Muscles Only Know How to Pull
Here's a fact that surprises most people the first time they learn it: no muscle in the human body can actively push. Every skeletal muscle works by contracting — shortening and pulling its two attachment points closer together — and that's the only trick it has. This single limitation explains why muscles almost always work in opposing pairs, called antagonists. The biceps brachii, on the front of the upper arm, pulls the forearm up to bend the elbow; the triceps brachii, on the back of the same arm, pulls it back down to straighten the elbow again. Neither muscle can reverse its own action — bending the elbow back straight requires an entirely separate muscle contracting on the opposite side.
This pairing shows up throughout the body. The quadriceps, a group of four muscles on the front of the thigh, straightens the knee; the hamstrings, three muscles on the back of the thigh, bend it back. The muscle that actually produces a given movement is called the agonist, or prime mover; the one relaxing to allow that movement is the antagonist; and muscles that assist the agonist without being the main force behind the movement are called synergists. Learning a muscle's name is only half the job — knowing its origin (the fixed attachment point that doesn't move) and its insertion (the point that gets pulled toward the origin) is what actually explains what a muscle does and why.
Not All Muscle Answers to You
Skeletal muscle — the kind attached to bone and covered in this game, from the deltoid at the shoulder to the gastrocnemius in the calf — is voluntary, meaning it only contracts when the nervous system deliberately tells it to. But it isn't the only type of muscle tissue in the body. Smooth muscle, found in the walls of the stomach, intestines, and blood vessels, works involuntarily, contracting in slow, wave-like patterns without any conscious input at all. Cardiac muscle, found only in the heart, is also involuntary, but it's built differently from smooth muscle: its cells are connected by structures called intercalated discs that let electrical signals spread rapidly from cell to cell, letting the entire heart contract in a single, coordinated beat rather than a slow ripple.
The diaphragm sits in an interesting middle ground. It's technically skeletal muscle, striped in appearance and capable of voluntary control — you can consciously hold your breath or breathe faster — but for most of your life it contracts automatically, driven by signals from the brainstem, without you ever thinking about it. That dual nature makes it one of the few muscles in the body that blurs the line between voluntary and involuntary control, and a useful reminder that muscle classification is about more than just where a muscle sits or what it's attached to.
Size and location also tell you something about a muscle's job. The gluteus maximus, in the buttocks, is the largest muscle in the body by mass, built to generate the powerful hip extension needed to climb stairs or push off the ground while running. The stapedius, deep in the middle ear and attached to the tiny stapes bone, is the smallest skeletal muscle in the body, doing nothing more strenuous than dampening the loudest sounds that reach the eardrum. Between those two extremes sit the muscles this game asks you to place correctly: the trapezius across the upper back, the rectus abdominis down the front of the abdomen, the tibialis anterior along the shin. Each one earns its name and location for a specific mechanical reason, and matching them correctly is the fastest way to start seeing the body as a connected system of levers rather than a list of isolated parts.
Source: National Institutes of Health (NIH) and Gray's Anatomy.