The main findings of this study were an increase in velocity and distance covered with the SSrecovery (and longest) recovery period condition (~ 3min20s) compared to the fixed 1min and 2min of recovery period. These differences could be explained by higher blood lactate concentration and RPE values found in the 1min recovery period condition compared to SSrecovery. In addition, the kinematic running pattern is modified regarding the recovery period, with a decrease in VO and SF in the 1min and 2min recovery period condition compared to SSrecovery. Finally, there were sex-related differences. Women displayed lower SL, FT, distance, velocity and heart rate recovery, higher CT, BLC and RPE, and less CMJ height loss than men in all conditions. These sex-differences reported in our study should not be related to intensity during the interval training sessions, because both men and women ran at similar intensity relative to the best 3000m time (Tables 2 and 3).
Previous studies have reported that IT do not produce substantial changes in the running kinematics of trained runners 14,33. However, these studies evaluated running kinematics during running to exhaustion exercise or time trials and these findings must not be compared with interval training sessions.
In the case of the recovery period, Collins et al. 20 compared the kinematics changes during a 10x400m interval training session with 60, 120 and 180s of the recovery period in a random order. The authors found that VO was higher with shorter recovery period, while SL remained unchanged, conversely to our results. However, the lack of agreement might be explained by the methodology used in both studies Collins et al 20 evaluated kinematics parameters using submaximal run bouts before and after the IT sessions, while our study they were measured during each repetition with a triaxial accelerometer.
Our results have shown significant differences in VO and SF, meaning that the fatigue achieved during recovery periods less than 2min produces changes of the kinematic model of running 34 with a lower VO of the body's centre of mass and a high SF. These kinematics modifications could be related to a reduction of velocity during this type of IT with recovery periods lower than 2min.
Regarding mechanical parameters, we found a significant loss of 7% on CMJ height in the 1min recovery period condition. Bachero-Mena & González-Badillo 14 evaluated the mechanical and metabolic responses of a high-intensity training session of 800m athletes (5x200m with 4min recovery) and they observed a progressive decrease in CMJ height of 27%. These results are different from our study because the exercise intensity of this type of IT (short IT) was higher than those observed in our study, which would imply a greater activation of anaerobic glycolysis and fatigue of type II muscle fibres.
The use of long IT like this (4x4min) is characterized by intensities at high percentage of VO2max (95%) 2,5 and a high dependency of the aerobic metabolism for ATP resynthesis during this type of IT has been found 23. Therefore, it seems that more than 1min of recovery period during long IT (4x4min) is sufficient to maintain the mechanical fatigue of endurance runners. Gorostiaga et al 35 suggested that at blood lactate levels not exceeding 8–12 mmol/L, CMJ height loss did not change significantly from peak values. In our case, the average values of blood lactate concentration remained between ~ 8–11 mmol/L, being significantly higher in the 1min recovery period condition with a 7% of CMJ height loss. Regarding the condition*sex interaction, significant differences were found in the CMJ height loss by males. It might be due to an increase in peripheral fatigue on neuromuscular system and a depletion of muscle glycogen levels. Likewise, the blood lactate concentration showed significant differences between 1min and SSrecovery conditions with values between 8–11 mmol/L. Previous studies 2,21,36 have found similar blood lactate values during IT sessions. Women had significantly higher blood lactate values with 1min of the recovery period (11.06 ± 1.41 mmol/L). Laurent et al 21 suggested that women may suffer a greater physiological load at a similar perceptual rating level due primarily to the estrogen levels, which may have a protective effect on skeletal muscle fatigue 37.
On the other hand, runners perceived the sessions as harder when the recovery period was 1min, although the velocity was only higher in SSrecovery condition compared to 2min condition. Therefore, the duration of the recovery period is a component key during IT design that it can modify the perception of effort even when performance is similar (as the velocity of the 1min and 2min conditions [18.43 ± 1.18 and 18.58 ± 0.96 km/h, respectively]). However, HR and HRR have not changed between the three conditions. This is in agreement with previous studies 2,3,21 that reported an increase in recovery time during IT results in a minor impact on cardiovascular responses. As a result, the HRR measured after the end of each repetition, which remains unchanged despite the variation in the duration of the recovery period. In this sense, 2 suggested that HRR is not useful (or at leats questionable) as a tool for determining recovery duration during IT.
Prescription of self-selected recovery periods enables individualization of IT to the athlete's requirements 4. Schoenmakers & Reed 23 and Seiler & Hetlelid 2 obtained with a similar protocol a shorter self-selected recovery period of 100 ± 34s and 118 ± 23s, respectively. In our study, the duration of the recovery period of SSrecovery was 198 ± 59s. These differences could be due to the type of recovery. The studies of Schoenmakers & Reed 23 and Seiler & Hetlelid 2 used active recovery while in our study it was passive. In addition, the study of Schoenmakers & Reed 23 used recreational runners while Seiler & Hetlelid 2 used well-trained runners as our study. A extended recovery period allows a greater recovery of phosphocreatine reserves and a greater neural impulse 4, which explains the higher performance shown by the subjects.
Finally, only one study analysed the gender differences during IT. Laurent et al 21 compared physiological responses by sex in trained runners with three session of IT consisting of 6x4min with 1, 2 and 4-min recovery period. The results showed as women produced different absolute running speeds and physiological and perceptual responses agreeing with our study. Besides, we also observed modifications in running kinematics, where men have greater SL, a higher FT, and women have a greater ground CT. These differences seem to be related to the different absolute running speeds during the IT training sessions, being higher for men compared to women, although the relative intensity was similar between both sexes.
The main limitation of this study was that IT sessions were completed in laboratory conditions. As a result, the changes in these parameters could be different in field conditions. For example, running kinematics during treadmill running is modified slightly compared to overground running 38.
From a practical viewpoint, we recommended to coaches and runners keep in mind that a longer recovery period (~ 190s) between repetitions during long IT results in a higher running speed and in a lower metabolic and neuromuscular fatigue. A greater duration of recovery period leads to a better recovery of phosphocreatine stores and a greater neural impulse in the next interval. Controversy, a short recovery period causes greater metabolic fatigue that is reflected in a reduced capacity for impulse in the push-off phase. Therefore, depending on the training phase and the training objective and stimulus, the recovery period can be increased or decreased by modifying the running speed or the desired mechanical or metabolic fatigue.
Women have a different running pattern than men due to their anatomical and physiological characteristics, so training should be structured taken these considerations in mind.