## 4 Discussion
The fact that in BP the presence of the eccentric phase increases barbell acceleration and that this acceleration is also produced earlier during the concentric movement, indicates, at least in theory, an improved ability to apply force over a short time (relationship RFD--time or force). In effect, data reported confirm that SSC enhances the capacity to produce force quickly, improving the performance in BP-EC, as in previous studies [24], but this effect of RFD or its production time (ms) was not observed for the MP. Indeed, the RFD is an important strength diagnostic variable especially linked to performance in resistance training [25], jumping [26], weightlifting [27], and throwing [28]. Obviously, if RFD is enhanced in BP-EC, it stands to reason that the force-velocity relationship (power) will be greater in each stage of the power output curve. Mean and peak power output were improved in BP-EC over BP-C (mean 13%, peak 10%), as reported in similar studies [11; 29] in which improvements in power by the SSC (8%-18%) were produced. In addition, power output produced up to peak velocity (peak velocity 9%, mean velocity 12%) and the braking phase (mean and peak approx. 11%) were increased in BP-EC, in proportion with the absolute values obtained in mean and peak power. However, power output levels were similar again between MP-EC and MP-C.  
Figure 5: Example of differences between eccentric–concentric (EC) vs. concentric (**C**) phases when lifting the same heavy 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. Our data also reflect increases in power output to reach approximately 50% of the concentric curve performed with light loads in BP-EC and BP-C (see Figure 4B); yet when heavier loads were used in BP and light and heavy loads in MP, power improvements were produced in close to 80% of the concentric curve (see Figure 4D, Figure 5B,D). In the BP-EC tests, the greater power output observed could be largely attributed to the use of elastic energy stored during the eccentric action of the countermovement [30; 31]. It has been described that during the eccentric phase of a countermovement plantar flexion, medial gastrocnemius muscle fibers contracted isometrically, and greater power output levels could be due to the elastic recoil of tendons tissues [32]. Probably, these mechanisms in BP-EC, as part of the negative work absorbed by the muscle-tendon unit when the barbell descends towards the chest (eccentric phase), could be stored by the series elastic components as elastic energy [33]. Conversely, the similar responses observed in MP-EC vs. MP-C may be explained by a longer transition period from the eccentric to concentric movement, causing a decrease in the reuse of stored elastic energy. This could determine a longer time spent by the bar in the sticking region, reducing the force in the acceleration phase up to peak velocity. The coupling time was not measured in our study; hence, a longer transition period between both muscle actions is purely speculative. With regards to the differences observed between BP and MP in our strength diagnosis (RFD, strength--1 RM, power), we propose they could be attributed to changes in muscle architecture and arm muscle moments produced by body position. A mechanical disadvantage as occurs in MP is produced when the arm is lifted to an angle over 90, because the deltoid muscle gradually shortens, and this induces a progressive loss in muscular strength [34]. In contrast, in BP, mechanical optimization is obtained with a glenohumeral abduction [35] of 90, which is the angle normally used in BP.