Note: This is the eighth chapter of my serialized book Healthy Movements for Human Animals. You can find an archive of previous chapters here.
How strong is strong enough? The answer depends on your goal. A football lineman, a runner, and an elderly person hoping to stay independent all need strength in different amounts and forms. But if your goal is to be a generally capable and healthy human animal, a short answer might be this: strong enough to climb a tall tree.
Climbing demands that the arms and legs control and move the body against the force of gravity at a wide variety of angles and leverages. There are many climbing movements, but the key building blocks are fundamental pushing and pulling movements: pulldowns, rows, and dips for the arms; squats and step-ups for the legs. These are the same movements that form the basis of most gym training, and that’s no coincidence. Gym exercises work because they approximate the demands that shaped human anatomy over 50 million years in the trees. If climbing were an exercise class, it might be called variable single-limb bodyweight resistance training. Such a class would provide baseline functional strength, balance, mobility, and overall body coordination.
In this chapter, we’ll explore the evolutionary and developmental history of climbing, examine how it appears in hunter-gatherer populations and children, and consider the physical and coordinative demands it places on the body. We’ll also compare climbing with lifting and carrying — the other primal movements most associated with strength — and make a case that climbing-style movements deserve priority. Finally, we’ll look at some simple ways to modify common bodyweight exercises to capture more of the variability and challenge that climbing naturally provides.
Evolution of climbing
Locomotion in the trees is far more complex than locomotion on the ground. In Ground World, a quadruped can go wherever it wants by repeating the same basic crawling gait cycle. Each step looks rather like the one before, with only minor variations needed to change direction and speed. Locomotion is cyclical, repetitive, and rhythmic.
Tree World is different, because the substrate is highly irregular, three-dimensional, and in most places isn’t even there. Every other step is a gap you can fall into. The branches that support body weight can be thick or thin, stiff or flexible, angled horizontally or vertically. To navigate this environment, primates evolved a wide variety of climbing techniques, along with the coordination to execute them and the perception to determine which ones are necessary when. These demands probably account for much of the explosion in primate intelligence.
The following sections review some of the major climbing techniques identified by primatologists, arranged roughly in the order they emerged. We’ll note which ones our ancestors relied on, and which remain relatively natural for human bodies today.
Pronograde quadrupedalism
This means crawling on all fours on top of a branch, with the spine horizontal. This was the main climbing technique of the earliest primates, who were roughly the size of squirrels, and therefore relatively small compared to branches. This allowed them to walk safely on top of branches using movement patterns that were not much different from the way they crawled over flat ground.
But these techniques became less useful with increasing body size. Larger primates are too heavy to walk confidently on top of most branches, and they began relying on a different approach: reaching overhead and hanging on branches to support their weight.
Suspensory Climbing
Suspension promotes a more extended, vertical posture compared to the more compact horizontal position used in crawling-style climbing. It also opens up new possibilities for sideways movement, including pendulum-like swinging and brachiation. Human ancestors certainly engaged in a lot of suspensory behavior and were capable of at least some brachiation, as discussed in the previous chapter.
Vertical climbing
Vertical climbing involves a wide variety of movements, but here we will distinguish two basic skills: (1) climbing with the aid of horizontal branches, and (2) ascending a vertical trunk without any horizontal branches for support. This is essentially the difference between climbing a ladder and climbing a pole. Humans are quite good at climbing ladders but struggle with poles. This reveals something interesting about how our anatomy changed since our ancestors descended from the trees. To understand these changes, we can consider why chimps climb poles better than humans.
Ascending a pole requires the climber to press inward against the trunk from opposing sides with the hands or feet or both. The climber is essentially pinching the tree and creating friction. Chimps can maintain this pinch with one hand and the opposite foot while using the other diagonal to reach upward and pull down. This results in a climbing pattern that looks somewhat like crawling.
Chimps can do this because of several anatomical features: long arms, short legs and flexible ankles keep their center of mass close to the trunk; prehensile feet grip the bark like hands (chimps basically have four hands); bowed legs allow the knees to spread wide and direct forces inward with the feet. Chimps are also strong, about 1.5 times as strong as humans.
Humans have traded most of these features for bipedal efficiency. Our legs are long, our arms are short, our ankles are stiff, our feet don’t grasp, and our hips and knees are configured to apply forces straight down into the ground, rather than inward against a vertical trunk. Our muscles have also evolved to be less powerful than chimps but better adapted for endurance activities like long-distance walking and running. As a result, climbing a bare vertical trunk is difficult for humans.
Human climbers who need to climb poles often replace some of these lost adaptations with tools, such as shoes that provide friction and straps that effectively lengthen our arms.
Clambering
Clambering means climbing at variable angles — horizontally, vertically, or diagonally — with movements that are irregular, relatively slow, and changing with each step. When humans climb trees or rocks, they are usually clambering, because each movement differs from the previous one.
This requires planning and perception to determine the right path and the right series of movements to navigate it. This cognitive demand may have been one of the pressures that drove the evolution of primate intelligence.
The transition (back) to the ground
After about fifty million years in the trees, our ancestors began transitioning to bipedal locomotion on the ground. The transition started about 7 million years ago, but proceeded slowly. After about 4 million years, the australopithecines were walking upright, but they still had long arms, curved fingers, and other features of a climbing anatomy. They probably continued to sleep in the trees. A full commitment to ground-based life probably didn’t occur until the emergence of Homo erectus, around two million years ago.
In adapting to bipedalism, our legs lengthened, our feet stiffened, and our arms shortened. These changes made us better walkers and runners but worse climbers. However, as we’ll see in the next section, modern hunter-gatherers still climb frequently, and children take to trees and climbing structures easily. Thus, although we think of ourselves as ground-based creatures, our bodies still “expect” regular visits to the trees, or at least activities that simulate the demands of doing so.
Climbing in hunter-gatherers
A recent cross-cultural analysis of over fifty hunter-gatherer societies found that nearly nine in ten engaged in tree climbing, and most of those did so with considerable skill. Climbing happens in a wide variety of environments, and is used to collect fruits, nuts, hunt arboreal animals, and to ambush prey from above. Perhaps most importantly, climbing is a way to collect honey, which is a significant food source for hunter-gatherers across the world.
Honey accounts for fifteen to twenty percent of the yearly calories of the Hadza in Tanzania. The Mbuti consume roughly eighty percent of their calories from honeycomb during the 3-month honey season in the Congo. The Efe, also of the Congo, devote a third of their foraging time to honey acquisition and regularly climb to heights exceeding 20 meters. Batek men in Malaysia climb trees over fifty meters tall on a daily basis in pursuit of honey and fruit. Among the Semai, also of Malaysia, traditional marriage nuptials involved three questions of the groom, one of which was: “are you a good climber?”
From this evidence, some researchers have concluded that the biological and adaptive significance of human climbing has been underestimated.
Children and climbing
Human infants acquire climbing skills quite early compared to other physical abilities. Before age two, babies can barely walk and cannot yet run or jump. But they can climb. YouTube has numerous videos of toddlers in diapers scaling vertical walls in bouldering gyms.
Kids of all ages climb whenever they see an opportunity, scrambling up furniture, playground structures, or trees. Many of their games involve the idea that the ground is hot lava. Among hunter-gatherers, children begin contributing to real climbing tasks by around age six.
Children climb well compared to modern adults. An athletic adult would have a clear advantage over an eight-year-old in contests of speed, strength, agility, or endurance. But in a test of climbing ability in trees, on playground structures, or a bouldering gym, the adult would not be nearly so confident. This suggests that modern adults are doing less climbing than is natural.
The biomechanics of climbing
What kind of fitness does tree climbing develop? A quick summary might be that it challenges the body in ways similar to variable unilateral bodyweight strength training.
