Do you know that biomechanics allows athletes to boost their performance by applying the laws of mechanics to living beings?
In this article we propose 5 experiments to be carried out in class or at home with your smartphone to understand what biomechanics is and use digital technology for inquiry based science education.
Table of content
What is biomechanics? - How to use a smartphone in biomechanics? - How does my smartphone calculate my daily steps count? - Where is a diver's center of gravity? - Can an accelerometer measure heart rate? - How can a pole vaulter jump higher? - Is it better to run barefoot or with sneakers?
What is biomechanics?
Biomechanics is a branch of science that studies the movements and forces that act on living organisms, using principles from physics, mechanics and biology.
Biomechanics is used in the field of health, for example to help the elderly to improve their autonomy, or people with physical disabilities. It is also used in sports to improve the performance of athletes.
To carry out his studies, the biomechanics researcher uses above all his intuition and his study of the subjects. As Leonardo da Vinci, the first biomechanist, wrote :
"The good painter must mainly paint two things, the person and his state of mind. The first is easy, the second difficult, because it must be represented through the gestures and movements of the limbs.
The modern biomechanist also uses all available scientific instruments to analyze the movements of each part of the body. For example accelerometers, gyroscopes can be attached to certain parts of the body to detect their movements.
The scientist will also use a number of instruments that allow him to analyze the functioning of muscles and tendons and their ability to create movement and transform energy. For example, the electromyogram (EMG) makes it possible to measure the muscular efforts, the mechanomyogram (MMG) makes it possible to measure the vibratory activity of the muscle during a contraction. These instruments make it possible to obtain the overall effort of the muscle during a contraction.
Finally, one of the great innovations in biomechanics has been the use of high-speed cameras which, thanks to kinematic analysis, make it possible to measure the displacements of each part of the body during a movement. We can use chronophotography, which consists of taking successive images at high speed, or video.
To discover more about biomechanics, visit the website of the biomechanics society.
Why should I use a smartphone in biomechanics?
Everyone can perform biomechanical analyzes with a smartphone. These powerful scientific instruments give you access to three types of data :
1. Smartphone's sensors data
2. Videos taken with the camera
3. External Bluetooth sensors
Smartphones integrate many sensors such as accelerometers, gyroscopes, magnetometers, which make it possible to know the position of a body or a part of a body with precision. The data acquisition frequency of these sensors is more than 150 data per second, therefore sufficient to make detailed analyses.
We can also use the smartphone camera to record videos of movement and then perform frame-by-frame analysis. FizziQ's kinematics module allows you to do this type of analysis directly with the laptop. Most phones can also shoot in slow motion, sometimes up to 240 frames per second, allowing the student to get even more accurate measurements.
Finally we can also connect external sensors, less cumbersome on the parts of the athlete that we wish to study. In the school or university setting, you can easily use micro-controllers like Arduino, Micro:Bit or M5 Stack. These devices will record data that can be transmitted to an acquisition device.
How does my smartphone calculate daily steps count and distance?
Have you ever wondered how your smartphone measures the distances you have walked?
In FizziQ, select the accelerometer to measure the absolute acceleration (also called acceleration with g), start recording and put your smartphone in your pocket. Walk a few steps and stop recording.
You see peaks that correspond to each step. Indeed, when you put your foot on the ground, the smartphone in your pocket suddenly stops moving, and the acceleration suddenly increases. If the software counts the number of peaks, it will be able to count the number of steps.
How do you go from the number of steps to the distance? Your smartphone required you for your height, in the health section, when you first logged in. From the height, the software deduces the typical step length for the smartphone owner and then a distance with the number of steps.
This measurement method is actually very accurate. This is how the Egyptians and Greeks of ancient times proceeded to measure distances. The bematists, or surveyors, were reputed to have a regular step and a good memory so as not to forget the number of steps. We have verified that the distance measurements they made were often less than 1% accurate compared to modern calculations for distances of 500 km and more!
By studying the graph more precisely, you will also be able to detect the different phases of the movement of your leg. For example, what movement corresponds to the plateau in the middle of the two peaks? Can a regularity of movement be detected? Could we detect that we are limping only by studying the graph? How does the smartphone also calculate the stairs you climb?
To carry out this experience on FizziQ, go to our activity: "Build your own pedometer"
Where is a diver's center of gravity?
If we throw a ball in the air and we neglect the friction, it will describe a parabola. But what about a diver performing a somersault? Does its center of gravity really describe a parabola?
To study this question we can go to the swimming pool and film an athlete diving. If you don't have a swimming pool near your home, you can use the diver's video from the kinematics video library.
Let's study the video in the kinematics module of FizziQ. The use of this module is described in this tutorial.
We make several successive measures. In the first we will point only the athlete's head. On a second we will point to his feet. If we draw curves in the experiment notebook, we see that these curves are extremely different. From the physical point of view it is very difficult to model these movements.
The laws of mechanics affirm on the other hand that the trajectory described by the center of gravity of a body in free fall without friction is a parabola. If we are able to plot the center of gravity for each image, we should obtain a resulting curve which is a parabola. Sometimes it is outside the athlete's body. Can you find it on each picture?
Can an accelerometer measure heart rate?
A beating heart creates regular pressure on the rib cage, but are these small shocks strong enough to be detected by the accelerometer of a smartphone? What can be deduced from the analysis?
Let's lie down, then select the transverse linear acceleration in the FizziQ app, press the record button, and place the laptop on our heart.
After 5 seconds, let's stop recording and study the graph. To see the beats, one can recalibrate the graph by pressing the scale button. You can clearly see the different beats and you can measure their frequency, the heart rate, with the magnifying glass.
We can also study the regularity of this rhythm. It is of course necessary to consult a doctor if the rhythm is irregular!
Amazingly, the accelerometer is also sufficiently sensitive to detect many other elements such as for example the T wave which is the second peak and which appears in the first third of the duration of the beat. This pulsation corresponds to the "repolarization of the ventricles", in other words, the ventricular myocytes "relax" and recharge in order to be able to depolarize again.
The heart is a complex machine but it is impressive that even with a commercial device, we can conduct analyzes on our health and better understand the functioning of our organs!
To know more about how to use the smartphone to analyse the human body using photoplethysmography.
How does a pole vaulter jumps higher?
One might think that the pole vault is a simple transformation of kinetic energy into potential energy, but the reality is quite different: by a movement of rotation then of push, the pole vaulter brings more than a third of energy in addition, which allows the athlete to go significantly higher.
To prove this statement, we conduct a kinematic analysis on the video of the pole vaulter which is in the video library.
The detailed analysis of the energy balance, which can be found in our article dedicated to pole vaulting, shows that there are three phases of energy input which will gradually be transformed into potential energy: kinetic energy of the run before take-off due to the run, the rollover at around 1.3s in which the athlete uses his abs to rotate and the rollover at around 1.8s where he extends vertically.
Each of these actions provides additional energy for the athlete to go higher. If we neglect the losses, we can estimate the elastic energy at the point of maximum compression: it is the difference between the initial mechanical energy and the mechanical energy at this point, i.e. approximately 1300 J. Potential energy necessary to pass from the turning point to the apogee being about 2000 J, we calculate the energy added by the athlete during the flight phase of at least 700 J, that is to say the equivalent of a gain of height of at least 1.30m!
The complexity of the movement of the pole vault is due to the fact that the performance requires both the good transformation of the race into elastic energy and its return into potential energy, but also to provide additional energy during the phase of flight to gain even more than a meter! Nice co-ordination!!
Is it better to run with trainers or barefoot?
Some athletes run barefoot but orthopedists do not recommend this practice, why?
To find out, let's use the accelerometer of our smartphone, which we can slip into our pocket. We select the absolute acceleration, we start the recording and on a road, or a track, we run barefoot with a regular stride for about ten seconds. After adding the data to the experiment notebook, we start again with sneakers.
Analyzing the data and comparing the two data graphs shows several interesting things, the first of which is that running is a very taxing exercise on the knees.
Indeed, the maximum accelerations are approximately 5 times the acceleration of gravity, this means that the knees take 5 times our weight with each stride, sometimes up to 7 times. If you weigh 75 kg, that can be half a ton! Isn't it incredible that our body can resist such efforts continuously?
If we now compare the two graphs, we see that the maximum values of acceleration when wearing sneakers are on average 15% lower than when running barefoot. There is therefore a significant advantage to using shoes that absorb shocks well to preserve the health of our joints!
In conclusion
Biomechanics, or the science of movements and the forces that act on living organisms, is an exciting branch of physics. Simple experiments that can be offered to students from secondary school make it possible to tackle subjects of the program or to open up the field of reflection of students through experimentation and the method of scientific investigation.
With the approach of the Olympic Games, here is one more tool to interest students in science!