Muscle contraction occurs as myosin heads grab onto actin filaments and pull them closer together, a process that increases the fiber's vertical size.

“what happens is you've got two these microfilaments called actin and myosin all right now what happens is they're overlap so they're not touching each other and you've got mein um kind of laying in the middle and it's this big thick tube and it's got these heads that flick off the top of it all right now these heads reach up and they extend again in 3D but if just think about in 2D they reach up and grab on what's called actin all right and so the idea when you contract the muscle is the myosin will reach up and they're going to reach out sort of outward so if you're watching this video you're seeing my hands kind of reach up and away from my body like I'm stretching my arms like I'm doing a big te if you will and my hands would then grab onto the actin and then if I were to squeeze my hands and bring my hands closer to my face that's the mein is actually then pulling the act in closer together so what actually happens in real life is those start stacking on top of each other and that's why when you squeeze your bicep it actually glows larger vertically because those muscle fibers are stacking on top of each other and that's actually elevating the size”