I have found that one of the best ways to teach this to someone is to have them perform stationary lunges with their feet spread to an appropriate distance, mark the floor at their feet with painters tape (peels up real easy), and then have them stand at one tape and step out or back to the other tape. Once they are able to do this with alternating steps in good form, we can move to walking lunges, and eventually to weighted walking lunges.
That is the same study I was going to write about, however, the date is
November 2008, Journal of Orthopedic & Sports Physical Therapy, volume 38, number 11, pp 681-686.
Studied conducted by Escamilla, R, Zheng, N, Macleod, T, Edwards, W, Hreljac, A, Fleisig, G, Wilk, K, Moorman, C, Imamura, R & Andrews, James conducted the study,
They were not looking at the forces on the ACL and PCL but the forces applied to the patellofemoral joint. Which is very relevant because I have seen in my career while working out at gym and when working as a trainer in gyms, that clients’ of personal trainers knees going past their toes.
The methods of the controlled laboratory study that examined 18 subjects using their 12 RM while performing a short and long step forward lunge with and without a stride.
Results: Patellofemoral joint forces and stress increased progressively as the knee flexion increased and decreased progressively as knee flexion decreased. Between 70 and 90 degrees of knee flexion patellofemoral joint force and stress was significantly greater when performing a forward lunge with a short step compared to a long step.
Conclusion: When the goal is to minimize patellofemoral joint force and stress during the forward lunge, it is important to watch not only a clients’ form but that their knee does not track past their toes. Due to the biomechanical demands supported by this study and other studies in the present literature.