If you bite into a sour apple, you feel a pain at the back of your jaw just underneath your ear. The muscles in your intestines also work in pairs. The muscle of your heart is also striped but it is involuntary. Cardiac muscle contracts and relaxes automatically without you having to think about it. When you take exercise your heart beats faster and with a bigger volume.
All muscles work in pairs.
Whether they are striped muscle, smooth muscle or cardiac muscle makes no difference, all muscles must work in pairs. This is because they can contract and relax but cannot push or stretch themselves. At the same time it also pulls the triceps to make it longer. So the triceps is stretched by the biceps pulling it. So these two muscles work together.