Goniometers, Potentiometers and Electrogoniometers

1. What is a Goniometer?

What is a Goniometer? A goniometer is a simple hand-held device for measur-ing joint angles. A goniometer is, in essence, a protrac-tor that features two arms: one that remains fixed and one which is positioned, accordingly allowing the desired angle to be measured. There are several types of goniometer, all giving a crude but useful measure of angles and range of motion. Clinically these allow a quick and useful assessment of static angles. However, these devices are of little use in measuring angles dynamically during different movement tasks. There are two main types of goniometer, the hinged 'ruler' and the fluid filled. The fluid-filled types are generally more versatile in the angles they can measure, but they measure relative to gravity so, if not used correctly, can be prone to errors if the subject moves whilst taking readings

2. What are Potentiometers and Electrogoniometers?

Potentiometers measure the change in linear or angular displacement by recording the change in voltage output. The potentiometer is positioned over a joint centre of rotation, with each arm orientated along the long axis of the proximal and distal segments. Changes in the angle of the joint will cause the voltage output of the device to change proportionally to the change in joint angle. Angular displacement potentiometers allow movement about one plane to be measured, and potentiometers are typically employed to measure sag-ittal plane movement only. Potentiometers are gener-ally a robust 'student proof' and low-cost method of collecting kinematic data, which can allow real-time  data to be observed. However, they are suitable only they for hinge joins and, if more than one joint is being can assessed at one time, then they may encumber the king movement slightly, thus reducing the validity of the data being acquired. One example of potentiometer is produced by MIE (Leeds, UK)

Electrogoniometers are very thin pieces of wire which are sensitive to bending. The amount of bending ltage changes the output voltage. These can be sensitive to joint angular movement in up to three planes simultane-the ously, although biaxial and uniaxial electrogoniome-nges ters are more common (biaxial referring to the measure rtput of flexion—extension and abduction—adduction of a  joint simultaneously). These are relatively inexpensive, accurate and reasonably unintrusive and so minimize and any gait modification.

3. Development of Electrogoniometers

 Electrogoniometers have been in development since time the early 1970s. Early devices were bulky potentiometers, sometimes referred to as (potentio-metric goniometers, which were attached to the body segments with rigid bars. Various studies were carried out on normal subjects but they were cumbersome to wear, especially if mul-tiple joints were to be studied. For these reasons the clinical use of such devices was limited. Nicol (1987) described a new flexible electrogoni-ometer which used strain gauge wire. This eliminated the use of rigid bars and made it possible for the elec-trogoniometers to be very light. This breakthrough in relatively cheap and unintrusive joint motion analysis gave rise to a plethora of publications on movement analysis of both normal subjects and patients on a variety of joints. These included studies of the hip and knee motion in pre- and postoperative hip-joint-replacement patients (Rowe et al., 1989), dynamic analysis of wrist circumduction in normal subjects and subjects with wrist disorders  the asessment of subtalar motion (Ball & Johnson, 1993) and active and passive ranges of movement of the ankle and knee motion in children with hemiplegic cerebral palsy (Hazlewood et al., 1994)

4. Accuracy of Electrogoniometers and Potentiometers

 It is not uncommon for the accuracy of angular displacement of video-based motion analysis to be assessed in comparison with electrogoniometric techniques. However, for this to be useful the possible sources for error using electrogoniometers need to be considered.

Possible sources of error when assessing human motion include:

■ Movement of the end blocks of the electrogoniometer on soft tissue around a joint;

■ Placement of the electrogoniometer in the correct plane of interest to eliminate crosstalk between two planes of motion, e.g. adduction and abduction, or rotation with sagittal plane movement; or

■ The limiting mechanical properties of the strain gauge wire giving a finite accuracy of angular displacement measurements.

 When used to study the accuracy of video-based motion analysis systems, if set up carefully, all but the last source of error should be eliminated.

Nicol (1987, 1989) reported that the error in the measurement of angular displacements due to the mechanical proper-ties of the strain gauge wire was within 1°. Therefore, accurate joint angular motion may be attained with careful placement of electrogoniometers