We originally designed UnbuckleMe to unbuckle the car seat without requiring the use of thumbs. However, since we have started rigorous testing of our latest 3D printed prototypes, we are watching people “pinch” the ends together with one hand, rather than use the “open palm” method that requires no thumbs. In fact, my mom realized that even she can use UnbuckleMe this way with her arthritic thumb. Two reasons why: 1. Leverage. Pinching at the furthest end of a type II lever arm creates maximum “effort” reduction, and 2. Because of the anatomy of the human hand…
Try this experiment: hold your hand up and spread your 5 fingers apart. Now move your index finger horizontally in the direction of your thumb (for your right hand, move it to the left, and vice versa). Watch what happens to your thumb: you can see it stabilizing at the CMC joint. In fact, it’s nearly impossible to move your index finger without creating stabilization in your thumb. Apparently, this is why the human hand is so good at pinching!
So the second reason why my mom is able to apply pinching pressure to the end of UnbuckleMe is because her index finger is free to help provide stability against her thumb. Think about how this differs from operating a standard buckle, when your index finger is fixed behind the buckle, and your thumb has to do all of the work. I thought this was pretty interesting.
Note: To read more about this, my mom referenced a recent study from the University of Minnesota, published in the Journal of Hand Therapy, where OTs actually directed patients to do the this index-finger exercise repeatedly to improve stability of the thumb (or more technically, “to prevent radial subluxation of the thumb metacarpal”) The study focused on people with normal hand anatomy, and is now being studied with the arthritic hand.