Grip strength is used as a measure of capacity in disability and personal injury claims. Although seemingly an objective measure, it is widely recognized that it is fallible to the effects of sub-maximal effort. We have developed an extended test protocol that uses the five positions of a Jamar dynamometer. The protocol mitigates the effect of inconsistency while retaining the ability to detect sub-maximal effort. Dynamometry was undertaken in 242 volunteers (male:female, 124:118) with a median age of 39 years (range 18–89, interquartile range 29–52). Normative values for consistency, curve deviation and variation from the expected curve pattern are provided. None of these measures were significantly affected by laterality, hand dominance, sex or age despite the effects of these variables on grip strength. The study defines the methodology and reference values to allow assessment of both the capacity of a patient and the reliability of the test for use in medicolegal practice. Level of evidence: II