An evaluation of inertial motion capture technology for use in the optimization of road cycling kinematics
John CockcroftUniversity of Stellenbosch
Overview of Presentation
• Background to inertial motion capture and road cycling
• Research motication objectives
• Experimental work
• Analysis of magnetic interference
• Analysis of cycling kinematics
• Conclusions
Inertial motion capture technology
• The MVN BIOMECH system
• Produced by Dutch company Xsens B.V • One of the fastest growing European technology companies• Track motion with accelerometers and gyroscopes attached to
the body with a body fitting Lycra suit
Portable storage
Wireless data transmission
Kinematic Analysis
Body-fit Lycra suit Digital model
Sensor to segmentcalculations
Sternum
Head
Forearm Hand
Upper leg
Lower leg
Upper arm
L3L5
Foot
T8T12
NeckShoulder
Toe
FULL BODY KINEMATICS:BIOMECHANICAL MODEL
23 Segments, 22 joints Anthropometrical scaling
INERTIAL MEASUREMENT UNITS
• Inertial navigation system
• Gyroscope provide angular data• Accelerometer provide linear data• Magnetometer provide heading in the global frame
• Errors in sensor kinematics
• Integration error in gyroscope and accelerometer data• Magnetic disturbances
ELIMINATING GYROSCOPE INTEGRATION ERROR
• Sensor drift
• Gyroscope offset errors are cumulative over time• Orientation error can become very large in just a few seconds
• Sensor fusion
• Accelerometer is used as an inclinometer to stabilize gyroscope data in the vertical plane
• Magnetometer data is used to stabilize gyroscope data in the horizontal plane
REDUCING ACCELEROMETER DRIFT ERROR
• Joint Updates
• Joint constraints reduce error in joint centre estimation
• Estimation of contact points
• Decreased drift of biomechanical model in global frame
REDUCING MAGNETIC INTERFERENCE
• Permanent constant interference
• E.g. prostheses, orthotics• Characterized as a priori by Kalman filter and removed
• Temporary constant/varying interference
• E.g. walking past a speaker or metal structure• Rejected by advanced Kalman filtering
• Permanent varying interference (>30s)
• E.g. metal beams in the floor• Kinetic Coupling algorithm for lower leg joint flexion
Road Cycling
• Goals of technique enhancement
• Minimize power demand by increasing aerodynamic efficiency• Maximize power production by increasing biomechanical
efficiency• Decreased risk of overuse injuries by improving technique• i.e. Optimize bike fit
Fore-aft position Handlebar adjustment
Saddle-heightadjustment
Down tube
Top tube
Seat tube
Seat tube angle
Objectives
• Objectives
• Perform measurements of outdoor cycling kinematics• Investigate level of magnetic interference from road bikes• Compare indoor and outdoor results• Investigate the link between rider kinematics and optimal bike fit
• Motivation
• Sports science research has been slow to adopt motion capture• There is very little sports related work being done with the MVN• Outdoor measurements of cycling kinematics not yet been
conducted
Test Protocol
• General suit setup
• 10 male sub-elite cyclists using own bicycles
• Two tests on separate days
• Indoor on stationary trainer• Outdoor followed by pursuit vehicle
• Three sessions per test
• Low, medium and high intensity (2, 3.5 and 5.5 W.kg-1)• 1min long steady-state recordings using the MVN suit
Test Protocol
Test Protocol
Test Protocol
Magnetic Measurement Results
• Test environments
• The indoor results show significant disturbances• Outdoor tests were undisturbed
• Road bicycles
• Hand sensors experienced worst interference• Chains, sprockets and pedals disturbed foot sensors
• Sensor fusion settings
• Kinematic Coupling algorithm is immune to interference• Can only be used for lower body when moving
0
20
40
60
80
1 2 3 4 5 6 7 8 9 10
MAg
netic
fiel
d in
tens
ity
Cyclists
Magnetic readings around left upper arm
Indoor
Outdoor
Undisturbed
0
20
40
60
80
1 2 3 4 5 6 7 8 9 10
MAg
netic
fiel
d in
tens
ity
Cyclists
Magnetic readings around left forearm
Indoor
Outdoor
Undisturbed
0
20
40
60
80
1 2 3 4 5 6 7 8 9 10
MAg
netic
fiel
d st
reng
th
Cyclists
Magnetic readings around left hand
Indoor
Outdoor
Undisturbed
ΘK
ΘH
Ankle
Knee
Hip
ΘA
Hip, knee and ankle flexion definitions
Overview of outdoor kinematics
• Flexion measurements are valid
• Joint excursions correlate well with past studies• Significant variability in flexion between cyclists
Maximum Minimum Range
Hip 75.5 ± 9.7(~90**)
23.5 ± 8.5(~45**)
52 ± 4.6(54 ± 4*) (~45**)
Knee 117 ± 7.7(~110**)
31.5 ± 7.7(30-60**)
85.5 ± 6.5(69 ± 4*) (~75**)
Ankle 12.1 ± 8.9(plantarflexion)
-9.8 ± 8.8(dorsiflexion)
21.9 ± 6.8(19 ± 4*)(~20**)
* (Bini RR, 2008)
** (R.J Gregor, 2000)
Comparison of indoor/outdoor tests
• Ecological validity of indoor testing
• Is laboratory testing on a trainer realistic?• No wind resistance, rigid wheel fixtures etc.
• Comparison of laboratory and road tests
• Hip and knee values in outdoor tests higher on average• Ankle values in outdoor tests lower on average• However, no clear trend between indoor and outdoor tests
Δ Maximum Δ Minimum Δ Range
Hip -4.7° -6.0° 1.3°
Knee -1.8° -3.9° 2.1°
Ankle 3.9° 4.0° -0.1°
1 2 3 4 5 6 7 8 9 100
20
40
60
80
100
120
140
Left Knee Maximum Flexion
Cyclists
Flex
ion
Angl
e[de
g]
Conclusions
• Can the MVN system measure accurate outdoor cycling kinematics?
• No. Road bikes cause unacceptable magnetic interference• Only lower body joint angles in the sagittal plane• Flexion values correlate well with other studies
• What was learned from the kinematic data?
• No clear difference between indoor and outdoor kinematics• High variability in hip, knee and ankle flexion suggest that
bicycle fit should not be based on anthropometrical data
THANK YOU
Motion Capture
• What is motion capture technology?
• Converting analogue marker tracking to a digital model• Applications in entertainment (e.g. movies and games)• Used in movement science (e.g. medical research of gait)
M
M
M
M
M
M
M
M
M
M
M
M
PHYSICAL SETUP MARKER TRACKING DIGITAL MODEL
OVERVIEW OF MOTION CAPTURE PROCESS:MVN SENSOR FUSION SCHEME
Orientation error θε
Gyroscope offset error bε
Magnetic disturbance error dε
QHM, Qd
QZM
QZG, QHG, Qb, Qθ
Accelerometer model
Gyroscope model
Magnetometer model
Kalman filter
Magnetometer signal
Gyroscope signal
Accelerometer signal +
+
_
_
VA
VG
HG
HM
GYROSCOPE SENSOR FUSION:ERROR-STATE KALMAN FILTER
SEGMENT KINEMATICS:CALIBRATIONS
N-pose T-pose Squat Hand-touch
ESTIMATING SEGMENT KINEMATICS
Effect of workload on kinematics
• Relationship between cycling power and rider kinematics
• Negligible change between low, medium and high power• Confirms the claims of current published literature
Δ Maximum Δ Minimum Δ Range
Hip 2.2 2.2 1.5
Knee 1.0 2.0 1.0
Ankle 2.9 4.1 3.6
Bilateral Asymmetry
• What is bilateral asymmetry?
• Difference between left and right sides of the body• Can be a difference in kinetics or kinematics• Can be caused by limb dominance, differences in joint
characteristics, anatomical differences, lateral pelvic tilt etc.
• Results
• Over 30% of the cyclists displayed significant asymmetry• Greatest asymmetry in hip and knee during downstroke• Greatest asymmetry for the ankles during the upstroke
-20
-15
-10
-5
0
5
10
15
20
1 2 3 4 5 6 7 8 9 10
Asym
met
ry in
flex
ion
[deg
rees
]
Cyclists
High Power
Medium Power
Low Power
Right more flexed
Left more flexed
Bilateral asymmetry of the knee joint at minimum flexion
Knee overuse injuries
• Knee overuse injuries very common in cycling
• Over 33% of knee injuries are to the patellofemoral joint (PFJ)
• Second most common is iliotibial band friction syndrome (ITBFS)
• Results from analysis
• 50% of the cyclists had knee flexion of over 120° and are at risk of PFJ pain
• 30% of the cyclists went under 25° flexion and are at high risk of ITBFS
Future Work
• Improvements to test protocol
• Design of ferromagnetic-free road bicycle• Protocol for clinical anthropometrical measurements
• Avenues for future studies
• Integration of MVN data with measurements of kinetic, neuromuscular and metabolic variables
• Analysis of biomechanical efficiency using 3D joint angles• Study of fatigue effects on upper body kinematics• Dynamic bicycle fit interventions for prevention of injury