## 4 Discussion
(**C**) Portion of the concentric phase (%) where acceleration up to the peak velocity was detected. (**D**) Portion of the concentric phase (%) where acceleration up to the braking phase was detected. Abbreviations: C = concentric; EC = eccentric–concentric. + Significantly different to BP-C (\(p\) = 0.001), MP-EC (\(p\) = 0.001), MP-C (\(p\) = 0.002). \(\forall\) Significantly different to BP-C and MP-EC (\(p\) < 0.05). \(\#\) Significantly different to MP-EC (\(p\) < 0.001) and MP-C (\(p\) = 0.021). \(f\) Significantly different to MP-EC (\(p\) = 0.003). this lack of effect of the countermovement on peak velocity, we propose that the recovery of stored elastic energy and agonist muscle innervation produced by the stretch reflex only increased the early stage of the concentric action, and its effect was diminished in later stages [11]. In contrast, the eccentric action produced prior the concentric phase resulted in a greater peak acceleration in the BP-EC exercise sequence compared to other exercises, and also this peak acceleration was detected earlier at the onset of the concentric phase (see Figure 2A,B). For example, the pre-stretch in BP-EC caused the faster acceleration of the barbell at the beginning of the concentric phase compared to BP-C. Hereafter, acceleration continued to increase to a point where it was slightly higher, lower, or similar to that recorded for BP-C. From here on, both acceleration curves progressed to reach peak velocity and the braking phase in both exercises (see Figure 4A). Although the purpose of this study was not to gain in depth knowledge of the muscle activation produced in several muscle actions, it is also tempting to speculate that the eccentric phase was crucial to improve concentric performance in the BP, presumably because the pre-stretch confers a state of maximum muscle activation before the concentric contraction begins [23], and at the onset of the same (50-100 ms) mainly in the triceps and pectoralis major [11]. Theoretically, muscle activation was the determinant for the earlier load acceleration produced during the concentric phase. Nevertheless, we observed that the eccentric action effect was modified depending on the load lifted. Hence, light and medium loads led to a greater peak acceleration as well as its earlier appearance in BP-EC (see Figure 4A) as opposed to heavy loads (see Figure 5A). In contrast, no significant changes in peak acceleration were observed between the EC and C sequences of the MP, although the same trend in the curves as in BP was observed. For light loads,  
Figure 4: Example of differences between eccentric–concentric (EC) vs. concentric (**C**) phases when lifting the same light load for a representative subject. (**A**) Acceleration and velocity curves in bench press. (**B**) Power output curves in bench press. (**C**) Acceleration and velocity curves in military press. (**D**) Power output curves in military press. Abbreviations used in A,C: AP = acceleration phase; DP = deceleration phase; MA = peak acceleration; PV = peak velocity. Abbreviations used in B,D: BP = braking phase; PP = propulsive phase. peak acceleration was produced slightly earlier in MP-EC than MP-C (see Figure 4C). When lifting heavy loads, acceleration curves were similar in both BP and MP (see Figure 5C). As in a previous study [11], our kinematic results suggest that peak velocity should not be considered a differential factor indicating the performance of resistance exercises of the upper limbs involving similar agonist muscle groups (e.g., BP pectoralis, triceps, deltoids, and MP deltoids, and triceps) irrespective of the muscle action involved. On the other hand, the acceleration produced up to the point when peak velocity is reached does seem to be a good diagnostic variable, since peak acceleration and the time at which it occurs during the concentric action is conditioned by the type of muscle action and the exercise executed.