To the best of our knowledge, this is the first study to evaluate the intra-trial, intra-, and inter-day reliability of vibration parameters during treadmill running. The findings demonstrated that most of the INPUT signal and GAS STV parameters, except for damping coefficient and setting time, reached excellent intra-trial and intra-day reliability from the first 10 steps. The reliability of all the parameters decreased significantly from intra- trial to intra- and inter-day comparison, indicating that artificial factors such as reequipping subjects in an identical manner remain challenging. Comparing the CV of all the parameters obtained with FFT and CWT tended to demonstrate very few differences. However, it is essential to note that the ICC calculated for a given parameter is comparable between the two methods. Intra- trial analysis of D1-A demonstrated good to excellent reliability for 9 INPUT variables, 10 GAS STV variables, and 4 VL STV variables (Table 1). All other calculated variables presented at least moderate reliability (ICC ≈ 0.5) except VL STV settling time. The INPUT and STV were reliably measured, given that the protocol does not require to unequip and reequip devices in a unique trial without changing the sensor's location. This finding gives complementary information to analyze the effects of different running shoes or equipment like compression garments on STV and the alterations in STV induced by fatigue and muscle soreness during a single trial. Interestingly, although the INPUT signal variable always indicated excellent reliability, the evolution of the reliability shows the tendency to be relatively reduced after 20 steps. It could be related to the fact that the subjects might change their running pattern to adopt a more shuffling gait when accustomed to this situation. Furthermore, the number of excellent ICC is lower for the VL STV compared to INPUT signal and GAS STV, which can be explained by the influence of the number of degrees of freedom that increases between the calf and the thigh, and the relative impact of the other body segments results in augmenting the magnitude of the STV variability. Interestingly damping coefficient and settling time were the less reliable parameters compared to others variables, whatever the measurement location (i.e., INPUT, GAS, and VL). This may be partly explained by the calculation method, which depends more on the amplitude and time of the spectrum-time curve (Wakeling and Nigg, 2001a). The intra-day reliability of D1 demonstrated that more than 75% of the INPUT and GAS STV variables presented good reliability (0.75 < ICC < 0.92, Table 2). Even though most of the other calculated parameters (2 for the INPUT, 1 for the GAS STV, and 8 for the VL STV) presented moderate reliability, it is important to observe that 6 STV variables showed poor reliability, especially for the peak acceleration, total energy STV irrespectively of the calculation method, and damping coefficient of VL STV (Table 2). The decrease in the reliability of the measurements between the two trials is related to de-equipping and reequipping the participant-induced confounding factors. This increased variability could create problems when attempting to test subjects before and after a race in an ecological situation when participants need to be re-equipped after the exercise. In this case, the decrease in reliability presents a problem because the differences induced by race will have to be very marked to become observable. This situation would mainly affect the vibration energy parameters of the VL muscle. Regarding the inter-day comparison, the number of STV parameters with good reliability continues to decrease compared to the intra-day reliability. Indeed, only 64% of the INPUT parameters, 53% of the GAS STV parameters, and 1 STV parameter of the VL reached a good reliability level. More interestingly, 13 STV parameters presented a poor inter-day reliability level, most concerning the STV parameters of the VL that 7 over 13 STV parameters with ICC less than 0.5 (Table 3). Once again, the total energy of VL STV and damping indicators for INPUT, GAS, and VL were the less reliable variables. In truth, studies examining inter-day reliability often report reduced ICC. As mentioned earlier, the fact that reliability declines from one day to the next can be explained by the fact that reequipping the subjects with the same sensor's location and cohesive band pressure is almost impossible. This may be amplified by the human coordination variability, which may induce different impacts and vibrations caused by variations in fitness and mild diffuse pain. It may also be possible that the runners produce different lower-limb movement patterns (i.e., kinetic and kinematic parameters) without being able to feel or verbalize it (Sundström et al., 2021). Previous research has shown that the soft-tissue vibration in the running was affected by the underlying internal mechanical properties of the tissues (muscle fat, connective tissue, vascular components, coupling between tissues, muscle activation, etc.) (Boyer and Nigg, 2004; Wakeling et al., 2002) and the interaction between the external factors (GRF, running velocity, foot strike pattern, angular velocity, surface and shoe type, etc.) (Ahn et al., 2014; Boyer and Nigg, 2007; Fu et al., 2013) and those tissue properties (Wakeling and Nigg, 2001b). Considering the participant in the present study were recreational runners, it could be supposed that each runner may have adopted a unique running style that could contribute to increases in inter-subject variability for some biomechanical variables of running as mentioned above; thus, it is customary to observe high CV for STV parameters. Comparing muscle vibrations over several days of manipulation seems more challenging, and interpreting the results should be cautious. Furthermore, it’s interesting to observe that the number of steps to achieve good reliability depends on the accelerometers’ location, STV parameters, and comparison. In line with the previous findings, it is not surprising that most of the STV parameters have good to excellent intra-trial reliability after only 10-steps (Arnold et al., 2019; Lavcanska et al., 2005; Oliveira and Pirscoveanu, 2021; Riazati et al., 2019; Riley et al., 2008). In contrast, almost half of STV parameters required more steps (range 50-90) to reach good inter-trial reliability compared to intra-trial reliability. As mentioned above, the artificial factor (e.g., accelerometers equipped/reequipping, re-adaptation of running gait) could affect the reliability level of STV parameters and the number of steps to achieve good reliability. In particular, the reliability of some STV parameters reveals an unstable trend (i.e., MNFFFT of intra-trial reliability both for INPUT and VL) varying in an unpredictable situation. Consequently, it is worth paying particular attention to checking the reliability of these STV parameters according to the number of steps within different trials/days, which permitted the researcher to acquire the minimal steps to reach a good reliability level of STV. Meanwhile, this study suffers from some limitations. Firstly, the current results are only relevant for treadmill running studies and, therefore, cannot be applied to other surfaces and may not be ideal for describing running conditions. Secondly, the runners performed running at a constant submaximal velocity with the different running shoes, which didn't report the influence of the running speed and shoe type on the STV reliability. Thirdly, it may be interesting to analyze the particular category population (e.g., elite runners, the overweight, and sex, the runner of different ages) because the STV measurement may be greatly affected by the mechanical properties of soft tissue. Further research should also look to quantify the kinematic, kinetic variations, high running speed, and neuromuscular changes associated with modifications in vibration parameters. |