The transfer of expertise to aerial skiing: Utility of an athletic profile in female athletes

Abstract

Australian aerial skiers are recruited from other acrobatic sports through a nationally coordinated transition program. However, there is limited understanding of how to train transferring athletes for both optimal performance and injury prevention. The aim of this study was to test whether an athletic profile could be used to identify which skills that gymnasts possess are more and less likely to transfer to aerial skiing based on similarities and differences in the athletes’ physical attributes. Six elite female aerial skiers and five state- and national-level female gymnasts completed a test protocol involving drop-landing, countermovement jump, isokinetic strength of the knee flexors and extensors, flexibility and anthropometrical profile. Aerial skiers and gymnasts had an anthropometrical profile ideal for acrobatic efficiency, but gymnasts had smaller thigh girth and greater knee range of motion. These results suggest that gymnasts are more likely to succeed in transferring to aerial skiing's acrobatic skills than skiing and landing skills. We suggest that gymnasts transferring to aerial skiing undertake pretransfer training designed to improve hamstring isometric strength and dynamic stability of the knee to reduce the risk of knee injury and improve their ability to maintain a crouched position.

Keywords

Acrobatic skills gymnastics,injury prevention isometric strength knee joint range of motion

Aerial skiing is a winter Olympic sport involving elements of skiing, mid-air acrobatics and landing.¹ Although some aerial ski athletes specialize early and train specifically from the beginning of their athletic development, others transfer at a later stage from acrobatic sports such as gymnastics and diving. In Australia, there is a nationally coordinated talent identification program through which athletes can transfer from elite artistic and rhythmic gymnastics to aerial skiing. Athletes enter the program from the ages of 12–16 yrs. Although they are taught fundamentals of skiing and landing on skis, it is assumed that their acrobatic skills and athletic ability will transfer from gymnastics to aerial skiing.² Whilst there is some evidence of broad transfer of expertise between sports,^3,4 there has been little consideration of transfer as a key factor in the development of overall expertise in a given sport.^5,6 Instead, researchers have investigated either transfer of isolated skills between sports, such as anticipation⁷ and motor execution,⁸ or transfer of coordination patterns within a sport between different contexts⁹ and tasks.¹⁰ From these studies, it seems that transfer of sport specific skills is difficult to predict and depends on the precise skill being transferred as well as the characteristics of the contexts and/or tasks that the skill is being transferred between. These findings are consistent with well-established theories such as Thorndike's¹¹ Identical Elements Theory. Researchers have only attempted to explicitly predict how the characteristics of the individual affect their capability to transfer skills more recently. Whilst there is evidence that a high level of expertise in their current sport facilitates athletes transferring skills to other sports,^6,7 it is unclear whether such transfer occurs because of adaptation of the skills themselves or of the abilities underlying the skills.⁵ In part, this is due to a lack of research examining the physical adaptations that support positive transfer of skill.¹² As a result, there is little evidence on how to train athletes transferring between sports to either optimize physical performance or prevent injury whilst facilitating the development of expertise in nontrained tasks.

Athletic profiling is one method of obtaining the evidence necessary to optimize athletic performance and reduce injury risk. Athletic profiling typically involves using a standardized battery of tests to benchmark the physical performance and attributes of an athlete.^13,14 Athletic profiling is an effective tool for identifying talented athletes because it aids in determining important performance indicators¹⁴ and provides a baseline for future testing to track development.¹⁵ Athletic profiling also aids in minimizing injury risk by revealing physical deficiencies and injury mechanisms.¹⁴ Hence, developing an athletic profile for aerial skiing could improve both talent identification and reduce the prevalence of injuries.

An athletic profile of aerial skiing would also be invaluable to assist athletes from other acrobatic sports such as gymnastics and diving to transfer to aerial skiing. For example, a close examination of the performance of a gymnast against metrics common to the athletic profiles of aerial skiing and gymnastic could be used to predict transfer¹⁴ and guide future transition protocols. The characteristics of an ideal acrobatic physique include low fat mass, broad shoulders, a narrow waist and a relatively short trunk, which are vital to achieve biomechanical efficiency through in-air twists and tumbles.^16,17 If elite aerial skiers possess a similar physique, it would be reasonable to predict positive transfer of acrobatic skills from gymnastics to aerial skiing. Successful transfer of these acrobatic skills is critical to optimizing the performance of a gymnast transferring to aerial skiing.

Since its inception in the late 1970s, aerial skiing has consistently reported very high injury rates. Aerial skiing produced the second highest injury rate of all sporting disciplines at the 2010 Winter Olympic Games, with 26% of participants incurring an injury.¹⁸ Between 2006 and 2009, more than half of World Cup aerial skiers sustained an injury. Many of these injuries were severe enough to result in more than 28 days absence from training and competition.¹⁹ The knee is the most common site of injury, with 47% of aerial skiers experiencing at least one major knee injury in their career.²⁰ Considering some aerial skiers transfer from sports like gymnastics or diving, there is a duty of care to ensure transferring athletes are appropriately trained and developed to minimize injury risk. As an athletic profile of an aerial skier has yet to be developed, the physical attributes important for injury prevention in aerial skiing are currently unknown.

The objective of this study was to investigate similarities and contrast differences between elite aerial skiers and gymnasts in a novel athletic profile of aerial skiing. It was hypothesized that significant differences in the athlete profile would be exhibited between aerial skiers and gymnasts, particularly with respect to physical attributes associated with skill in skiing.

Methods

Participants

For this study, we recruited 11 female participants in two cohorts. The first cohort comprised six Australian female aerial skiers (17.2 ± 1.0 years, 161.8 ± 2.5 cm, 57.0 ± 3.6 kg, experience 1.7 ± 0.4 years). These participants were members of the national aerial skiing team with funding and support from the Olympic Winter Institute Australia, Victorian Institute of Sport and Australian Institute of Sport and were being groomed to represent Australia at the World Championships and Olympic games. The second cohort comprised five state- and national-level female rhythmic gymnasts (17.8 ± 1.4 years, 157.2 ± 3.6 cm, 52.8 ± 3.3 kg, experience: 12.8 ± 1.3 years). All participants provided informed consent, and ethical approval was obtained from the Deakin University Human Ethics Advisory Group (Health), approval number HEAG-H 26_2011.

Testing procedures

An athletic profile for aerial skiers does not exist. So, we reviewed sports sharing common athletic requirements and similar injury profiles as aerial skiing to develop an appropriate battery of tests. Our protocol included a drop landing task,²¹ a countermovement jump (CMJ),²² isokinetic strength tests of the knee flexors and extensors,²³ flexibility tests and anthropometric measures.²⁴

Anthropometric measures

We recorded the following anthropometric measures: standing height (cm); body mass (kg); sum-of-seven skinfold thickness (triceps, subscapular, biceps, supraspinale, abdominal, frontal thigh and medial calf; cm); shoulder breadth, waist, hip and thigh girth and arm and leg length (all in cm). Standing height was measured to the nearest 0.1 cm using a stadiometer (Seca-220, Seca, Birmingham, United Kingdom). Body mass was measured to the nearest 0.05 kg using an electronic digital scale (UC-321, A&D Company, Tokyo, Japan), with participants wearing only shorts and a singlet. Skinfold sites were marked according to International Society for the Advancement of Kinathropometry (ISAK) guidelines,²⁵ and the thickness of each skinfold was measured twice with a Harpenden caliper (Baty Ltd, West Sussex, United Kingdom). If a difference of more than 5% was observed between trials, we took a third measurement. If only two skinfolds were taken, we averaged the measurements. However, if three measurements were taken, the closest two measures were averaged. Breadths were measured using a Harpenden long-sliding calliper (Holtain Ltd, Crymuch, United Kingdom), and girths and lengths were measured using a steel measuring tape (KDS, Kyoto, Japan), in accordance with ISAK guidelines.

Flexibility tests

The flexibility tests we chose were an ankle range of motion (ROM) task and a sit-and-reach test For the ankle ROM test, participants began with their knees touching the wall and feet flat on the ground. We defined maximum ROM as the furthest point away from the wall to which participants could move their heel whilst keeping both the heel on the ground and the knee touching the wall. Participants completed this test on both limbs.

We followed the American College of Sports Medicine procedure for the sit-and-reach test.²⁶ The sit-and-reach test required participants to sit with their feet placed 10 cm apart and flat against the end of a flexibility test box (Flex-Tester, Novel Products, Rockton, IL, USA). Sit-and-reach was recorded as the most distant point reached with arms straight and palms down, whilst maintaining straight legs. Participants completed three repetitions of each flexibility test, with the highest value recorded.

Isokinetic strength tests

We measured isokinetic strength of the knee flexors and extensors using a calibrated isokinetic dynamometer (Biodex System 4-Pro, Biodex Medical Systems, Shirley, NY, USA). Participants completed two sets of five repetitions of maximal flexion and extension at both 60°/s and 180°/s on both limbs with 120 s recovery between each set. All data were recorded using the Biodex Advantage 4.0 software (Biodex System 4-Pro, Biodex Medical Systems, Shirley, NY, USA) and smoothed with a fourth order, low pass, Butterworth digital filter with a cutoff frequency of 5 Hz. Peak torque was recorded for each repetition, and we calculated hamstrings to quadriceps (H:Q) ratio.

Dropping landing

For our drop-landing protocol, participants performed six repetitions of a six near-vertical drop-landings from a 0.9 m platform with ∼1 min rest between drops. We recorded their three-dimensional hip and knee kinematics using a 12-camera, high speed, three-dimensional motion capture system (Raptor-E, Motion Analysis Corporation, Santa Rosa, CA, USA). The cameras surrounded a 2 × 1 × 2.5 m capture volume. Data were collected and processed using the manufacturer-supplied software (Cortex Kintools 3.0.1, Motion Analysis Corporation, Santa Rosa, CA, USA) on a laptop computer (Hewlett Packard, Palo Alto, CA, USA). Static and dynamic calibration of the motion analysis system was performed prior to each data collection session in accordance with the manufacturer's guidelines. Kinematic data were sampled at 500 Hz, which has been validated as a suitable speed for analyzing dropping and jumping tasks.²¹ Each participant was fitted with 23 retroreflective markers (Medical Motion, Cardiff, CA, USA) according to a modified Helen Hayes marker set, involving bilateral markers at the shoulder, anterior superior iliac spine anterior thigh, medial and lateral knee, anterior shank, medial and lateral ankle, heel, and toe as well as markers at the C7 vertebrate, right scapula, and sacrum. Virtual markers were created at the center of the ankle, knee and hip joints and origin of the pelvis. All marker locations were filtered with a fourth order, low-pass Butterworth digital filter set at a cutoff frequency of 6 Hz. Maximum angles of hip flexion, knee flexion and knee valgus upon impact during the drop-landing were obtained using angles between the pelvis, thigh and shank segments.

Counter movement jump

Our CMJ protocol required participants to complete six jumps with ∼30 s rest between repetitions. Jumps were completed with no arm swing. We measured jump kinematics using the same system and marker set as the drop landing task. Jump height was calculated using the maximum displacement of the sacral marker.

In addition to these kinematic data, we also measured counter movement jump kinetics by having the participants perform each jump on an in-ground force plate (AMTI Force and Motion, Watertown MA, USA) sampling at 500 Hz. The force plate data were collected using manufacturer supplied software (NetForce, AMTI Force and Motion, Watertown MA, USA) on a desktop computer (Lenovo, Beijing, China). A customized program (LabView ver7.1, National Instruments, Texas, USA) allowed us to calculate maximum vertical ground reaction force (GRF) from the maximum force identified as well as the rate of force development during power production.

Data processing and statistical analyses

Our preliminary analysis comprised three tests. First, a Shapiro-Wilk test was performed to test for normality. This test was not statistically significant suggesting that our data were normally distributed and appropriate to be examined using parametric tests. Second, because we tested the aerial skiers over two sessions, a one-way analysis of variance (ANOVA) was used examine differences between testing sessions. This test revealed no significant difference between sessions. Thus, the profile was considered stable across sessions and all variables were averaged and included for analysis. Third, a series of group (aerial skiers, gymnasts) × trial ANOVA tests were conducted to examine differences between trials within and between groups. These tests revealed no main effect for trial or interaction between trial × group for any of the variables. So, we pooled all trials and removed trial as a factor.

For the main analysis, we used a series of one-way ANOVAs to examine differences in the 43 parameters measured between the two athlete groups (aerial skiers, gymnasts). All statistical analyses were completed using the Statistical Package for the Social Sciences software (version 20.0, IBM Corporation, Armonk, NY, USA) with the significance level set at p < 0.05.

Results

Statistically significant differences between aerial ski and gymnastic participants were observed in two of the 43 parameters measured. Thigh girth was significantly greater in aerial skiers compared with gymnasts (p = 0.047; Table 1), whilst gymnasts recorded a significantly longer sit-and-reach compared with aerial skiers (p < 0.001; Table 1).

Table 1.

Differences between aerial skiers and rhythmic gymnasts.

	Aerial skiers	Rhythmic gymnasts	Difference
Drop landing: maximum angle (°)
Right
Knee valgus	11.25 ± 1.80	10.66 ± 2.74	0.59
Knee flexion	87.80 ± 6.91	95.76 ± 8.81	0.91
Hip flexion	87.71 ± 8.01	93.62 ± 3.28	−7.96
Left
Knee valgus	14.03 ± 3.50	13.12 ± 3.13	−7.26
Knee flexion	86.30 ± 7.15	93.56 ± 9.83	−5.91
Hip flexion	88.28 ± 8.95	94.74 ± 3.68	−6.46
CMJ
Height (m)	0.347 ± 0.02	0.322 ± 0.03	2.56
RFD (N/s)	3775 ± 1884	2870 ± 1224	905
GRF (N)	1711 ± 558	1985 ± 68	−274
Peak torque (Nm)
Right (60°/s)
Extension	139 ± 36.2	132.7 ± 14.4	6.4
Flexion	69.9 ± 13.5	74.6 ± 8.4	−4.7
Right (180°/s)
Extension	93.1 ± 24	91.4 ± 15.4	1.7
Flexion	49.7 ± 10.3	57.0 ± 11.23	−7.3
Left (60°/s)
Extension	133.7 ± 33.5	125.3 ± 28.4	8.3
Flexion	69 ± 17.6	72.2 ± 2.3	−3.2
Left (180°/s)
Extension	93.5 ± 14.8	87.2 ± 26.3	6.3
Flexion	46.8 ± 11.6	56.5 ± 4.7	−9.6
H:Q ratio
Right
60°/s	0.51 ± 0.04	0.57 ± 0.10	−0.06
180°/s	0.54 ± 0.09	0.64 ± 0.16	−0.10
Left
60°/s	0.52 ± 0.05	0.60 ± 0.14	−0.08
180°/s	0.51 ± 0.13	0.69 ± 0.18	−0.18
Overall	0.52 ± 0.06	0.62 ± 0.14	−0.10
Anthropometry
Height (cm)	161.8 ± 6.1	157.2 ± 8.0	4.6
Weight (kg)	57.0 ± 8.7	52.8 ± 7.3	4.2
Skinfolds (cm)
Triceps	12.0 ± 1.9	10.8 ± 1.8	1.2
Subscapular	8.3 ± 1.9	9.6 ± 1.4	−1.3
Biceps	5.8 ± 2.4	6.0 ± 1.6	−0.2
Supraspinale	8.0 ± 2.3	8.4 ± 1.6	−0.4
Abdominal	13.5 ± 4.8	11.5 ± 2.9	2.0
Front thigh	19.6 ± 3.3	16.4 ± 4.7	3.2
Medial calf	9.7 ± 2.4	9.8 ± 4.0	−0.1
Sum-of-seven	76.8 ± 16.2	72.6 ± 14.7	4.2
Girths (cm)
Waist	68.8 ± 4.1	69.5 ± 5.3	−0.7
Hips	91.8 ± 5.8	87.7 ± 5.1	4.1
Thigh	51.7 ± 3.2	47.5 ± 2.7	4.2*
Calf	35.0 ± 2.6	33.6 ± 2.0	1.6
Upper arm	27.9 ± 2.3	26.9 ± 2.4	1.0
Breadths and lengths (cm)
Shoulder breadth	35.9 ± 2.2	36.1 ± 1.7	−0.2
Arm length	51.2 ± 2.3	52.8 ± 3.6	−1.6
Leg length	81.5 ± 2.4	80.1 ± 5.0	1.4
Flexibility (cm)
Sit-and-reach	41.8 ± 3.4	47.8 ± 2.0	−6.0**
Ankle ROM
Right	34.3 ± 1.8	32.3 ± 5.2	2.1
Left	35.2 ± 1.7	33.5 ± 4.3	1.7

CMJ: countermovement jump; GRF: ground reaction force; H:Q: hamstrings to quadriceps; RFD: rate of force development; ROM: range of motion.

All values are mean ± SD, n = 11 (aerial skiers n = 6, gymnasts n = 5), *p < 0.05, ***p < 0.001.

Discussion

The aim of our study was to investigate similarities and contrast differences between elite aerial skiers and gymnasts. Results were not consistent with our hypothesis that there would be significant differences in the athlete profiles of aerial skiers and gymnasts. Rather, there were significant differences between aerial skiers and gymnasts for only two of the 43 parameters of the athlete profile that we developed. Comparison of our aerial skiing athletic profile with established profiles from other sports reveals similarities with the profile of female acrobatic athletes. The aerial skiers that we tested had similar skinfold measurements and body dimensions to high level female gymnasts.²⁴ This suggests that elite aerial skiers possess a physique that is advantageous for acrobatic performance in general. Therefore, we predict that positive transfer of acrobatic skills would occur from gymnastics to aerial skiing because the physical attributes that are advantageous for acrobatic performance are common across sports.

In contrast, the difference that we uncovered between the aerial skiers’ and gymnasts’ thigh girth is telling for gymnasts’ ability to adapt to aerial skiing. The aerial skiers’ greater thigh girth was probably due to muscle hypertrophy resulting from higher volumes of isometric contraction required because it was not accompanied by differences in strength or power. Skiers are required to make relatively large isometric contractions to maintain a crouch position,^27,28 high volumes of which will result in hypertrophy without gains in power or rate of force production.²⁹ The ability to maintain a crouch position for long durations is a known determinant of elite performance in alpine skiing.²⁸ However, the ability is less important in aerial skiing where a crouch position is held for a relatively short period of time as the skier approaches the take-off ramp. One possibility is that the aerial skiers are accumulating high volumes of isometric contraction incidentally by participating in alpine skiing either for recreation or as a means of cross-training. Such participation in alpine skiing may help to develop and maintain aerial skiers skiing skills. However, we cannot be sure of this conclusion because we neither tested isometric leg strength nor collected data on whether the aerial skiers participated in alpine skiing. Nevertheless, consideration should be given to including high volumes of isometric contraction of thigh muscles in the training programs of gymnasts transferring to aerial skiing. This will support development of transferring athletes skiing skills by helping them to attain and maintain a crouched position.

The other possible reason for the greater thigh girth of the aerial skiers was that the skiers had greater thigh adiposity. However, this explanation is not supported by the results because the mid-thigh skinfold of the skiers was not significantly different to that of the gymnasts.

It is common for aerial skiers to suffer landing related knee injuries. This makes both a stable knee joint stability and good landing kinematics critical for aerial skiers to reduce their risk of landing related knee injuries. The results of the sit-and reach tests suggest that the gymnasts had greater static hamstring flexibility and ROM at the knee joint. A larger ROM is typically thought to be accompanied by poorer joint stability.³⁰ However, the drop landing task revealed that not only the gymnasts displayed dynamic knee instability but also the skiers. Female athletes (soccer and basketball players) have been reported to generally demonstrate 9° or less of knee valgus upon landing. In comparison, the aerial skiers and gymnasts in our study demonstrated knee valgus of 11.25 ± 1.80° and 10.66 ± 2.74 on the right leg and 14.03 ± 3.50° and 13.12 ± 3.13 on the left, respectively. Increased knee valgus is a result of decreased neuromuscular control of the lower limb in the coronal plane, with incorrect co-contraction of the quadriceps.³¹ Knee valgus torques are the single most significant predictors of peak anterior cruciate ligament (ACL) force upon landing³² with knee valgus of more than 9° associated with greater risk of ACL injury.²¹ Due to the high incidence of landing-related knee injuries amongst aerial skiers, a drop-landing task holds promise for injury prediction in the sport, and thus future research should correlate drop-landing kinematic data with aerial skiing injury data. However, it is important to determine if hypermobility further increases the risk of knee injury for gymnasts transferring to aerial skiing.

Since the participants in our study were national- and international-level athletes, their poor landing kinematics alludes to insufficient physical preparation rather than a lack of skill. Both the aerial skiers and the gymnasts were not as strong or powerful as female athletes from other sports involving a power component. Their CMJ height was lower than that observed in athletes from karate³³ or lacrosse³⁴ whilst they also exhibited less peak torque during knee extension and flexion than karate athletes.³³ This could suggest a power deficiency amongst the participants in our study, which would limit performance and successful transfer to aerial skiing due to the established power component of the sport.³⁵ However, further research is required to determine whether power-predicting tests are accurate indicators of current and future performance in aerial skiing.

The findings of this study are limited by the small sample size; however, due to the elite nature of the aerial skiers, there were no other athletes that could be tested, as the entire aerial skiing team volunteered for the study. Furthermore, no performance and injury data were collected, and as such, the predictive ability of each test with respect to performance or injury cannot be determined. Similarly, electromyography (EMG) and kinetics were not recorded during the drop-landing task. As such, muscle activation patterns and eccentric muscle actions could not be quantified. Further, we did not collect eccentric muscle strength, muscle/tendon thickness or cross-sectional area data. This may be an area for future research especially in sports where optimal landing is paramount for performance and injury prevention.

Based on our findings, we suggest that training for gymnasts transferring to aerial skiing should focus on increasing hamstring strength, improving dynamic stability of the knee and developing landing kinematics. The program could involve dynamic neuromuscular training methods such as plyometric training, which has been shown to significantly increase hamstring muscle peak torque and H:Q ratio and reduce knee valgus in female athletes from jumping sports, over a six-week training program.^32,36 Similarly, functional eccentric training has been reported to improve hamstring strength, causing resultant increases in H:Q ratio amongst females.³⁷ Therefore, a transition package involving both plyometric and functional eccentric training holds promise for minimizing injury risk when landing training commences.

Footnotes

Acknowledgements

The authors would like to acknowledge the Australian Winter Olympic Institute for their support of the project especially the coaching staff for the Australian Aerial Skiing Team.

In January 2021, School of Physiotherapy and Exercise Science, Curtin University, became Curtin School of Allied Health.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

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

ORCID iDs

Simon M Rosalie

Paul Gastin

Kevin Netto

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