This study aimed to compare the effect of ISSG and CSSG on the bio-motor abilities of young soccer players. Our results showed that aerobic power (i.e., VO2max and Yo-Yo), and anaerobic power (RAST) improved in both groups. In contrast, the results of Illinois, 30-meter, body fat percentage, and body weight showed no improvements. Finally, the intensity measures of HRmean, and RPE were lower in ISSG than CSSG, while HRmax was higher in ISSG than in CSSG groups.
Aerobic power
In the present study, it was found that there is no significant difference between the two groups of ISSG and CSSG in VO
2max and Yo-Yo test results. However, after comparing the pre-test and post-test in both, it was found that there was a significant increase only in the ISSG training group. Previous research was consistent with the results of the present study [
31]. That study compared traditional (running) with SSG training, in which SSG training was performed in the form of intervals in 4 sets of 4 minutes with a maximum heart rate of 90 to 95% and 3 minutes of recovery between the sets. The results of that study showed that the sub-maximal lactate response and VO
2max were improved in those players. Also, with the increase in aerobic abilities of the players, their displacement in the game increased by 571 meters [
31]. Another study compared ISSG and CSSG (2 vs 2, 3 vs 3 and 4 vs 4) training with 6–12 minutes of duration and found similar physiological responses in both groups, including improvement in VO
2max [
32]. Another study found that SSG (5 vs 5) training performed with 85% HR improved aerobic power [
33]. However, other research found that SSG training did not affect participants’ aerobic capacity compared to non-ball speed training. Still, it is important to highlight that floaters were used and could justify the different results [
34]. Besides, the number of players and the field size could also justify the different results from the present study.
Anaerobic capacity, heart rate, maximum heart rate and RPE
The present study found that anaerobic power in the ISSG training group was higher than in the CSSG group. Also, after comparing the pre-test and post-test in both groups, it was found that there was a significant increase only in the ISSG training group. On the other hand, the HR
mean in the CSSG group was significantly higher than in the ISSG training. Also, there was a significant increase in the HR
max in ISSG compared to CSSG. CSSG allowed players to sustain a higher work rate over time. Despite the higher training volume, the locomotor demands tend to be low, producing lower fatigue [
35]. CSSG seems to allow a better recovery for a strength training day, contributing to better readiness in the following days [
36]. Also, this training type can contribute to tactical complexity improvements [
5].
In the present study, heart rate (HR) was used to evaluate the effect of CSSG and ISSG training on the physical fitness of young soccer players. CSSG training produced a higher HR response than ISSG training, consistent with previous studies using a similar training design [
14]. The consistent intensity of CSSG training is the main contributor to this effect. On the other hand, ISSG training is characterized by constantly varying exercise intensity over a short period, causing HR to peak and then decline. This exercise pattern resulted in a significant increase in mean HR of the ISSG group compared to the CSSG group. In addition, the ratings of perceived exertion (RPE) of the CSSG group were significantly higher than those of the ISSG group, indicating that the CSSG exercise was more intense. Further research could explore these different training methods’ potential advantages and disadvantages in more detail.
In line with the results of the present study, we can refer to a study where the authors compared the two types of ISSG training (3 6-minute attempts with 3 minutes recovery between) and CSSG (an 18-minute effort without recovery). The authors concluded similar improvements for the anaerobic power index, while HR, RPE and blood lactate were identical in both groups [
37]. In another study, the effects of CSSG (2 vs 2, 3 vs 3, and 4 vs 4 with a duration of 6, 9, and 12 minutes, respectively) and ISSG (2 vs 2, 3 vs 3, and 4 vs 4 with a duration of 2, 3, and 6 minutes, respectively) were compared. The results indicated a significant improvement in anaerobic power for all three protocols [
32]. In another study, the ISSG (4 vs 4 with 3 of 6-minute attempts with varying recovery times between at-tempts) observed a significant improvement in anaerobic power in the post-test compared to the pre-test [
38].
The literature shows subtle differences in training programs, age, and player ability. Based on the research background, the simultaneous increase in the number of players and the size of the field in SSG increases the training intensity. For example, Rampini et al. examined the effects of a simultaneous increase in the number of players and pitch on HR
max, blood lactate and RPE in 20 amateur soccer players. The results showed that an increase would follow the training intensity in the named variables [
31]. On the other hand, they found that when the number of players increased, they reported a decrease in the HR
max percentage. The heart rate increased as the number of players decreased [
12]. In contrast, other studies did not show significant results in HR changes with decreased players [
32,
39,
40].
Most research suggests that as the field size increases, RPE, HR, and lactate concentration increase [
12,
41,
42]. A study of the effect of ground dimensions on HR showed that with increasing ground dimensions, heart rate increased during activity [
31,
43]. In contrast, a previous study has yet to achieve significant results [
12]. Two studies also observed higher RPE with increasing field dimensions [
31,
43].
Recent studies have shown that different numbers of players obtain different physiological and technical responses. With the number of players decreasing, heart rate, fatigue index and lactate concentration increase, but technical activities decrease. The relationship between the parameters of small-sided game training and the players’ ratio to the field’s size is also important [
36].
Body mass and fat, speed and agility
In the present study, the changes in soccer players’ body mass and fat percentage decreased in both training programs, but this decrease was not significant. It seems that the reason for the non-significance of these changes goes back to the initial body weight and the level of physical fitness of these players, and also the insufficient training period (eight weeks), which is in line with previous a previous study that analysed the effect after a detraining period of four weeks plus a training period of another four weeks [
44]. Indeed, another study showed positive effects only after 11 weeks [
45]. Also, in the variables of speed and agility, the results showed that the speed was not significantly different between the two groups of ISSG and CSSG. An increase in speed was observed in both groups, which was greater in the ISSG training group; however, none of the results related to this variable was significant. In the analysis of agility, no significant difference was observed between pre-test and post-test values for each group. Among the reasons why speed and agility in soccer players were not significant in this study could be related to the short training period and the players’ initial level of physical fitness.
The present study has some limitations that could affect the interpretation of the results. One limitation is the small sample size, which may not represent the young soccer players’ population. This limitation may affect the external validity of the study results, as the findings may not be generalizable to other people or contexts. Nonetheless, the sample size calculation showed 88.96% of power with 16 participants. Additionally, only 4vs4 SSG were considered for analysis, which may limit the generalizability of the results to other SSG formats. Furthermore, the study only analyzed a short period of eight weeks of the pre-season, which may not capture the long-term effects of the different SSG interventions. Finally, the lack of control over confounding variables, such as dietary habits or other physical activity outside the training program could be considered another limitation. These factors could potentially bias the results of the study in either direction.
Despite these limitations, the present study has practical implications for coaches and trainers who work with young soccer players. The findings suggest that both ISSG and CSSG can effectively enhance the bio-motor abilities of young soccer players, but with different physiological responses. Coaches and trainers can use this information to tailor their training programs to their athletes’ specific needs and goals. Future research should address the present study’s limitations by using larger sample sizes, longer intervention periods, and more comprehensive measures of physical activity and dietary habits. Additionally, future studies could investigate the effects of SSG interventions on other outcomes, such as technical and tactical performance, injury risk, and psychological factors. The present study provides a foundation for further research on SSG interventions in young soccer players.
Overall, the results of this study help coaches and their staff to optimize their training plan and periodization by providing highlights with CSSG and ISSG with total durations between 25 to 40 minutes. Additionally, this study provides relevant information on two possibilities of SSG in different formats (intermittent and continuous), which can be chosen according to the training’s aim and objectives and the period of the season.