You can’t push on a rope.
This is why you typically need two muscles to get things done. Muscles only shorten; if you flex a joint, you can’t expand your muscles to push that joint back to its original position. You have to pull a different muscle, with different insertion points, to get that limb back to where it was. For instance, you have biceps to flex your forearm, and triceps to extend it.
Spiders have always been something of a puzzle, because many of their limb joints have unpaired muscles. This is particularly true of the joints far from the body; the joints close to, and on, the body have more usual paired muscles.
On the face of it, this should mean that their joints should only be able to go int one direction. But spiders are agile predators and their limbs are moving back and forth rapidly.
Many spiders use hydraulic pressures to snap their limbs back into position after a muscle has moved it. This has been quite well investigated in a few small species, but Weihmann and colleagues reckoned it was worth re-investigating in a larger spider. They took a big spider species, Ancylomete concolor (pictured), and studied the forces the legs exerted when this spider jumped.
Some of the math and methodology is a little hairy (no pun intended), but this picture helps:
In short, if the spiders are using hydraulics (as small spiders do), the forces from the tip of the leg should be directed forward. If the spiders are mainly using the paired muscles in the joints close to the body, the forces should be directed much more upward.
Wiehmann and company find that their results are much more in line with the jump being powered by muscular contraction than hydraulic pumping. They’re not saying it’s entirely muscle, though, just that muscles are contributing more than the hydraulic factors.
The team briefly takes a stab at the bigger question: why mess around with all the hydraulics in the first place? Why do spiders not have paired muscles all the way through their legs, like sensible insects and crustaceans? Weihmann and company speculate that because spiders are obligate, active predators, that the loss of extensor muscles means that there’s more room for big, powerful flexord muscles – just the things to grab and grapple and subdue prey.
Weihmann T, Gunther M, Blickhan R. 2012. Hydraulic leg extension is not necessarily the main drive in large spiders. The Journal of Experimental Biology 215(4): 578-583. DOI: 10.1242/jeb.054585
Photo from here.