An experimental analysis of the sources of inaccuracy occurring in hip strength measurements conducted by hand held dynamometry

Abstract

Background Hand held dynamometers (HHDs) are often the preferred choice for the clinical evaluation of muscle strength, although they can be inaccurate. The aim of this study was to quantitatively investigate the causes of deviation and error in the strength assessment of the hip joint. Methods Twenty-two healthy adult participants were tested for hip extension and hip flexion strength by a ‘make test’ method. Forces and moments were measured by a multi component load cell held by an expert operator. Position and motion were concurrently recorded by an optoelectronic system. Ideally, neither the sensor nor the limbs should move during the measurements. The setup allowed to quantify the angular displacement of the sensor and the undesired motion. From such measurements, an estimation of accuracy and quality was derived. Results Observed accuracy was <4% for forces and <14% for moments. The overall quality of the measurements was ∼80% for both flexion and extension. The lower accuracy observed in hip moment was attributed to the method used to measure the lever arm. Conclusions The quantitative measurement of the sources of inaccuracy showed that HHD hip assessment is reliable for both flexion and extension. Correct HHD positioning and correct measure of the lever arm are crucial.