The effects of motor imagery and action observation-based plyometric training on vastus lateralis and biceps femoris muscle architecture of adolescent soccer players

Lower limb muscle-tendon injuries are the most common injuries in adolescent soccer players. Although it is known that plyometric training (PT) improves muscle function, the effects of neuroscience-based PT on the muscle macroscopic structure are unclear. The primary aim of this study was to examine the effects of motor imagery and action observation-based (MI+AO) PT on the pennation angle (PA), muscle thickness (MT), and fiber length (FL) of the vastus lateralis (VL) and biceps femoris long head (BFlh). The secondary aim was to evaluate the effects of training on jump performance. Thirty-six healthy adolescent soccer players (age: 13.91±1.05 years, height: 162.22±10.20 cm, weight: 52.09±8.33 kg) were randomly assigned to PT (n=17) or control (n=19) groups. Athletes in both groups continued their routine training programs. PT group also participated in 3 sessions of training per week for 8 weeks. The PT consisted of 10 different exercises and a total of 3480 jumps. Athletes watched the prepared videos and imagined jumping. Muscle images were taken using the Soundcam mobile ultrasound probe, and then analyzed with MicroDicom (V 2022.2 Bulgarian) software. The single step jump test was used to evaluate both sides jump performance. Outcomes were assessed at baseline, 4, 8, and 12 weeks. There was no significant change in the muscle architecture parameters in the control group during 8 weeks (p>0.05). The VL PA was higher on the dominant (mean difference=2.40±3.45°, p=0.01), and non-dominant side (mean difference=3.32±3.75°, p=0.01) in the PT group at week 4. Similarly, there was a significant increase in dominant (mean difference=0.15±0.19 cm, p=0.03) and non-dominant side (mean difference=0.24±0.23 cm, p=0.01) VL MT. Except for the dominant side BFlh MT (mean difference=0.21±0.31 cm, p=0.03), there were no changes in VL FL, and other BFlh muscle architectural parameters on both sides (p>0.05). The increases in non-dominant side VL MT and dominant side BFub MT were higher in the training group compared to the control group at the end of the 4th and 8th weeks (p<0.05). Both sides jump performance was significantly better in the PT group at weeks 4 and 8 compared to baseline (mean difference=16.71±11.76 to 22.88±12.19 cm, p<0.01). The non-dominant side jump performance of the athletes in the control group increased significantly at the end of the 8th week (mean difference=10.12±12.82 cm, p=0.03). Muscle architecture and jump performance of the athletes in the training group did not change significantly during the follow-up period (p>0.05). This research provides the first evidence that MI+HG-based PE improves muscle architecture. It is recommended that future research be conducted in injured athletes and using neuroimaging tools.