Comfortable Measurement of Ski-turn Skill

using Inertial and Plantar-pressure Sensors

Seiji Matsumura, Ken Ohta and Toshitaka Kimura

Communication Science Laboratories, Nippon Telegraph and Telephone Corporation, Atsugi, Kanagawa, Japan

Keywords:

Realistic Evaluation, Ski, Carved Turn, Inertial Sensor, Plantar-Pressure Sensor.

Abstract:

This paper proposes a comfortable and easy way to measure skill in ski turns with inertial and plantar-pressure

sensors. The inertial sensors were used to log the motion of skis, and the plantar-pressure sensors were used

to measure plantar-pressure distribution and analyze how the feet transmit force to skis. One intermediate

skier and one expert skier, both adult males, participated i n this experiment. They skied in short turns in six

trials totally. According to the results of the experiment, the data for skidded and carved turns were clearly

different. Therefore, we consider that our proposed measurement method will enable skiers to conveniently

analyze their ski-turn skills on their own.

1 INTRODUCTION

In recent years, carved turns have become mainstream

in skiing because they are less decelerative than skid-

ded turns. It is especially important for intermediate

skiers to acquire skill in carved turns. However, it is

hard for skiers to analyze their own tu rns while actu-

ally skiing. Realistic evaluation on the ski slope is an

effective way for skiers to autodidactically learn h ow

to execute turns. For this purpose, a simpliﬁed measu-

rement method that doesn’t disturb skiing action is re-

quired. In addition, the system should be comfortable

to wear and allow easy measurem ent. Thus, it should

be compact, and measurement items must be kept to

a minimum. In this study, we measured the motion

of skis and the force added to skis from th e feet. The

motion of skis was measured with inertial sensors (Fa-

sel et al., 2017), and the plantar-pressure d istribution,

which shows wh ich area of the plantar applies a load

to skis, was measured with plantar-pressure sensors

(Falda-Buscaiot et al., 2017).

This paper proposes a comfortable and easy way

to measure skill in ski turns with these sensors.

2 METHODS

2.1 Participants

One intermediate skier and one expert skier, both

adult males, participated in th e experiment. The in-

termediate skier skis w ith skidded turns. On the o ther

hand, the expert skier skis with carved turns. Before

the experim ent, they we re provid ed with an outline

of the general purpose of the study, which also infor-

med them that they could withdraw at any time wit-

hout penalty. All methods used in this study were ap-

proved by the Ethics and Safety Committees of NTT

Communica tion Science Laboratories a nd in accor-

dance with the Declaration of Helsinki. The protocol

number of the Ethics and Safety Committees of NTT

Communica tion Science Laboratories is H3 0-002 .

2.2 Measurement Procedure

The participants skied down the slope several times to

warm up before the experiment. There were six trials

in total, and they skied in short turns in each trial.

They were asked to try to keep the rhythm of their

turns constant. In the experiment, they wore insole-

type plantar-pressure sensors in their ski boots. An

inertial sensor was mounted on e a ch ski. Both sensors

recorde d data synchronously during each trial. Since

these sensors are easy to wear and set up, it would be

easy for skiers to set them up on their own. Each turn

in each trial was extracted from the turn start to its end

as one phase. We also shot movie s at 60 fps from the

top of the slope to verify the relationship between the

body motion and sensor data.

Matsumura, S., Ohta, K. and Kimura, T.

Comfortable Measurement of Ski-turn Skill using Inertial and Plantar-pressure Sensors.

DOI: 10.5220/0006959301450148

In Proceedings of the 6th International Congress on Sport Sciences Research and Technology Support (icSPORTS 2018), pages 145-148

ISBN: 978-989-758-325-4

145

2.2.1 Inertial Sensors

We used two nine-axis wireless inertial sensors (LP-

WS1105, LOGICAL PRODUCT). The sensors were

mounted on the skis in front of the bindings, as shown

in Fig. 1. The x axis was per pendicular to the ski-

ing direction, the y axis was alon g the direction of

skiing, and the z axis pointed towards the ground.

Right-handed screw rotation in the positive direction

was positive rotation. The sampling rate was 1,000

Hz, and data were logged in each sensor’s memory.

We measured acceleration, rotation, and geomagne-

tism for all three axes to analyze the motion of skis.

The data were passed through a 10 -Hz low-pass ﬁlter

for prepro cessing. In this study, we focused on the

angular velocity proﬁles based on the rotation data.

Figure 1: Inertial sensors (l eft) were mounted in front of the

bindings.

2.2.2 Plantar-pressure Sensor

We used the pedar system (novel.de) as a plantar-

pressure sensor. Th is system d ivides the plantar into

99 sections and measures the pressure in e ach section.

The sampling rate was 100 Hz. In this study, we di-

vided the plantar into fo ur areas as shown in Fig. 2.

The sensor’s values in each area were summed up.

Medial

Forefoot

(MF)

Lateral

Heel

(LH)

Medial

Heel

(MH)

Lateral

Forefoot

(LF)

Figure 2: The plantar sensor and the four plantar areas.

Pressure was calculated in each area. Right foot is shown.

3 RESULTS

3.1 Angular Velocity on Skis

Each turn was extracted from th e turn start to end as

one p hase. According to the inertial sensor data, the

time series of angular velocity differed between skid-

ded and carved turns.

Figure 3( a ) and (b) show the angular velocities of

both skis along the x axis, whic h indicates ﬂuttering

of the skis perpendicular to the skiing direction. Large

ﬂuttering was observed in the middle of the turn phase

in the intermediate skier, while stable behavior during

the w hole phase in th e expert skier.

Figure 3(c) and (d) show the angular velocities al-

ong the y axis, which indicates tilting of the skies in

the skiing direction. The results demonstrate that the

changes in tilted velocities inward or outward were

larger and jaggier in the inter mediate than in the ex-

pert skier. Notably, the expert’s turn s stopped tilting

the skis, and the tilt angle stayed the same in the

middle phase.

The z axis indicates skidding of the skis [Fig. 3 (e)

and (f)]. The interm ediate skier’s turns showed the

minimum peak in the midd le of the turn phase. On

the other han d, the expert’s tur ns were stable, and the

amount of change was also small. The amplitude of

the angular velocity in skidded turns was larger than

in carved turns.

These angular velocity results indicate that the in-

termediate skier skied with skidded turns, while the

expert executed turns with very little skiddin g, w hich

are characterized as car ved turns.

3.2 Plantar Pressure Distribution

Figure 4 shows the plantar pressure distribution in

each planter area. The in termediate skier mostly used

the outside leg to load the ski ( upper m ap of Fig. 4),

while the inside ski was little loaded. In particular, the

medial forefoot of the o utside leg loaded the ski in the

ﬁrst half of the phase, and then the lateral forefoot of

the outside leg worked to load the ski in the last half

of the phase.

On the other hand, the expert used both legs to

load the skis (lower map of Fig. 4). The medial fo-

refoot of the insid e leg worked stron gly in the ﬁrst

half of the phase. In the last half of the phase, the

medial heel of the inside leg was used and then me-

dial heel of the outside leg was used. The leg put-

ting the most load on the skis was switched from the

inside to the outside leg during the last half of the

phase, which was largely different from the interme-

diate skier’s pattern.

icSPORTS 2018 - 6th International Congress on Sport Sciences Research and Technology Support

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(a) X axis in the interme diate skier.

(b) X axis in th e expert skier.

(d) Y axis in th e expert skier.

(e) Z axis in the intermediate skier.

(f) Z axis in the expert skier.

Figure 3: Angular velocity along three axes in skis. The horizontal axis shows the phase of a turn. The blue line shows the

outside ski, and the orange line shows t he inside ski.

Figure 4: Plantar pressure in the four areas of both feet during a turn. The horizontal axis shows the phase of a turn. The

upper map is for the intermediate shier; the lower map is for the expert skier. Meanings of the acronyms on the vertical axis

are as follows. Out: outside leg for a turn. In: inside leg for a turn. MF: medial forefoot. LF : lateral forefoot. MH: medial

heel. LH: lateral heel. E.g., Out:LF means ”lateral forefoot of the outside leg for a turn”.

Comfortable Measurement of Ski-turn Skill using Inertial and Plantar-pressure Sensors

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4 DISCUSSION

In the experim e nt, we found that skidded turns in the

intermediate skier were executed mostly with the out-

side ski. Moreover, the minimum peaks in the middle

of the turn phase o n the rotation of x and z axis indi-

cated the ﬂuttering and skidding of the skis.

On the other hand, the carved turns of the expert

skier were executed with both skis. Ski motion in the

carved tur ns was moderate and stable. This means

that they were executed with a steady posture.

5 CONCLUSION

The proposed measurement system, consisting of in-

ertial and plantar-pressure sensors, is comfortable to

wear and makes it easy to me a sure ski-turn sk ill. Ac-

cording to the results of the experiment, data for skid-

ded and carved turns were clearly different. There-

fore, we consider that our pr oposed mea surement will

enable skiers to conveniently analyze their ski-turn

skills on their own.

REFERENCES

Falda-Buscaiot, T., Hintzy, F., Rougier, P., Lacouture, P.,

and Coulmy, N . (2017). Inﬂuence of slope steepness,

foot position and turn phase on plantar pressure distri-

bution during giant slalom alpine ski racing. In PLoS

ONE.

Fasel, B., Sp¨orri, J., Sch¨utz, P., Lorenzetti, S., and Ami-

nian, K. (2017). An inertial sensor-based method for

estimating the athlete’s relative joint center positions

and center of mass kinematics in alpine ski racing. In

frontiers in Physiology.

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