Sloth and human hands work the opposite way.

In a relaxed state, human hands are most likely to have uncurled fingers, while a clenched fist usually signals tension. But it’s the opposite for sloths: Their hands naturally curl into a hooklike grip, a useful adaptation for an animal that spends about 90% of its life hanging upside down from branches. 

Sloth hands are built to grab hold, with long, curved claws and specialized flexor tendons that help keep their unique digits curled around branches without much muscular effort. To let go, however, a sloth has to actively extend its fingers, using muscle energy to stretch through the natural pull of the flexors.