I'm not a biologist or an engineer. How do I design legs for a large animal that leaps very long distances?

Question

For designing fictional animals, I'm trying to design one with four appendages that is meant to jump large distances vertically and length-wise and is as large or larger than a human, with hands or hand-like extremities that can both grasp objects, claw at other animals and help posture their body.

Though there are some semi-examples in nature, far as I know, there is not an animal that quite fits this criteria. How do I shape the legs, their proportions, angles and muscle sizes and overall body posture to realistically fit this? Some type of frog-kangaroo-human hybrid?

Answer 1

Copy Nature

Take a real animal that leaps long distances and copy the proportions. For example frogs, grasshoppers, or the housecat. All these are small of course so their methods may not scale up.

The largest pouncing animal I know is the lynx. They weigh 20-30 kg and can jump two metres vertically.

The skeleton looks like this:

compared to a housecat:

and a lion:

It looks like the long hind feet make a big difference for pouncing. They are proportionally longer than the lion's foot bones. Also the lion's back legs go straight down while the smaller cats' legs go diagonally. I suspect the lion is better at running long distances but the cats have a wider range of motion.

Steal those bone ratios for your animal! As for muscle thickness, make them the right thickness to allow leaps. You don't need to know the diameter in milimetres. Just "1 Lynx" diameter is enough.

Answer 2

Definitely not using only legs.

Your typical flea or grasshopper use the tensile strength of their exoskeleton as a spring to create the bursts of speed needed for jumping. However, as you get bigger, body mass becomes higher. You’ve surely heard of the square-cube-law. On top of that bones can only support so much weight. Indeed gravity can be a cruel mistress. Inertia too as a matter of fact. Larger bodies can’t accelerate as fast as smaller ones without losing integrity. You need more time to accelerate.

Ok, so how do we make an elephant jump?

Well we can’t. Because at that point it won’t be anything like an elephant anymore. You can increase the size of legs all you want, you still won’t have the power necessary to jump far. And if you do, the landing will be harsh.

This calls for a complete redesign of the quadruped body plan. Legs normally contribute most of the jumping power, but for a large animal well need the whole body to put in some work. The spine should look like a zigzagging mechanism like in this zig zag corkscrew:

To jump the predator would need to shorten its body and bend its legs. It would take a stance similar to a frog. As it begins to jump, large sets of muscles around the spine squeeze to extend the spinal cord, propelling the animal forward. Then midair it shortens again. When landing it should intercept the ground with its forelimbs and roll to dissipate that momentum like what stuntmen do.

Answer 3

Factors in Jumping

The focus here is on lowering mass over volume, increasing elasticity and tensile energy, and using effective proportional structures in creating leaping force and lowering stresses from impacts.

Mass

Larger animals will need adaptations that lower the weight of their bodies overall. Birds, even large birds, are known to have hollow bones to lower mass and assist with flight. Jumping may have similar considerations, but note that jumping introduces physical stresses that are not seen with flight.

Elasticity and Tension

The other side of the bone density consideration is that highly elastic muscles are important in jumping, so bone density must be high where the elastic tension and landing impacts are highest. The flea, one of the highest jumpers of the insect world has highly elastic muscle proteins. Frogs have really stretchy tendons to help with their jumps. The muscle structure must add tension either to itself, or to nearby tendons and then release the energy suddenly.

Proportion and Structure

The key factor here is keeping the upper part of the body small (perhaps tending towards rounded or oval for 4 legged creature) and keeping at least the back legs long. The examples of the frog and the flea are excellent for this. The counterpoint is that the front legs must be able to withstand impacts. The kangaroo is an example of a body type which creates a workaround to this, as the leaping and impacts both occur in the lower body. Frog legs fold neatly out to either side of themselves, indicating that the upper part of the hind leg is a similar length to the lower leg. They also have very long toes, esp. the species that jump best. Front legs are smaller, and structured well for impact, thick, fleshy, and short.

Frog (South African Sharp Nosed):

• length of body (not including legs): 2.1 inch (53.34 mm)
• hind legs proportions: (.7 (upper leg) + .8 (lower leg) + .8(feet)) = 2.3 times the body length (about 5 inches)
• proportions source
• jump length: 130 inches

"The tibia bone of the hind leg (shin bone) is twice as long as the femur >(thigh bone) and creates a sort of "z" shape to the hind legs which act like >two giant springs, capable of propelling the kangaroo up to 30 feet in a >single bound. This unusual form of locomotion is called saltation." source

Kangaroo (Eastern Grey):

• nose to rump: about 4 feet
• tail length: about 4 feet
• leg length: not yet available, still looking for source material
• jump length: about 35 feet
• jump height: up to 10 feet