Concurrent Validity by Comparing EMG Activity between Manual Muscle Testing, Handheld Dynamometer, and Stationary Dynamometer in Testing of Maximal Isometric Quadriceps Contraction

[Purpose] The purpose of this study was to establish the concurrent validity of electromyography (EMG) activity and muscle strength scores obtained by manual muscle testing (MMT), handheld dynamometer (HHD), and the gold standard stationary dynamometer (SD). [Subjects] Forty healthy young adults (17 men, 23 women) volunteered to participate in this study. [Methods] Subjects performed maximal voluntary isometric contraction (MVIC) of the quadriceps for 5 seconds in three successive trials. Surface EMG was used for recording the muscle activity during the maximal MMT, HHD, and SD measurements. [Results] The mean and peak EMG amplitudes (ICC3,k = 0.968 to 0.980) of the three MVIC methods and muscle strength scores (Pearson r = 0.868) measured by HHD and SD showed good concurrent validity. The intra-trial reliability of mean EMG activity was also good (ICC3,1 = 0.960 to 0.989), reflecting consistent measurement between trials. [Conclusion] Our EMG results imply that MMT and HHD are useful and simple methods of measuring MVIC for EMG normalization. Considering cost, time, and clinical utility, we suggest that the use of HHD to measure MVIC for EMG normalization is highly valuable and should generate more meaningful information associated with pathological conditions or interventions for all patients.