Effect of Different Onset Thresholds on Isometric Midthigh Pull Force-Time Variables

Dos'Santos, T, Jones, PA, Comfort, P, and Thomas, C. Effect of different onset thresholds on isometric midthigh pull force-time variables. J Strength Cond Res 31(12): 3463-3473, 2017-Various thresholds have been used to identify the onset of contraction during isometric midthigh pull (IMTP); however, no agreed onset threshold exists for this assessment. The purpose of this study was to compare relative body weight (BW) and arbitrary onset thresholds to a criterion onset threshold 5 SDs of BW for IMTP force-time variables; force at each threshold, peak force, time-specific force values (100, 150, and 200 ms), and rate of force development (RFD) during 0-100, 0-150, 0-200 ms. Academy rugby league players (n = 9, age: 18.5 ± 0.4 years; height: 1.82 ± 0.09 m; mass: 91.2 ± 13.1 kg) performed 2 IMTP trials on a force platform sampling at 1,000 Hz. The neutral force-time data pool (18 trials) was analyzed with five different thresholds and compared with criterion threshold to determine any variance in force-time variables. Five SDs of BW were significantly lower than 10% BW and 75 N for threshold force which led to significantly greater time-specific force values at 100 and 150 ms and unacceptable limits of agreements (LOA) for all force-time variables. No significant differences (p > 0.05) were observed between 2.5% and 5 SDs of BW; and between 5% and 5 SDs of BW for threshold force and all force-time variables with acceptable LOA. The 5 SDs of BW and 2.5% BW onset thresholds consistently resulted in the lowest values for threshold force, time-specific force values, and RFD, attributed to a lower onset bias. Therefore, scientists and practitioners are recommended to use a 5 SD of BW onset threshold for time-specific force values and RFD for accurate data because it accounts for signal noise during the weighing period. Subsequently, there is greater certainty that the onset of contraction identifies a true meaningful change in force, in contrast to relative BW thresholds.