Ground reaction force bilateral asymmetries during submaximal sagittal plane lifting from the floor

Twenty-eight healthy men (n=14n=14) and women (n=14n=14) lifted a milk crate in the sagittal plane from floor to knuckle height ten times using a symmetric stance in order to determine if asymmetries were both present and consistent within a lifting bout. Mean ground reaction forces under each foot were compared over the entire lift, an initial unweighting phase during the countermovement, a weighting phase, and a final unweighting phase when coming to a stop in the upright standing position. Paired t  -tests with p<0.05p<0.05 were used to determine individual asymmetries in vertical, anterior/posterior, and medial/lateral forces. After verifying that there were no significant differences between genders, subjects were grouped to determine if kicking leg was related to the asymmetry. Over the entire lift, 75.0% of the subjects were statistically asymmetric in the vertical direction, 7.1% in the anterior/posterior, and 96.4% in the medial/lateral. Reduced levels of asymmetry were observed in the phases in the vertical and medial/lateral directions while slightly more were asymmetric in the anterior/posterior direction. Separating by kicking leg yielded significant differences only in the medial/lateral forces (all phases as well as the entire lift). In conclusion, even in subjects with no reason to be asymmetric, asymmetries in the ground reaction forces are more likely the norm rather than the exception. However, assessing based on kicking leg alone will most likely not predict the side with greater forces.Relevance to industryUnilateral injuries have recently been shown to cause changes in spinal kinematics which may prove to be a risk factor for future low-back pain and injury. Based on the findings of the current investigation, if asymmetries are a risk factor, even healthy people with certain asymmetries may be predisposed to future pain and injury.