Tuesday, March 23, 2021 · 16 min read
Force Measurements on Trampoline Jumpers
This project was initiated by a pure curiosity about the forces that might work on the acrobats. We set the goal to measure the actual force on the acrobat's body while jumping on a trampoline - and teamed up with one of the best acrobat teams there are, the Dunking devils. The initial prediction was: The bigger the trampoline, the smaller the forces that act on the acrobat – but is it, the greater the force, the greater the joy?
Since Dewesoft has a lot of experience in car crash-tests we thought it could be interesting to compare the forces that act on the athlete jumping on the different sized trampolines to the forces the human body endures during a car crash. Besides the curiosity part, our aim was also to see if such jumping might be (too) dangerous for the athletes.
See for yourself in this video - Measuring the actual force on the acrobat's body while jumping on a trampoline - DD Squad x Dewesoft:
Trampolines are fairly simple tools or devices designed for gymnastics or recreational use. A Trampoline consists of a piece of taut, strong fabric stretched anchored by coiled springs within a steel frame. The fabric used for jumping up and down is not in itself elastic; the elasticity is offered by the springs that store potential energy applied by the jump.
The springs are what make things happen – based on two laws of physics:
Robert Hooke’s Law of elasticity: the force needed to extend a spring is proportional to the length of the extension.
Isaac Newton’s third law of motion: for every action, there is an equal and opposite reaction. When you jump down on the trampoline, the springs are pushing back an equal and opposite reaction – you bounce back into the air.
As a sports trampoline jumping was born in 1936 when the American gymnast George Nissen developed the modern trampoline. The first official American championships took place in 1954 and 10 years later the first world championship was held in England. The same year officials from participating countries formed the International Trampoline Association. Since 2000, trampoline gymnastics has been part of the Olympics program.
We set out to compare force measurements on three different trampolines:
Mini-trampolines, also known as rebounders, are trampolines with a rebound mat that is less than 1 meter - 3 ft 3 in - in diameter and raised about 30 centimeters (12 in) off the ground.
Olympic trampolines are often referred to as being 17 feet long and 10 feet wide. However, the official-size trampoline is defined in the metric system, and is precisely 5m long x 3m wide x 1.15m high – that is 16.5682 feet x 9.54724 feet x 3.78937 feet.
These trampolines have larger mats with a lower spring count which ensures that there is less tension in the jump mat and therefore not quite that much force required to get a strong bounce. The size allows more jumpers to interact and join forces on the mat.
The Dunking Devils
The natural choice of performers for the trampoline jumping was the Dunking Devils – the DD Squad – a Slovenian group of world-famous acrobats performing acrobatic basketball with trampolines. From Ljubljana, their road to success began in 2004 when a group of 4 friends passionate about acrobatic dunking decided to share our love of the trampoline sport with the world.
Over the years, the group has grown to include more than 70 acrobats, 25 of which are professional performers. In addition to being two-time world champions in acrobatic dunking, they also hold two Guinness World Record titles – one for the highest front flip slam dunk and the second for the longest slam dunk from a trampoline.
Although the application is unique, the measurement itself is nothing new or extraordinary. The problem was to set up a proper lightweight and small-size configuration that was able to measure the forces acting on the athlete’s body.
We ended up equipping the acrobats with a backpack containing the needed measurement equipment. We used a small backpack in which we placed a tablet computer to which we connected the DS-IMU1 sensor and the GPS Antenna.
The DS-IMU1 inertial navigation system measures the GPS position, height, and roll-pitch-jaw. and acceleration forces. The GPS Antenna provides the necessary GPS signal for good accuracy. The GPS Antenna is connected to the IMU device, which is connected to the PC via a USB cable. The data are stored directly on the tablet computer using DewesoftX data acquisition software.
The Dewesoft DS-IMU1 is a combination of multiple sensors; a gyroscope, accelerometer, magnetometer, pressure sensor, and a high-speed GNSS receiver. Coupled with sophisticated algorithms it delivers accurate position, velocity, acceleration, and orientation even under the most demanding conditions.
Before the start of the measurement, we aligned the sensor axis and set up remote desktop access to the tablet so we were able to observe data being captured during the measurement.
Although it is very hard to compare the joy of jumping on a mini-trampoline to that of the supernova, our predictions were somewhat confirmed. Keep in mind, that the g-force of 1G is defined as that of gravitational acceleration on Earth - 9.8 m/s².
On the mini-trampoline the force of the jump reached 6.9G on the x-axis and 8G on the Z-axis – the combined g-forces at take-off are equivalent to those of a car crashing into a wall at 25 km/h.
On the Olympic trampoline, there is only the Z-axis. At the highest jump G-forces of 8.4 were reached, similar to the car crashing the wall at 20 km/h.
On the supernova trampoline, the forces from the bodies of more acrobats can be transferred to the jumper – it’s designed for maximum height. The biggest forces are on the x-axis since the jumper is bouncing on his back and here a force of 14.6G was reached, which is the same force as a car crash at 30 km/h.
As described in the video there was a variety of G-forces acting on the athlete’s body depending on the size of the trampoline. The forces varied from 6 to 14G which is similar to a 25-30km/h car crash. We were surprised to see the high intensity of the forces acting against the athletes.
Concerning the danger aspect, we concluded that although the G-forces are quite high and unexpected, jumping the trampoline is safe since the deceleration force is still much lower than in a car crash, because of its flexible impact surface.
So, how about, the greater the force, the greater the joy? It’s probably fair to say that in this case; size doesn’t matter after all - jumping a trampoline is fun anyhow.