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.