## 4 Discussion  
This study aimed to analyze the effects of 20 weeks of regular endurance and strength training on RE and physiological, spatiotemporal, and neuromuscular variables in national long-distance runners. The main findings of our study were as follows: (1) a 3.5% deterioration in RE; (2) a 5% improvement in VO2 max relative to body mass; (3) an improvement in MAS, VT1, and VT2 speeds; and (4) an increase in %VO2 max of VT1 but not of VT2. In addition, to assess biomechanical variables, we used fixed speeds during the RE test and the MAS during the pre-test. However, there were no associated biomechanical changes at these speeds. Our results indicate an improvement in VO2 max, and MAS that occurred simultaneously with a decrease in RE. In that sense, previous studies have found that VO2 max and exercise economy are inversely related [31,32]. For this reason, the increase in speed of VT1 (9.4%), VT2 (3.7%), and MAS (2.8%) after the training program in our study was accompanied by a higher oxygen cost/energy cost (worse RE) during GXT (9.1, 5.9, and 4.6% for VT1, VT2, and MAS, respectively) and the fixed speed in the RE test (4.1 and 3.5% for oxygen and energy cost, respectively). In our study, the improvement in VO2 max was \(\sim\)5%. However, well-trained runners improved their running performance (800 m and 2 miles) during 8 weeks of regular training without changes in VO2 max or RE [33]. In a longitudinal study with female and male elite middle-distance and distance runners [34], VO2 max remained unchanged after one, two, and three years of training, while running performance improved significantly in the male runners. A possible explanation for this phenomenon could be that highly trained/elite runners could have reached a biological limit in VO2 max after several years of consistent training [34], while in our study, the participants (well-trained but not elite runners) could have not reached that biological limit, and only 20 weeks of training were sufficient to find some changes in that parameter. Due to the linear relationship between oxygen uptake and intensity during a GXT [35], it is expected that the increase in VO2 max will be accompanied by a loss of efficiency (higher oxygen/energy cost), as we found in our study (Figure 2). In addition, it is possible that in the last weeks of training during our study, the training capacity that was more developed was aerobic power due to the proximity to some cross-country competitions. Therefore, it appears that physiological adaptations after a period of regular endurance and strength training are dependent on the most developed capacities at the times closest to the laboratory evaluations performed. Figure 2: Changes in VO2 during the GXT after the training program. This physiological adaptation mentioned above could be explained by the training stimulus applied during the training period. The participants displayed a pyramidal TID during the 20 weeks of training, with 6-8% of training in z3 close to and higher than the intensity of VO\({}_{\text{2 max}}\)/MAS. These improvements could have been induced by increased time at high VO\({}_{\text{2 max}}\) and distance completed at higher speeds [36; 37]. A high fraction (%) of VO\({}_{\text{2 max}}\) during high-intensity sessions plays a decisive role in the improvements of parameters such as VO\({}_{\text{2 max}}\), so the inclusion of interval training sessions close to 90% and above 105% of MAS could have resulted in the high %VO\({}_{\text{2 max}}\) during these sessions. Previous studies have shown that continuous and interval training promotes significant improvements in VO\({}_{\text{2 max}}\)[38; 39; 40]. [PERSON] et al. [41] found that a 6-week high-intensity interval training program increases MAS by 7.9% in recreational runners, and [PERSON] et al. [42] claim that 4 weeks of high-intensity interval training increases MAS by 3.6% in well-trained runners. Therefore, sufficient stimulus around VO\({}_{\text{2 max}}\) (or speed--MAS) is necessary to improve this parameter.