39 Friction in Joints

Synovial Joint Friction

Static and kinetic friction are both present in joints. must be overcome, by either muscle or , in order to move. Once moving, acts to oppose motion, cause wear on joint surfaces, generate thermal energy, and make the body less efficient. (We will examine the efficiency of the body later in this textbook.)  The body uses various methods to decrease friction in joints, including synovial fluid, which serves as a lubricant to decrease the between bone surfaces in synovial joints (the majority of joints in the body). Bone surfaces in synovial joints are also covered with a layer of articular cartilage which acts with the synovial fluid to reduce friction and provides something other than the bone surface to wear away over time[1]. We ignored friction when analyzing our forearm as a lever because the frictional forces are relatively small and because they acted inside the joint, very close to the pivot point so they caused negligible torque.

This figure shows a synovial joint. The cavity between two bones contains the synovial fluid which lubricates the two joints.
Synovial joints allow for smooth movements between the adjacent bones. The joint is surrounded by an articular capsule that defines a joint cavity filled with synovial fluid. The articulating surfaces of the bones are covered by a thin layer of articular cartilage. Ligaments support the joint by holding the bones together and resisting excess or abnormal joint motions. Image Credit: OpenStax Anatomy & Physiology

Reinforcement Exercises

The cartilage-on-cartilage kinetic friction coefficient within synovial joints has been measured to be as low as 0.002[2].


  1. OpenStax, Anatomy & Physiology. OpenStax CNX. Jun 25, 2018 http://cnx.org/contents/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@10.1.
  2. Farshid Guilak, ARTHRITIS & RHEUMATISM Vol. 52, No. 6, June 2005, pp 1632–1633 DOI 10.1002/art.21051

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Body Physics 2.0 by Lawrence Davis is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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