How strength training changes your muscle/fat percentage

Want to get stronger? Well, you’ll need to start increasing your amount of lean muscle mass – and to do that, you’re best off lifting heavy rather than running long. We explain exactly what body composition is, why you should eat more if muscle is your goal and why it’s OK to discuss body fat Vs muscle percentage.

Most of us come to strength training for one main goal: to get stronger. We want to build lean muscle mass, lift heavier and last longer. But what does that actually mean on a physiological level and besides being able to perform better, what physical changes happen to our bodies when we strength train?

Our bodies are made up of many different elements but for the purposes of training, we’re concerned with the balance between body fat and lean muscle mass. This isn’t about fat shaming; body composition is simply about looking at how much muscle we have and what kinds of energy storage our body carries to support that muscle. While most of us won’t have the tools to measure these things accurately (and really, there’s no need to get that geeky about it), thinking about body composition and lean muscle mass – the amount of skeletal muscle you’ve got – allows you to concentrate on building strength without the hangups that come with using size or weight as measurements. It’s not just that the more muscle mass you have, the lower your body fat percentage; according to a 2018 review published in the journal, Annals of Medicine, high muscle mass can make you stronger, energised, more mobile and more healthy overall. 

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“Body composition matters because if we have a healthy balance between muscle and fat, we are more likely to have more energy, higher levels of self-esteem, confidence and it will also mean we are at less risk of heart disease, diabetes and other issues,” explains strength coach and founder of FeelFit. “That healthy balance increases longevity – after all, we’re all aiming to train for life! But that really does mean getting a good balance between the two; some body fat is healthy especially for us women as being too lean can lead to serious health issues.” 


Our muscles account for up to 60% of our overall bodyweight – so the more lean muscle you have (i.e the stronger you are), the heavier you’ll be. As we get older, we naturally start to lose muscle; once we hit 40, that process speeds up dramatically with around 8% being lost per decade. Muscles are important because they keep our organs functioning properly, help to keep our skin intact, maintain a high level of immunity and obviously have us staying strong. 

Forget the calipers or body scans, all you need to do is perform a few simple exercises every so often to check on your progress and basic mobility:

  • Can you do 10 press-ups?
  • Can you hold a plank for 30 seconds?
  • Can you do 20 reverse lunges?
  • Can you jog for 20 minutes?
  • Can you deadlift 10kg?

You might not be able to do all of these initially but as you build muscle mass, they should become a lot easier. Once you’ve mastered them, increase the intensity – going on to 20 full push-ups, a minute plank etc. Even better, why not work your way through one of our training plans and then see how much stronger you feel at the end?

Deciding what ‘strong’ looks like to you is the best way to set your goals


If you’re looking to build muscle, you couldn’t do much better than to start a strength training programme. While cardio is great for giving your heart a good workout, strength training allows you to hit specific muscles in a way that gets them growing.

“Strength training can help change body composition by maintaining or promoting an increase in muscle mass and when combined with a nutrition intervention, a decrease in fat mass. The appearance of muscle is what we often refer to as ‘looking toned’,” explains Joshua Peters, PT at SIX3NINE.

Hypertrophy tends to be the most common type of strength training for converting body fat into lean muscle mass. Our skeletal muscles are made up of bundles of muscle fibres, known as myocytes. Each myocyte contains myofibrils that allow the muscle to contract; hypertrophy is when the number of those myofibrils increases, causing the muscle to increase in strength and density. Slowly increasing the strain on muscles causes them to become damaged – which the body then repairs. That process causes the muscles to adapt by growing in size and strength.


The reason that strength training is so good for building lean muscles is that you can put that stress on particular muscles and slowly increase the load over a period of time – without other factors interfering. Running, by comparison, is a lot harder to chart; you can easily increase the weight in a deadlift by going up to the next size dumbbell but unless you’ve got a treadmill, it can be hard to know how and when you’re increasing the intensity of a run. Joshua explains: “Strength training is a far better strategy for increasing your muscle mass as it provides the necessary stimulus for your body to change. Cardiovascular training generally won’t provide the stimulus for building muscle mass. However, low-intensity cardio sessions can be used to help increase your body’s ability to recover from hard bouts of training, which in turn, can be used to aid your strength training.” He goes on to say that strength training also helps to maintain current levels of muscle mass if body fat reduction is your goal.

However, Ellie stresses that cardio plays a really crucial role in building fitness and changing body composition. “I wouldn’t say (strength training) is better than cardio – I believe that we should have both in our exercise programmes. Strength training will change your shape, yes, but having a good level of cardio fitness is also key to maintaining that balance we are all looking for. They are different but both equally as important.”

So, do we really need to think about our body composition if strength is our goal? It’s important to note that “strength” can mean different things to different people – so you’ve got to decide what being strong means to you. If, for example, your goal is to be able to do your first pull-up, then having lower levels of fat and higher lean muscle mass would help you achieve that… but if you simply want to feel strong, “then ratios between muscle mass and fat mass are less important,” Joshua says. “Most of the strongest people in the world have higher levels of fat mass but they will have an exceptional amount of muscle mass too.”

Ellie agrees: “Being strong isn’t about shape or size. We often get caught up in how strong should look and the fitness industry has forced a body ideal onto what that strength looks like, but it looks and feels different for us all.” Having said that, however, she goes onto say that body composition is important because increasing lean muscle mass can help us to feel stronger and more powerful. “Having strength as your goal is good if you are following the right programme because you’ll see both increases in physical strength and as a byproduct, changes in your shape.” 


The (slightly problematic) saying goes: abs are built in the kitchen, not the gym. The truth is, however, that nutrition really does play a huge role in the way our bodies look, feel and function, and food is often the missing factor when it comes to fuelling and benefitting from strength workouts or tweaking body composition.

Again, nutrition is something that is “individual to us all as there really isn’t one plan that fits all and it should be kept simple,” Ellie stresses. “As a coach, I often find that people are under-eating when it comes to building strength and lean muscle. The role of nutrition in changing your body composition and building strength is a very big one. We can easily get this wrong and then wonder why we are seeing results or are extremely tired.” She advises avoiding any kind of plan that advocates restricting food groups.

To increase or maintain muscle mass, Joshua says that you will need to consume enough protein. “If you’ve been training for a long time, you may also require a small surplus of calories to build muscle.” If you’re looking to reduce body fat slightly, a small energy deficit can be useful as “exercise alone is not very efficient”. That’s because often, exercise doesn’t use a lot of energy and Joshua warns that “fitness trackers tend to grossly overestimate how much energy we burn in our training sessions.” Adjustments to your daily nutrition choices will have a much bigger impact on how you perform and feel, whether that means upping your protein consumption, reducing the number of processed foods you eat every day or concentrating on eating over your five portions of fruit and veg a day to ensure that you’re getting a wide range of vitamins. 

Build more lean muscle mass with a classic kettlebell deadlift – working the posterior chain and those big powerhouse muscles like the glutes and hamstrings.

Ready to work up a sweat? Hop on over to the SWTC video library where you’ll find a range of 30-50 minute workouts, led by our very own SWTC trainers. 

Images: Getty

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