Effect of Low-Pass Filtering on Isometric Midthigh Pull Kinetics

Dos'Santos, T, Lake, J, Jones, PA, and Comfort, P. Effect of low-pass filtering on isometric midthigh pull kinetics. J Strength Cond Res 32(4): 983-989, 2018-The purpose of this study was to investigate the effect of low-pass filtering on isometric midthigh pull (IMTP) kinetics, including body weight (BW), onset threshold force, time-specific force values (50, 100, 150, and 200 ms), and peak force (PF). Forty IMTP trials from 24 collegiate athletes (age: 21.2 ± 1.8 years, height: 1.72 ± 0.09 m, mass: 79.4 ± 8.2 kg) were analyzed and compared using unfiltered (UF) and low-pass filtered (LPF) (fourth-order Butterworth) with cutoff frequencies of 10 (LPF10) and 100 (LPF100) Hz. Significantly lower (p < 0.001, g = -0.43 to -0.99) onset threshold forces were produced when force data were LPF. This led to significant (p < 0.001, g = 0.05-0.21) underestimations of time-specific force values when LPF10 compared with UF, displaying unacceptable percentage differences (1.2-3.3%) and unacceptable limits of agreement (LOA) (-25.4 to 100.3 N). Although significantly different (p ≤ 0.049), trivial (g ≤ 0.04) and acceptable percentage differences (≤0.8%) and acceptable LOA (-28.0 to 46.2 N) in time-specific force values were observed between UF and LPF100. Statistically significant (p < 0.001), yet trivial (g ≤ 0.03), and acceptable percentage differences (≤0.7%) and acceptable LOA (-4.7 to 33.9 N) were demonstrated in PF between filtering conditions. No significant differences (p = 1.000) and identical BW values were observed between filtering conditions. Low-pass filtering results in underestimations in IMTP kinetics; however, these differences are acceptable between LPF100 and UF but unacceptable between LPF10 and UF (excluding PF). Filtering procedures should be standardized when longitudinally monitoring changes in IMTP force-time characteristics to allow valid comparisons, with analysis of UF data recommended.