Case Study

Proc IMechE Part P:J Sports Engineering and Technology2020, Vol. 234(4) 337–346� IMechE 2020Article reuse guidelines:sagepub.com/journals-permissionsDOI: 10.1177/1754337120918037journals.sagepub.com/home/pip

A biomechanical field testing approachin snow sports: Case studies toward adetailed analysis of joint loading

Nicolas Kurpiers1 , Paul McAlpine2 and Uwe G Kersting3,4

AbstractIn this study, kinematic and kinetic measurements were combined to assess the effects of removing the stiff shaft from aski boot. It was hypothesized that joint flexion at the ankle, knee and hip increase and reduce joint loading specifically atthe knee. A previously developed force sensor was combined with a high-speed camera system for data collection of 6degrees of freedom ground reaction forces and three-dimensional marker data in the field on a wave slope. The col-lected data were used as input to a musculoskeletal model for the estimation of joint kinematics and joint moments andcontact forces in the ankle and knee. The force sensor, which was previously used for skiing, had experienced wear andtear and was thus prone to breakage. As a result, joint loading could only be analyzed for two skiers. These two skiersdid not use the added range of ankle flexion to its full extent, but showed substantial reductions in joint moments andjoint contact forces (e.g. knee compression force from 85 to 57N/kg). Only one of the five experienced skiers testedwas able to adopt the anticipated movement pattern by substantially increased maximum ankle joint flexion angle (from10� to 37�) and knee joint flexion angle (from 93� to 105�) and the respective ranges of motion when skiing through awave course. The study provides information on possible individual adaptations to ski boot modifications. The mechani-cal construction of the force sensor will need to be modified to withstand the high forces expected during freestyle ski-ing. The study also supports the future use of this measurement setup for comprehensive studies in snow sports,provided that a sufficient training period is given.

KeywordsFreestyle skiing, ground reaction forces, joint loading, computer modeling, ski boot

Date received: 11 September 2018; accepted: 8 March 2020

Introduction

Freestyle skiing is a growing discipline in young peopleand has been associated with a comparably high risk ofknee injuries.1 The total prevalence of injuries in alpineskiing is approximately two to three injuries per 1000skier days2 and has decreased over the decades.However, severe knee injuries have tripled since 1970.3

In freestyle skiing, epidemiological data are only avail-able from competitions which show the knee to beinvolved in approximately 50% of all injuries. In com-parison, handball players show a lower incidence ofknee injuries with approximately 11%, where a com-parison of exposure time remains difficult due to fun-damental organizational differences of these sports.4

The indication of knee injury prevalence according tosports differs remarkably depending on the reference.Thus, basketball indications range from approximately8%5 to 33%,6 while soccer indications range from15%7 to more than 35%.8

In freestyle mogul skiing, knee injuries are predomi-nant and can have devastating consequences for theskier.9 Previously, a ski boot modification was sug-gested to allow for a more natural, combined knee andankle flexion, like in a squat movement without skiboots, to better negotiate bumps and potentially reducethe load on the knee joint.10 A previous study assessedthe effect of ski boot hardness on wave course skiing

1Department of Sport Science, University of Hildesheim, Hildesheim,

Germany2High Performance Sport New Zealand, Auckland, New Zealand3Institute of Biomechanics and Orthopaedics, German Sport University

Cologne, Cologne, Germany4Sport Sciences, Department of Health Science and Technology, Aalborg

University, Aalborg, Denmark

Corresponding author:

Nicolas Kurpiers, Department of Sport Science, University of Hildesheim,

Universitatsplatz 1, W-411, 31141 Hildesheim, Germany.

Email: kurpiers@uni-hildesheim.de

by a two-dimensional video approach, but found noalterations for commercially available soft and hardboots. It has been concluded that the range of ski boothardness is not sufficiently large to allow for kinematicalterations.11 A different intervention, where the wholeshaft of the ski boot was removed, has previously beenutilized for balance training in competitive alpineskiers. This particular kind of boot intervention needsto be conceived as a coordination training tool, ratherthan an applicable modification, for competition orregular use. However, it may be considered as a simplemodification and may be used in a concept validationexperiment.

It is vital to assess training methods or equipmentinterventions in order to reduce the number of injuriesin specific sports. While this has been done in indoorsports where potentially risky situations can be matchedin a laboratory setup,12,13 the situation becomes morechallenging in winter sports. In particular, the requiredattire of the participants to perform under low tempera-tures is making sensor and marker attachments a chal-lenge. Previously, only a few investigations havemeasured kinematics in freestyle skiing,14,15 whereasothers have included force and pressure measurementsin skiing and freestyle skiing.10,16 Studies where kine-matic and kinetic data sets were combined and used tocalculate joint loading in skiing are still rare,17 whilesuch an approach is required to fully investigate andunderstand injury mechanisms, joint loading and theeffects of specific alterations to skiing equipment, inparticular. Establishing a protocol in regard to snowsport, a quantitative analysis of freestyle-specific move-ments with specifically matched measurement equip-ment will allow for investigations of mechanical loadingexperienced by athletes during freestyle skiing.

The purpose of this study was twofold. First, thestudy was aimed at implementing a methodologicalapproach combining video-based, three-dimensional(3D) movement data with kinetic data from a force sen-sor mounted between the ski and boot. These data werethen imported into a musculoskeletal modeling systemto estimate knee joint moments and joint contact forces.Second, the mentioned ski boot modification, with andwithout the shaft attached to the boot, was comparedto verify that this approach is suitable to identifyequipment-related changes in knee joint loading duringskiing on a wave course. Accordingly, the first hypoth-esis was that the chosen measurement components weresuitable for application to mogul skiing for trainingpurposes. Mogul skiing versus skiing on a wave slope inthis study is different as the latter does not includeturns. However, the bending and straightening move-ments of the legs are the major component of mogulskiing that is restricted by the stiff ski boot shaft, for-cing the skier into a potentially so-called ‘‘backseatposition.’’ This part of the complex movement can beassessed in a wave slope under simplified conditions,that is, turns excluded and an extreme modification.This extreme modification, as adopted from alpine ski

racing, was used to assess the possibility of altering themotion patterns. The modification would be implemen-ted in a less extreme nature for follow-up investigationsin a mogul course including turns in order to approxi-mate the actual mogul skiing technique and avoid harmto the ankle joints. Therefore, the second hypothesiswas that the flexible (no shaft) boot would allow for anincreased knee and ankle flexion on a wave course. Thethird hypothesis was that this change in techniquewould be accompanied by a reduced knee joint loading.

Method

Five male skiers (27.86 11.9 years) were recruited forthis study with their skill level ranging from ‘‘advancedrecreational’’ to ‘‘expert.’’ Ethical approval wasobtained from the University of Auckland, NewZealand. Participants were interviewed about theirtechnical skills, competitive level and training statusprior to participation. They had to be injury free for atleast 2months prior to the study. Subsequently, theywere introduced to the training boot and signedinformed consent if they agreed to engage in the study.All testing took place during the fall on the Zermatt skifield, Switzerland, at an altitude of approximately3500m above sea level with temperatures ranging from215 �C to 25 �C.

Participants were asked to ski with the bindingsadjusted to fit the test boots by an experienced ski ser-vice worker. They initially trained for at least 4 h withthe new boots on the normal slope as well as on thewave course. They were instructed to practice bendingand straightening movements in order to get accus-tomed to the altered flexibility of the boots and activelyuse the extended range of ankle movement, which isnot possible with regular ski boots. The familiarizationphase was conducted progressively, first with just themodified ski boot and, after 2 h, with both the newboot and a mock-up of the force plate18 to practice thetesting situation.

The tests were carried out on a wave slope of anaverage inclination of 19� with six bumps of approxi-mately 0.5m height situated at a distance of 4m apartfrom each other. Bumps number three and four framedthe measurement area in order to collect the data withina fluent motion cycle. Snow fell twice on the testingarea between the testing days, such that reconstructionof the test slope was necessary. The visibility variedwithin each testing day from slightly foggy to sunnyand the wind intensity changed as well. However, eventhough the environment changed between runs, onlymarginal changes in climate or snow conditionsoccurred during individual testing sessions, whichlasted for approximately 1 h.

For modification of the boot, the ski boot’s shaftwas removed and the residual material at the back ofthe boot was made more deformable by milling two80mm vertical cuts into the rear spoiler, resulting in a

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noticeably greater flexibility in both anterior and pos-terior directions (Figure 1). Three pairs of boots (HeadRaptor Super Shape, Head and AUT) were made avail-able for this study in sizes 28.0, 28.5 and 29.0 cm.

For acquisition of kinetic data, two 6 degrees of free-dom force sensors, developed specifically for skiingmeasurements, were utilized.19 The dynamometer partswere milled from aluminum, assembled and providedwith strain gauges resulting in a weight of approxi-mately 2 kg per piece, a height of 36mm and a width of62mm.19 Each sensor consisted of three different com-ponents, specifically the boot sole adapter, load cellsand binding adapter (Figure 2). The ski boot wasattached to the adapter with a buckle. Force data wereamplified by custom-built strain-gauge amplifiers,sampled at 500Hz, A/D converted (model: USB4716,Advantech, USA) and stored onto a minicomputerusing commercially available measurement software(Dasylab V.5; Measurement and ComputingCorporation, USA). The coordinate system of thedynamometers was defined with the x-axis anteropos-terior (alongside the ski), y-axis medio-lateral (acrossthe ski) and z-axis vertical to the ski. The midpoint ofthe sensor assembly was the origin of the associatedcoordinate system.

Four synchronized high-speed cameras (BaslerA602f) and a Simi Motion System (Version 266; SimiReality Motion Systems, Germany) were used to collectvideos of the performance in a 103 3m area at a fre-quency of 100Hz. Two upper cameras and two lowercameras were situated aside the test slope on tripods,which stood in ice buckets to prevent any disruptivemovements. The angle between any two optical axeswas 60� or more and the cameras had a distance of 7–10 m to the recording volume.

A set of 27 black markers (19 dynamic and 8 static)were fixed to the racing suit of the skier bilaterally onbony landmarks to define the body parts (Table 1). A

pole calibration method with a total of 36 calibrationpoints was applied within the area of recording. Theirpositions were identified using a theodolite(Geodimeter� System 600, Germany) and entered intothe Simi Motion System.

Participants were filmed during three runs wearingthe modified flexible ski boot with no shaft (FL), per-forming bending and straightening movements in thewave slope, while ground reaction force (GRF) andassociated moments were collected simultaneously.Subsequently, three runs with a standard ski boot (ST)were recorded. Synchronization of video and forcerecordings was managed off-line by including two shorthops at the start of the test runs into the recordings.

All data were processed off-line for the kinematicsusing Simi Motion Software. The consecutive worksteps in Simi Motion comprised the camera calibration,specification of the markers for digitization, the digiti-zation itself and the calculation of the 3D marker coor-dinates using the direct linear transformation (DLT)method.20

Force data were converted using experimentallydetermined calibration factors19 in a customizedMATLAB (V.2013; The MathWorks, USA) routine,resampled, saved as text files and imported into theSimi Motion Software. For synchronization in SimiMotion, the reference hop visible in one of the cameraswas used. A low-pass, recursive zero-time-lag second-order Butterworth filter with a cut off-frequency of10Hz was applied to the kinematic data and 100Hz forthe force data.21

Subsequently, the preprocessed data served as theinput to a musculoskeletal model. The AnyBodyModeling System (AnyBody Technology, Denmark)was used, which resolves joint moments in a first stepand subsequently estimates muscle activations by

Figure 1. Ski boot modification (Head Raptor Super Shape,Head and AUT): (a) greater flexibility in anterior direction and(b) 80mm cut in rear spoiler to remove resistance in theposterior direction.

Figure 2. (a) Force sensor attached between ski boot andbinding. (b) Three parts of the force sensor.

Kurpiers et al. 339

applying an optimization criterion; in this case, theoverall muscle activation was to be minimized. The cur-rent body model is a modified full body model includ-ing the Twente lower extremity model,22 which hasbeen adopted from the AnyBody Repository for thecurrent application. Segment lengths were scaled to themarker data using the scaling algorithm proposed byAndersen et al.23

From the model output, the maximum and mini-mum ankle and knee flexion and extension angles,GRF and knee joint forces and moments wereextracted. For this study, forces and moments at theknee joint were calculated using the local coordinatesystem of the tibia as it was common in other studieswith x-direction anteroposterior, y-direction verticaland z-direction medio-lateral.24–26

The modified boot in this study was considered hav-ing no constraints to the ankle joint, whereas the nor-mal boot did restrict the movement. The calculatedankle stiffness values for both conditions have beenincluded in the ‘‘Anybody model’’ as an angle-dependent resisting moment at the ankle joint. Prior to

this investigation, a separate boot stiffness test was con-ducted in order to gain specific ankle resistance stiff-ness for the stiff boot.27 This laboratory-based test wasconducted using a phantom leg and foot, including amechanical ankle joint in the boot attached to a ropewith a force transducer and several markers. Theapplied load by pulling the rope, the displacement ofthe top of the prosthesis, as well as the moment arm ofthe load with respect to the ankle joint center weremeasured over the expected range of movement duringskiing.

Only descriptive statistics were applied since onlythe data sets for two skiers included kinematic andkinetic results. In the graphs and the table, the meansand standard deviations (SDs) over the three trials arepresented.

Results

Observations

All participants felt challenged to ski freely on theslopes in the boot modification. When asked to per-form test runs through the wave course, they managedwithout major problems, as it appeared easier for themto negotiate the waves. However, visual inspectionrevealed that the majority of the participants posi-tioned themselves rather backward over the ski, con-trary to the instructions given.

During the testing, the function and integrity of thebinding sensors were thoroughly checked after eachrun. In several instances, metal connectors to the indi-vidual load cells were observed to be damaged or thefunction of the sensor was compromised by icing up.Overall, only two participants completed all test runswithout sensor damage. Based on these design flaws,actions were taken to correct the issues following thisexperiment to provide a more robust construction forfuture studies.

Kinematics

The results show changes in joint angles consistently forthe ankle joint. A greater maximum ankle joint flexionangle, as well as a greater ankle joint range of move-ment, was found when wearing the modified ski boot.The mean (L/R) maximum ankle dorsiflexion increasedfrom 18� to 14� for skier 1 and 15� to 3� for skier 2 fromthe standard boot to the modified boot, respectively.The minimum dorsiflexion changed from 4� to 27� and25� to 211� for skier 1 and skier 2, respectively. Theresulting ranges of motion were increased for skier 1,but decreased for skier 2. Both skiers were more dorsi-flexed in their ankle joints in the stiff boot, but chosedifferent ranges. Note that the angles reported hererefer to the foot being flat on the ground in a neutralupright stance. In a ski boot, the foot points downward,which is accounted for in the model.

Table 1. Anatomical landmarks.

Body segment Marker name Location

Trunk CLAV Cranial end of thesternum

Hip RASI Right anterior superioriliac spine

LASI Left anterior superioriliac spine

RPSI Right posterior superioriliac spine

LPSI Left posterior superioriliac spine

Right thigh RTHIPROX Right quadriceps proximalRTHIDIST Right quadriceps distal

Left thigh LTHIPROX Left quadriceps proximalLTHIDIST Left quadriceps distal

Right knee RKNELAT Lateral epicondyle offemur

RKNEMED Medial epicondyle offemur (static)

Left knee LKNELAT Lateral epicondyle offemur

LKNEMED Medial epicondyle offemur (static)

Right shank RTIB Medial to tibiaLeft shank LTIB Medial to tibiaRight foot RTOE Second phalanges distales

RHEE Posterior to calcaneusRANKLAT Lateral malleolusRANKMED Medial malleolus (static)

Left foot LTOE Second phalanges distalesLHEE Posterior to calcaneusLANKLAT Lateral malleolusLANKMED Medial malleolus (static)

Equipmentmarkers

RFRONTBIN Right front binding

RREARBIN Right rear bindingLFRONTBIN Left front bindingLREARBIN Left rear binding

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In regard to knee flexion, a greater maximum flexionwith only slightly increased minimum flexion wasachieved by skier 1. Therefore, he increased the range ofmotion in the knee joints by about 12� from the stan-dard to the modified condition, while keeping hip flex-ion unchanged. Skier 2 used slightly straighter kneesand hips when skiing with the FL boot, while the rangesof motion remained virtually unchanged (Figure 3).

Kinetics

The magnitudes of the ankle joint moments were con-sistently reduced, while the minimum and maximumknee moments varied somewhat in direction andamount of change (Figure 4). The maximum normalbinding forces were only slightly reduced. The kneecompression forces applied to the tibial plateauwere overall reduced by 20%. The maximum forcebetween patella and femoral condyle was generallysmaller, resulting in a reduction of 30% on average(Table 2).

Ground reaction forces were reported in the localcoordinate system of the ski force plate. Mean GRF

and moments acting at the boot soles of both legs fortwo participants are displayed in Table 2 for both theFL and the ST conditions. The normal force (Fz) is themain component of the resultant GRF, as the perfor-mance only comprised vertical movements of the legs.The GRF is approximately 1% larger in the ST condi-tion with 10.1N/kg compared to 9.9N/kg in the FLcondition for skier 1, while the GRF is approximately18% larger in the ST condition with 13.4N/kg com-pared to 10.9N/kg in the FL condition for skier 2.

For the knee joint, an average reduction of maxi-mum compression forces along the long axis of thetibia was shown, which was not symmetric and mainlyaffected the right knee. The minimum compressionforces were symmetrical and on average reduced by70% when comparing the FL boot to ST boot (Table2). Finally, as the model contains a simplified kneemechanism, including the patella, an estimate was givenfor the force which the patella is experiencing when thequadriceps muscle is active. Apparently, this load var-ies from complete offloading to approximately 30N/kgfor the FL boot on both legs, but increases by approxi-mately 70% when skiing with the ST boot (Table 2).

Figure 3. Kinematic results for skiers 1 and 2: (a) minimum and maximum hip flexion angles for the ‘‘flexible’’ (FL) and the‘‘standard’’ (ST) condition (skier 1); (b) minimum and maximum knee flexion angles (skier 1); (c) minimum and maximum ankleflexion angles (skier 1); (d) minimum and maximum hip flexion angles (skier 2); (e) minimum and maximum knee flexion angles (skier2); and (f) minimum and maximum ankle flexion angles (skier 2).

Kurpiers et al. 341

Discussion

This study had a twofold aim: exploring that a full 3Dinverse dynamics analysis with the given testing devicesis possible and illustrating this by assessing the effectsof a ski boot intervention in a wave course. Five partici-pants were tested (Table 3), but only two full data setscould be analyzed due to issues with connectors to theload cells within the force sensor assembly, resulting inunrecorded data.19 Thus, results from only two casesare presented here. For these two participants, compar-ably small kinematic changes were shown, except forthe ankle joint with a 9� increase in flexion on average.Kinetic changes were substantial at not only the anklejoints with up to 500% increase in the ankle momentson one side (Figure 4) but also affecting the knee withup to 70% increase on one side, even though the bootmodification primarily altered the ankle movement.For comparison, Liebl et al.28 found ankle moments ofapproximately 2Nm/kg in runners and Funken et al.29

assessed knee moments of approximately 4.6Nm/kg inlong jump. It may be noted that the gastrocnemius mus-cle activation increased up to 1100% in the ST condi-tion, which suggests a strong correlation to the kineticchanges and thus the boot modification. The underlyingidea of this boot alteration as a training tool was to

allow for a natural squatting movement when flexingthe legs to absorb a mogul and simultaneously reducethe joint loading on the knee, which only was the casein skier 2.

The increase in ankle flexion for the two skiers chan-ged by 11� and 3�, which indicates that these two sub-jects adapted to the ski boots by increasing both themaximum ankle joint flexion angle as well as the rangeof movement, whereas the knee joint angles onlyshowed marginal changes. A study on a task similar tothe current project reported comparable results; how-ever, some differences were also observed.11 Theauthors tested a hard ski boot compared to a soft skiboot, which were both commercially available. Thetask for the testing procedure was the same as in thisstudy, specifically skiing down a wave slope withoutturns absorbing the bumps. The study found minimaldifferences in joint angles between shoe types. In viewof the current results, it can be proposed that substan-tial changes in boot hardness are associated with dra-matic alterations to joint loading. The knee angles(KAs) were greater in this study in the extension direc-tions but not in flexion. This could be explained bypossible differences in the initial joint angle definitions,since the ranges of movement were similar in bothstudies.

Table 2. Minimal and maximal force components for skiers 1 and 2.

Force component (N/kg) Left SD Right SD Left SD Right SDSkier 1

ST FL

Normal force (min./max.) 0.3/8.5 1.8/0.5 2.3/11.7 0.3/0.6 1.7/9.3 0.1/0.5 2/10.6 0.4/0.5Knee compressionforce (min./max.)

43.4/73.9 3.6/0.8 42.4/95 17.1/16.5 13.3/90.8 1.8/14.2 9.6/63.5 2.8/5.9

Patella compressionforce (min./max.)

0.1/15.1 0.1/6.2 0.4/11.3 0.6/3.5 0/9.5 0/0.1 0/11.8 0/1.1

Skier 2

ST FL

Normal force (min./max.) 4.3/14.5 1.4/3.8 1.4/12.4 0.5/0.6 1.3/12.1 0.5/3.5 1.3/9.7 0.1/1.9Knee compressionforce (min./max.)

24.7/75 18.5/6.1 18/95.9 3.5/16.5 6.7/56.8 2.4/8.7 5.4/57.3 0.2/7.8

Patella compressionforce (min./max.)

2.7/68 0.7/15.5 2.8/36.8 0.6/7.5 0.1/36.4 0.1/11 0/23.8 0/6.8

ST: standard; FL: flexible; SD: standard deviation.

Table 3. Mean values kinematics (angles in degrees, n = 5).

ST FL

Left SD Right SD Left SD Right SD

Hip flexion (min./max.) 40/107.3 12.6/17.4 40.2/106.8 12.4/16.8 38/102.4 10.2/12 39.2/100.6 12/12.6Knee flexion (min./max.) 31.9/78.7 9.8/21.2 36.8/83.6 10.4/18.3 31.5/78.7 4/17.9 36.9/85.6 6.3/11.1Ankle flexion (min./max.) 27/6.6 7.5/6.2 27.6/11.8 8.1/10.3 24.8/17.7 9.2/12.9 0.2/19.9 6.8/12.8

ST: standard; FL: flexible; SD: standard deviation.

342 Proc IMechE Part P: J Sports Engineering and Technology 234(4)

It has to be noted that the kinematic results for theankle joint in the FL condition appear to be slightlyasymmetric and more variable for the FL boot (Figure3(c)), which may be an effect of an uneven constructionof the wave course. Due to the fact that the ski is com-parably loosely linked to the leg in the FL condition,small variations in curvature of the snow surface mayhave generated this kinematic difference. While kneeand hip joint kinematics remain relatively unchangedand symmetric, it is possible that this slight alterationhas a substantial effect on the symmetry of muscle andjoint loading parameters reported in this study. This isof particular interest as the future intention will be toinvestigate asymmetric tasks; however, in this experi-ment, only movements in the sagittal plane wereexpected. Such variations may be substantial, but una-voidable in field studies such as this one.

Despite the substantial alterations in joint loadingand muscle activations, quite a large inter-individualvariation was observed. This leads to the suspicion thatthe training period was likely insufficient for most par-ticipants to make full use of the greater forward flexibil-ity of the boot, resulting in a different technique. Mostparticipants, including the two presented here, skied ina fashion very similar to their conventional movementpattern. However, the training period with the modifiedequipment turned out to be vital for representativeresults because the greater dorsiflexion and less stabilityrequired adaptation to a different orientation on theskis. Neurophysiological studies indicate that changing

the engram which leads to the execution of a learned‘‘movement program’’30 is challenging to alter, depend-ing on the difficulty level of the task. In the case ofhighly automatized and complex movements, a suffi-cient training period is required to establish a newmovement strategy.31 In mogul skiing–related move-ments, a period of 3 days for familiarization has beenshown to be useful to get accustomed to a differentstance on the ski.32

In this study, only one participant was able toengage in such an extended training period due to timelimitations and changeable weather conditions. Visualinspection revealed which participants showed a greateradaptation with an increased maximum ankle angle(AA) of 37� in the FL condition compared to 10� in theST condition. Furthermore, a two times greater rangeof movement in the ankle joint was exhibited, corre-sponding to up to 10� greater knee joint range ofmotion, as well as maximum angle and a more distinctforward lean indicated by a 13 cm more anterior projec-tion of the center of the hip/mass with respect to apoint halfway between the ankle joints18 (Figure 5).The change in maximum knee range of motion suggestsa more active vertical absorbing movement with themodified boot, which is crucial for mogul skiing tech-nique. However, as no kinetic data were collected forthis subject, no implications for the resulting changes injoint loading can be made.

A strict standardization for the training period wasnot considered crucial as the major goal of this study

Figure 4. Selected kinetic results: (a) minimum and maximum knee moments for the FL and ST conditions (skier 1); (b) minimumand maximum ankle moments for the FL and ST conditions (skier 1); (c) minimum and maximum knee moments for the FL and STconditions (skier 2); and (d) minimum and maximum ankle moments for the FL and ST conditions (skier 2).

Kurpiers et al. 343

was the verification of the method. However, this aspectwas considered vital for the implementation of follow-up studies. Participant 4 (Figure 5) was the only partici-pant who trained for a full period of 3 days with themodified exercise boot. This is presumably reflected inthe results that differ discernably in an intra-subject aswell as in an inter-subject comparison. A greater ante-riorly directed ankle range of movement may havecaused the forward shifted center of mass during thetest runs in the FL condition. This may have caused areduction in the load on the passive structures of theknee. Such distinct changes were not found in otherparticipants.

It is assumed that a lack of forward lean resistanceled to a tendency to lean back at the instant of bumpabsorption to maintain balance and avoid falling.Thus, the boot modification may have been too drasticto gain a natural squatting movement above the baseof support. No clear kinematic alterations were foundfor most participants and parameters; however, bothparticipants showed a small, but consistent, decrease inpeak GRF. Changes for joint forces and moments didnot show a consistent pattern. A possible explanationis that the lower body kinematic strategy was adjustedby some participants in the way that an additionalimpact buffer was shared between the ankle, knee andhip joint almost evenly. Therefore, only slight changesto each joint in conjunction with a more active

absorption by quickly pulling up the legs could theore-tically have that effect. The knee compression forcessupport this explanation.

There are several limitations of this study.Differences in measurements can be encountered dueto differences in the initial position of the model, theheight of the moguls or the inclination of the testingslope. Regarding joint angles, some researchers deter-mined zero in the static trial when the participant isalready in a slightly flexed position and other research-ers determined an upright position without boots as the0� or 180�.33 Other aspects that lead to differences inthe results of the two projects are camera adjustments,methods for digitization, accuracy of the force plates,calculation matrix for the GRF, choice of the modeland model specifications. Several validation studieshave been conducted on specific parameters used in thecomputer model for gait and cycling and showed goodreliability of the current model predictions.34

The methodological differences need to be consid-ered for the interpretation of the current results andresults, in general. The movement task was simplifiedin this study to facilitate its execution within a shorttraining period, but failed in four out of five partici-pants. Performance in a wave slope is related to thevertical movements of the legs during mogul skiing.However, as mogul skiing includes turns, there are pre-sumably different joint motions and loads transferred

Figure 5. Test runs of participant 4 after three full days of familiarization with leg flexion on the bump and leg extension in thevalley between bumps: (a) standard ski boot with shaft (ST) and (b) modified ski boot with no shaft (FL) (AA: ankle angle and KA:knee angle).

344 Proc IMechE Part P: J Sports Engineering and Technology 234(4)

to the knee joint, which need to be investigated infollow-up studies, including turns. The varying resultsof this study can be explained by the lack of trainingtime and changes in stability of the boot intervention.

Conclusion

In conclusion, this study successfully verified the testingprocess, including motion analysis in an outdoor skiingenvironment, a rather drastic boot modification andthe utilization of the computer model. This conclusionis valid, provided a further improvement loop of thesystem guarantees the avoidance of icing of the loadcells and the damage of specific parts of the force sen-sors by reinforcement. By the time this article is pub-lished, this requirement had been met. The removal ofthe shaft impedes the adjustments of joint movementsby changes in muscle activity if no sufficient trainingtime is given. In this study, only one participant man-aged to kinematically make full use of the boot altera-tion. In the future, this specific training tool should stillbe considered for several skiing disciplines on differentskill levels; however, caution should be used as ankleprotection by the boot is not maintained. Hence, fur-ther data collection is required with sufficient trainingtime and an improved ski boot modification in follow-up studies.

Acknowledgements

We want to thank Zermatt Bergbahnen AG, JurgBiner, Veit Senner, Andreas Kiefmann, PeterSpitzenpfeil and all participants.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interestwith respect to the research, authorship, and/or publi-cation of this article.

Funding

The author(s) received no financial support for theresearch, authorship, and/or publication of this article.

ORCID iD

Nicolas Kurpiers https://orcid.org/0000-0002-3622-5282

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