The force plate has been the most commonly used device to assess leg extensor muscle function under stretch-shortening cycle conditions and is regarded as the gold standard in the measurement of vertical jump performance. Nonetheless, the cost and technical complexities of such a device precludes its routine use within many sporting environments spawning the development of many proposed alternatives. However, of these alternatives only contact mat devices can assess the full spectrum of muscle function due to their ability to quantify both jump height and ground contact time. PURPOSE: To determine the concurrent validity of a portable device using a force plate as a criterion reference. METHODS: Four men (body mass 74.4 ± 10.7 kg), competing in power sports (athletics, basketball, rugby) performed ten squat (SJ), countermovement (CMJ) and drop jumps (DJ) on a contact mat placed on a force plate. A force threshold of 5N was used to determine flight and contact phases. To ensure that measurements were not influenced by the weight of the contact mat the force plate was zeroed with the contact mat placed on the force plate. Flight time was used to quantify jump height using standard methods and the reactive strength index (RSI) was calculated as jump height divided by contact time. To examine the validity of the portable device, the method of comparison as described by Bland and Altman was used. Differences were calculated by subtracting force plate measurements from those of the portable device. RESULTS: Systematic bias was evident for all measures of jump performance (p<0.01), namely jump height and RSI. The existence of non-significant correlations (p>0.05) between the absolute differences in jump height and the means revealed no evidence of heteroscedasticity for both SJ and CMJ. The resulting error interval was 1.93 ± 1.87 cm and 1.92 ± 2.09 cm for SJ and CMJ heights, respectively. A significant correlation (r = 0.44; p<0.01) between the absolute differences in RSI and the means revealed evidence of heteroscedasticity. Therefore, the differences in RSI were expressed as a percentage of the mean and revealed no further evidence of heteroscedasticity. The resulting error interval was 12.5 ± 7.1 %. CONCLUSIONS: The results from the portable device cannot be compared directly with other similar devices or used interchangeably with force plate measurements due to the significant bias evident. The bias may partially be explained by a lower sensitivity to force in the portable device when compared to a force plate. The measurement precision established for the portable device should be carefully considered when interpreting changes in vertical jump performance. PRACTICAL APPLICATIONS: The portable device can be recommended to monitor adaptations to training in vertical jump performance as it possesses high applicability within field based sports testing. Changes in performance should be considered in absolute terms for SJ and CMJ whereas it is strongly advised that changes in DJ performance be expressed in relative terms as a percentage to account for the heteroscedasticity revealed in RSI.