When you think of exercise, what immediately comes to mind?

Going out for a jog? Sweating it out on the spin bike at the gym? Or maybe loading up weight at the squat rack? What about a resistance band workout?

All of those classify as exercise, but they serve different purposes. If you want to increase your squat 1-repetition maximum by 50 pounds, a daily cycling class won’t get you there.

It’s clear that your body adapts differently to different types of exercise, but how does that happen and what does it mean for your health?

This article will break down the benefits of the two primary forms of exercise training – aerobic (commonly referred to as cardio) and resistance, or weight training.

What does aerobic exercise do for your health?

Aerobic, cardio, endurance – these terms all refer to exercise that simulates heart rate and breathing rate to provide your muscles with oxygenated blood (this differs from anaerobic exercise, which we’ll discuss shortly). The energy that powers such exercise is produced in muscle cells primarily via an oxidative pathway, meaning oxygen is required.

That explains all the heavy breathing when you go out for a run, doesn’t it?

That oxygen is delivered via blood being pumped from your heart, through your arteries, and returning to the heart through your veins.

So, it’s apparent that aerobic exercise primarily works two systems: energy production in your muscle cells and blood delivery in your cardiovascular system.

You’ll get to learn more about the energy production system soon, but first it’s necessary to understand cardiovascular adaptations that impact your health.

CARDIAC ADAPTATIONS

You probably understand that your heart is a muscle, responsible for contracting and relaxing, day in and day out, to pump blood through your body. In that way, it’s a totally unique muscle.

As long as you live, your heart never, ever gets a break. That’s pretty amazing when you realize that all your other muscles get to rest on and off throughout the day. But you can take steps to make sure your heart is up to the challenge.

As an analogy, consider two people: a professional football player, and a high school track athlete. Each is given a 10 pound dumbbell and told to perform 70 arm curls per minute, until their arm gives out.

You’d expect the football player to last a lot longer than the high school athlete – his curls are more efficient since he does them several times per week in the gym, and he’s stronger.

Similarly, aerobic exercise trains the heart to be stronger and more efficient at circulating blood. With aerobic exercise, the chamber of the heart (left ventricle) that pumps blood to the rest of the body literally gets larger and squeezes out more blood per pump, which means its stroke volume is increased.

This results in an improved capacity for cardiac output, which is the quantity of blood pumped by the heart per minute.

If you’ve heard of hypertrophic cardiomyopathy (enlarged heart), it may seem counterintuitive that a large left ventricle muscle is a beneficial adaptation to aerobic exercise. But, important characteristics differentiate an enlarged left ventricle due to healthy aerobic exercise training and one resulting from disease.

A strong, efficient heart is exactly what you want in order to live a long and healthy life. To understand why the left ventricle’s output, and stroke volume in particular, is such a big deal, consider patients with chronic heart failure.

These patients’ hearts pump out less blood per heartbeat than normal, meaning their stroke volume is reduced. Exercise rehabilitation aims to increase the amount of blood they pump with each heartbeat, and it works.

If your heart is bigger and stronger, pumping more blood per beat, it doesn’t have to beat as rapidly. That’s why you often hear of elite endurance athletes with resting heart rates in the 30’s and 40’s. This is more important than it may seem: lower resting heart rate is associated with reduced risk of cardiovascular disease.

All these cardiac adaptations are aided by an increase in blood volume that occurs with aerobic exercise training. Without getting too technical, the expanded blood volume improves the heart’s contractility and filling capacity, allowing it to pump more blood per beat.

Although the heart is a different type of muscle than what’s in your arms or legs, it’s still subject to a related function. It contracts in order to move blood throughout the body. In addition to making it stronger and more efficient, you can also lighten the heart’s load by decreasing the resistance it faces.

VASCULAR ADAPTATIONS

Each time the heart beats, blood shoots out from the left ventricle into a big artery called the aorta, and then flows into a vast network of vessels branching off. Every artery in the body provides resistance to the blood flowing through it which the heart pushes against.

The resistance provided by arteries is variable, though. Aerobic exercise training reduces the heart’s workload by reducing arterial stiffness.

When you perform aerobic exercise, your heart rate increases, pushing more blood through your arteries than at rest. The inner wall of your arteries feel the increased blood flow, and through a series of mechanisms, causes your arteries to widen.

As you train and your arteries experience this regularly, they become more effective at expanding to accept each rush of blood with each heartbeat, every day. If you don’t regularly do aerobic exercise, your arteries never experience this stretch and they literally stiffen up.

It’s much harder for your heart to pump blood through a stiff tube than a stretchy one – imagine forcing water through a steel pipe versus a rubber tube.

If the upstream cardiac effects of improved arterial health aren’t enough to convince you exercise is important, consider this: increased arterial stiffness is associated with coronary artery plaque development, the stuff that causes heart attacks.

Aerobic exercise also impacts your vascular system by promoting capillary growth. Capillaries are the microscopic vessels where oxygen diffuses from red blood cells to muscle (and other) cells.

Aerobic exercise requires increased oxygen delivery to the muscle to produce energy, so your body grows more capillaries to be able to better handle the energy demand. It does so by stimulating a molecule called vascular endothelial growth factor, and it’s never too late to benefit: older individuals have a similar response as younger.

METABOLIC ADAPTATIONS

Along with cardiovascular adaptation, aerobic exercise substantially impacts your muscles’ energy production system. Once blood delivers oxygen to the muscle cells, they still have to use it to produce energy that powers all the exercise you’re doing

As mentioned earlier, aerobic exercise primarily relies on oxidative energy production. This takes place in tiny energy generators within cells called mitochondria. Aerobic exercise also relies to a great extent on breaking down fat molecules for energy, which can only happen within mitochondria.

Consequently, aerobic exercise training drastically improves your muscle cells’ ability to burn fat by generating more mitochondria and improving their functionality. Particularly in the hours following each training session, your body burns more fat than usual.

Along with a greater quality and quantity of fat-burning machinery, aerobic training can increase your resting metabolic rate, resulting in more calories burned each day.

High intensity aerobic exercise also increases your excess post-exercise oxygen consumption (EPOC), resulting in increased calorie burn after training sessions in addition to what you burned while exercising. While the extra expenditure may seem minimal at around 10 – 20 extra calories per hour, the cumulative effect could impact your long term body composition goals greatly.

Over time, your body will adapt to the training you do. To maintain EPOC as you gain fitness, make sure to gradually progress your training intensity.

MUSCULAR ADAPTATION

The cardiovascular and metabolic systems get all the attention when it comes to aerobic exercise, but your muscles also adapt as a result of aerobic training.

Muscles are made up of several fiber types. Aerobic exercise training primarily influences type I fibers, also known as ‘slow-twitch’ fibers.

They get the name from the proteins within that are responsible for their contractions. Relative to type IIa fibers (‘fast-twitch’), type I fibers contract more slowly but have a far greater capacity to contract over and over, for a sustained period of time. Perfect for aerobic exercise like running, cross-country skiing, and cycling.

It should come as no surprise, then, that aerobic training results in primarily hypertrophy of type I muscle fibers. They adapt by adding more ‘slow-twitch’ contractile proteins.

SO WHAT?

No matter who you are, all the adaptations you just read about are relevant to you. They are exactly what fuel endurance performance and cardio-metabolic health alike.

How? You saw that these adaptations reduce the risk of conditions like heart failure, heart and vascular disease, and chronic metabolic disease. But one factor ties them all together nicely.

Cardiorespiratory fitness is strongly associated with risk of mortality from all causes. Cardiorespiratory fitness is measured by your maximal oxygen consumption during an exercise stress test (it’s also commonly known as VO2max).

VO2max is such a strong indicator of your health and mortality risk because it’s determined by all the beneficial adaptations to aerobic exercise discussed so far: your heart’s ability to pump oxygenated blood through arteries, blood supply at the capillary level, and mitochondrial machinery to use the oxygen your muscles receive.

Metabolic diseases are particularly associated with mitochondrial defects, which can be prevented or reversed with aerobic exercise. And the muscular adaptation mentioned? That allows you to progress your aerobic training and continue making cardiovascular and metabolic improvements.

What does resistance exercise do for your health?

Resistance exercise is training that progressively overloads your muscles to promote muscle strength, power, anaerobic endurance, and size.. Traditional weightlifting, bodyweight exercises like pushups and pullups, and resistance band exercises are all examples of resistance training meant to make your muscles bigger, stronger, more powerful, and more functional.

Specific adaptations to resistance training begin within the muscle cells. However, you’ll still get systemic benefits ranging from muscle growth to cardiovascular benefit.

To gain a deeper understanding of the whole-body performance and health effects of resistance training, read into how resistance exercise affects muscle at the microscopic level.

MICRO ADAPTATIONS

The point of resistance training is to make muscles function more effectively. This all starts with the contractile proteins that act to control muscle shortening and lengthening.

When you do resistance exercise, some of those proteins get yanked apart. That, along with the stress your muscle experienced, is the stimulus for your muscle to rebuild – this time bigger, stronger, or more powerful than before.

After resistance exercise, your muscle synthesizes proteins (this is aided by nutritional stimuli i.e. protein consumption). Special cells known as satellite cells also spring into action to help build up the broken down muscle. They normally lie quietly adjacent to muscle cells, but resistance exercise tells them to get to work.

Satellite cells combine with the muscle cells that were strained and damaged during your resistance training session. In doing so, they lend their molecular machinery to support protein synthesis that leads to muscle hypertrophy.

As for fiber type, resistance training with loads over 60% of your 1-repetition maximum results in hypertrophy of primarily type IIa fibers (‘fast-twitch’). These fibers, compared to type I fibers, are capable of rapid contraction with high force, but tire more easily. That makes sense – your body adapts to meet the challenge it’s presented with (e.g. lifting a difficult weight for eight repetitions).

Not only do type IIa fibers have the greatest growth, but a third fiber type called IIx (formerly known as IIb) can be converted into IIa fibers. Type IIx fibers, before they’ve converted, have properties that blend those of type I and type IIa.

These micro- level adaptations matter to athletes and the general population alike. When you make measurable gains in muscle mass, strength, or power, you can thank the protein synthesis and fiber-specific adaptations that occurred within your muscle cells.

MACRO ADAPTATIONS

All those microscopic adaptations add up to cause changes that are easier to grasp. Resistance training at the proper intensity leads to measurable muscle hypertrophy.

Strength improves in part due to changes to the neuromuscular system. Control over your muscles is typically a balance between competing neural signals. Some of those signals tell the muscle to contract, while others prevent contraction.

Regular resistance training can reduce neural inhibition that normally limits the strength and/or endurance of the muscle.

Muscle accounts for roughly 20% of resting energy expenditure, so it’s impact on calorie burn and body composition is meaningful. Not only that, but you can’t increase the mass of most of the other organs that account for resting energy expenditure, like the liver, heart, brain, and kidney. Muscle is different because it hypertrophies, growing larger and expending more 

calories.

By packing on muscle, not only do you increase strength, power, and function, but you also raise your basal metabolic rate. And by doing so, you’ll see an increase in your metabolism and an improvement in your health.

CARDIORESPIRATORY ADAPTATIONS

If you’ve ever lifted weights or done resistance exercise, you’ve probably felt your heart pounding with the exertion.

Does that mean you’re getting cardiovascular and metabolic adaptations like you would with aerobic training?

Maybe not.

Resistance exercise does raise your energy expenditure. But it does so differently, and to a lesser extent, than aerobic exercise.

The primary fuel sources for high intensity weight lifting are phosphocreatine and ATP, two molecules that are readily available to power short, quick movements like weight lifting. This means resistance exercise trains your energy production systems, but has less impact on the aerobic energy systems.

When it comes to cardiovascular benefits, isometric resistance training can help you maintain healthy blood pressure. Isometric exercises involve contracting your muscles against resistance that doesn’t move, so it’s different than traditional resistance training.

To get the best of both worlds, some people turn to circuit training, which consists of lifting higher repetitions of lighter weights and moving rapidly between exercises, to maintain a higher heart rate and metabolic demand.

SO WHAT?

Large-scale and small-scale alike, these adaptations to resistance exercise impact your health and physical performance. Your muscles carry you through the day, allowing you to do everything from climbing the stairs at work to picking up your two-year-old for a hug when you get back home. If you’re an athlete, they are essential to performance during training and on game-day.

Breaking down and building up muscle through resistance training is essential to maintaining function as you age. Loss of muscle mass even threatens some people’s capacity to live independently.

In terms of body composition, muscle mass is not only an important component to maintain, but it also contributes to your resting metabolism, helping you maintain a healthy energy balance.

By now, you should have a solid understanding of what aerobic and resistance training each do. To recap, aerobic causes the cardiorespiratory system to adapt. It maintains heart function and health, and keeps your energy metabolism system running. Resistance training still benefits the cardiovascular system, but its role is mainly for muscle gain and function.

Is aerobic, or resistance training better for fat loss?

So, aerobic and resistance exercise each have enough benefits to be heralded as a ‘miracle drug’ by some medical groups.

But what if your specific goal is to shed a few pounds of fat? Should you focus on aerobic or resistance training?

The answer is straightforward: aerobic. Read on to find out why.

It’s true that resistance training that increases your skeletal muscle mass will boost your basal metabolic rate, causing you to burn a few more calories each day. But, the most substantial increase to your calorie burning will be thanks to the exercise you do.

On the other hand, aerobic exercise boosts the muscle’s fat-burning machinery, the mitochondria. It also burns substantially more calories per session than resistance exercise, and results in study indicates RT had greater EPOC than steady state aerobic so only HIIT was similar or greater.

Most importantly, if you have fat to lose, aerobic is probably most efficient for both visceral and subcutaneous fat loss.

Visceral fat is the nasty, metabolically active fat your body stores in your abdominal cavity, surrounding the internal organs. It’s what you should care about if you’re concerned about your risk for cardiovascular disease, diabetes, or a host of other health problems. Subcutaneous fat is what your body stores just under the skin. It’s more visible, but less detrimental to your health.

If you’re truly going after substantial fat loss, and dedicated to keeping it off, you need to strongly consider adding a dietary component to your weight loss plan.

To illustrate, take a goal of 1 pound of weight loss per week. That equates to a 500 calorie deficit per day, or roughly one hour of moderate intensity aerobic exercise every day of the week. Not only is that unlikely to be sustainable, but you’d probably injure yourself by taking on an intensive program like that.

To reach the same calorie deficit, you could eliminate 500 calories from your daily energy requirement. That could mean simply reducing your portion size at each meal.

The good news is that diet and exercise go hand-in-hand for losing weight. By obtaining a calorie deficit from both exercise and diet, neither has to be as drastic. Instead you can combine 250 calories of exercise with a 250 calorie reduction in energy intake each day.

Combining diet and exercise even improves your chances at maintaining your weight loss over time, and keeps up your metabolism despite your energy deficit. If that isn’t enough for you, it may be better than either exercise or diet alone for reducing visceral fat.

Pulling It All Together

Before reading, you probably already knew that exercise is key to your health and body composition. Now, you should have a much clearer idea of why that’s the case.

Put simply, aerobic and resistance training each tell your body to adapt in different ways. That’s exactly why they can’t be substituted for each other. If cardiovascular health is your primary concern, resistance training won’t result in the same blood vessel adaptations you would get from cycling training. Likewise, you can’t expect to gain much strength or power from running. It won’t tell your type IIa muscle fibers to hypertrophy!

Both are important for a healthy body composition. While aerobic is best for losing fat (especially visceral fat), resistance builds muscle that keeps you functioning all day long.

All the adaptations you’ve read about – from strengthening your heart and making your arteries more flexible, to synthesizing muscle contractile proteins – directly impact your health and physical function. What medications can you name that have such a widespread and profound impact?

Maybe those scientists who call exercise a miracle drug are right.