# Motor Imagery and Sport Performance: A Systematic Review on the PETTLEP Model
## 3 Results
### Combining MI with Physical Practice  
Physical practice (PP) is more effective in increasing strength compared to MI alone and comparison between MI+PP vs. PP alone leads to unclear results [104]. However, [PERSON] and [PERSON] [54] found that when using a PETTLEP approach, the strength gains were similar to those acquired by the PP-alone group.  
When making a comparison between MI and PP, it must be noted that PP activates both the muscles and the neural circuit controlling the motor action, resulting in an optimal training of both the peripheral and central system by the PP [1,106]. Furthermore, although similar, the neural networks underlying MI and PP are not identical, probably due to the inhibition of different sensorimotor output, which is required during MI [6,107].  
The different ability of individuals to perform the MI task could lead to suboptimal activation of the control network, leading to increased variability in the results [30,108, 109, 110]. Cerebral activity in motor-related areas was found to be elicited more by the combination of MI and PP than by either one of the two alone [111,112]. Accordingly, greater benefits were experienced after MI+PP than after PP alone, both in healthy subjects [54,113] and symptomatic populations [114,115]. The study of [PERSON] et al. [113] was particularly interesting because it introduced MI training during rest periods. Over a 6-week program, subjects performed both the bench press and sled leg press. The MI group experienced a higher increase in strength for the sled leg press but not for the bench press. These results could be explained by findings showing that closed kinetic chain exercises (such as the sled leg press) resulted in increased strength gains when compared with open kinetic chain exercises such as the bench press [116].  
Nonetheless, there are other factors that could have affected the results. First of all, subjects were all involved in different sports but had no experience with resistance training. Additionally, contrarily to how subjects were instructed in the study [113], the bench press must be considered a compound movement that involves more than lowering the barbell to the chest and pushing it back up. A correct execution of the bench press requires scapula retraction, involvement of the legs, and the execution of a specific barbell path [117].  
To avoid issues related to proper execution of exercises, for research purposes, it is preferable to use isometric contractions in order to limit variability in the results. A study which used a similar protocol of [PERSON] et al. [113] was conducted by [PERSON] et al. [118], resulting in the highest strength gains, and MI of muscle relaxation was also better than passive recovery [118]. [PERSON] et al. [119] showed that sport practice enhances MI brain patterns, probably because vividness of the imagery is a strong variable affecting the results of a MI intervention and suggesting that MI could be more effective when performed after PP.  
These findings suggest that a combination of MI and PP could be the optimal method for increasing strength. However, more studies in which MI is performed in addition to PP (without reducing time devoted to PP) are required. A crucial problem in this field of research is that imagery abilities play a key role in the effectiveness of this practice: if participants are not able to clearly imagine the motor task, their benefits will certainly be limited [109,110]. Nonetheless, during MI, greater activity of motor-related areas is associated to the experience of the subject in performing the motor task [120] and to the adoption of a first-person perspective during the imagined task [121]. With this in mind, implementation of MI protocols in strength training is suggested. However, in some cases, its effectiveness could be limited to technical improvements and increased motivation [113; 122].