The effects of action observation in the lower limb landing biomechanics: a systematic review and meta-analysis

Introduction
Non-contact ACL injuries usually occurs when an athlete lands from a jump and, therefore, landing training is frequently used as an injury prevention strategy. This systematic review aimed to investigate the effects of raction observation in the motor learning of a proper landing technique of healthy adults.

Material and methods
Randomized controlled trials were deemed eligible if they included athletically active healthy adults without history of lower limb injuries and if they compared action observation, in the form of direct observation or video feedback, to control. The outcome measures were lower extremity biomechanical parameters: sagittal plane flexion angles, dynamic knee valgus and vertical ground reaction force.

Results
Six trials were included. Two trials were rated at low risk of bias, one trial with some concerns and three trials with high risk of bias. Pooled data analysis indicated that action observation improves peak knee flexion (MD 15.95, 95% CI 3.53 to 28.38, I2 = 92%) and initial contact knee flexion (MD 4.05, 95% CI 1.62 to 6.48, I2 = 0%), may improve peak hip flexion (MD 18.16, 95% CI -1.71 to 38.03, I2 = 92%) and dynamic knee valgus (SMD 0.52, 95% CI -0.31 to 1.34, I2 = 75%) and has no effect on vertical ground reaction force (SMD -0.04, 95% CI -0.68 to 0.61, I2 = 62%) compared to control.

Conclusions
In conclusion we can state that action observation is a potential strategy to enhance motor learning of a proper landing technique in healthy individuals.