The Physics
Factbook
An encyclopedia of scientific essays

Coefficients of Friction of Human Joints

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Bibliographic Entry Result
(w/surrounding text)
Standardized
Result
Serway, Raymond, and Jerry Faughn. College Physics. 6th edition. Canada: Brooks/Cole-Thomson Learning, 2003: 101.
  μs μk
synovial joints in humans 0.01 0.003
μs = 0.01
μk = 0.003
Table of Friction Coefficients. Encarta. 2007. 23 May 2007.
  static friction kinetic friction
synovial joints (in humans) 0.01 0.01
μs = 0.01
μk = 0.01
Williams, Mark. SNOW HYDROLOGY (GEOG 4321):SKIING AT THE TRIPLE POINT. University of Colorado, Boulder. 23 May 2007.
Surfaces in Contact Static μs Kinetic μk
Human Joints (Synovial Lubricant) 0.01 0.003
μs = 0.01
μk = 0.003
Friction Center Coefficient Database. 2005. Southern Illinois University. 23 May 2007.
Friction Couples Static Coefficient Kinetic Coefficient
synovial joints (humans) 0.01 0.003
μs = 0.01
μk = 0.003
Linn, Frank C. Lubrication of Animal Joints: I. The Arthrotripsometer. Journal of Bone and Joint Surgery. Vol. 49 (1967): 1079-1098. [see table 1 below] μ = 0.0053
Hills, and Butler. Surfactants Identified in Synovial Fluid and their Ability to Act as Boundary Lubricants. Annals of the Rheumatic Diseases (1984): 641-648. "These absorbed monolayers of synovial surfactant were then found to be excellent boundary lubricants in vitro, reducing the coefficient of kinetic friction (mu) in the dry state and under physiological loading by up to 97% for extracts and 99% for PC alone, reaching mu = 0.01." μ = 0.01
Table 1: Reproducibility of Friction Data, Sliding Coefficient of Friction
  Day Tested
Condition before Mounting Lubricant 1 2 3 4
Fresh (2)
Frozen (1)
Synovial fluid 0.0048±0.0012
(3)
0.0050±0.0008
(3)
0.006±0.0007
(3)
0.0053±0.0008
(3)

We run, we walk, and we swing our arms back and forth each day. Behind our movements lie synovial joints that allow most of our motions. The term, Synovial, comes from the word Synovium, which has a Latin origin meaning "with egg" because the Synovial fluid is similar to egg white. The entire joint is surrounded by a joint capsule. Within the joint capsule, we have articular cartilage, which provides a smooth surface for the ends of our bones to slide during movement. There are also layers of fibro-cartilage in regions where the joints don't fit well together. The Synovial membrane that surrounds the joint and lies beneath the capsule secretes Synovial fluid. Synovial fluid reduces friction in our joints and helps to maintain and lubricate the articular cartilage to allow us to move freely.

Friction is the force that opposes the direction of motion. There are two types of friction, static and kinetic. There is static friction when objects are at rest. When objects move against each other, there is kinetic friction. Static friction tends to be greater than kinetic friction.

The coefficient of friction is the ratio of friction over normal force and is represented by the Greek letter mu, μ.

μ = f/N

Each individual material has its own coefficients of static and kinetic frictions. The coefficient can also change due to factors such as temperature, the magnitude of the normal force, and speed. Coefficients of friction are always positive and most tend to range from 0 to 1. Low coefficients of friction indicate that things slide off the material easily and high coefficients of friction indicate that things don't. The coefficient of Synovial joints is similar for both humans and dogs. In 1968, it was reported to be 0.0044. Currently, the coefficient of friction is approximately 0.01, which is low since our joints have to be able to glide easily when we move.

Connie Qiu -- 2007