Reliability and validity of an enhanced paper grip test; A simple clinical test for assessing lower limb strength

Background: The paper-grip-test (PGT) involves pulling a small card from underneath the participant's foot while asking them to grip with their hallux. The PGT is shown to be effective in detecting foot muscle-weakening but its outcome is operator-dependent. To overcome this limitation, an enhanced PGT (EPGT) is proposed that replaces the pass/fail outcome of the PGT with a continuous measurement of the pulling force that is needed to remove the card (EPGT-force). Research question: Is the EPGT-force an accurate, reliable and clinically applicable measurement of strength? Methods: Reliability and clinical applicability were examined in two ways. Firstly, two examiners measured EPGT-force for twenty healthy volunteers in a test/retest set-up. EPGT force was measured using a dynamometer, the hallux grip force was measured using a pressure mat. The clinical applicability of the EPGT was tested in ten people with diabetes. Postural sway was also measured. Results: Interclass correlation coefficients (ICC) revealed excellent inter-rater reliability (ICC > 0.75). Intra-rater reliability was excellent for the first examiner (ICC = 0.795) and good for the second (ICC = 0.703). Linear regression analysis indicated that hallux grip force accounted (on average) for 83 %±4 % of the variability in EPGT force. This strong relationship between EPGT force and hallux grip force remained when the test was performed in a clinical setting with the latter accounting for 88 % in EPGT force variability. Spearman rank order correlation showed that people with diabetes with a higher difference in EPGT force between limbs swayed more. Significance: EPGT force is a reliable and accurate measurement of hallux grip force. Hallux grip force was previously found to be strongly correlated to the strength of all muscle groups of the foot and ankle and to the ability to maintain balance. The proposed EPGT could be used to monitor muscle weakness in clinics for better falls-risk assessment.