Patient, observer and instrument variation in the measurement of strength of shoulder abductor muscles in patients with rheumatoid arthritis using a modified sphygmomanometer.

We describe the use of a modified sphygmomanometer to measure the strength of shoulder abductor muscles. Using a 5 x 5 Greco-Latin square design we compared the measurements of 5 physical therapists taken on 5 patients with rheumatoid arthritis with 2 cuff adaptations. The design variables explained 96.4% of the pressure readings, with patients accounting for 76.6%. There was no significant variation due to order of testing, cuff preparations or observers. The method is simple, portable, inexpensive, comfortable and safe to use. It can also be applied to 26 different muscle groups.