As the world is mired in epidemics of chronic diseases like obesity, type 2 diabetes, and various forms of cancer, the average human today has gotten progressively more sedentary compared to previous generations. As the economies of the first world have shifted to predominantly those based on information and technology, the amount of physical labor by the average person has dropped precipitously.  Thus, unless you work in construction or another physically demanding occupation, chances are a structured exercise training program is perhaps the only real opportunity you have to engage in physical activity to counteract the ills of a modern, sedentary lifestyle.  “Sitting is the new smoking,” which intends to emphasize the importance of not becoming a victim of too little physical movement.

If you are contemplating a new exercise program, you have probably heard about all of the platitudes of exercise, e.g., reduced risk of: all-cause and coronary heart disease mortality, diabetes, hypertension, colon cancer, sarcopenia (muscle wasting), metabolic syndrome, osteoporosis, osteoarthritis, disability, depression, and anxiety.  Thus, if you could package all of the benefits of exercise into one pill, you would have the most valuable medication in human history.  With all of the benefits attributed to exercise, you may not know exactly how exercise affects our body, including our organ systems.  For this particular article, we are going to focus on the two primary organ systems that are affected by exercise:  the musculoskeletal and cardiovascular organ systems.  Of course, all organ systems are affected by exercise, both directly and indirectly through the interaction of those systems with each other.

Before we discuss the effects of exercise on these two crucial organ systems, we need to first understand the principles of overload, specificity, and adaptation as they apply to exercise training. Overload refers to stressing the body beyond what it considers to be normal, and the body’s response is to maintain homeostasis.  To do this, it must overcompensate and become stronger, so that next time the stimulus will not be as disruptive to maintaining homeostasis.  The stressors have to be increased (progression in overload) to continue to improve.  Additionally, the body is specific in its response to stressors.  Specific exercises elicit specific adaptations creating specific training effects.  What exercise or activity your perform is what will improve, i.e., bench pressing will improve bench pressing, not running.  The adaptation to the exercise is particular to the type of activity.  Thus, the body’s adaptation(s) will be driven by the goals of your exercise training program.

With that said, we can now discuss what the benefits are to your musculoskeletal and cardiovascular organ systems in response to a structured exercise training program. When you engage in a progressive weight training program, you can expect to achieve one very important benefit over most others:  strength!  Your muscles will initially increase in strength slowly as your body accommodates to the program (the first 2-3 weeks, which is more nervous system adaptation than purely musculoskeletal adaptation), then accelerate rapidly (the first 3-4 months), and then finally strength increases slowly or levels off.  The lower your initial strength, the greater you can expect your percentage of improvement as you progress over time.  In addition to strength, hypertrophy, or growth, of muscle is perhaps the most obvious result of weight training.  Transient, or temporary, hypertrophy occurs during, and for several hours after, exercise and happens because of the accumulation of fluid in the interstitial and intracellular spaces of the muscle.  Permanent hypertrophy is a result of three primary factors:  (1) increase in the number and size of myofibrils per muscle fiber, (2) proliferations in the amounts of contractile proteins (actin and myosin), and (3) to a lesser extent augmented amounts of connective, tendinous, and ligamentous tissues.  Additionally, strength and cross-sectional area of muscle fibers are strongly associated with each other, as the larger the muscle fiber the greater the number of actin myosin filaments, which allows for a more cross bridges to produce muscular force during contractions.  Weight training also creates a selective hypertrophy of fast twitch muscle fiber, resulting in an increase in the fast twitch to slow twitch ratio of fiber area.

Hypertrophy and strength can be gained simultaneously, but their increases do not occur in an entirely linear fashion. A bodybuilding routine (higher repetitions per set, less rest between sets, lower weight lifted, and greater volume of training) will ultimately produce more hypertrophy, whereas powerlifting (very low repetitions per set, maximal weight lifted, less volume of training, and more rest between sets) will produce greater increases in strength.  Finally, strength training also has a dynamic effect on bone mineral density, it reduces mitochondrial density in the muscle cell, and it increases concentrations of creatine phosphate, adenosine triphosphate, and glycogen.  As a general guideline, try to exercise all major muscle groups 1-2 times per week, 2-4 exercises per body part, 3 sets per exercise, 8-15 repetitions per set, 12-15 total sets per body part, and no more than 1.5 minutes of rest between sets.  Make sure to do at least a couple of sets of warm-up activity before engaging in work sets.

The other primary form of exercise is aerobic or cardiovascular activity. This can be done with any rhythmic activity done for a period of time, e.g., 20-60 minutes for steady state activity and 10-20 minutes for interval training.  Activities can include walking, jogging, running, sprinting, swimming, rowing, aerobics classes, cycling, and the use of hybrid movers like stair steppers, gliders, and ellipticals, among others.  The main cardiovascular and cardiorespiratory benefits of aerobic training evolved out of early cardiac rehabilitation studies several decades ago.  Since then, these benefits have been validated in thousands of studies all over the world.  As with weight training, the initial increases in cardiovascular functioning in response to aerobic training is at least partially due to improved functioning in the nervous system.  Aerobic training has consistently been found to improve aerobic power (VO₂ max = the maximum oxygen uptake), which simply stated improves the efficiency of the heart.  This means less oxygen is required to perform the same amount of work (as measured by a reduced rate of oxygen consumption for the heart muscle during exercise).  Aerobic exercise lowers resting heart rate and blood pressure, and it also causes lower heart rate and blood pressure responses during exercise as you become more fit.  Along with the improvement in aerobic power, aerobic exercise causes these cardiorespiratory system increases:  heart size and volume, blood volume and total hemoglobin, stroke volume (at rest and during exercise), the amount of oxygen extracted from blood, lung volume, and maximum cardiac output.  Musculoskeletal system improvements in response to aerobic exercise include:  the number and size of the mitochondria in the cells, the storage of myoglobin and triglycerides in the cells, oxidative phosphorylation, and the tensile strength of the connective tissues.  Improvements due to aerobic exercise that have a direct effect on cardiovascular health include an increase in HDL cholesterol and decreases in total and LDL cholesterol.  Aerobic training also typically increases slow twitch fiber, unless you are doing high-intensity interval training, such as 100 meter sprints.  Bone mineral density is not known to be significantly impacted by aerobic exercise, particularly for light or non-weight bearing activity, such as walking and swimming, respectively.  Aerobic exercise confers benefits optimally by doing at least 20 minutes at moderate or high intensity most of the days of the week.

Regardless what type of exercise you like, you should optimize your health by incorporating both weight training and aerobics to achieve the benefits of both. You can incorporate both forms of exercise during the same session, and it is best to do weight training before aerobic activity to maximize the use of glycogen stores in the muscles.  You should also endeavor to exercise every day to gain the most health benefits.  In other words, the more you exercise, the more benefit you will derive.  So, get active and reap the benefits of an exercise program that includes lifting weights and incorporating aerobic activity, too.