Anatomy in Motion #4: Anticipations and Vertical Jump

In the previous posts, we've superficially explored the anatomy of the body as it relates to movement. For the sake of clarity, we've focused on simple, confined movements that might not be the most common nor the most interesting. 

In everyday life, rarely does the body act in such an isolated manner, muscles that you wouldn't think have an impact on your actions all relate to each other and, at least in a skilled person, help each other out, synchronizing their efforts and moving the body in the desired direction. That's going to be what today's topic focuses on. We'll analyze one simple action, jumping, and I'm going to use that action to highlight some useful concepts. All things put together, I would like to cover why we tend to anticipate actions. Why does going backward help us move forward, why does going down help us go up, and how does this counterintuitive idea of going in the opposite direction make us move more efficiently? 

MOVEMENT ANALYSIS: VERTICAL JUMP

Movements that require a powerful output of energy like throwing a baseball, a golf swing, or in this case a vertical jump can generally be divided into three phases: the eccentric, the isometric, and the concentric phase.

Throughout these phases, there are certain main elements we want to pay attention to: the use of the elastic energy in the tendons, the segmental motion of the body and building on previous momentum, and the manipulation of the center of mass.

This is the video I will be using for reference:

 ECCENTRIC PHASE

During the first part of the jump, we start from a neutral position with extended legs, and by relaxing our leg muscles we let gravity drop our body weight and bend our legs. During this phase we can cover three different events.

Firstly, muscle contraction alone does not produce movement, it’s the manipulation of the joints through the use of muscle that does. So if the force of the vertical jump comes from extending the legs quickly, we obviously need to bend the legs first so they can be extended.

Second, as gravity takes its course, the muscles that we require for jumping (notably the quadriceps, the calf, the glutes, and the anterior part of the deltoid) are stretched. This stretch builds elastic energy in the tendons that connect these muscles to the bone. In an efficient vertical jump, our center of mass should drop (and raise) vertically. 

Third, during the last part of the descent, the stretching muscles start contracting eccentrically to slow down the descent (so we don't fully collapse to the ground). Among other things, since the pushing part of the high jump is limited in time, starting to activate the muscle on the descent could mean that these muscles will be in peak pushing power while pushing up instead of starting from zero.

ISOMETRIC HOLD

When the body reaches its lowest position there is a small moment of pause as the contraction of muscle transitions from slowing down the fall to extending the legs. This moment reaches the peak stretch of the tendons and shouldn’t last more than a fraction of a second. We want to use the elastic energy we've built for recoil on pushing up, but if the hold lasts too long the same elastic energy will be dissipated as heat in the body. 

That's why starting the jump from a standing position feels more effective than simply holding the bent position at the bottom.

CONCENTRIC PHASE

The spring-like mechanisms we've cranked up until now get released during the concentric phase. Everything works together in the body with the purpose of building enough upward momentum to lift our body mass up in the air and escape gravity for but a few moments. Our arms (which you wouldn't think are necessary for a jump) are the first to start swinging upward, lifting our center of mass and helping initiate the lift.

Second, our glutes and quadriceps contract with as much force as they can to extend the upper and lower part of the leg together with the torso. This is building upon the momentum that the swing of the arms is creating and is the major force created for the jump.

Third, towards the end of the extension, the calf muscles contract to extend the heels downward and give the body one last push before detaching from the ground.

The built-up momentum keeps pushing the body upward even after our feet leave the ground until the constant force of gravity slows us down and inevitably brings us back to earth.

A quick digression for you to consider. The sentence "legs extend, pushing the body upward" is a slight over-simplification. Pushing the body upward can only happen because of the resistance offered by the ground. When the legs extend, they push down, but since the ground is sturdy and generally doesn't move easily, our pushing force is reflected and lifts our body up instead. Things could be very different if we tried to perform the same action on a very loose net or very soft surface. In those cases, we would see that extending the leg would actually push the ground downward and our body would not move. When analyzing body mechanics it's important to take into consideration the context and the environment our actions operate in, because sometimes those actions can have very different results.

This stretch-and-contraction action of the muscle is obviously not limited to the vertical jump. Many forms of locomotion are cyclical by nature and use the elastic energy of the muscle that builds up and releases over and over again to make movement sustainable for long distances. Walking and running (which you could consider a series of small jumps), are prime examples of this, together with cycling.

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What does this mean for animation?

As we've seen before, our bodies always try to produce the maximum force with minimal effort. That was the case for a more micro level of levers and muscle architectures, and it's also the case here. Anticipations don't exist simply for storytelling purposes, they come from a place of mechanical need. The stronger the movement needs to be, the bigger the anticipation usually is. The smaller or lighter the action, the more minimal (or absent) the anticipation would need to be. Just like in a golf swing, anticipating a movement gives the body the room necessary to reach peak velocity before the moment of impact, together with the extra power that comes from the elastic effect of our muscles. What an anticipation will look like will depend a lot on what the action is, but trying to define what the primary muscle movers of your action are will help figure out which muscles might need to be stretched before the contract. One thing that often remains true is that, for the reasons we’ve seen, anticipations happen in the opposite direction of our main action.

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