This study was set up to test our hypothesis that using the Jamar dynamometer a voluntary submaximal effort has a greater variability compared to a real maximal effort, especially when there is a long interval between different measurements. We tested 32 volunteers without a history of upper limb injury or operation in our hospital, with an interval of four to five weeks between tests. Each volunteer was asked to grip first right-handed then left-handed, first with a real maximal effort and then with a submaximal effort. This test was performed twice during the same session. The same individuals were seen again one month later and were studied using the same protocol. Sensitivities and specificities for the grip test were calculated using six different criteria to indicate a submaximal effort. Repeated grip testing with a time interval of one month cannot reliably detect voluntary submaximal effort in healthy volunteers.