At the end of last semester, we completed our experiment to find the "strength curve" for a pull-up. We sought after this so we could better model our design to achieve the maximum compatibility with a real pull-up. We first drew and machined a load cell and attached strain gages to it. When the load cell receives some axial force, the strain gages send a signal to an instrument (commonly called a "lunchbox") which gives a reading in microstrains. By knowing the Young's modulus of the load cell (made of aluminum), we can compute a stress level. We compared this value to length of arm reach, to achieve a curve.
To perform the experiment, we attached the load cell to the hoist in the GEL high-bay. We then attached a pull-up bar to the load cell. The load cell is attached to the lunchbox using a wheatstone bridge configuration. Once we calibrated the strain gages using measured weights, we performed the experiment. One of us would stand on two weights, which we attached to his feet. This prevented him from lifting himself off the ground. We didn't want this because the strain read would simply translate to his weight. We would then measure the distance from the bar to a fixed point on his body. He would pull on the bar and the maximum strain reading would be taken. We would then change the distance and repeat. We ran the experiment both by moving the bar down and up, each time recording the distance.
Our results can be found here. As you can see, it's not really a curve, but more of a linear relationship. This was different than what we expected, as most muscles behave in a parabolic fashion. These results indicate that it is hardest to pull when the bar is closest to your shoulders, and it becomes easier in a linear fashion as the bar is raised. Our goal is to model our revised pull-up machine to follow this curve.
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