Have you ever been up in Sarvangasana (Shoulderstand), feeling as though your legs weigh a ton? Your arms won't support you, your chest is collapsed, and your jaws are tight? Wish you had a magic skyhook to pull you up against gravity? Lacking a skyhook or a helper in the ceiling, you'll have to use your muscles to lift up—actually, to push up—so that you don't collapse.
It's worth doing a bit of work, though, to train those muscles. When they're doing their job, you'll no longer have to struggle against gravity—instead, your pose will feel light. According to yoga tradition, every cell in your body will benefit from the reverse effects of gravity on your circulatory system. Your heart and lungs will feel spacious rather than compressed, and you'll be able to savor the quiet introspection that the pose brings.
Ultimately, Sarvangasana's lift comes from its foundation, most of which is formed by the shoulders and upper arms. Most of the upward push comes from the strength of the muscles in the shoulders and of those that move the shoulder blades. It's likely that you're weak in just these areas: Unless you're a weightlifter or rower, you probably haven't conditioned your body to push backward with your arms. But with a little informed practice, you can strengthen and coordinate these muscles to work as you want them to.