Three types of muscle contractions occur during pushups.
The push-up is, quite rightly, praised as one of the best exercises for strengthening your chest muscles. But that's not all that's happening when you do push-ups, which are classified as a compound movement because they involve multiple muscles and joints moving at once. At the same time your chest muscles are pressing your body up, your triceps, shoulders, core and even your back muscles are activating to help keep your body stable and complete the motion.
As a general rule, your pecs at the primary movers - or at least one of the primary movers - during any exercise that involves pushing, except for exercises that involve pushing straight overhead. In that case, your shoulder muscles take over as the primary movers.
Pecs Are the Primary Mover
Your chest muscle - or more specifically, the pectoralis major - is classified as the primary mover during push-ups. That means that your pecs are taking most of the load during push-ups, although they're far from the only muscle that's working.
Your Shoulders Act, Too
Your deltoids - the shoulder muscle that sits atop your arm like a cap sleeve - are divided into three distinct groups of muscle fibers. Of these, the anterior (front-facing) muscle fibers help out during push-ups, activating to help swing your upper arms in front of your shoulders as a result of the pressing motion.
Your Triceps Help Out
Also active during push-ups are the triceps, the big, meaty muscle on the back of your upper arm. The triceps kick in powerfully to help straighten your arm at the elbow, a vital part of the push-up motion.
Your hand placement during push-ups determines how much of the movement originates with your triceps muscles. If you place your hands close together in a diamond shape, right under your nose, this shifts emphasis away from your chest muscles and onto your triceps. Doing push-ups with your elbows tucked close against your body, hands positioned under your ribs instead of under your shoulders, shifts emphasis to the triceps as well. In fact, the latter exercise is sometimes known as triceps push-ups.
Stabilize with your Core
While your chest, shoulders and arms power the motion of push-ups, your core works throughout the motion to keep your body stable and straight from head to heels. In particular, your rectus abdominus or "six-pack muscle," your obliques, and the erector spinae (a series of muscles that run up and down the length of your spine) all work together to stabilize your core.
Work Up to Push-Ups
Although people who are used to them can make push-ups look easy, they're actually a very challenging, complex motion to master. If you're working up to doing push-ups, consider starting with knee push-ups. Instead of doing push-ups while balanced face-down on your hands and the balls of your feet, bend your knees and allow them to rest on the floor. You can also do bench push-ups, resting your hands on a weight bench or even the kitchen counter - this makes the motion much easier - or do free weight exercises like chest presses to help strengthen your chest, shoulder and arm muscles for push-ups.