Effect of ski boot settings on tibio-femoral abduction and rotation during standing and simulated skiing

https://doi.org/10.1016/j.jbiomech.2007.10.019Get rights and content

Abstract

Ski boots are designed to transfer high forces from the skier to the ski. For this purpose they are made of stiff materials and constrain the leg of the skier to an unnatural position. To overcome the problem of unnatural knee posture, the ski boots can be adjusted in the frontal plane as well as in the horizontal plane by the canting mechanism and the “v-position”, respectively. Canting enables lateral and medial orientation of the shaft with respect to the base of the boot. The “v-position” is a pronounced outward rotation of the boot's base with respect to the ski's long axis. The purpose of this study is to investigate the effect of different foot rotations and ski boot canting settings on knee kinematics during standing and simulated skiing. Knee kinematics was measured by means of motion analysis and with the help of skin-mounted markers on 20 subjects.

The ski boots in their standard settings significantly constrained the skier to an unnatural valgus position. Ski boot base rotation had a significant effect on internal external knee rotation, whereas canting had an effect on varus–valgus angles during standing. However, for the simulated skiing position no effects were observed. The study suggests that the constraints of the ski boots result in a clinically relevant valgus misalignment. Canting settings reduced the misalignment but only by about 10%. Increased ski boot canting settings would therefore be desirable. Knee kinematics showed that rotational misalignment could not be linked to any significant increase in injury risk.

Introduction

Ski boots are made of stiff materials to transfer high forces from the skier to the ski (Maxwell and Hull, 1989). The drawbacks of stiff ski boots are that they constrain the skier's leg to an unnatural position, e.g. a skier with natural varus leg alignment is forced to a valgus movement in order to preserve parallelism of the ski to the ground (Corazza and Cobelli, 2005). Possible consequences of knee varus or valgus misalignment might be overuse injuries (Sharma et al., 2001, Teichtahl et al., 2006). Tibia-femoral rotation also alters the tibio-femoral articular cartilage contact points. This might change contact stress distributions in the cartilage and predispose the joint to degenerative changes (Li et al., 2006, Andriacchi et al., 2006). These long-term overuse effects of skiing in an unnatural position are difficult to assess, but deviations from normal posture are commonly taken as a risk factor for overuse injuries (Issa and Sharma, 2006). In addition to the risk of long-term overuse injuries, the risk of acute ACL ligament ruptures might be increased through knee misalignment. In dynamic landing movements, it has been shown that neutral limb alignment compared to varus or valgus reduces the possibility of ACL rupture through a valgus or varus opening mechanism (Chaudhari and Andriacci, 2006).

A less severe but very common knee injury is the patellofemoral pain syndrome (PFPS). Muscular imbalance of the lower extremity is one of the major contributing factors (Thomee et al., 1999). It might be speculated that knee misalignment, caused by the ski boot, places extra pressure on the muscles around the knee and possibly exaggerates muscle imbalances and patella grinding.

One solution to overcome the problem of unnatural knee posture is the canting mechanism offered by the majority of ski boot manufacturers. Canting enables lateral and medial tilting of the shaft with respect to the base of the boot (Fig. 1). Canting is supposed to adjust to individual varus or valgus posture. Another possibility to adjust the ski boot in the frontal plane was suggested by Corazza and Cobelli (2005). They designed a system that allows rotating the sole of the boot about the anterior posterior axis relative to the shell and cuff. This system is integrated into the sole and is locked into the desired position before skiing. The main drawback of this system is the large amount of local deformation that affects the stiffness of the overall system.

As previously mentioned, tibio-femoral malrotation is another important risk factor. A pronounced outward rotation of the base of the boot with respect to the ski direction would correspond more to the natural outward rotation of the foot (Schwarz et al., 1974). This so-called “v-position” with different pivoting points at the heel or at the mid-foot (Fig. 2) is enjoying increasing popularity among ski manufacturers.

During the turn, edging the ski into the snow causes external rotation of the tibia with respect to the femur of up to 5° and a valgus angle of 12° from neutral, measured by Yoneyama et al. (2000) and Greenwald et al. (1997), respectively. It is speculated that ski boot canting and base rotation reduces the observed valgization and internal rotation, respectively. To our knowledge, no investigations exist on the effect of ski boot canting setting or base rotation on knee kinematics; therefore the following two hypotheses were assessed in this study:

  • 1.

    The ski boot in its standard setting (neutral canting and no base rotation) causes a significant statistical misalignment in the knee angles—both varus–valgus (VV) and internal–external rotation (IRER)—compared to the natural barefoot situation.

  • 2.

    Canting and boot rotation have a significant statistical effect on knee VV and IRER angles, respectively.

The hypotheses were tested for both standing and simulated skiing situations.

Section snippets

Experimental setup

Twenty subjects (15 m, 29±6 years, 77±9 kg, 175±6 cm) participated in the study. Prior to participation all subjects were informed about the nature of the experiment and signed an informed consent. The subjects were all physically active and asymptomatic with no history of lower limb-, spinal- or neurological injury. The VV angles (measured via Q-angles) of the subjects during standing were in the range of values reported for normal subjects (Livingston and Mandigo, 1999).

First, the subjects had to

Barefoot knee kinematics during squatting

Regarding the VV and IRER angles at the respective knee flexion angle during the eight FE cycles measured (Fig. 6), the graphs suggested a linear relationship between these parameters. The linear relationship was confirmed with the median of the absolute correlation coefficient, which was greater than 0.9 for VV and IRER (Fig. 6). For the VV angles, the majority of the subjects showed a negative correlation coefficient—this means an increased varus with increasing knee flexion. Only two

Discussion

Comparing knee kinematics of the simulated skiing positions to those measured during outdoor conditions; mean knee flexion angles of 45.8±8.9° were close to the average knee angles of 50°, measured by Yoneyama et al. (2000) at the outside leg during a left turn. In this study, external rotation of the shank with respect to the thigh was 2.5±1.8° relative to the situation without ski boots. The absolute values relative to upright standing were 7.0±3.4°, which is within the range of rotation

Conflict of interest statement

Both authors do not have any financial and personal relationships with other people or organizations that inappropriately influence the work performed.

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