Isometric load cell systems provide more limited information since strength is measured only at a specific point in the range of motion. Such equipment is usually impractical in the clinical setting, however, due to expense, space requirements, specialized technical training, and a limited number of muscle groups which can be reliably tested. The HHD consists of a simple hand-held device equipped with a small internal load cell capable of measuring muscular force. The maximum force is then recorded. The first critical step in evaluating a measurement method is to determine its reliability.
Reliability refers to the ability of the instrument to repeatably measure the same number.
A reliable instrument is consistent. There are two kinds of reliability. One is intratester reliability and the other is intertester reliability. Intratester reliability refers to whether the device is reliable when testing is administered repeatably by the same person. Intertester reliability refers to whether the device is reliable when testing is administered by different persons.