Smartphone-based accelerometry is a valid tool for measuring dynamic changes in knee extension range of motion

Measurement of static joint range of motion is used extensively in orthopaedic and rehabilitative communities to benchmark treatment efficacy. Static measures are, however, insufficient in providing detailed information about patient impairments. Dynamic range of motion measures could provide more detailed information about patient impairments thus leading to better clinical assessments. Reliable and valid methods are available, but due to limitations in the present technology, dynamic measures are seldom performed in clinical settings. The objective of this study was to determine the validity of smartphone-based accelerometry measuring the dynamic range of motion of the knee joint during a passively executed extension movement.

Dynamic knee extension range of motion was examined three consecutive times in twenty-one healthy male subjects utilising an isokinetic dynamometer to generate passively the extension motion. Measurements of joint angles in dynamic knee extension were performed using two methods: (i) isokinetic dynamometer (gold-standard method, Biodex System 4 Pro) and (ii) smartphone (iPhone 6, attached to the tibia) accelerometry data.

Tests of validity showed excellent correlation (rs=0.899) between methods, with a low standard error of measurement of 0.62deg. and limits of agreement ranging from -9.1 to 8.8deg. Interclass correlation coefficients showed excellent between-measures reliability (ICC>0.862) for both methods.

Smartphone-based accelerometry is a valid tool for measuring the range of motion at the knee joint during dynamic extension movements. This method enables the clinician to carry out simple, low cost, and valid clinical measurements of dynamic knee extension range of motion.