Abstract: Objective To quantize the synergistic force in movement of upper limbs between shoulders and elbows. Methods The transverse forces of elbows and shoulders during movement were recorded in a healthy adult with an upper-limb-force-measuring plate form which comprised of 2 three-dimensional force sensors, respectively. Then he performed shoulder abduction/adduction and elbow extension/ flexion at 100%, 75%, 50% and 25% of the maximum contraction force, respectively. The ratio of the active action force and the joint action force (named assessment index) was used to assess the synergistic degree of the forearm and the upper arm. Results In the shoulder abduction motion, the assessment index decreased as the strength of active action decreased, meant interference of joint action increased. However, it was almost stable in the shoulder adduction, increased in the elbow extensionas, and was irregular in the elbow flexion, as the active action strength decreased, respectively. Conclusion It may be more difficult to control upper arm than the forearm.

Key words: associated movement, synergistic movement, shoulder, elbow, assessment index