Does the Position of the Hinge in Cross-country Ski Bindings Affect

Muscle Activation in Skating?

Conor M. Bolger

, Øyvind Sandbakk

, Gertjan Ettema

and Peter Federolf

1,2

Center for Elite Sports Research,

Department of Neuroscience, Norwegian University of Science and Technology,

Trondheim, Norway

Department of Physical Performance, Norwegian School of Sport Sciences, Oslo, Norway

Keywords: Efficiency, Kinematics, Principle Component Analysis, EMG.

Abstract: The objective of this study is to examine effect of changed hinge positioning in cross-country ski bindings

on efficiency, kinematics and muscle activation patterns. Differences in muscle activation were investigated

using a principle component analysis (PCA) due to the higher sensitivity of this method. Participants

performed three tests utilizing varying hinge positions: front (toe attachment 0 cm), middle (4 cm behind

toe), back (8 cm behind toe). The skiers performed 4 minutes at moderate intensity in G3 skating technique

with all hinge positions. All tests were performed at 5% incline and 3.89 m/s. Conclusion: The greatest

differences between skating and classical, and between genders were found on flat and uphill terrain, but

were not associated with variations in heart rate. In all athletes, the hinge position affected (F (2,87) > 5.71,

p < 0.005) the scores ξ3,i of the third eigenvector v3 in which represented 7.0% of the entire variability of

the dataset (EV3 = 0.0699). The current pilot-study indicates differences in gross efficiency and kinematics,

and revealed significant differences in muscle coordinative patterns with varying hinge positions.

1 INTRODUCTION

In cross-country ski skating the movement is

performed in a zigzag fashion with the skis angled

towards the average direction of travel and the

corresponding leg push-off is performed

perpendicular to the ski (Sandbakk et al., 2013). The

ski boot is attached to the ski via a binding

mechanism, normally creating a pivot location in

front of the toe. Therefore a relatively soft boot that

flexes is required to fully utilize plantar flexion. The

leg push-off in ski skating is comparable to speed

skating, but the klapskate introduced a more

backward located mechanical hinge where a stiffer

boot could rotate about. This innovative technology

promotes plantar flexion and knee extension

throughout the push-off phase (Houdijk et al., 2000).

Due to better mechanical efficiency the

introduction of the klapskate resulted in a slight

increase in total power output and an increase in

speed skating velocities by 3-5% (de Koning et al.,

2000). However, no differences in muscle activation

patterns, neither in timing nor in amplitude, between

the conventional skate and the klapskate could be

identified using standard methods for analysing

electromyogram (EMG) signals (Houdijk, 2000).

Although a previous study showed that the klapskate

system might be more effective than this current

system also in cross-country skiing (Stoggl et al.,

2010), the effects of hinge positioning on such a

system has never been examined. Therefore, the

objective of this study is to examine effect of

changed hinge positioning in cross-country ski

bindings on efficiency, kinematics and muscle

activation patterns. Differences in muscle activation

were investigated using a principle component

analysis (PCA) due to the higher sensitivity of this

method (von Tscharner, 2002).

2 METHODS

2.1 Subjects

Three male Norwegian elite cross-country skiers

participated in the study (26 ± 3 years, body height

182 ± 7 cm, body mass 74 ± 7 kg). The skiers

participated at a national and international level and

were familiar with roller skiing as part of their daily

summer training.

M. Bolger C., Sandbakk Ø., Ettema G. and Federolf P..

Does the Position of the Hinge in Cross-country Ski Bindings Affect Muscle Activation in Skating?.

 2014 SCITEPRESS (Science and Technology Publications, Lda.)

2.2 Instruments and Materials

Treadmill tests were performed on a 5x3 m motor

driven treadmill (Forcelink B.V., Culemborg, the

Netherlands). All skiers used a modified Madshus

Nano Carbon skate boot (Madshus/K2, Biri,

Norways) that was fit to a prototype binding

designed specifically for this project, (IDT Sports,

Lena, Norway). All skiers used the same pair of

roller skis (IDT Skate, IDT Sports, Lena, Norway).

Ventilatory variables were assessed employing

open-circuit indirect calorimetry with an Oxycon

Pro apparatus (Jaeger GmbH, Hoechberg,

Germany). The instruments were calibrated against

ambient air and commercial gas with known

concentrations (16.00 ± 0.04% O

and 5.00 ± 0.1%

, Riessner-Gase GmbH and Co, Lichtenfels,

Germany).

Movement kinematics were monitored via an

Oqus (Qualisys AB, Gothenburg, Sweden) infrared

camera-system with a 250Hz sample rate and where

synchronized with EMG, sampled at 1500Hz. EMG-

electrodes were attached to the skiers’ dominant

skiing-side leg on the bellies of m. semitendenosus,

m. vastus medialis, m. vastus lateralis, m. rectus

femoris, m. lateral gastrocnemius, and m. tibialis

anterior. The placement of the electrodes followed

the recommendations of the Surface

Electromyography for the Non-Invasive Assessment

of Muscles (SENIAM). The electrodes were bipolar,

disposable pre-gelled Ag/AgCl surface electrodes

(Noraxon USA, Inc., Scottsdale, AZ) with 20 mm

inter-electrode distance.

2.3 Protocols and Procedures

Participants were prepped with retrorflective

markers along with EMG electrodes and sensors

prior to a 20-minute ski specific warm-up with the

normal roller ski boots and bindings on the roller-ski

treadmill. Participants performed three tests utilizing

varying hinge positions: front (toe attachment 0 cm),

middle (4 cm behind toe), back (8 cm behind toe).

Skiers performed 4 min at moderate intensity in G3

skating technique with all hinge positions. All tests

were performed at 5% incline and 3.89 m/s.

Ventilatory variables where continuously monitored

through the entire test. A 15-minute break was given

between hinge tests to minimize sweating and

fatigue.

A skate cycle was defined as one right and one

left leg push-off. The beginning and end of each

cycle was defined as ski lift-off of the left ski. Cycle

rate was calculated as the number of cycles per

second. Cycle length was the covered distance on

the treadmill during one cycle, which was calculated

as speed times cycle time.

Work rate was calculated, in accordance with

Sandbakk et al. (Asan Grasaas et al., 2014),

as the

sum of power against gravity P

g = m ·g · sin α · v

and friction Pf = m · g · cos α · μ · v where m is the

body mass of the skier, g the gravitational

acceleration, α the angle of treadmill incline, v the

speed of the treadmill belt, μ the frictional

coefficient (.026). The aerobic metabolic rate was

calculated as the product of VO

2 and the oxygen

energetic equivalent using the associated respiratory

exchange ratio and standard conversion tables.

Gross

efficiency was calculated as the external work rate

performed by the entire body divided by the aerobic

metabolic rate, presented as a percentage.

The EMG analysis was conducted with custom-

written Matlab

(The MathWorks Inc., Natwick,

MA) codes and included the following steps: For

each trial, 30 stride cycles were determined using the

acceleration signal with the ski takeoff serving as

trigger points. A wavelet transformation (von

Tscharner, 2000) yielded the intensity of the EMG

signal. This intensity was resampled such that 501

data points represented each cycle and normalized to

unit intensity per cycle. The normalized waveforms

of the 6 muscles were then concatenated into a 3506-

dimensional vector. All vectors from the 3 subjects

skating with 3 different hinge positions formed a

270 x 3501 matrix which was submitted to a

principal component analysis (PCA). The PCA

yielded (a) eigenvectors v

that quantified correlated

deviations from the mean waveform, (b) eigenvalues

quantifying how much of the variability was

represented by each eigenvector, (c) scores ξ

k,i

that

quantified how much the deviations of each cycle i

from the mean waveform was represented by the

eigenvectors v

. The scores ξ

k,i

were analyzed to

determine if the hinge position caused correlated

deviations from the mean multi-muscle EMG

waveform. Due to the small number of participants,

the statistical analysis was only conducted to

determine intra-subject effects of the hinge position,

i.e. for each subject a 1-way ANOVA was calculated

and Student T-tests were used for the post-hoc

comparison between hinge positions.

3 RESULTS

The mean cycle lengths for the front, middle and

back hinge positions were 8.0 m, 8.0 m, and 7.8 m

respectively, with corresponding cycle rates of

0.49s, 0.48s, and 0.50s. Gross efficiency for the

front, middle, and back hinge positions were 16.1%,

16.6% and 15.9% respectively.

In all athletes, the hinge position affected (F

(2,87) > 5.71, p < 0.005) the scores ξ

3,i

of the third

eigenvector v

(Figure 1). This eigenvector

represented 7.0% of the entire variability of the

dataset (EV

= 0.0699). A visual representation of

how v

changed the muscle activation pattern is

displayed in Figure 2.

Figure 1: Boxplots representing the distribution of the

scores ξ

obtained for the 30 skating cycles analysed for

each condition. The asterisk * indicates significance in the

post-hoc test.

Figure 2: Mean muscle activation pattern of the 6 muscles

plus the deviation from the mean represented by v

: the

blue curve indicates the back hinge position, the red line

the front hinge position. The skating cycle is represented

from ski take-off (0%) to the following ski take-off

(100%).

4 DISCUSSION

The current pilot-study indicates trends in gross

efficiency and kinematics, and revealed significant

differences in muscle coordinative patterns with

varying hinge positions. Although differences in

efficiency and kinematics were expected due to

previous literature on the effects of the klapskate, the

effect on EMG patterns has not been shown before.

The employment of PCA to determine hinge

position differences is a novel approach that

investigated correlated deviations from the mean

multi-muscle EMG waveform and could validly

reveal small changes in muscle activation.

The EMG waveforms seen in Figure 2 represent

the skate push-off for the front and back hinge

position. With the back hinge position the

gastrocnemius activation indicates an increased

plantar flexion during the push-off phase and the

tibialis anterior activation indicates a counter

dorsiflexion movement during the swing phase. This

suggest increased hip and knee extension within the

skate push-off proposing that the skiers’ hips are

held in an anterior position as the knee extends

dorsolateral to the movement direction, which may

have resulted in the shorter cycle length and

decreased mechanical efficiency. Conversely,

increased activity in the vastus lateralis, vastus

medialis and rectus femoris during the ski-plant

phase with the front hinge position may suggest

decreased stability of the system and increased

muscle recruitment required to balance.

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