Most people go through their physical training life thinking that running and big muscles are completely incompatible. The popular images that support this misconception come from media reports that show musclebound bodybuilders only engage in lifting heavy weights and seemingly bone-thin marathon runners doing nothing but run long distances in incredibly fast times. 

Science however looks beyond media images and what we know of how the body works argues against both these stereotypes. Before we go any further it’s worth examining however just what it is that makes both the above popular images work. Bodybuilders are, indeed, heavily muscled and generally avoid running and marathon runners don’t generally go for lifting weights as part of their regular training. The reason for this lies in specialization. 

If we are doing bodybuilding our measure for success lies in size. We really need to build our muscles to the largest possible size to feel we are succeeding in our sport. So, size rather than strength or speed or explosiveness is what we strive for and running is not something that would help us gain that. Similarly, marathon runners are interested in endurance. The measure of success of their chosen sport is long distances run fast. They have zero interest in having large biceps or heavily built pectorals and having massive quads, in this case, is something unnecessary. 

Specialization in a particular sport determines the focus of training for that sport. At that point the body’s adaptive responses kick in and what we get in terms of visual, physical transformation is exactly what we would expect: bodybuilders with massively heavy muscles and marathon runners with thin, stringy bodies. 

Body Adaptation and Performance Goals

As long as particular, very specific, performance goals are being aimed for, specialization makes perfect sense. No one takes up bodybuilding, for instance, in order to have mediocre sized muscles. No one starts to do marathon running so they can just run 3km. The moment we stop chasing a sports-specific goal however and begin to look at the body’s capabilities as a performance goal, the picture changes drastically. 

A systematic review of fourteen different studies on cardio training showed that when running or cycling are added to the mix of other training (what is known as concurrent training) the body’s adaptation mechanism only increases strength without any appreciable loss of muscle mass. Concurrent training however does inhibit the hypertrophy response in muscle tissues. In other words, you can run and lift and do bodyweight exercises without worrying that you will lose muscle mass. Your strength will increase. But if you are aiming for size and simply want to grow large muscles then that combination is not going to help you do it. 

This makes sense at performance level too. The body adapts to what is being asked of it. If your goal is to go faster and faster over long distances, for example, then the body will shed every gram of excess muscle it has because it is expensive to transport and expensive to maintain. If however you want to be able to run long distances and still look like a gladiator, the body will adapt for that too. The classic example of that kind of adaptation at a professional level are decathletes who excel at ten different types of sports that appear to be countering each other from a physical point of view (like 100m sprints and 1,500m runs) and still look like they could physically walk through walls. A similar point is made by Super Middleweight World boxing Champion, Carl Froch, who says that professional boxers do so much running that they may as well be semi-pro runners. Not only does it not affect their muscle mass, it helps them maintain sharpness in the muscle they carry.   

While long distance runners and cyclists usually never even consider adding strength training to their routines, studies have shown that resistance training improves joint stability and muscle density and can seriously help improve endurance for long distance runners. Those who do resistance training could also benefit with greater muscle density and improved range of motion and strength if they added some cardio training (running or cycling) to their fitness routine. 

Finally from a purely functional strength point of view running offers one of the easiest ways to build a robust platform to maintain fitness levels. 

Summary

Contrary to popular belief regular cardio training, whether that is running or cycling can help those who train with weights experience improved gains in muscle strength and endurance. Runners and cyclists who add resistance training to their fitness routines also experience marked increases in their performance both in terms of explosive power and endurance. Running and other cardio activities, like cycling, can help us build muscle as long as excessive muscle growth is not our primary goal. 

Sources

The Effects of Exercise Training in Addition to Energy Restriction on Functional Capacities and Body Composition in Obese Adults during Weight Loss: A Systematic Review
Explosive-strength training improves 5-km running time by improving running economy and muscle power
High-intensity interval training increases SIRT1 activity in human skeletal muscle.