## 4 Discussion
However, in other studies comparing indoor machines designed to work the same muscle group, it has been found that at similar percentages of 1 RM, the EMG activity of the main muscles involved is similar [19], confirming the results of the present study. In light of these results, it can be affirmed, along the same lines as previous studies, that the amplitude of the EMG was greatly dependent on intensity rather than on the execution technique for both traditional SLP and BIOFIT-LP machines [42]. However, a significantly greater activation was observed in the BIOFIT-LP machine as compared to the SLP machine for the tibialis anterior at both intensities in the full stroke and concentric phase, but not the eccentric one. In contrast, the gastrocnemius showed a greater activation in the SLP as compared to the BIOFIT-LP machine at both intensities and all phases, and the biceps femoris showed greater activation in the SLP at 75% 1 RM during the eccentric phase. As observed in several studies, the position of the selector of the intensity of the exercise and ergonomic and biomechanical characteristics of the equipment with respect to the joint levers condition the participation of certain muscle groups [3, 43, 44]. In this sense, the findings for the SLP machine are in line with what was found in previous studies, where it was shown that in this machine, an increased activation of the biceps femoris and gastrocnemius was generated, especially in the push-off phase [4], due to the co-contraction of these muscle groups, together with the extensor musculature of the knee, to improve the anteroposterior stability of the joint in the angulations of greater instability [4]. Another possible explanation is that the projection of the intensity of the exercise in the SLP falls forward with respect to the base of support and the support of the feet, so that the co-activation of the biceps femoris and gastrocnemius is vital for knee stabilization [4, 45]. In contrast, in the BIOFIT-LP machine, the intensity of the exercise projection falls further back with respect to the base of support and foot support, which could favor the activation of the tibialis anterior as a stabilizer of the ankle joint [46]. For all of the above, in general terms, it can be stated that with the new BIOFIT-LP design, an EMG similar to that achieved with the traditional fitness room machines, such as the SLP, could be achieved. Therefore, this new machine design could provide an alternative for effective strength training, albeit outdoors and free of charge, with the advantages that this could have for the health of the population [8]. However, the lack of previous studies analyzing this issue with OFE, and the fact that the studies that have analyzed EMG activity with traditional SLP have included fewer muscle groups than in the present investigation [19], makes it necessary to conduct future research that contrasts the results of the present investigation. Another relevant result of the present investigation was that when comparing the EMG of the BIOFIT-LP and SLP, with respect to the OFE-SLP, the machines with an external selector for the intensity of the exercise showed significantly higher values in all muscle groups, regardless of the intensity of the exercise, with a few exceptions. Despite the popularity of OFE in recent years [8, 24, 25], no previous study was found that analyzed EMG activity on this type of machine, so that its efficacy is somewhat uncertain [47]. However, previous studies have pointed out that intensity selection conditions fiber recruitment, and therefore the recorded signal [48, 49]. Considering that the classic OFE line only allows working with self-weight [24], the results of the present investigation could be due to the fact that the subjects' own weight did not represent an intensity close to their 60% or 75% of the 1 RM during the use of the OFE-SLP. Therefore, in light of the results of the present investigation, the OFE-SLP machine may have a limited efficacy, given the EMG activity it generates [47].