## 4 Discussion  
The main findings of this study are that: (a) the effect of the individual training improves muscle tone in the EG more than in the CG in both muscles (BF and ST). (b) The tone improvement is more relevant in the semitendinosus than in the biceps femoris in the right leg. (c) The 90\({}^{\circ}\)degree COD worsens the execution time in the control group and is maintained in the experimental group. Also, during the research, the experimental group did not have any hamstring injuries from the beginning of the research until the end of the season, but there were hamstring injuries in the control group in four football players. The time of contraction in milliseconds, measured with TMG, reflects the explosiveness of the muscle. A muscle with a short time of contraction indicates high explosiveness, and a muscle with a long time of contraction shows a slow muscle [29]. The Tc also depends on the percentage of rapid or slow fibres in the evaluated muscle. When the percentage of rapid fibres increases, the Tc decreases [19]. Muscular displacement, measured by TMG, is a parameter that reflects muscle tone and stiffness [19]. If the stimulated muscle has great displacement, it indicates a lack of muscular tone; when the muscle has little displacement, it shows stiffness in the muscle. On the contrary, if the muscle's displacement is adequate, it demonstrates good muscle tone. In our study, there are significant differences in the Tc on the right leg in the time-muscle-group interaction. In addition, these differences occur in ST and BF between pre- and post-test, and these changes are different in both groups (Tables 3-5). The changes in BF are greater in the EG than in the CG. In the control group, contraction time decreases, and in the EG, the contraction time increases (right leg CG: pre-test 26.02 \(\pm\) 8.54 ms, post-test 25.57 \(\pm\) 18.47 ms and in the EG pre-test 29.9 \(\pm\) 7.68 ms, post-test 37.5 \(\pm\) 14.89 ms), which means that they are slower. This could be an indicator of fatigue because the muscle takes longer to contract, maybe due to the increased competitive density in EG (two match per week) and the intervention programme. It can be further explained by factors related to the central nervous system, such as changes at the spinal level [30] or by peripheral factors associated with the muscle, such as failure of transmission at the neuromuscular junction [31]. In addition, the biceps femoris is a muscle that is more fatigued than the ST [32]. Specifically, it has been observed that the acute effects of training generate a decrease in Tc [33; 34] and that the chronic effects, more sustained over time, generate an increase in Tc. Our results disagreed with [PERSON] et al. [35], in which after an 8-week training programme, the BF reduced the contraction time (pre-test 24.5 \(\pm\) 10.6 ms, post-test 19.6 \(\pm\) 9.3 ms). The athletes completed, on average, eight training sessions per week, which consisted of \(\approx\) 6 technical and tactical (game-based training) and \(\approx\) 2 strength-power (e.g., unloaded vertical and horizontal jumps, and squat jump) training sessions. The period was implemented in the transition between two championships, so no official matches had been played. Moreover, they did not individualise the specific training programme and had no control group. However, the increment of time contraction in BF in our study also appears in the study of [PERSON] et al. [36], (pre: 28.8 \(\pm\) 5.9 ms, post: 29.8 \(\pm\) 4.6 ms). This study also carried out individualised training on a group of football players and compared with participants who were physically active, non-football playing males who had not undergone any systematic training process, after a 7-week preseason with a high load and a 10-week gap between tests. This could imply that in periods of higher load, the contraction time of the BF increases; this is a fatigue indicator. However, further studies are needed to confirm these findings.