Hand synergies: how the brain tames the complexity of grasping

The control problem

Count the joints of your hand and wrist and you arrive at well over twenty possible movements. If the brain had to calculate every joint angle for every grasp, picking up a cup would be a large computing task. Yet people grasp unfamiliar objects quickly and reliably, even in the dark.

Neuroscientists have long suspected that the nervous system uses shortcuts. Instead of steering every joint, it seems to combine a handful of building blocks. These building blocks are called synergies.

What a synergy is

A synergy is a fixed relationship between several joints or muscles that are activated together. The simplest example is closing the hand: all fingers flex at the same time, each joint a little more or less, in a characteristic proportion. Another pattern spreads and rotates the fingers to adapt the hand to a wide or narrow object.

Researchers distinguish two kinds:

• Postural synergies describe typical combinations of joint angles, that is, the shapes the hand takes.
• Muscle or motor synergies describe groups of muscles that are activated together with a fixed timing.

Both share the same idea: a few control signals combine to produce many different movements.

What the classic experiments showed

In a much-cited study from 1998, Marco Santello, Martha Flanders and John Soechting recorded the hand postures of people who were asked to imagine grasping a large number of familiar objects, from an apple to a pencil. Using principal component analysis, a standard statistical method, they found that the first two components alone explained the large majority of the variation in hand shape. The first pattern roughly opens and closes the hand; the second adjusts it to object size and shape.

Later studies with real objects, with muscle recordings and with brain imaging found related patterns. They also showed that additional, smaller components carry the fine detail that makes a grasp precise. In short: the basic shape comes from a few synergies, the finishing touch comes from many small corrections, often guided by touch.

From neuroscience to robot hands

Synergies offered engineers a way out of the dilemma between simple grippers and complex anthropomorphic hands. If a few patterns explain most human grasps, a robot hand could be built to produce exactly those patterns with very few motors.

Two ways of using the idea have developed:

1. Synergies in software. A fully actuated hand is controlled through a small number of synergy commands instead of individual joint angles. This makes grasp planning faster and the motions look more natural.
2. Synergies in hardware. The pattern is built into the mechanics, for example with tendons, pulleys and elastic joints, so that one motor drives the whole hand along the first synergy. Combined with compliance, the hand then adapts its shape to the object. Our explainer on soft and underactuated robotic hands describes these designs.

Researchers coined the term adaptive synergy for this combination: the motor sets the main pattern, and the softness of the joints lets the hand deviate from it where the object pushes back.

Synergies and touch

Synergies also play a role in perception. When we explore an object by touch, the hand moves in typical patterns, and the brain interprets the resulting sensations in the context of those movements. Some research groups have studied whether touch feedback can be organised along synergies too, for example to give users of haptic devices or prostheses a simpler, more intuitive signal. More on how touch works and how it can be recreated technically is on our page about haptic perception and touch feedback.

Why it matters for prosthetic hands

A person using a myoelectric prosthesis can usually produce only a few reliable control signals with the muscles of the residual limb. A hand that needs a separate command for every finger would be exhausting to use. Synergy-based designs let a single “open” and “close” signal produce a natural, adaptive grasp, and some prosthetic hands are based on this principle. Our guide to bionic and myoelectric prosthetic hands explains the control options in more detail.

Limits of the synergy idea

Synergies are a powerful simplification, not a complete theory of the hand. Skilled tasks such as playing a violin, writing or manipulating small objects use finely tuned, individual finger movements that go well beyond the first few patterns. Scientists also debate whether synergies are built into the nervous system or simply reflect the tasks and mechanics of the hand. For robot designers this debate matters less: the pattern is useful either way.

Summary

Hand synergies describe how a few coordinated patterns produce most everyday grasps. The idea links neuroscience with engineering: it explains how the brain keeps grasping manageable and shows how robot and prosthetic hands can grasp well with very few motors. For the bigger picture, return to the overview of robot hands and the sense of touch.