ACL Injury Risk in Elite Alpine Ski Racing: A Review

Alpine ski racing is an Olympic winter sport that is both physiologically demanding and highly technical in nature (1). The combination of these factors introduces a considerable risk of injury to athletes, with injury to the anterior cruciate ligament (ACL) being one of the most significant.

This article aims to serve as a review of ACL injury in alpine ski racing, highlighting the prevalence and burden of injury, providing an overview of the primary mechanisms of injury and the associated risk factors, before discussing strategies for ACL injury mitigation based on the findings presented.

 

ACL Injury in Alpine Ski Racing – The Numbers:

Alpine ski racing involves sustained, high-force muscular contractions repeated over durations ranging between 45s – 150s, at speeds reaching 120km/h (2, 3, 4). The ischemic environment created in the working musculature by those contractions promotes significant levels of central and peripheral fatigue (2). This fatigue, combined with the variable and complex environment ski racing is performed in, expose athletes to a high risk of injury (5, 6). Injury to the knee is the most common, with complete rupture to the ACL being the most frequently diagnosed of those injuries, accounting for as much as 20% of all ski related injuries, experienced at a rate of ~5 per season per 100 athletes in elite level competition (3, 7-9). Rupture to the ACL is one of the most severe knee injuries suffered by athletes, with an injury burden in alpine ski racing of at least 7 months lost to sport and/or competition (10, 11). Unfortunately, due to residual neuromuscular deficits, impaired knee function and aberrant movement patterns post ACL reconstruction, re-injury rates to both the ipsilateral and contralateral limb remain high, despite extensive rehabilitation (12).

 

How do ACL injuries occur in alpine ski racing?

Injuries to the ACL in alpine ski racing are often non-contact in nature, and occur when an athlete experiences rapid knee flexion, with the addition/combination of anterior translation and external/internal rotation of the tibia (7, 13). Causation is by result of three primary mechanisms: the dynamic snowplow, a back-weighted landing, and the slip-catch (Figure 1). These mechanisms are often witnessed in the later stages of a race, suggesting that ACL injury risk is elevated when athletes are in a fatigued state (9).

Figure 1.

Non-contact mechanisms of ACL injury in alpine ski racing.

Note. Adapted from (7). 1.1 = Dynamic snowplow, 1.2 = Back-weighted landing, 1.3 = Slip-catch.

 

The Dynamic Snowplow: The dynamic snowplow occurs when a skier is out of balance backwards and loses ski contact with the inside ski during a turn (1.1a). With uneven pressure on each ski, the outer ski drifts outwards from the body’s centre of mass, and the inside ski rolls from outside edge to inside edge (1.1b). The inside edge of the inside ski reengages and catches the snow surface, forcing the skier into deep ranges of hip/knee flexion, causing valgus and rapid tibial internal rotation (1.1c).

The Back Weighted Landing: The back-weighted landing mechanism occurs when a skier gets of balance during flight phase of a jump (1.2a), and lands backwards with the tails of the skis contacting the ground first (1.2b). They then try to recover from this back leaning position (1.2c), before falling backwards, resulting in compression of the tibiofemoral joint combined with anterior shear on the tibia (1.2d).

The Slip-Catch: The slip-catch mechanism (considered to be the most common), occurs when the skier is out of balance backwards and inwards during the turning or ‘steering’ phase (1.3a), and the outer ski moves away from the body’s centre of mass, extending the skier’s knee as they try to regain control (1.3b). The outer ski then reengages, initiating a carved turn across the body, causing knee valgus and internal rotation (1.3c), the skier then falls backwards and inwards (1.3d).

 

What risk factors are associated with ACL injury in alpine ski racing?

Injuries in sport are complex and multi-factorial in nature, comprising of both modifiable and non-modifiable, intrinsic and extrinsic risk factors.

Intrinsic risk factors: Refer to internal factors specific to the athlete, and relate to the athlete’s physical, physiological and psychological characteristics. Examples of which include age, gender, training age, injury history, body composition and capacity related limitations (such as strength, mobility etc.). Research indicates that intrinsic risk factors associated with an elevated risk of ACL injury in alpine ski racing include:

·   Insufficient dynamic stabilisation of the trunk (9)

·   Low and/or impeded levels of neuromuscular control of the lower leg (14)

·   Hamstring and quadriceps muscle strength imbalances, inter-muscle co-activity and rates of torque development (15)

Extrinsic risk factors: Extrinsic risk factors on the other hand are external to the athlete, and relate to the environment, equipment, or sport-specific contexts and constraints. Examples for alpine ski racing include equipment (skis, ski boots and bindings), environmental conditions (extreme cold, wet etc.), course conditions (gate distance, slope angle, ice etc.), training load and quality of coaching. Extrinsic factors associated with ACL injury risk include equipment, weather and variable course settings (16). Equipment is certainly an area that has received a lot of attention, particularly the ski-boot-binding set up. The ski-boot-binding system is designed to have two primary functions, to release (release the ski from the boot under circumstances where twisting or bending forces are being experienced), and retain (retain a rigid coupling between ski and boot to allow efficient force transfer). However, due to the speeds and extremely high forces witnessed in alpine ski racing, it is not uncommon for athletes to maximise their binding retention settings to augment force transfer and reduce the potential for pre-release, which largely negates the release function of the binding system, allowing the skis to be retained during crashes and potential injury events. This retention allows the skis to act as levers, predisposing the knee to higher forces and torque in exposed and compromised ranges of motion during injury mechanism events (5). Additionally, modern carving skis are designed with more aggressive side cuts and sharper edges, which can increase control and performance during a well-executed ski turn, whilst simultaneously increasing the forces required for the skier to maintain balance and edge control, exposing lower limb joints to an increased level of injury risk (17).

 

What does all this mean for practitioner’s working with alpine ski racing athletes?

Bringing this altogether, the outlook for mitigating ACL injury in alpine ski racing seems bleak, and as practitioner’s, it seems we have little influence over injury outcomes for our athlete’s. This is in part due to the biomechanical nature of the sport, and the non-modifiable extrinsic factors associated with it (variable weather, inconsistent conditions etc.). And whilst an attempt could be made to try and address some of these factors to lower an athlete’s injury risk, such as modifying equipment (more sensitive ski release settings, less aggressive ski side cut), these adjustments come at a cost to performance, which is a trade-off that is not often desired, or tolerated, in a high performance setting.

Additionally, as is the nature with ‘injury prevention’ strategies and training interventions (with ‘injury prevention’ being a contentious term in and of itself), they require time, which is often something we do not have a great deal of as S&C coaches. Therefore, the important question is, how much time as S&C coaches should we dedicate towards trying to prevent injury? (If injuries can even be prevented in the first place). With the limited time we have available with our athletes, isn’t it more important we focus on training the qualities that best enhance performance?

Ultimately, I believe the answer to this question is both personal (to the S&C coach) and specific (to the context and environment they are operating in). Personal, as each practitioner will have their own set of philosophies and guiding heuristics that govern their decision making, and specific, as depending upon the context, these decisions might make themselves, based upon the demands of the team, the culture of the programme, as well as things like individual athlete age/training age, time of the competitive season etc. The intersection between the personal and the specific in this situation dictates how low or high risk a programme may be, or, more simply, how conservative or aggressive it may be (Figure 2).  

Figure 2.

Factors influencing programme risk.

I first began thinking of approaching my coaching and programme design in this way after coming across the ‘Barbell Strategy’ in Nassim Taleb’s, ‘Skin in the Game’, which was developed and applied through a training intervention and adaptation based lense in Mladen Jovanovic’s ‘Agile Periodisation’ (Figure 3).

Figure 3.

The ‘Barbell Strategy’

Note. (20)

In this context, the ‘Barbell Strategy’ offers a decision-making strategy that can help guide programme design, based on the two opposite ends of the barbell, one which is focused on protecting from the downsides (low risk, conservative), and the other pursuing the upsides (high risk, aggressive). The distribution between the two ends is not equal, and never will be, as each situation (/athlete) will require a different approach (as per the personal vs specific heuristic). Therefore, and circling back to our discussion on how to approach ACL injury mitigation strategies for alpine ski racers, practitioner’s need to attempt to balance the performance-injury trade off (ratio of time spent protecting from the downsides vs pursuing the upsides) on a case-by-case basis. However, traditional methods of training that elicit ‘general’ physiological adaptations (increased maximal force production, increased metabolite buffering capacity), can often serve a dual purpose within the prevention vs performance conversation, by both protecting against injury whilst simultaneously helping towards increasing performance outcomes. In other words, by trying to mitigate ACL injury (protecting from the downsides), we may be able to increase performance outcomes (pursue the upsides), and vice versa. It is within this realm where the art of coaching meets the science, and where a practitioner can truly maximise the utility of their programme and have the most influence.

Strategies for mitigating ACL injury risk in alpine skiing – combining performance and prevention:

Of course, this requires an in-depth knowledge of the sport, the athlete and the mechanism of injury. For the purposes of alpine ski racing, research suggests that key areas for adaptation (that can elicit multiple levels of training transfer), include maximal lower limb force production, increased quadriceps and hamstring coactivation and hamstring rate of torque development, and increased levels of lower body mobility.

Maximal force production: Maximal strength increases an athlete’s ability to produce, and importantly for ski racing, withstand, large forces. Lower body maximal strength is key to maintaining control throughout a carved ski turn and reducing the potential for out of balance scenarios prevalent with the mechanisms of ACL injury in the sport. Additionally, increased maximal strength increases an athlete’s strength reserve, which can reduce the amount of relative strength required per ski turn. Seeing as ACL injuries in alpine ski racing are linked to increased levels of fatigue, an increase in movement economy and a reduction in metabolite accumulation brought about by a higher strength reserve could serve to offset fatigue and slow down the loss of motor control throughout a ski race.

Quadriceps and hamstring coactivation and hamstring rate of torque development (RTD): Hamstring and quadriceps coactivation can increase knee joint stability and reduce the excessive and rapid knee joint moments that are common in ACL injuries in ski racing. The hamstring muscle group serve as synergists for the ACL, helping reduce anterior translation of the tibia, therefore, increasing hamstring strength and muscle co-activation helps protect the ACL by providing additional support and stability to the knee joint during knee flexion and extension. Furthermore, injury to the ACL in sport occurs at <50ms (19), therefore, hamstring training should focus not only on maximal force production properties but should also aim to target rate of torque development qualities (at a variety of lever lengths), to reduce ACL loading during moments of rapid knee instability in response to compromised ranges of motion and unexpected perturbations.

Lower body mobility: Adequate levels of hip and lower body mobility can afford an athlete the ability to effectively absorb and distribute forces in otherwise compromised positions. Considering all three of the primary mechanisms of ACL injury in alpine ski racing involve the skier being out of balance backwards (and/or inwards) at the moment of injury, being able to maintain control and structural integrity in these vulnerable positions is highly advantageous.

PROACTIVE >REACTIVE - Conclusions for the S&C practitioner:

In summary, there is an inherent risk of ACL injury alpine ski racing athletes are predisposed to that cannot be mitigated. Research indicates there is not one single solution that can prevent ACL injuries from occurring, however, S&C practitioners can aim to be proactive in their programme design by targeting adaptations that balance the performance-injury trade off, considering the situational context and constraints they are operating within. Using decision making strategies or heuristics, such as the barbell strategy, can help systemise the decision-making process and help provide a clear rationalisation for the chosen approach.

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