A two-toed sloth hanging beneath a branch in a green Costa Rican rainforest canopy

How Can a Sloth Hang All Day With So Little Muscle?

A sloth can spend hours hanging beneath a branch with a body that contains surprisingly little muscle. That sounds like a design flaw. In reality, it is a brilliant lesson in doing more with less.

Sloths do not solve gravity by becoming furry bodybuilders. Their limbs, muscle fibres and joints are arranged to produce the kind of force that matters in the canopy while keeping energy costs low. New studies published in 2026 have now mapped the forelimb muscles of Hoffmann's two-toed sloths in exceptional detail, adding fresh evidence to a long-running question: how can such a low-energy animal be so secure beneath a branch?

The answer is not one super-muscle. It is a complete system built around pulling, gripping and avoiding unnecessary effort.

Sloths really do have less muscle

The contrast is genuine. The Smithsonian's two-toed sloth fact sheet explains that these animals have reduced muscle mass alongside a metabolic rate far below what would be expected for their body weight. That combination helps conserve energy, but it also means a sloth cannot afford wasteful movement.

Reduced muscle does not mean weak everywhere. It means the available muscle has to be placed and used carefully. A sloth's everyday world is mostly below the branch, where the limbs work in tension as the body hangs underneath. That is different from a dog, cat or human pushing down against the ground.

For a suspensory animal, pulling muscles, secure joints and curved claws matter more than the explosive power needed for sprinting. The whole body can therefore be tuned for dependable support rather than speed.

A new map revealed 59 forelimb muscles

In the first of two companion studies, researchers produced a detailed map of the forelimb of Hoffmann's two-toed sloths. The 2026 Journal of Anatomy study documented the origins, attachment points, actions and fibre directions of 59 muscles.

The team found several arrangements that differ from those described in three-toed sloths. These included a connected chain of muscles spanning the shoulder and elbow, plus extra muscle bellies in some areas. The researchers interpreted that complexity as consistent with the two-toed sloth's frequent use of prolonged hanging and below-branch walking.

This distinction matters. Two-toed and three-toed sloths have evolved many similar features, but they are not interchangeable models. The new work focused on Choloepus hoffmanni, so its exact measurements should not be casually applied to every living sloth species.

The flexors do the heavy pulling

The companion muscle architecture study examined how the shape and arrangement of those muscles affect force, movement and leverage. Across the forelimb, flexor muscles were larger than their extensor partners.

That pattern makes sense beneath a branch. Flexors help pull the limb and digits into a secure position. Strong shoulder and elbow flexors can also generate substantial turning force at the joints, known as torque. Long moment arms give some of these muscles useful mechanical advantage, allowing the sloth to produce effective joint force without simply adding more muscle mass.

Other muscles favour a longer range of movement. In other words, the limb is not built around one setting. Different muscle groups balance stability, leverage and controlled motion, giving the sloth both a strong hold and enough reach to move through an uneven canopy.

The grip estimate is remarkable

The 2026 architecture study estimated that the digital flexors in Hoffmann's two-toed sloths could produce a combined maximum force of about 2.2 times body weight. This is an anatomical estimate, not a claim that every sloth grips every branch at maximum force all day. It does show how much force-producing capacity is concentrated where the animal needs it most.

The claws add another advantage. They hook around branches, so the sloth is not relying on fingertip friction in the way a person would while hanging from a bar. Muscles, tendons, joints and claws share the work as one system.

This also explains why a resting sloth should not be mistaken for a helpless animal. Slow movement and low muscle mass are part of an efficient design, not evidence that the animal lacks strength.

Hanging still can be surprisingly cheap

An earlier experiment looked at what the muscles do while a sloth is actually hanging. Researchers recorded electrical activity in the forelimbs of six brown-throated three-toed sloths. The study found very low muscle activation during stationary hanging, with more activity needed for below-branch walking and vertical climbing.

The experiment involved a different sloth genus, so it should not be treated as a direct test of the Hoffmann's sloths in the 2026 anatomy work. Together, however, the studies reveal a useful pattern. Sloths are not constantly straining at full effort. Their bodies can maintain a hanging posture with relatively little active muscle work, then recruit more support when movement demands it.

That is crucial for an animal living on a tight energy budget. Saving a small amount of effort every minute becomes a major advantage over hours spent resting, feeding and travelling beneath branches.

Slow fibres help resist fatigue

Muscle quality matters as much as quantity. A study of two-toed and three-toed sloth forelimbs found a broad distribution of large, slow muscle fibres alongside smaller fast fibres. The researchers linked these properties with sustained force and fatigue resistance during suspension.

That does not mean the chemistry is simple. The work also found an unusual mix of metabolic features, showing that labels such as “slow muscle” only tell part of the story. Sloth muscles are specialised tissues with their own balance of fibre types and energy pathways.

The safest summary is that sloths combine reduced muscle mass with strategic architecture, strong flexors, useful leverage and muscle properties suited to long periods of support. They are economical rather than underpowered.

A body built for one very particular world

A sloth's strength makes sense only in context. On the forest floor, the same long limbs and curved claws make movement awkward. Beneath connected rainforest branches, they become an elegant suspension system.

That connection between anatomy and habitat is also a conservation lesson. A body perfected for moving through a continuous canopy cannot simply switch to roads, lawns or power lines when trees are removed. Protecting and reconnecting rainforest canopy preserves the physical world that sloth muscles, joints and claws evolved to use.

So, how does a sloth hang all day with so little muscle? By wasting almost nothing, putting strength in the right places and letting anatomy do as much of the work as possible.

Sources and image credit

Featured image: Koen Swiers via Pexels. The Pexels photo page and licence were checked before publication.

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