A pneumatic muscle is good in tension. Like the muscles in your body it cannot "push". You need a spring, gravity or a 2nd muscle to pull in the opposite direction. They are very durable and have no sliding seals to leak. Traditional cylinders are somewhat delicate, especially long stroke cylinders in the extended position. Pneumatic muscles don't have much stroke (only 25% of it's overall length) but can pull with a lot of force. For example a 20mm diameter actuator can lift 230 lbs at 60 psi. A 20mm traditional cylinder can only lift about 29 lbs.
They aren't simple rubber tubes. Then have diagonal tensile members that can't stretch. So when the diameter of the rubber sleeve increases due to increased air pressure, the length must shrink. Check out this cool
Festo humanoid video.
