This study examines the hypothesis that force/distance curve variability distinguishes submaximal from maximal efforts in isokinetic trunk and lifting strength tests. Thirty normal subjects were tested on the Cybex Trunk Extension/Flexion (TEF) and Liftask (LT) machines during maximal (100%) and submaximal (50%) efforts. Considering each test separately, visual assessments of curve variability were indeterminate of degree of effort in 28% of TEF and 34% of LT tests. Measurement models of curve variability were more clearly discriminating. When a given subject's test curves were considered together, scaled visual assessments identified the degree of effort in 91% of TEF and 86% of LT results. The measurement models were accurate 90-92% of TEF and 79-92% of LT results. Clinical judgment is required in evaluating effort during tests of isokinetic trunk and lifting strength.