Running might not seem like the most muscle-working of activities. It works the leg muscles, but you may be surprised that it strengthens other parts of your body, too.
But exactly what muscles does running work? It sounds like it should have a simple answer. But the truth is, it’s a little more complicated than just a list of muscles.
Whatever your reason for running, there’s no denying it’s a great form of cardio exercise.
And while you’re not likely to build a lot of muscle just from running, there’s plenty more muscle activation than you may realize!
The Two Main Phases of a Runner’s Gait
To understand muscle activation during a run, it’s important to understand the two phases of a runner’s gait cycle.
Muscle activation is different in each phase. The stance phase starts when your foot hits the ground, and includes the heel-to-toe transition. When you lift your foot off the ground, the swing phase begins. These two phases repeat one after the other throughout your entire run.
The stance phase can be broken down further into five different stages:
Every part of this phase has the foot on the ground; during this phase, the foot absorbs the shock that comes with your landing.
It’s worth noting that some people land with the midfoot or the forefoot. This doesn’t change the stance phase, but it may remove a few of the stages above.
The swing phase is when your foot leaves the ground during the toe-off. As it lifts, the foot flexes and supinates, and the knee flexes to allow the foot to clear the ground properly as the leg comes forward. This phase of the gait cycle sets you up for the stance phase.
It’s important to understand that each foot is going through each stage at opposite times. When the left foot enters the stance phase, the right foot is in the mid-swing phase, and vice versa.
There’s a brief period of time when both feet are off the ground in the swing phase. This is known as the float phase. Elite runners spend about 11% more time in this phase than casual runners!
Understanding the Two Types of Skeletal Muscle Fibers
Most muscles contain both slow-twitch and fast-twitch muscle fibers. The exact mix of the two is genetically determined, but it’s around 50/50 for most people.
While each muscle contains some of both of these types of fibers, each fiber is used for a different purpose and a different type of movement.
Slow-twitch muscle fibers—known as type 1—are the most used of the two. They’re used in endurance activities—everything from standing for long periods of time to cycling for a long distance utilizes these muscle fibers.
They burn through their energy supply slowly and steadily, giving them the energy to work for longer periods of time but at a lower intensity. They run on oxygen—an aerobic energy system.
When Do You Use Your Slow-Twitch Muscles?
You use your slow-twitch muscles in almost every movement when it starts off slowly. Standing, walking, jogging, bouncing… They’re the starting point for every relaxed, low-intensity muscle movement.
Fast-Twitch Muscle Fibers
Fast-twitch muscle fibers—type 2 muscle fibers—are used for short, sharp bursts of movement. They use up their energy source much faster than slow-twitch muscles, because they expend more of it at a time in order to provide fast, powerful movements.
Technically, they’re split into two groups—type 2a and type 2b. Type 2a muscle fibers can fire with or without oxygen. Type 2b fibers don’t require oxygen to fire, which means they run on an anaerobic energy system.
When Do You Use the Fast-Twitch Muscles?
Fast-twitch fibers only really come into play when you need to do a quick, sudden movement. Sprinting, jumping, and plyometric exercises employ the fast-twitch muscle fibers to power them.
Which Muscles Are Used in Running?
The gluteus maximus and gluteus minimus are important stabilizing and propelling muscles. They’re especially helpful for stabilizing the pelvis during the float phase of a runner’s gait, when neither of the feet are on the ground.
While everyone is different, the glutes typically are split 50/50 to 70/30 slow-twitch to fast-twitch fibers. If you think about it, it makes sense—they’re stabilizers designed to keep us upright, help us rise from a seated position, and things like that, rather than being explosive muscles.
The quads tend to be slightly more slow-twitch than fast-twitch in ratio. Again, they’re large, stabilizing muscles that are often used for helping us stay upright, supporting our body weight when we walk, and helping us to get up from seated or kneeling positions.
Many runners are quad-dominant, which means those slow-twitch fibers are going to be more prominent. While all runners use their quadriceps in the running process, quad-dominant runners tend to longer distances.
#3: Hamstring Muscles
In contrast to the quads, the hamstring muscles contain more fast-twitch fibers than slow-twitch. They produce the explosive force needed for the push-off phase of your gait.
The hamstrings do less work in quad-dominant runners as the quadriceps take over. It’s a good idea to work on strengthening your hamstrings in the gym if you want to avoid or fix muscle imbalances!
#4: Hip Flexors
The hip flexors are made up of the iliopsoas, which includes the psoas major and iliacus, and the rectus femoris. They’re quite evenly split between fast and slow-twitch muscles.
Located at the front of the hip, they play a large role in the push-off, work with the glutes to stabilize the pelvis, and even help to stabilize the knees.
The calf muscles are more slow-twitch than fast-twitch. Again, they perform support and stabilizing functions in most cases, and they perform less of an explosive function than you may expect on the push-off.
They also take quite a bit of force as your feet hit the ground, as the impact vibrations travel up the kinetic chain.
#6: Upper Body Muscles
Don’t assume that running is all about the lower body muscles! Your upper body muscles do take part in the movement of running. Interestingly, many upper body muscles tend to be split with more fast-twitch fibers than slow-twitch fibers.
Your arms constantly move, helping to create momentum as you move forward. The upper back muscles, shoulders, and lats help to maintain your posture as you run.
#7: Core Muscles
The core contains a relatively equal number of fast-twitch and slow-twitch muscle fibers. Your core acts as a strong stabilizer when you run, playing an important role in maintaining your posture during a movement that would otherwise cause instability in the spine.
A weak core can hamper your running form and lead to injury. Whatever kind of runner you are, you would most likely benefit from strengthening your core muscles in the gym.
Running Uphill and Downhill
Muscle activation is slightly different when you aren’t running on even ground. You’ll change your body position slightly, but stay focused on your form to reduce your chance of injury.
The hip extensors and quadriceps work harder during a downhill run. The other muscles in the legs, hips, and ankles also do more work than on a flat surface.
Bear in mind that you’ll naturally adopt more of a heel strike when running downhill. This could strain different muscles from what you’re used to, especially the shins.
Your quads will activate more strongly when you’re running up a hill, in order to work against gravity and propel you upwards.
As you’ll naturally switch to more of a forefoot or midfoot strike in order to push yourself up the hill, your calf muscles will also be more activated.