Note: This is the seventh chapter of my serialized book Healthy Movements for Human Animals. You can find an archive of previous chapters here.
Human babies can hang from overhead supports within months after birth. This surprising ability hints at something fundamental about our species: grasping objects and pulling the body toward them is one of our most basic physical skills.
This is because our primate ancestors spent tens of millions of years hanging from branches while living in the trees. For them, suspension was a routine resting posture, especially for feeding.
Children show an innate attraction to hanging from overhead supports, as shown by their interest in climbing trees or playground simulations of trees like monkey bars. Hanging and swinging from them develops grip strength, upper body mobility, and functional pulling strength.
And yet modern adults rarely hang as an exercise. They may do pull-ups or rows, but these more complex movements can miss some of the specific benefits available from simply hanging out. It’s a bit like practicing squats without ever spending any time simply standing.
This chapter explores the evolutionary and developmental history of hanging, how it appears in natural settings versus modern life, and provides exercises to reintroduce this fundamental movement.
The evolution of hanging
Many of the evolutionary adaptations that allow hanging (such as grasping hands, mobile shoulders, and upright trunk posture) were discussed in the previous chapter on reaching. This chapter focuses on the specific anatomical changes that allowed hanging.
The first primates didn’t need to hang because they were very small (about the size of squirrels) and could obtain food by simply walking along small branches of trees. However, as some primate lineages evolved into the first apes, their bodies became too heavy to walk safely on the thin outer branches where most fruits were located. So they developed a new strategy to help support their body weight: hanging from overhead branches.
Suspension created a new relationship to gravity, and opened a whole new world of possible movements and postures. Body weight could now be supported by pulling from above instead of just pushing from below. Several anatomical specializations evolved to make this possible.
The hand
In primates that hang frequently, the finger bones are elongated and curved, and the joints naturally bend into the shape of a hook.
Although human hands eventually shortened and straightened to allow more skilled manipulation of objects, their original hook-like design remains evident. Even with the hand at rest, it looks ready to grab a branch.
This shape explains why the handles we design for maximum grip security—barbells, pull-up bars, suitcases, hammers—are shaped like tree branches.
Grip strength and endurance
Maintaining a hook-shaped hand against the pull of gravity requires sustained work from the muscles that flex the fingers, which are located in the forearm. Suspensory primates evolved well-developed forearm flexor muscles for this purpose.
The wrist also became more mobile to allow the body to shift position under a stable grip. Suspensory primates have far more wrist mobility than quadrupeds, especially into rotation, but also in flexion, extension, and both radial and ulnar deviation. As with the shoulder, this enhanced mobility means that control must be provided actively by muscles, further increasing demands on forearm strength.
The elbow
Hanging is most efficient when the elbow is straight, which allows body weight to be supported more passively. Most quadrupedal mammals can’t fully extend the elbow because there is a large bony projection at the back (the olecranon process). This gives the triceps leverage to extend the arm. Primates evolved a shortened olecranon process, sacrificing some strength into extension, but allowing a full straightening of the elbow to make hanging more efficient.
The lats
The lats tend to be stronger in tree-dwelling mammals like gibbons, chimpanzees, and orangutans. In ground-based mammals, the lats work mainly as a forelimb retractor, pulling the arm backward during walking or running. But in suspensory primates, it becomes a sling that connects the arms to the back and pelvis.
Humans retain large and powerful lats, even though we no longer brachiate for a living. As such, it plays a major role in movements that integrate the shoulder and pelvis, including walking, running, throwing, and climbing.
Swinging and brachiation
Hanging originated as a static posture for feeding, but it eventually opened the door to new forms of locomotion. When the body is suspended from above, the lower half is free to swing like a pendulum, which is an efficient way to move the body through space.
The simplest version is swinging with both hands, like a trapeze artist, to bring the feet into contact with a distant branch. Brachiation is more complex, and involves the tarzan-like movement of swinging with one hand while the other reaches for the next branch.
Human ancestors were likely capable of at least some brachiation, and modern humans retain at least some of this ability. Children commonly brachiate across monkey bars on playgrounds without any formal instruction.
Development of hanging
As noted in the intro, babies can hang from overhead supported at astonishingly young ages.
This ability depends on the palmar grasp reflex, which causes babies to close their fingers around objects that are stroked across the palm. This reflex is present in other primates, and probably evolved to help babies hold onto Mom’s fur as she climbs through the canopy.
The naturalness of hanging and swinging is easily seen by watching young children on playgrounds, who start doing these movements spontaneously, and develop skills without instruction. If there is a bar within reach, they will start hanging from it, often while bringing the knees up into a position that aligns and stabilizes the trunk.
