Key result
Low-pass filtering at 10 Hz significantly underestimated time-specific force values compared to unfiltered data (p<0.001), whereas 100 Hz filtering produced acceptable differences.
Low-pass filtering of isometric midthigh pull force data results in underestimations of kinetics, suggesting that unfiltered data should be used for valid longitudinal comparisons.
Unfiltered isometric force data may be needed for accurate kinetics; leaves open optimal filtering standards for longitudinal strength research.
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.
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Dos’Santos et al. (2018) studied Healthy collegiate athletes (n=24). Low-pass filtering (10 Hz and 100 Hz) vs. Unfiltered data was evaluated on Onset threshold forces and time-specific force values (g = -0.43 to -0.99, p=<0.001). Low-pass filtering at 10 Hz significantly underestimated time-specific force values compared to unfiltered data (p<0.001), whereas 100 Hz filtering produced acceptable differences.
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