Joints and muscles of hands and paws

Abstract

The joints of the human hand allow about 24 degrees of freedom of relative movement, about the same as in the fore feet of most mammals. Only ungulates and whales have substantially fewer degrees of freedom. Humans, cercopithecoid monkeys and primitive mammals such as Didelphis all seem to have 40–50 muscles, or independently-usable parts of muscles, in their hands or fore feet. Dogs, ungulates and whales have fewer. The extrinsic muscles of the fore foot are larger fractions of body mass in cercopithecoid monkeys than in Didelphis or in fissipedes. Their muscle fibres are longer (relative to ulna length) in humans and monkeys than in cursorial fissipedes, but only flexor digitorum superficialis has markedly longer fibres than in Didelphis and Meles (a non-cursorial fissipede). Ratios of muscle fibre length to moment arm indicate capacity for producing angular movements. They are larger for various hand muscles of humans, a monkey (Macaca) and a wallaby (Macropus), than for dogs and sheep. The difference is attributed to conflicting requirements for prehension and running.