Training Programs for ACL Injury Prevention

A copy an article written for the summer 2013 ACMG newsletter on neuromuscular training for ACL injury prevention in skiers:

Training Programs for ACL Injury Prevention

The most recent data on ACL injury prevention indicates that neuromuscular training programs are effective in reducing ACL injuries at rates of 50-85%[1]. As most winters see one or more guides experiencing sometimes season-ending ACL injuries, early season strength and conditioning programs can be an important tool in injury prevention. While the “best” training program will depend on the genetics, strength, and body control of the individual, a few elements are common across all effective training programs.

ACL Anatomy and Function: The anterior cruciate ligament attaches to the posterior lateral (rear, outer mid-line aspect) of the femur and the anterior medial (front, inner mid-line aspect) of the tibia. Its main functions are to prevent anterior translation (forward shift) and internal rotation of the tibia on the thigh. The posterior cruciate ligament prevents the opposite motion: posterior translation of the tibia (and external rotation), while the two collateral ligaments prevent lateral motion.

Injury Mechanism in Skiing: Two main injury mechanisms appear in advanced skiers: “slip and catch” and “back-weighted landings.” The slip and catch is a hard-snow mechanism that involves an outside ski that is drifting sideways (the “slip”) followed by a sudden re-grip (the “catch”). The steering angle of the inside edge causes the ski to move inside the skier’s direction of travel while the re-grip increases torque at the knee joint. The result is a knee that is forced into dynamic flexion and a valgus (knock-kneed) orientation while the tibia internally rotates on the femur.
            Back weighted landings create an anterior stress on the knee: during dissipation of the landing forces, the tibia is pulled forward on the femur as the knee joint flexes. Back weighted, or“backseat,”  skiing increases the base-line stress on the ACL creating a higher risk in injury from low-force slip and catch events.
            In addition, guides are prone to injury from twisting forces due to the increased upper-body inertia caused by skiing with a pack, combined with higher lower-body torsion forces encountered in heavy or variable snow or due to small sluffs and avalanches.

Training for Injury Prevention: The main risk factors for ACL injury are weak hamstrings relative to quadriceps, poor fore-aft and lateral knee stabilization during flexion, and poor truck stability. A simple test for neuromuscular control and the knee joint is the plyometric vertical drop: the athlete drops from height and immediately rebounds upward. Valgus (knock-kneed) orientation indicates a higher risk of ACL injury.
Common features of the training programs effective at reducing ACL injuries are: increasing hamstring strength relative to quadriceps strength, improving lateral stabilization at the knee joint, improving trunk stabilization, and integrating strength and stabilization dynamically with plyometric training.
Strength should be the base of the training program, as stabilization training won’t be effective without it. A simple test for hamstring:quadriceps strength ratio is to test a 3 rep maximum weight on both leg extensions and leg curls: the max leg curl weight divided by the max extension weight is the H:Q ratio. For most people this will be considerably less than one, but having a 1:1 ratio is a good goal to shoot for in training. Increasing the relative hamstring strength requires targeted hamstring training involving exercises such as stability ball leg curls, hip extensions/glute-ham raises, Russian leg curls, Romanian deadlifts, good-mornings. Hamstring contraction also needs to be integrated with quadriceps contraction in movements that mimic the knee flexion involved in skiing: squat and lunge variants. The goal is to be able to extend (stand up) from a flexed knee and hip position using co-contraction of both hamstrings and quadriceps. Successful integration of the two muscle groups will result in a lower leg that stays relatively vertical and a knee that stays above the ankle. Failure to properly engage the hamstrings will result in forward shift of the knee and lower leg towards the toes (and associated increase in ACL loading). A simple exercise to increase hamstring activation while ski touring is to keep your heel-lifters low enough that you heels are always slightly below your toes on climbs and then focus pulling or dragging your lead foot backward rather than stepping forward and standing up. Shorter strides and increased cadence may help.
Lateral stability is primarily a combination of hip abductor strength and proprioception, allowing the skier to resist valgus movement at the knee. Clamshell exercises can enhance hip abductor engagement, but training should focus primarily on maintaining lateral knee stability (i.e. no inward or outward shift from a plane running through the middle of the foot and the hip) during knee flexion and extension. Single legged squats and lunge variants stress the lateral stabilizers and instability from “wobble boards” can be added as skill increases.
Good trunk stabilization means that a skier can control their upper body motion, allowing them to resist positions of increased load on the knee. Exercises that stress the ability to stabilize the upper trunk under load have the greatest applicability to skiing. Variations on plank holds, especially with added instability from wobble boards and BOSUs can be a good place to start. Overhead squats require excellent trunk stability at even light loads, while regular squats, deadlifts and good mornings emphasize trunk stabilization at increasing loads. Medicine ball catches and throws from a seated position, stability ball/BOSU, or glute-ham machine emphasize trunk stabilization under dynamic loading.
Finally, plyometrics (or “rebound” training) integrate both the strength and stability skills with dynamic loading more similar to the requirements of skiing. Variations on high jumps, long jumps, box jumps, rebounds and single legged jumps will all stress the ability to dynamically stabilize the knee joint.
Overall, an ACL injury prevention training program should progress from a combination of strength and stabilization exercises with good control of knee position, to plyometrics, which emphasize stabilization under dynamic load.
For an intermediate trainee, familiar with all of the exercises, a sample program might consist of:
·      Hamstring strengthà ONE of: Russian leg curls, good mornings, Romanian deadlifts.
·      Fontal plane stabilizationà ONE of: Front Squat, Back Squat.
·      Lateral Stabilizationà Clamshells, then ONE of: wobble board lunge, single legged squat, weighted lunge.
·      Trunk Stabilizationà Med ball throws from glute/ham machine, then ONE of overhead squats, weighted overhead walking lunge, deadlift.
·      Plyometricsà Box jumps
Training correct movement patterns and muscle engagement are far more important than load or volume. A good coach or personal trainer can personalize the broad components presented here into an individual program the focuses on your specific weakness. Ongoing coaching on correct form will improve the effectiveness of the exercises and help to prevent injury at higher loads and with more complex movements.
General pre-season endurance training, combined with attention to good ski technique with the joints “stacked” over the center of the ski, good hydration, and proper DIN settings for the conditions, will all also help to reduce the risk of season-ending ACL injury.
Thanks to Lucinda Jagger, High Performance Advisor at Own The Podium and Dr. Mark Heard, Orthopedic Surgeon at BanffSport Medicine, for their input in reviewing this article.

[1] Gagnier JJ, Morgenstern H, Chess L. Interventions Designed to Prevent Anterior Cruciate Ligament Injuries in Adolescents and Adults: A Systematic Review and Meta-analysis. Am J Sports Med. 2012 Sep 12.
Sadoghi P, Keudell von A, Vavken P. Effectiveness of anterior cruciate ligament injury prevention training programs. J Bone Joint Surg Am. 2012 May 2;94(9):769–76.