Shadow Robot Company Ltd.
251 Liverpool Road
N1 1LX

The Air Muscle is an extraordinary actuator that is small, light, simple and 'friendly'. It is soft, has no stiction, is easily controllable and exceptionally powerful.

Advantages of the Air Muscle

Lightweight Air Muscles weigh as little as 10 grammes - particularly useful for weight-critical applications.
Lower Cost Air Muscles are cheaper to buy and install than other actuators and pneumatic cylinders.
Smooth Air Muscles have no 'stiction' and have an immediate response. This results in smooth and natural movement.
Flexible Air Muscles can be operated when twisted axially, bent round a corner, and need no precise aligning.
Powerful Air Muscles produce an incredible force especially when fully stretched.
Damped Air Muscles are self-dampening when contracting (speed of motion tends to zero), and their flexible material makes them inherently cushioned when extending.
Compliant Being a soft actuator, Air Muscles systems are inherently compliant.

A Shadow 30mm Air Muscle ready to bend a nailA Shadow 30mm Air Muscle bending a nail

The Shadow Air Muscle is a simple yet powerful device for providing a pulling force. It behaves in a very similar way to a biological muscle. When actuated with a supply of compressed air, they contract by up to 40% of its original length. The force it provides decreases as it contracts, and the first few percent of the contraction is very powerful indeed.

Animation of the Shadow Air Muscle - by Mark Worsdall
The simplest use of an Air Muscle is to move a lever. One muscle will pull the lever in one direction, and a spring can return it. Two muscles will allow the lever to be pulled in either direction, with considerable force. Because the muscle contracts over a known distance, it can be used to provide a safe movement: there is no need to ensure that the lever is not going to be rotated beyond its end-stop, because the muscle will only move the lever to its set up angle.

Image showing the adjustment of lever and muscle


Air Muscle Construction
The Air Muscle consists of a rubber tube covered in tough plastic netting which shortens in length like a human muscle when inflated with compressed air at low pressure. An Air Muscle has a power-to-weight ratio as high as 400:1, vastly outperforming both pneumatic cylinders and DC motors that can attain a ratio of only about 16:1. It has been in continuous development for advanced robotics work by Shadow since 1982, and is now available for use in a variety of applications as a powerful, lightweight actuator. Air Muscles are normally operated using compressed air in the 0-60psi (0-4 bar) range.

The Core of an Air Muscle is a rubber tube....

Rubber Tube infini-d 4.5

....wrapped in a tough plastic weave....

Rubber tube with weave - infini-d 4.5

....which shortens in a scissor action when pulled out, just like a Chinese finger puzzle. As the rubber tube fills with air it is forced to expand.

A small Air Muscle, at just 6mm in diameter, has the strength, speed and fine stroke of a finger muscle in a human hand. An Air Muscle 30mm in diameter is capable of lifting more than 70 Kg at a pressure of only four bar, while a large muscle (50mm) has enough power to pull down a brick wall.

Most of the robots built by the Shadow Project are driven by Air muscles, and looking at these will provide much insight on the possible uses of the Air Muscle.

Shadow Air Muscle Range
Thumbnails / Products Braid
fitting size
Pull at
6mm Thumbnail  6mm  150mm
 4mm  3 kg  7kg
20 mm Thumbnail  20mm  210mm
 4mm  12 kg  20kg
30 mm Thumbnail  30mm  290mm
 6mm  35 kg  70kg
 Please note: Muscle lengths differing from the list below can also be supplied.


Length measurements fully stretched: are between the attachment points when the muscle is fully stretched and therefore include the muscle ends which do not contract.

Operating restrictions: when not under load, muscles should not be inflated beyond 2 bar. When under load, muscle life may be reduced if pressures greater than 4 bar are used.

Air Muscles are not designed, intended, authorised or warranted for use in life support applications, devices or other critical applications.

Fully stretched 20mm Air Muscle
A 20mm Air Muscle fully stretched to 210mm

Fully stretched 30mm Air Muscle
A 30mm Air Muscle fully stretched to 290mm

A 30mm Air Muscle Header
A 30mm Air Muscle header

The Shadow Starter Kit

Shadow Air mMuscle Starter Kit
Shadow Air Muscle Starter Kit
  • One standard 15cm (stretched) air muscle.
  • Three-way-valve for controlling the air flow.
  • Bottle cap adapter for attaching an air reservoir.
  • Fizzy drinks bottle to be used as the air reservoir.
  • Footpump adapter for connecting a car footpump to the 3mm air line.
  • Tee piece, in case you want to connect one footpump to more than one system.
  • Two releasable nylon cable ties, to attach the muscle to your device.
  • Plus a length of 3mm air line which can be cut and push-fitted into the other components.
  • NB You will need to supply a foot pump for charging the reservoir.
© Copyright Shadow Robot Company Ltd.

Phone: +44 (0)207 700 2487
251 Liverpool Road, London, N1 1LX, UK

View Legal Information