic health. This concept was first proposed
in 1954 by Mayer et al. (28), who demonstrated that a minimal
amount of physical activity was required to maintain appropriate appetite (food intake) regulation and stable body mass in
rats (Fig. 2). Above this minimal level of physical activity,
decreases in activity levels are matched with decreased food
intake, resulting in stable body mass, but below this minimal
level of necessary physical activity, further decreases were met
with an increase (instead of a decrease) in food intake, resulting in increased body mass. The importance of physical activity on weight management is often dismissed as inferior to that
of diet; however, calorie-equivalent amounts of reduction in
diet or increases in physical activity result in similar amounts
of weight loss (37). Also, physical activity is an especially
important factor in preventing a regain in body mass and
visceral adiposity after diet-induced weight loss (18). Finally,
increasing physical activity may be more effective at selectively losing adipose tissue (and maintaining lean body tissue)
than is diet alone (37).
Several observational studies have demonstrated the importance of regular physical activity for weight maintenance (12,
Fig. 2. “Mayer’s Hypothesis”—The critical physical activity concept. A
critical minimal amount of physical activity exists for individuals, below which
there is weight gain. Above this critical minimal amount of physical activity
there is weight stability, until a critical maximal amount of physical activity is
reached, after which there is weight loss.
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lack of deterioration was likely due to an observed increase in
physical activity over the control period. At months 1, 2, 3, and
4, the inactive comparison group had a statistically significant
greater number of steps than one or more of the exercise
training groups. As a result, the true effects of continued
physical inactivity could not be adequately evaluated in this
study.
Evidence of short-term cardiometabolic deterioration was
also replicated in the second STRRIDE study (29). This study
was designed to compare the effects of aerobic training (AT),
resistance training (RT), and combined aerobic and resistance
training (AT/RT) on cardiometabolic risk factors in a population of sedentary, overweight to obese men and women
(STRRIDE-AT/RT). In this study, all subjects (n ⫽ 214)
underwent a 4-mo control period of continued physical inactivity before being randomized into one of the three exercise
training groups. While the cardiometabolic changes over the
control period have yet to be fully explored, preliminary
analyses suggest similar findings as those observed in
STRRIDE: the control group in STRRIDE-AT/RT had increases in weight and measures of adiposity (Table 2). While
the absolute changes in metabolic parameters related to physical inactivity in these studies were small in size, they were
impressive in that they developed over periods of only 4 – 6 mo
and, if extrapolated over the course of years of continued
physical inactivity, would lead to very significant metabolic
deterioration.
Extrapolating our data over 10 yr suggests that continued
physical inactivity would result in a 40-cm increase in waist
circumference in subjects aged 40 – 65 years old; however,
these estimations assume individuals do not modify their dietary or physical activity patterns over time. Based on a
cross-sectional analysis of body composition across 10-yr age
groups of a heterogeneous population (both physically active
and inactive), increases in waist circumference over time
would be expected to be smaller (1–5 cm every 10 yr in
subjects aged 20 –70 yr; Ref. 14). In a 20-yr longitudinal
analysis of the Coronary Artery Disease in Young Adults
study, physically inactive subjects (initially 18 –30 yr old)
increased their waist circumference by 7 cm every 10 yr (17).
Based on these discrepant and incomplete findings, greater
investigation is needed of the magnitude of metabolic deterioration related to long-term physical inactivity, as well as its
interaction with age, secular trends, and changes in dietary and
physical activity patterns. In addition, greater investigation is
also needed to understand whether long-term physical inactivity is related to an individual’s response to a physical activity
intervention.
While the degree of metabolic deterioration attributable to
continued physical inactivity over the long term (on the order
of years) remains unclear, the progressive metabolic deterio-
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METABOLIC DETERIORATION OF THE SEDENTARY CONTROL GROUP
physical activity that is needed for the general population for a
variety of parameters. Indeed, this was the challenge tackled by
the 2008 Physical Activity Guideline committee, who extensively reviewed the scientific literature on the effects of physical activity on not only cardiometabolic parameters, but also
other outcomes such as cardiovascular disease and time to
mortality (33). Methodological differences in physical activity
measurement and study design also added to the challenge of
determining the minimal amount of physical activity required
for stability over time in various health measures. The consensus recommendation from this committee was that some physical activity is better than none, but that 150 min of moderate
intensity physical activity is needed for average improvements
in a broad array of health outcomes and that additional benefits
are obtained with additional physical activity. Interestingly,
this amount of physical activity is similar to the amount of
physical activity needed to prevent metabolic deterioration in
STRRIDE, providing a mechanistic link to the long-term
cardiometabolic benefits associated with this amount of physical activity in the long-term (41).
Several important questions still remain with respect to the
optimal physical activity regimen required to prevent metabolic deterioration (34). For instance, while vigorous intensity
physical activity results in greater improvements in cardiorespiratory fitness than does moderate intensity physical activity
(10), some evidence suggests that moderate intensity physical
activity may be more efficacious for improvements in certain
metabolic parameters, such as triglycerides, metabolic syndrome, and insulin action (20, 47). Also, based on recent
mechanistic animal studies (3, 13, 55) and observational longitudinal studies (21, 32, 50), specific sedentary behaviors,
such as prolonged periods of sitting, may be uniquely related to
cardiometabolic risk factors; these findings raise concerns
about the need for more frequent light-intensity physical activity throughout the day to break up periods of prolonged
sitting.
While questions persist regarding the optimal frequency and
intensity of exercise for a population, a large body of evidence
supports current physical activity recommendations and greater
adherence to these recommendations will result in significant
improvements to public health. Unfortunately, a large proportion of the population still does not meet these recommendations. In an analysis by Trojano et al. (51) based on accelerometer data, less than 5% of US adults perform 30 min of
physical activity per day, suggesting, by inference, that the vast
majority of US adults are experiencing some degree of metabolic deterioration due to inadequate physical activity levels.
As a result, a greater focus on improving adherence to current
physical activity recommendations, at both the individual and
population levels, needs to be made by the medical and
research communities, and society in general.
MOVING TOWARD PERSONALIZED RECOMMENDATIONS
FOR PHYSICAL ACTIVITY
Fig. 3. Changes in body mass with continued physical activity vs. different
amounts (expressed in km/wk) of exercise training regimens. A curvilinear
model is fitted to the group data indicating that the theoretical minimal exercise
amount required for weight maintenance (x-intercept) is ⬃12.0 km/wk. Findings are from an analysis of STRRIDE (46). The equation for the curvilinear
model is y ⫽ ⫺0.004x2 ⫺ 0.04x ⫹ 1.03 and the R2 ⫽ 0.995. The equation for
the linear model was y ⫽ ⫺0.14x ⫹ 1.3, with an R2 ⫽ 0.938.
J Appl Physiol • VOL
Current recommendations have established physical activity
goals that will curb the metabolic deterioration related to
physical inactivity in the population; however, there is great
variability in the physiological response to the identical physical activity exposure for any given individual (7). A similar
variability will be observed with respect to the pathophysio-
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24, 39); however, debate remains over the amount of activity
that is needed for this goal. In 2002, the Institute of Medicine
(IOM) issued a report recommending 60 min of moderate
intensity physical activity every day to maintain weight based
on findings from cross-sectional analyses of doubly labeled
water studies (19); however, since the IOM report, the claims
drawn from these analyses have largely been refuted due to a
lack of longitudinal study design to support a causal relation, as
well as similar physical activity amounts observed in normal
weight, compared with overweight or obese subjects (6).
Based on findings from STRRIDE, 6 mo of continued
physical inactivity results in ⬃1.0-kg gain in body mass,
whereas 6 mo of a low amount of exercise training (⬃19
km/wk) results in ⬃1.0-kg loss in body mass and a high
amount (⬃32 km/wk) results in 3.0-kg loss of body mass (46).
A curvilinear line can be fitted into the data that suggests that
⬃12.0 km/wk (8.0 miles) of physical activity is required to
maintain body mass (48; Fig. 3)
While understanding the amount of physical activity needed
to prevent weight gain is important, so is understanding the
amount of physical activity needed to prevent the deterioration
of other metabolic parameters. However, the amount of physical activity needed varies depending on the particular variable
being considered. For instance, extrapolating from the
STRRIDE findings with a best-fit curvilinear line, between 11
to 19 km (7 to 12 miles) of physical activity are needed to
prevent worsening of LDL, HDL, and visceral fat parameters(22, 46, 48). It is also important to note that amounts of
physical activity required to improve cardiorespiratory fitness
are not good determinants for the amount of physical activity
needed to prevent metabolic deterioration in sedentary individuals. For instance in the DREW study, small amounts of
physical activity (4, 8, and 12 kcal·kg⫺1·wk⫺1) were sufficient
to result in a dose-response increase in cardiorespiratory fitness
(when measured as a percentage of baseline levels in this
highly detrained and unfit population), but none of these
amounts of activity were sufficient to reduce blood pressure in
hypertensive women (9).
Considering the variable amounts of physical activity
needed to prevent various parameters of metabolic deterioration, it is difficult to determine the precise minimal amount of
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METABOLIC DETERIORATION OF THE SEDENTARY CONTROL GROUP
mendations will remain an important approach to prevent
weight gain and metabolic deterioration. As a result, greater
consideration and assessment of caloric consumption is needed
to provide optimal physical activity recommendations for individuals.
Another important consideration in maintaining energy balance is the amount of time spent performing light-intensity
physical activity, sometimes referred to as nonexercise activity
thermogenesis (NEAT; 27). A large proportion of an individual’s daily physical activity energy expenditure is determined
by NEAT, and this amount is highly variable and influenced by
multiple things such as an individual’s age, occupation, built
environment, and leisure-time activities (27). Better accounting
for nonexercise physical activity patterns will allow for more
optimal personal physical activity recommendations, as those
individuals with particularly low amounts of NEAT will require additional physical activity beyond levels currently recommended for the population to maintain energy balance if no
adjustments are made in caloric consumption.
In addition to energy balance, significant variability exists
with respect to how individual cardiometabolic risk factors
respond to physical activity. In the HERITAGE Family Study,
only 15.5% of the variability of the HDL cholesterol response
to exercise training was explained by demographic variables
and other quantifiable influences in multivariate regression
Fig. 4. Variability in changes in peak oxygen consumption (V̇O2 in ml·kg⫺1·min⫺1) among individuals exposed to similar periods of continued physical inactivity
(Control) and similar regimens of low amount and moderate intensity exercise training (Mild), low amount and vigorous intensity (Moderate) exercise training,
and high amount and vigorous intensity (High) exercise training. Findings are from an analysis of STRRIDE.
J Appl Physiol • VOL
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logical responses to physical inactivity. Extrapolating from
findings from STRRIDE, while eight miles a week of moderate
or vigorous intensity physical activity was adequate to maintain a stable body mass for this study population, theoretically
only 50% of these individuals would actually obtain the goal of
weight stability. As a result, much greater understanding is
desirable with respect to the factors that determine individual
differences in the degree with which metabolic factors deteriorate with physical inactivity. Similar variability is observed to
physical inactivity and various levels of activity for a number
of other physiological variables, such as cardiorespiratory
fitness, lipid levels, and glucose control (see Fig. 4 for changes
in peak V̇O2, with special reference to variability in the control
group). This heterogeneity in physiological responses to different physical activity regimens was also observed in the
DREW study (44). Clearly a better understanding of the
mediators of this variability is desirable when counseling
individuals on the response to changes in physical activity.
One obvious area of individual variability in physical activity requirements relates to the maintenance of energy balance.
People that consume more calories will require greater
amounts of physical activity to maintain a stable weight. While
reducing caloric intake is certainly critical in cases with particularly excessive caloric consumption, increasing physical
activity to amounts beyond current physical activity recom-
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CONCLUSIONS
Clinical trials of exercise training in sedentary individuals
have provided important insights into the mechanistic link
between the health effects of physical activity and inactivity in
the short term with the longer-term effects observed in epidemiologic studies. These clinical trials have demonstrated that
even short periods (4 – 6 mo) of continuing a physical inactive
lifestyle result in progressive deterioration of several cardiometabolic parameters and that only modest amounts of physical activity are required to prevent this metabolic deterioration.
Similar amounts of physical activity are currently recomJ Appl Physiol • VOL
mended for the general population and, if followed, these
recommendations will likely result in significant improvements
in public health. That being said, there exists significant variability in how individuals respond to periods of physical
inactivity or to defined physical activity regimens. As a result,
much greater work is needed to determine how much physical
activity is required to prevent metabolic deterioration in any
given individual. Progress in this area will allow the field to
move toward more personalized physical activity recommendations.
DISCLOSURES
No conflicts of interest, financial or otherwise, are declared by the authors.
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analyses (26). Recent evidence also suggests that some of this
variability may be explained by genetic polymorphisms of
cholesterol ester transfer protein (1, 2, 53), lipoprotein lipase
(1, 40), and hepatic lipase (1, 11). Also, physical activity
attenuates the adverse effects associated with polymorphisms
in the fat mass- and obesity-associated gene (FTO) on measures of adiposity (38, 52). Greater understanding of the
gene-physical activity interactions on changes in cardiometabolic risk factors will allow for more precise determinations of
the amounts of physical activity needed to avoid the metabolic
deterioration associated with physical inactivity in individuals.
Finally, another important step toward personalized physical
activity recommendations will be made with acquiring a
greater understanding of the interactions between physical
activity and medications used to treat chronic disease. While
there is a paucity of literature on this topic in general, there is
a particularly great need for understanding these interactions,
not only given the highly prevalent and increasing utilization of
these medications in the population, but also because they
share mechanisms of action or targets. For example, 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (statins) have unique and perhaps antagonistic effects on skeletal
muscle (30). While some of the beneficial metabolic adaptations to physical activity involve adaptations in skeletal muscle
mitochondria, the pathophysiology of statin-induced myopathies may be mediated by mitochondrial injury (36). Furthermore, statin-induced myopathies are more common in individuals that are more physically active (43). On the other hand,
some drugs may have a complementary or synergistic relationship with physical activity on cardiometabolic parameters. For
instance, the beneficial effects of physical activity and thiazolidinediones are at least partly mediated by their effects on
isoforms of the nuclear transcription factor peroxisome proliferator-activated receptor (PPAR) in the skeletal muscle; furthermore, exercise training and thiazolidinediones have complementary roles in improving insulin sensitivity (54).
Clearly, many known and unknown factors influence the
amount of physical activity any individual will need to prevent
metabolic deterioration. As a result, the research community
will need to strive for a greater understanding of all these
factors and attempt to integrate them to determine optimal
physical activity prescriptions for individuals. While this monumental undertaking continues, more practical efforts can also
be used to personalize physical activity recommendations.
Similar to the fashion in which drug dosages are up-titrated or
down-titrated to elicit an optimal physiological response for an
individual, physical activity prescriptions should also be modified based upon how individuals respond to interventions.
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