## 4 Discussion
They concluded that a longer duration in the eccentric phase negatively impacts mechanical power and barbell velocity. However, their study did not examine the kinematics of the eccentric phase, which would be necessary to fully understand these effects. Importantly, their focus was solely on the concentric phase, neglecting the prior eccentric movement. [PERSON] et al. [58] evaluated the reliability of the GymAware PowerTool system for measuring mechanical power and velocity. Yet, like many others, they overlooked the eccentric phase of the movement, missing the significant role it plays in muscle strength and performance. Similarly to what occurs in phase 2, there is a decrease in both kinematic and mechanical parameters as the intensity of the load increases during phase 3. In line with current study findings, [PERSON] et al. [56] determined that two different subphases can be identified within the concentric phase, referred to as the propulsive and braking phases, respectively. The propulsive phase is defined as the portion of the concentric phase in which the acceleration of the barbell exceeds the acceleration of gravity (\(\geq\) -9.81 m\(\cdot\)s\({}^{-2}\)), while the braking phase occurs when the force applied is lower than the barbell's weight [46]. Considering that the kinematics of the barbell reflects the outcome of muscular action during the exercise, the results show that both propulsive [19] and mean velocity decrease as the load increases. These findings are supported by previous studies, which also observed a decline in velocity as the load increased [43]. These findings highlight the need to consider both mechanical and kinematic variables when designing training protocols based on movement analysis (such as velocity, acceleration or power). Concerning phases 1 and 4 of the BP, these should be analysed using a more specific methodology, since an external analysis of the dynamics of the subject-barbell system, evaluating the behaviour of the barbell, is not specific enough to determine what proportion of the movement depends on muscle activity and how much on the action of gravity. To address this, an analysis of the internal dynamics of the system would be required, incorporating the characterisation of joint mechanics and electromyographic activity. Nevertheless, some limitations of the present study should be acknowledged. Firstly, the sample size is relatively small. It would be valuable to conduct a similar study using the methodology outlined here with a larger sample to validate the findings. Additionally,the distribution of load intensity intervals is relatively broad. Although participants were instructed to push the barbell as fast as possible during each trial, this may have influenced the acceleration profile of the barbell, potentially deviating from the optimal acceleration curve principle [46]. Future research should consider defining the intensity intervals in advance and selecting narrower ranges of load intensity to achieve more precise results. Regarding the methodological contribution of this study, to the best of our knowledge, this is the first study to use Xsens Link technology for BP analysis. The Xsens MVN Link system stands out as an advanced tool in biomechanical analysis due to its precision, versatility and ease of use, surpassing traditional optical systems and linear transducers. Unlike optical systems that require a calibrated space and are susceptible to marker occlusion, Xsens uses portable inertial sensors that function in uncontrolled environments, allowing for its application in real training conditions. Moreover, it has demonstrated high similarity in measured angles and positions of the segments with optical systems in dynamic tasks such as walking [16] (CMC \(>\) 0.96) and gymnastic activities. [59]. Therefore, it could be interesting to incorporate joint kinematics analysis of the BP using the methodology proposed in this study, as it provides a better understanding of the BP exercise.