Carrier’s previous research had trained fighters hitting a punching bag as hard as they could, using a variety of strikes. The scientists calculated that fist strikes carry 1.7 to 3 times greater force per area compared to slaps, and that buttressing the fist with the thumb more than doubles the hand’s ability to transmit punching force.

The new experiment builds on that study. In an email to Popular Science, Carrier explained that the metacarpal bones (the ones in your palm, that connect your fingers to your wrist) are most likely to break during a fight. The team wanted to measure how much strain is put on those bones when a punch is buttressed by the thumb versus not. Those measurements require attaching a transducer directly to the bone, which is why they couldn’t do the test in living subjects.

Instead, the team bought 8 arms from two body donor programs. They stripped the hands down to muscle and tendon, and attached fishing line to some of the tendons. On the other end of the strings, guitar tuners allowed the researchers to tighten and loosen the tendons, to make the cadaver fists clench and unclench.