Position and Displacement
In the previous article, we look at the forces that apply to a dancer's body and how we can use them to describe how a dancer can move around in space. To describe the movement of the body stimulated by the forces, we can use Position and Displacement concepts.
For illustrative purposes, we will use this wonderful picture of Victor Fung and Anastasia performing a small leap in Quickstep. (credit: Sam's Photography). To illustrate displacement, we will use the jump's starting point (Position 1) and the jump's presumed endpoint (Position 2).
Simply said, displacement is the difference between position one and position two; let's say 4 feet. We can say that Victor and Anastasia have displaced their bodies from P1 to P2 4ft in length. The term distance is often used to describe displacement, but they are not precisely interchangeable since displacement always has direction. In this case, Victor has displaced 4ft to the LEFT.
Since the distance doesn't have direction, we can say that the arch on the picture represents the distance traveled from P1 to P2- it might be 5-6ft.
Distance traveled 5-6ft
Displaced by 4ft to the LEFT.
Dance is often referred to as a movement through space and time with Energy. These Space, Time, and Energy concepts are directly related to Velocity, which is considered the first derivative of displacement. Confusing? Let’s untangle the mystery.
Remember, displacement deals with space- Victor has displaced 4ft to the right. Velocity, on the other hand, needs the factor of time to be accounted for.
Victor’s displacement from the example above was 4ft to the right. That is the space the dancer has moved through. If we measure the time that it took Victor and Anastasia to displace their bodies from P1 to P2, let’s say it took them 1s; we would be able to determine Velocity.
Velocity = Displacement / Time
Velocity= 4ft/ 1s= 4ft/s
This sounds very similar to the bike computer that tells you how fast you are riding. For example, 22m/h. But there is one difference. Speed is to Velocity as Distance is to Displacement. Speed does not have a direction, whereas Velocity has a direction- Victor’s Velocity is 4ft/s to the Left.
The concept of Energy
If a dancer is moving and has a velocity, it immediately means it has Energy. For the dancer to be moving around in space, he has to have Energy in the body. Now that Energy comes from a force that is applied to the object either internal (muscle force) or external (your partner’s influence).
Acceleration is the second derivative of Displacement. It describes the movement of the body through space and provides this connection between movement and force. Pushing the car's accelerator makes the engine use energy and makes the car's speedometer increase, giving the car some velocity. The energy that the car is using is producing force.
We can simplify the process like this- Acceleration—> Energy—> Force—> Velocity
Let's have a look at Victor and Anastasia's picture again. For them to jump up we know that they have to produce a ground reaction force by pushing down to the ground. This causes an acceleration up, which means that we have a velocity. However, everything that goes up must eventually come down because we have the force of gravity, causing acceleration down.
The second that Victor produces his forced and the ground reaction force has pushed him up off the stage; gravity starts to act to accelerate him back down, which means that the velocity will slow down. At the top of the jump, the couple will have 0 velocity, therefore, starting to come down to the floor.
As mentioned above, correct biomechanics provide efficient movement and may reduce the risk of injury. In dance and any other sport, it is always good to consider abnormal or faulty biomechanics as a possible cause of injury. These abnormal biomechanics can be due to anatomical or functional abnormalities. Anatomical abnormalities such as leg length discrepancies cannot be changed, but the secondary effects can be addressed, such as a shoe build-up or orthotics. Functional abnormalities that can occur can be muscle imbalances after a long period of immobilization. For example, broken limbs can take a long time to recover, and the body can start building mechanisms to adjust for the new "reality" and cause the body to move irregularly.
In biomechanics, the different planes of motion and axes are often referred to. Have a look at this video to become familiar and imagine how you can use them in dance.
Incorrect technique can cause abnormal biomechanics, which can lead to injuries. Below are just a few examples of the relationship between faulty technique and associated injuries.
- Wrong fitting shoes can cause pain, inhibit bunions' development, and uneven weight distribution over the foot.
- Incorrect footwork can also cause an uneven distribution of weight.
- Allowing the weight to roll on the outside of the foot can cause injuries of the ankles and knees.
- Misalignment of the lower and upper body can affect your personal balance and your partner's posture. It can also cause injuries.
- Misalignment of the joints in flexion ankle, knee, and hip can cause loss of balance and ligament tear.
- Not counterbalancing a body part that works in extension away from the base of support.
Biomechanics is an incredibly complex topic. The laws, concepts, and principles we have covered are the core basics of it. Every sport discipline has a lot of studies conducted on the biomechanics to perfect and tune the technique for the most efficient, powerful, and injury-free movement. Read and understand the basics, then try to discover for yourself how all of those principles apply to a particular movement. It is fun and can make you a better dancer. Happy Dancing!