## 4 Discussion
Therefore, it would have little capacity to generate adaptations, especially in terms of hypertrophy in most of the population, as it is lacking the increased muscle recruitment that can potentially provide a more anabolic stimulus [6], being able to only provide a sufficient stimulus in populations with low fitness levels, such as older detrained populations [23]. However, when comparing the EMG of the OFE-SLP with respect to the other two machines, no differences were found in the tibialis anterior as compared to the SLP for the full stroke and eccentric phase; for the gastrocnemius as compared to the BIOFIT-LP for the full stroke, and the concentric and eccentric phases; and for the gastrocnemius as compared to the SLP at 60% 1 RM in the eccentric phase. The discussion of this section is limited, as only one article was found that analyzed the activity of gastrocnemius and tibialis anterior at the same time during the use of guided machines for lower limb training [40], developed on classic fitness center machinery, without including any type of OFE in the analysis. In any case, the function of the gastrocnemius and tibialis anterior muscle groups in this type of exercise is stabilization [4; 19], so the lack of differences in EMG between machines may be due to the fact that the execution of these exercises, when performed with guided machines, may not be greatly dependent on these stabilizers [4]. The second objective of this work was to determine the differences in the EMG of the muscle groups of the lower limbs at 60 and 75% of the 1 RM on machines that allow selection of the intensity of the exercise (SLP and BIOFIT-LP). The results showed an increase in the EMG activity of all the muscle groups analyzed when performing at an intensity of 75% of the 1 RM, compared to 60% of the 1 RM, both for the full stroke and for the concentric and eccentric phases in the SLP. The results are in line with those found in previous studies, in which an increase in EMG was found with an increasing intensity [22]. This is because at higher intensities, the recruitment and firing frequency of the recruited motor units increases [48; 49]. In contrast, the BIOFIT-LP machine showed significant differences with increasing intensity only in the vastus lateralis for the full stroke and concentric phase, in the biceps femoris for the full stroke and concentric and eccentric phases, and in the gastrocnemius for the eccentric phase. No differences were found in any of the EMG cases on this machine with an increasing intensity for the rectus femoris and tibialis anterior. The fact that differences were found in the posterior musculature in the eccentric phase could be due to the fact that this phase in this exercise is performed in favor of gravity, and the participants were asked to perform it at a controlled speed, so that in the face of an increase in the intensity, it is possible that there would be an increase in EMG activation responsible for slowing down the movement, i.e., posterior musculature [14; 50]. In contrast, the concentric phase is performed against gravity, which might require a greater activation of the biceps femoris musculature as the main knee stabilizer [14]. Regarding the vastus lateralis and rectus femoris, the intensity could have been insufficient for the recruitment of more fibers by the rectus femoris, showing differences only in the vastus lateralis. Previous studies have indicated that at high intensity (90% of the 1 RM), the vastus lateralis presents greater activations than the rectus femoris [22]. For this same reason, the signal recorded in the tibialis anterior did not present significant differences even when increasing the intensity, indicating the lack of a different stimulus in terms of joint stabilization [46]. Based on the results of the present investigation, in general terms, an increase in the intensity could imply an increase in the recruitment of fibers of the main musculature involved, and therefore, of the adaptations that will be achieved by training with such machinery [22].