The EMG, NIRS, and RPE responses to two turn transition techniques in alpine skiing

Summary

Objective

Two techniques facilitate the transition in an alpine ski turn, extension (EXT) and flexion (FLEX). The purpose of this study is to compare NIRS, EMG, and RPE responses between EXT and FLEX in expert level skiers.

Design

24 Professional Ski Instructors of America Level III alpine ski instructors, examiners, and National Demonstration Team members completed one standardized run each of EXT and FLEX. A turn cycle, made of a right and left turn, was divided into four Phases.

Methods

NIRS assessed HbO2 desaturation in the rectus femoris (RF) and vastus lateralis (VL). EMG was collected from the RF, VL, and gluteus medius (GM). Rating of Perceived Exertion (RPE, 0–10) assessed subjective exertion.

Results

FLEX resulted in greater HbO2 desaturation in RF (−77.9 ​± ​21% vs. −67.5 ​± ​23%, p ​< ​0.001), VL (−69.8 ​± ​23.1% vs. −61.1 ​± ​24.6%, p ​< ​0.001) and RPE (6.0 ​± ​2.3 vs. 5.2 ​± ​2.0, p ​< ​0.001) compared to EXT. FLEX generally resulted in greater EMG activity in the steering and transition phases for RF and VL (p ​< ​0.001) while EXT resulted in greater activity GM on the inside leg.

Conclusions

Caution should be exercised with recreational skiers as the FLEX transition resulted in significantly greater physiological stress compared to EXT. Localized hypoxia appears to be the focal point of this stress as HbO₂ desaturation was ∼15% greater along with 15% greater RPE during FLEX. FLEX also produced greater EMG activity than EXT in the majority of the turn phases.

1. Introduction

While the ski turn has been previously analyzed, the turn transition by itself has not received scientific attention. To make a carved turn, the skis need to be on their edges. Going from one set of edges to the new set of edges requires that the skis are flat against the snow if only momentarily. This flat ski period on the snow is known as the transition. The turn transition, sometimes called ‘turn switch’ or ‘crossover’, is the movement pattern that links left and right ski turns. A turn transition is the demarcation between two connecting turns and is usually on the path of travel halfway between the two turns. This location has been defined in many ways [[1], [2], [3]]. These movements are flexion and extension primarily of the lower body and are distinguished by the order in which they occur. For the skier to be in balance during a ski turn, their center-of-mass must be in a line-of-action over their base-of-support while the ski is on edge. This leaning or tipping of the ski and skier creates a ski/snow interaction resulting in a ground-reaction-force with the snow resulting in the ski turning. Transitioning from one turn to another involves the skiers the center-of-mass moving from the inside of one turn to the inside of the upcoming turn.

Among skiing professionals, the techniques for the transition have been a popular topic of discussion and debate [4]. The two most prevalent techniques are a transition with extension and a transition while flexed. Both have other names, but the use of ‘flexion’ (FLEX) and ‘extension’ (EXT) describes the actions of the lower body without colloquial confusion. The FLEX transition is also known as a retraction turn, turn on a flexion, and suck-switch. FLEX is becoming more popular among professional and recreational skiers as it is a strategy often used by elite ski racers. If the leg stays flexed during the transition, it is termed a flexed transition. There is very little vertical movement of the center of mass. However, EXT is the conventional or contemporary technique. When the new outside leg extends during this transition period to promote weight transfer from one ski to the other. Compared to a FLEX transition, there is substantially more vertical movement of the center of mass. Hence, the term of an extension transition.

A major difference between the EXT and FLEX transitions are the movements of the lower limbs, which ultimately affects center of mass. During the transition the legs are either extending, as in the EXT transition, or very flexed, with up to 90° each of knee, hip, and trunk flexion in the FLEX transition. Differing physiological responses would be expected for each transition technique. The action of leg extension requires muscular effort to raise the skier's center-of-mass, while the flexed legs and more static position of the FLEX transition resulting in a muscular response consistent with an extreme flexion angle leading to a more compressive movement.

When ski racers skied with more flexion in the lower body, Szmedra, Im, Nioka et al. [5] found, using near-infrared spectrophotometry (NIRS), reduced blood flow and a larger oxygen hemoglobin (HbO₂) desaturation in the vastus lateralis compared to when they skied in a more upright posture. This is consistent with Rundell, Nioka, Chance [6] who reported significant ischemia, HbO₂ desaturation, and reduced oxygen delivery in speed skaters when skating in a low crouched position. It is well known that persistent ischemia leads to hypoxia which may also result in reduced force production and premature fatigue [7]. In contrast, the greater extension–flexion activity exerted by muscle during the EXT transition may increase arterial and venous blood flow to the working muscles via the skeletal muscle-pump mechanism when compared to the lower posture FLEX technique [8].

From a mechanical perspective, the joint lever arm is altered and the quadriceps muscles lengths change when the knee flexes. As the knee angle increases, this third-class lever arm is shortened increasing the amount of force needed for extension. The amount of force needed is also increased with increased knee joint flexion by altering the length–tension relationship of the skeletal muscle fibers [9,10]. When the sarcomere is shortened, or lengthened, to the outer limits, the capacity of the sarcomere to form crossbridges decreases which results in decreased force production.

To our knowledge, physiological responses to the alpine ski turn transitions have not been measured to date. With the popularity of the FLEX transition challenging the contemporary EXT transition, the skiing population's declining fitness level, high speed lifts which reduces rest time on the chairlift ride, and the desire of the recreational skier to ski more throughout the day, understanding the physiological responses are important for skier enjoyment and safety. The extreme posture of the FLEX transition, relative to the contemporary EXT transition, has less efficient lower body joint angles resulting in a disadvantageous length–tension relationship of muscle and less muscle-pump activity. With this in mind, the purpose of this study is to measure lower body muscle activity (EMG) and oxygen desaturation (NIRS) utilizing the FLEX and EXT turn techniques. We hypothesize that because of body position, FLEX will lead to greater HbO₂ desaturation, greater electrical activity of the muscles, and greater perceived exertion compared to EXT.

2. Methods

Twenty-four Professional Ski Instructors of America (PSIA) Level III alpine ski instructors, examiners, and National Demonstration Team members provided informed consent. Study procedures were approved by the Montana State University Institutional Review Board. There were nine females (mean age ±SD: 46.4 ​± ​5.9 ​y) and 15 males (mean age: 50.3 ​± ​9.3 ​y). All skiers are top level skiers and instructors who routinely demonstrate the given transitions during instruction and exams. Skiers used their own skis which had a mean underfoot ski width of 70 ​± ​5 ​mm. Testing took place at Bridger Bowl Ski Area, Bozeman, MT and Palisades Tahoe, Olympic Valley, CA.

All skiers skied a course of 24 standardized turns. The first two turns and last turn of each run were dropped from the analysis as skiers had not yet reached a stable speed and the last turn was not a complete turn. Thus, 21 turns were used in the analysis, 11 outside leg turns (right foot turns to the left) and 10 inside leg turns (left foot turns to the right). Turn indicator markers were set at 18 ​m diagonal distance from one another with 4.5 ​m offset on a run with a moderate pitch that averaged 12°. The run was machine groomed nightly. Researchers slipped the course after each run and salt was spread on the snow to minimize rut development and maintain snow consistency.

Skiers were given one run to warmup with both transitions. Skiers completed one run using the EXT technique in the transition phase in all turns and one run of the FLEX technique in all turns. Interventions were administered in a counter-balanced fashion and data were analysed as a within subject design. Skiers were instructed to ski each run as consistently as possible.

Transition and steering data of the present study were assessed according to the turn phases reported by Kröll et al. [11]. Those authors presented a turn in four phases, Initiation, Steering I, Steering II, and Turn Completion. Based on their calculations, the combined Initiation and Turn Completion phases account for 41% of the turn time. For purposes of the present study, transition was defined as the combination of Initiation and Turn Completion phases. The start of the Initiation phase was defined as when the skis were flat on the snow and coincident with the center of mass centered over the two skis. According to Kröll et al. [11], the Initiation phase accounts for 23% of the turn. Turn completion was defined from the start of Initiation and backing up 18% of preceding turn time. Movements and ski position were confirmed by video analysis. The skiers were filmed from behind with a Panasonic HCV770 during each run. Recording frequency was 60 ​Hz. This video was transferred to Camtasia version 2023 (TechSmith, East Lansing, MI). Additionally, at least two uninvolved PSIA Certified Examiners viewed the video to ensure the skiers were properly executing the transition techniques and turns according to PSIA standards.

For turn analyses, a turn cycle was defined as consecutive right foot and left foot turns. A right foot turn is a turn to the left where the right leg is the outside leg. A left foot turn is where the right leg is now the inside leg. Each turn cycle was divided into two phases, steering and transition (Fig. 1). Phases Ia and Ib, in Fig. 1, Fig. 2, Fig. 3, Fig. 4, refer to the right foot turn (outside leg) steering and transition phases, respectively. Phases IIa and IIb refer to the left foot turn (inside leg) steering and transition phases, respectively.

Fig. 1.

Schematic of a turn cycle with four Phases. Ia: Right foot turn, steering phase; Ib: Right foot turn, transition phase; IIa: Left foot turn, steering phase; IIb: Left foot turn, transition phase; X: Turning gates.

Fig. 2.

Rectus femoris activity of right and left foot turns.

Fig. 3.

Vastus lateralis activity from right and left foot turns.

Fig. 4.

Gluteus medius activity from right and left foot turns.

Near infrared spectroscopy was used to assess mean HbO₂ desaturation across the entire run (Moxy, Fortiori Designs LLC, Hutchinson, MN, USA). The reliability and accuracy of the indirect assessment using near-infrared spectroscopy has been previously reported [12]. This method evaluates the ratio of the oxyhemoglobin to total hemoglobin in the muscle, expressed relatively as a percentage [13]. Data was collected at a sampling frequency of 2 ​Hz from the rectus femoris (RF) and vastus lateralis (VL) of the left leg. Each location was shaved of any hair and cleaned with isopropyl alcohol. Sensors were then placed on the skin over the belly of each muscle and secured with stretchable tape. Skiers were seated for 7 ​min before each run to allow for baseline data collection. NIRS data were averaged over the last 15 ​s of the seated rest to derive a baseline value. HbO₂ data are presented as % change from baseline to the lowest point at the end of the run.

Electromyography data were collected from the RF, VL, and gluteus medius (GM) of the right leg according to Rainoldi et al. [14]. The sensor sites were shaved of any hair, the skin cleaned with isopropyl alcohol wipes, the sensors were then attached to the skin with two-sided tape, and surgical tape was used to secure the sensors in place. Data were recorded on a portable data logger until processing (Trigno Personal Monitor, Delsys, Inc., Boston, MA). A 125 ​ms interval was used in the RMS calculation. The EMG signal was filtered with a second order Butterworth filter (10–400 ​Hz). Data were processed via RMS calculation and scripts based on turn time normalization (Delsys, Inc., Boston, MA, USA). Sampling frequency was 1926 ​Hz.

As a relative index of EMG signal intensity, the amplitude range and mean signals for each turn were measured and corresponding percentage of the maximal range calculated with the average being reported.

The Borg Rating of Perceived Exertion scale (RPE, 0–10) was used to assess subjective exertion at the completion of each run.

2.1. Statistical analysis

Data were analysed using IBM SPSS Statistical Software. Due to lack of homogeneity of variance and normal distribution of the data, the Wilcoxon Signed Rank Test was used to analyze all data. Statistical significance was established at p ​< ​0.05. Cohen's D test for effect size was calculated where 0.2 is a small effect, 0.5 medium effect, and >0.8 a large effect.

The authors declare the financial support of MSU Kreighbaum Foundation and Palisades Tahoe for this project. Neither entity was involved in project design, data collection, data analyses, or manuscript preparation.

3. Results

Data from one skier was dropped due to equipment malfunction and loss of their data. Thus, the final analysis was performed with 23 skiers. All skiers completed both of their runs without incident. There was no statistical difference between EXT and FLEX transitions for mean turn time (p ​= ​0.34). Mean turn time for EXT was 1.53 ​± ​0.16 ​s versus 1.54 ​± ​0.17 ​s for FLEX.

Performing the FLEX technique in the transitions resulted in greater HbO₂ desaturation from baseline in RF (−77.9 ​± ​21% vs. −67.5 ​± ​23%, p ​< ​0.001) and VL (−69.8 ​± ​23.1% vs. −61.1 ​± ​24.6%, p ​< ​0.001) compared to EXT. The differences between techniques for RF and VL desaturations amount to 15% and 14%, respectively. Rating of Perceived Exertion was 15% greater in FLEX than EXT, 6.0 ​± ​2.3 vs. 5.2 ​± ​2.0 (p ​< ​0.001).

Table 1 contains the EMG voltages for all muscles and phases while Fig. 2, Fig. 3, Fig. 4 contain EMG responses for each muscle during a full turn cycle. For the outside leg turns, the FLEX transition technique resulted in significantly greater VL EMG activity than EXT during both steering and transition phases by 2% and 10%, respectively. The EXT transition resulted in significantly greater RF EMG activity during the steering phase by 3% over FLEX. However, FLEX technique resulted in greater RF EMG activity than EXT during the transition phase by 31%. The FLEX technique also resulted in 3% greater GM activity during the steering phase than EXT. No statistical difference was observed between techniques during the transition phase for GM.

Table 1.

Mean (±SD) EMG signal per turn phase.

		Vastus Lateralis			Rectus Femoris			Gluteus Medius
Phase		EXT	FLEX	Effect Size	EXT	FLEX	Effect Size	EXT	FLEX	Effect Size
Outside Leg	Steering	0.141 (0.017) ∗	0.144 (0.014)	0.2	0.098 (0.015) ∗	0.095 (0.009)	0.3	0.088 (0.009) ∗	0.091 (0.006)	0.3
	Transition	0.078 (0.01) ∗	0.086 (0.011)	0.8	0.062 (0.01) ∗	0.081 (0.005)	2.3	0.063 (0.008)	0.064 (0.006)	0.1
Inside Leg	Steering	0.118 (0.007) ∗	0.108 (0.009)	1.3	0.101 (0.011) ∗	0.113 (0.014)	4.0	0.076 (0.003) ∗	0.063 (0.003)	4.3
	Transition	0.095 (0.01) ∗	0.101 (0.01)	0.6	0.058 (0.006) ∗	0.067 (0.006)	1.5	0.081 (0.003) ∗	0.071 (0.006)	2.0

Data listed in mV; EXT: Extension transition; FLEX: Flexion transition; ∗: significantly different from FLEX (p ​< ​0.001); Effect size: Cohen's D.

When the EMG activity of the inside leg was assessed, VL activity was significantly greater for EXT than FLEX by 8% during the steering phase. However, FLEX resulted in 6% greater VL activity compared to EXT during the transition phase. The FLEX technique resulted in significantly greater RF EMG activity during steering and transition phases, by 12% and 16% respectively, compared to EXT. Gluteus medius activity was significantly greater for EXT during both steering and transition phases by 17% and 12%, respectively.

No differences for amplitude range and percent of maximum range for the mean signal were observed between transition techniques for any of the muscles (Table 2).

Table 2.

Amplitude range (V) and percent of maximum signal.

	Mean Maximal Amplitude Range		Mean as a % of Maximum Amplitude
	Extension	Flexion	Extension	Flexion
Gluteus medius	0.0020 (0.002)	0.0018 (0.001)	44.5 (9.1)	45.3 (6.5)
Rectus femoris	0.0022 (0.0014)	0.0025 (0.002)	48.5 (6.9)	48.3 (8.0)
Vastus lateralis	0.0035 (0.0026)	0.0037 (0.0029)	49.3 (7.3)	50.0 (8.8)

4. Discussion

This study analyzed the influence of FLEX and EXT transition techniques on physiological responses during alpine ski turns. We hypothesized that because of body position, FLEX will lead to greater HbO₂ desaturation, electrical activity of the muscles, and perceived exertion than EXT. Generally, our hypotheses were met.

The EXT transition has deep roots in alpine skiing technique [15]. The skier transfers weight to the new turning ski and while extending the legs. The FLEX transition gained its popularity from World Cup ski racers [4,16]. While it is a technique, it also evolved as a ski racing tactic to reduce the path length of the skier's center-of-mass. The ski racer minimizes, or eliminates, the vertical extension (EXT) between turns which reduces the length of the parabolic arc of their center-of-mass by the flexed (FLEX) lower body resulting in a shorter line through a racecourse.

Elite Alpine instructors were recruited because they could perform both FLEX and EXT transitions consistently according to PSIA standards [16] and which allowed for the comparison within the same subject. Their consistency at maintaining similar velocity can be seen in the average time-per-turn data (EXT 1.53 ​± ​0.16 ​s, FLEX 1.54 ​± ​0.17 ​s). Video was reviewed by two PSIA Examiners post run to verify that transition techniques skiers performed the FLEX transition with sufficient knee and hip flexion at the level of the required PSIA technical standards. Conversely, there was knee and hip extension to conform to the requisite PSIA technical standard [16].

Based on EMG data, Table 1 and Fig. 2, Fig. 3, Fig. 4, the FLEX technique resulted in greater RF and VL activity in the outside leg compared to the EXT technique, especially during the transition phase. A possible explanation for this is the influence of body posture when transitioning from one ski to the other. With a high amount of hip and knee flexion seen with FLEX, the biarticulate RF would be lengthened where it may have been taken out of its optimal force-producing length according to the length–tension relationship or that the crouched position did not allow for efficient hip and knee movements to take place. At this point, greater stimulation of the active motor units must occur, or more motor units have to be recruited, to complete the task. If those muscles of the outside leg can't produce sufficient force at the right time, then the inside leg must increase its activity to assist in performing movement. This explanation might also be applicable to the VL but for stabilizing the knee joint. Interestingly, the GM, which is an important muscle for femoral rotation and stabilization, had activity that was variable between turn phases and legs. There was minimal difference between techniques noted for the outside leg, but large differences between techniques for the inside leg where EXT resulted greater activity than FLEX.

Our hypothesis was confirmed with the NIRS measurements from RF and VL and support the findings reported previously [5,6] that the FLEX technique resulted in greater desaturation. These muscles showed 15% and 14% greater oxygen desaturation, respectively, in the FLEX trial compared to EXT. Our hypothesis was also confirmed with EMG, where the FLEX technique generally showed greater RF and VL activity, suggesting more effort was possibly due to the inefficiency of a more flexed posture that affects sarcomere length and potentially the mechanical advantage. This interpretation of the HbO₂ - EMG relationship is plausible as it has been reported that there is an inverse relationship between decreased oxygen content and an increased EMG signal [17]. That relationship suggests that the smaller lower body joint angle may have reduced venous or arterial blood flow limiting oxygen transport or lessened the mechanical advantage of the joint angles which then required greater effort on the part of the working muscle(s) or both in the present study. The static nature of the FLEX transition also would minimize any advantage that the skeletal muscle pump may provide. It should also be noted that there is extra effort involved in the EXT transition, which takes energy, as shown in some of the EMG results and the decrease in HbO₂.

In conclusion, the importance of greater effort per turn has ramifications for the skiing public. While the well-conditioned skiing professionals have little difficulty performing the FLEX transition technique, this extra effort and resulting fatigue may limit the length of enjoyable time and possibly leave the skier less able to avoid an unforeseen out-of-balance incident. Additionally, when the hips are behind the knees, which is often found in an increased knee flexion stance, there is an increased risk of anterior cruciate ligament injury which is the most common injury in Alpine skiing [18]. A more flexed knee angle has also been shown to increase both tibiofemoral and patellofemoral compression [[19], [20], [21]] along with quadricep tendon forces [20] and mean peak shear forces [22] resulting in increased stress to the knee joint. The skiing professional should be aware of consequences of teaching a flexed transition to clientele that may not be ready physically and skill-wise. It should be noted that the FLEX transition technique does provide a steppingstone to more advanced terrain, such as moguls, where the FLEX transition technique is used to absorb the large terrain undulation keeping the skier in more ski-to-snow contact.

Practical applications

• •Ski teaching professionals should be aware of the physiological differences in the two transitions.
• •The FLEX transition technique was perceived to be more difficult physiologically than EXT. The implication being that for less fit skiers or beginner or intermediate level skiers may be subjected to premature fatigue. The fatigue may lead to increased risk of injury.
• •Increased knee and hip flexion increased muscle electrical activity and decreases oxygen desaturation.
• •The FLEX transition is fast for the ski racer and can be used in ungroomed terrain.

Limitations

• •A more objective measure (i.e., motion capture) could be used to quantify body position.
• •The specific start and end of the transition are not well defined in the literature.
• •Lack of quantification of snow conditions and hill topography.
• •Did not control for skis used by the skiers as skiers used their own skis.

Funding sources

We acknowledge the financial support of MSU Kreighbaum Foundation and Palisades Tahoe for this project. Neither entity was involved in project design, data collection, data analyses, or manuscript preparation. This has been declared in the Methods section under Statistical Analyses.

Declaration of competing interest

The authors declare this project was supported by the Ellen Kreighbaum Foundation at Montana State University and Palisades Tahoe. The funding entities did not have input on project design, data collection, manuscript preparation, or manuscript approval. The authors further state that there is no conflict of interest.