Concurrent validity of an isokinetic lift test used for admission to the Swedish Armed Forces

The aim of this study was to assess the concurrent validity of the IsoKai isokinetic lift test peak force (IsoKaiPeak) in comparison to a submaximal 5-10RM deadlift test (5-10RMDL), and to develop an equation for converting the IsoKaiPeak in Newton (N) to an estimated 1RM (1RMest) deadlift load in kilograms (kg). The participants included 28 males and 16 female employees in the Swedish Armed Forces (20–59 years). Each participant conducted the IsoKai lift test, followed by the 5-10RMDL test at one occasion. The Pearson′s correlation coefficient, with a 95% confidence interval was calculated to evaluate the validity between the IsoKaiPeak and the 1RMest deadlift load derived from the 5-10RMDL test. Univariate and multivariable linear regressions were used to derive the equation for calculating the 1RMest deadlift load based on the IsoKaiPeak. The IsoKaiPeak showed good- to-excellent correlation with the 1RMest deadlift weight with a correlation coefficient of 0.84 (0.72–0.91) for the total sample, and 0.65 (0.37–0.83) and 0.81 (0.53–0.93) in males and females, respectively. The final equation, 1RMest deadlift weight (kg) = -51.63 + (0.08 x IsoKaiPeak) + (2.28 x BMI), explained 72% (adjusted R2 = 0.72) of the total variance in the 1RMest, and had a standard error of the estimate (SEE) of 16.57 kg. In conclusion, the IsoKai isokinetic lift test could be considered a highly valid measure of maximal dynamic muscular strength in comparison to the 5-10RMDL. The equation can be used to convert the IsoKai lift test (N) results to an 1RMest deadlift load (kg), but with consideration of the relative large SEE.