A degree of freedom is a plane of movement. The concept of degrees of freedom is central to kinematics.
Consider a ship moving. It is moving in three dimensions.
- First, up and down in a vertical plane
- Second, left and right in a horizontal plane;
- Third backward and forward, also in a horizontal plane.
Each of these corresponds to one degree of freedom.
Within each of these planes, or axes of movement, an object can rotate thus adding a degree of freedom in each plane and making a total of 3 rotational degrees of freedom.
The human upper limb has been studied in detail and degrees of freedom are important for constructing upper limb prosthetics especially the robotic prosthetics.
Before we go further, let us understand the basic definitions of movements
- Roll – a rotational movement.
- Yaw – a sideways movement in a horizontal plane
- Pitch – an up and down movement in a vertical plane
Human upper limb, excluding the hand, has seven degrees of freedom.
Here are those 7 degrees of freedom. Three of these degrees of freedom are in the shoulder, one in the elbow and again three in the wrist.
First Degree: Shoulder Pitch
To find shoulder pitch, point your entire arm straight and move your shoulder up and down.
Second Degree: Arm Yaw
Point your entire arm straight and move your entire arm from side to side.
Third Degree: Shoulder Roll
Point your entire arm straight and roll your entire arm from the shoulder, as if you were screwing in a light bulb.
Fourth Degree: Elbow Pitch
Point your entire arm straight and then bend only your elbow. The forearm can move up and down.
Fifth Degree: Wrist Pitch
Point your entire arm straight and without moving your shoulder or elbow, flex your wrist up and down.
Sixth Degree: Wrist Yaw
Point your entire arm straight and without moving your shoulder or elbow, flex your wrist from side to side.
Seventh Degree: Wrist Roll
Point your entire arm straight and without moving your shoulder or elbow, rotate your wrist, as if you were turning a doorknob.
Distal to wrist hand is said to have 23 degrees of freedom as shown in the following image.
Each digit except thumb has 4 degrees of freedom, one for each PIP, PIP and MCP joints and one for abduction-adduction.
Thumb due to its complex motion possesses 5 degrees of freedom. Carpometacarpal joints of 4th and 5th digit are said to have one degree of freedom each.
- Whole Hand Input. Dissertation by David Joel Sturman M.S., Rutgers University (1981). Massachusetts Institute of Technology (1979)