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How Should You Train?

Updated: Nov 17, 2020

Learn about cellular energy systems, how different stresses lead to different adaptations, and why you need them all...


When it comes to finding out the truth about how you should physically train, there is a lot of misinformation out there. Should you stay in the fat-burning or carb-burning zone? Should you lift high-rep and low-weight, or low-rep and high-weight? Is strength more important than cardio or vice versa? The list goes on and on… The truth is in the physiology of how our mitochondria, cells, and organ systems work, and this does take a bit of explaining, but there are some things we do know. Your body has an amazing capacity to adapt to the training (or de-training) stimuli you are giving it, and it’s often quality over quantity that will get you results (whether it’s aerobic endurance, muscular strength, or muscular size) more efficiently. Everyone is starved for time, so let’s take a look at how we can make the most of our physiology to get the adaptations we want.


First, let’s lay out some basic definitions. When we are talking about physical adaptations to training, we are generally referring to changes in the musculoskeletal, myofascial, and cardiovascular and cardiorespiratory systems, for the most part. Strength usually means the ability of a muscle to exert maximal force against resistance (cite). Conditioning is a little more vague and can mean both cardiovascular conditioning (aka “endurance”) and muscular conditioning (high-rep work). Strength is a component of all activities, but good examples include weight-lifting, body-weight training, and discus/shot-put events.

Let’s go over some properties of skeletal muscle. We already defined strength, where the goal is maximal force output; the ability to contract and generate force underlies all movements, hence it is very important. Power is how much work/energy a muscle can exert per unit time (such as explosive movements), where the speed of the contraction is key. Good examples of power sports include power-lifting, wrestling, and gymnastics. Muscular endurance is the ability of a muscle to do repetitive contractions for a longer period of time, where the goal is to resist fatigue.

The heart, or myocardium, is itself a muscle, and repeated use will result in thickening and strengthening of the ventricular walls (Hellsten and Nyberg 2016). We generally refer to cardiovascular capacity as aerobic condition. The heart is an endurance organ by the very nature that it beats the entire time you are alive! Aerobic conditioning is the same as endurance, where the goal is to resist fatigue on a whole-body and central nervous system level (rather than just the local muscular level). This will be trained to some degree no matter what type of physical work you are doing, but particularly if you are running, cycling, swimming, and other rhythmic activities that involve recruiting large muscle mass (which is directly proportional to cardiovascular work) for a long period of time. And the heart will obviously be involved in short bursts of activity as well, although the dynamics of blood flow and pressure will be different than in long duration activity.


As mentioned earlier, your body has the capacity to adapt in different ways to different stimuli. If you are going to be sitting at work and then lying on the couch, your body is going to adapt to the disuse by de-training at a muscular and cardiovascular level. If you start to be active, some of the same general adaptations will happen from any activity, but at a certain point of progression your body will specialize. The variables include the volume of work, the intensity, the frequency, the muscle groups used, if the contractions are concentric (shortening with contraction) or eccentric (lengthening with contraction), and if the contractions are more isometric (static) or rhythmic movement.

Beneath all of these variables are unique cellular energy systems that run the show, with the goal of converting food molecules into adenosine triphosphate (or ATP), a usable form of energy. The majority of our energy is made from oxidative phosphorylation. This pathway is in the electron transport chain of the mitochondria (powerhouse organelle inside cells). It is efficient, requires oxygen to work, and can go for a long time – it’s tapped into from aerobic endurance activities (i.e. walking, running, etc.). Type 1 slow oxidative muscle fibers use predominately this pathway.

For activities that are moderately intense at the muscular level and are limited to a few minutes, the cell uses a different pathway for more immediate energy, aerobic and anaerobic glycolysis. Oxygen may be present but isn’t needed here, and not as much ATP is produced. This is tapped into during moderately intense weight lifting and middle-distance racing like 400-800m running. Type II a fast oxidative and type II b fast glycolytic muscle fibers use predominately this pathway.

When all-out speed and power are needed, the cell uses the phosphagen system. This uses creatine phosphate stored in the muscles to donate phosphate to ADP to resynthesize ATP. It’s very limited but very fast and doesn’t use oxygen. This is tapped into for one-rep-max lift or power lift, or a running sprint. Type II b fast glycolytic fibers use predominately this pathway (in addition to the previous system) (Baker et al, 2010).

This can be confusing because in reality, all of these systems are used all the time, but in varying proportions (Human Kinetics). There are health benefits to engaging in all of these systems to be well-rounded, from both a fitness and metabolic standpoint.


Cardiovascular conditioning, utilizing primarily oxidative phosphorylation, is the most important because your body’s primary job is to move and circulate its blood and lymph to feed cells nutrients and remove their waste products. A strong heart muscle and responsive blood vessels that can dilate and relax are also key to performance, basic functionality and longevity. And from an energy balance perspective, this is the system capable of doing the most work and burning the most calories over time, which is very important to those trying to lose weight.

Intermediate to very intense bursts of activity, utilizing primarily the glycolytic and phosphagen systems, are also important. Activating these systems allows access to some of the hormonal pathways necessary for muscle growth, testosterone and growth hormone maintenance, and insulin sensitivity (Kraemer and Ratamess 2005). And if you don’t use it, you lose it – the physical stress placed on the muscle, bone and fascia from lifting and other intense movements is the very signal to your cells to keep those tissues developed and healthy, which is critical past middle age when these tissues are more likely to atrophy.


Now to come full-circle back to the subject of “conditioning”, let’s add some clarification. Both the skeletal muscles and the heart muscle are “conditioned” from physical activity, to varying degrees depending on the intensity and type of movement. Sometimes both systems are targeted, as in high-intensity interval training (HIIT), boot-camp, and any high-energy full-body activity. Sometimes the heart is the primary target and the large lower body muscles are a vehicle for this work, as in endurance running, cycling, swimming, rowing, etc. And other times the primary target is the muscles themselves, as in weight lifting. This can be further differentiated into muscular endurance conditioning (high reps and low percentage of max lift), hypertrophic (“size”) conditioning (high total volume and intermediate percentage of max lift), and pure strength training (low reps, high percentage of max lift).


The unsung contributor to of all of this is the nervous system. It interfaces with the heart, blood vessels, muscles, and fascia, expertly coordinating their functions. The nervous system is involved with everything, but has a special role in the development of pure strength (Folland and Williams 2007), so let’s explore that…

When you are beginning a muscular strength and conditioning program, any combination of reps and load will cause adaptation for both size and strength. As you progress in your fitness, a certain base will have been created and further specialization will be required to tell the cells to go in one of two main directions: size or strength. Muscle hypertrophy is created by giving the muscle a lot of work to do, with little rest, and eating enough calories to support the growth. Bodybuilders train in this way. This is functional up to a point.

Training for strength might be considered a more functional, less egoic way to train! A muscle can generate a lot of force without looking huge. To optimize strength, the nervous system is heavily recruited. The amount of force exerted by a muscle during a voluntary contraction depends on the number of motor units recruited, and the rates at which the associated motor neurons send the firing impulses (Enoka and Duchateau 2017). This sheds light on the mechanism behind muscles that are inhibited – the muscle tissue itself might be normal, but if the brain is not adequately communicating with that muscle through the motor neurons, it will be useless. Thus, from both a training and rehabilitative standpoint, strength matters.

It is also important to realize that any nervous system gains must be reinforced. You can temporarily increase a muscle’s range of motion and also it’s force generation capacity, but these will be only short-term adaptations if the regimen is not practiced, almost like a window of opportunity that needs to be passed through to truly keep going. The neural responses to strength training actually happen pretty quickly, but the structural tissue accretion that solidifies these adaptations takes a bit longer to attain (Moritani and DeVries 1979).


The take-home message here is that the body is plastic and moldable -- we can use this to our advantage if we train it to do the things we want (huge realm from physical training to psychology and beyond!). Cells respond to different stimuli in different ways, and it’s up to us to decide which physical signals we want to send to them to give them a clear direction for adaptation. The realms of health and fitness track in parallel for some time, and ultimately diverge if one overspecializes. Some people want to be a “pure endurance athlete” or “pure bodybuilder”, and although we can indeed train this way, the truth is often along the middle road. Incorporate some cardiovascular conditioning, some hypertrophy, some heavy lifts – push ALL the buttons – and you will continue to select all those complementary cellular pathways to keep working. Sure, you have goals, refine yourself a bit in a particular direction… but don’t deviate too far from the healthy, well-rounded, functional human being.

© Sabine Harrington 2020, all rights reserved.

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