The 14th IFToMM World Congress, Taipei, Taiwan, October 25-30, 2015 DOI Number: 10.6567/IFToMM.14TH.WC.PS1.010

Effects of Seat Position on Joint Angles and Moments of the Lower Extremities During Cycling

Jia-Da Li1, Hsing-Po Huang1, Hsuan-Lun Lu1, Francois Liang2 and Tung-Wu Lu1,3* Institute of Biomedical Engineering, National Taiwan University, Taipei, Taiwan1

Cycling & Health Tech Industry R&D Center, Taichung, Taiwan2 Department of Orthopaedic Surgery, School of Medicine, National Taiwan University, Taipei, Taiwan3

e-mail: twlu@ntu.edu.tw

Abstract: Knowledge of the kinematics and loadings at the lower limb joints at different seat positions during cycling are helpful for the selection of the best seat position for rehabilitation purposes. This study aimed to develop an instrumented cycle ergometer to measure the joint angular and moment changes during cycling at different seat positions. Fifteen healthy young subjects participated in the current study and performed cycling in nine different seat positions. The results showed that seat positions, especially horizontal ones, affected joint loadings. Moving the seat anteriorly loaded the hip flexors and knee extensors while moving the seat posteriorly loaded the hip extensors, knee flexors and ankle plantarflexors. These results will help therapists to decide seat positions most suitable for the rehabilitation of individual patients and for ergonomic bicycle design.

Keywords: Cycling, joint kinematics, kinetics, rehabilitation

1 Introduction Cycling is one of the most popular tools widely used

for transportation, recreation, sport, and rehabilitation. For rehabilitation purposes, a cycle that enables the adjustment of joint angles and loadings for strengthening specific muscles without imposing too much loads to the injured or diseased joints to prevent further complaints, is more important than identifying bike parameters that optimize output efficiency or muscle endurance [1-3]. For this purpose, changing seat positions can be useful in manipulating loading distribution among the lower limb joints during cycling. A complete biomechanical analysis of how changing seat positions can affect the kinematics and kinetics of the lower limb joints is necessary and will help therapists to decide seat positions most suitable for the rehabilitation of individual patients, e.g., cardiopulmonary or muscular strength training, according to the joint conditions of individual patients. However, this information is not available because previous studies have mostly focused on the adjustment of bike parameters to achieve maximum mechanical output efficiency or muscle endurance for the road cyclists. Three-dimensional joint loadings changes at different seat position remain unexplored. Therefore, the current study aimed to bridge this gap.

The purposes of the study were (1) to develop an instrumented ergometer with position-adjustable seats and (2) to measure the joint angles and moments at different seat positions during cycling.

2 Materials and Methods 2.1 Subjects

Fifteen healthy young adults (age: 25.0 ± 4.4 years, height: 171.4 ± 5.5 cm, mass: 72.1 ± 16.5 kg) without any neuromusculoskeletal dysfunction, participated in this study with written informed consent as approved by the Institutional Research Board. Each subject wore twenty-eight markers on the pelvic anterior superior iliac spine, posterior superior iliac spine and bilateral side of greater trochanter, middle thigh, lateral/medial epicondyles, fibula head, tibial tuberosity, lateral/medial malleolus, second metatarsal bone head, tuberosity of fifth metatarsal bone, navicular tuberosity, and foot heel. Anatomical segmental reference frames were defined via a static subject calibration.

In dynamic trials (Fig. 1), three horizontal seat positions were defined as the ones with the horizontal distances between crotch of the rider and the axle center of crank arm to be 17%, 22% and 27% of the subject’s leg length. Three vertical seat positions were also included and defined as the ones with the vertical distances between crotch of the rider and the axle center of crank arm to be 74%, 79% and 84% of subject’s leg length. The sequence of the three seat horizontal and three vertical positions were randomized for each subject while performing cycling. A metronome was used to control the cycling speed to be at about 35 rpm. At least five complete crank cycles were recorded at each seat position.

2.2 Instrumentation Each subject performed cycling on a stationary

ergometer at an average resistance of 20Nm mimicking rehabilitation conditions (Fig. 1). The pedals of the ergometer were instrumented with 6-component load-cells (FS6-5000-PT, AMTI, USA) for measuring pedal reaction forces during cycling. The 3D marker trajectories were measured using an 8-camera motion capture system (MX T40, Vicon Motion Systems Ltd., UK) at a sampling rate of 120Hz.

2.3 Data Analysis A 7-link chain model of the human body, composed of

the pelvis, bilateral thighs, shanks and feet, was built with six ball-and-socket joints (i.e., bilateral hip, knee and ankle) so the model was fully described by a total of 24 degrees of freedom (21 rotations plus 3 spatial translation). A global optimization method (GOM) was used to reduce skin marker movement artifacts relative to the underlying bone [4]. With the marker and pedal reaction force data, inverse dynamic analysis was performed to obtain the joint moments. A Cardanic rotation sequence (z-x-y) was used to represent inter-segment rotation, so called “joint

angles”. Definition of segment-embedded coordinate system was following ISB recommendation [5-6]. Inertial properties, namely mass, position of center of mass and moment of inertia were obtained using an optimization method [7]. Initial guess was given using Dempster’s coefficients [8]. Newton-Euler equations were used for solving for joint resultant forces and moments. Crank angle was defined as 0 degree while the crank arm was positioned at the top dead centre (TDC) and the angles increased while the crank arm rotated forward. Crank angles equaled to 180 degrees when the crank arm was positioned at the bottom dead centre (BDC) and at 360 degrees when the crank arm was rotated back to the TDC again. The GCVSPL algorithm [9] was used to interpolate the curves of the joint angles and moments within one crank cycle and to get 361 data points for the crank angles. Maximum and minimum joint moments were extracted for statistical analysis. One-way ANOVA was used to test the differences between the 3 seat positions for vertical and horizontal position changes respectively. Polynomial trend detection was performed if a main effect was found. The significant level was set at 0.05.

Fig. 1. A subject was performing cycling. Red line represents seat vertical position and yellow line represent seat horizontal position.

3 Results The flexion angles of the hip, knee and ankle were

found to decrease significantly with the seat moving upward (p<0.05), except for the ankle dorsi-flexion (Figs. 2-5). All the three joints of the lower limb were affected significantly with the horizontal seat positions (p<0.05). While the seat was moved posteriorly, the hip flexion and the ankle plantarflexion increased significantly but the knee flexion was decreased significantly (p<0.05).

With the seat ascending in height, the hip extensor moments decreased significantly (p<0.05) but the hip flexor moments were not affected significantly (p<0.05,

Figs. 6-9). In contrast, the knee extensor moments were not affected, but the knee flexor moments were increased significantly (p<0.05). The ankle moments were not affected by seat height. The horizontal seat position affected the loadings at the hip, knee and ankle significantly (p<0.05). While the seat was moved posteriorly, the hip extensor moments increased but hip flexor moments decreased (p<0.05). The knee moments were in contrary to the hip, i.e., decreased extensor moments and increased flexor moments (p<0.05). The ankle plantarflexor moments were increased significantly (p<0.05).

4 Discussion An instrumented cycle ergometer with position-

adjustable seat was developed and a complete biomechanical analysis on the effects of seat position on the joint kinematics and kinetics of the lower extremities during cycling was performed in the current study. According to the results, seat positions indeed changed the joint kinematics of the lower extremities and also induced different joint load distributions during cycling.

For the hip joint, the loadings of the extensors and flexors were comparable. Moving the seat upward decreased the extensor moments, while moving the seat forward also induced lower extensor moments. Therefore, if the cyclist needs to relieve the gluteus maximus but load the iliopsoas, he/she should choose a higher and more anterior seat position, and vice versa.

For the knee joint, the extensor loading was mostly greater than that of the flexors. Although lower seat height increased knee flexion angle significantly, the knee extensor moments did not seem to be affected. The knee extensor moments were mostly affected by horizontal seat positions. According to the results, if the cyclist needs to relieve the quadriceps but load the hamstrings, he/she should move the seat posteriorly, and vice versa.

For the ankle joint, the plantarflexors were loaded during most of the crank cycle but the dorsiflexors were not loaded. The results showed that the gastrocnemius and soleus muscles were loaded at rear seat positions and the loads reduced while the seat was moved forward.

Since the joint loadings were more sensitive to horizontal seat positions than the vertical positions, it is suggested that horizontal seat positions can be determined first followed by the vertical positions while the therapist is trying to find a proper joint loading condition for rehabilitation purpose. The recreational bicycles on the current market mostly allowed only the seat position to be adjusted along the longitudinal direction. From an ergonomics perspective, including the function of anterior/posterior seat position adjustment is needed for the user to choose the optimum position.

The current study was the first to report a complete 3D joint kinematics and kinetics analysis at different seat positions during cycling. The current results will be helpful for better muscle training and injury prevention [10].

5 Conclusion Joint kinematics and kinetics of the lower extremities

were sensitive to seat positions, especially the horizontal position. Moving the seat anteriorly loaded the hip flexors and knee extensors while moving the seat posteriorly

loaded the hip extensors, knee flexors and ankle plantarflexors. These results will help therapists to decide seat positions most suitable for the rehabilitation of individual patients without imposing too much loads to the injured or diseased joints, and for ergonomic bicycle design for injury prevention. References [1] de Vey Mestdagh, K. Personal perspective: in search of an

optimum cycling posture. Applied Ergonomics 29(5), pp. 325-334, 1998.

[2] Silberman, M. R., Webner, D., Collina, S. and Shiple, B. J., Road bicycle fit, Clinical Journal of Sport Medicine 15(4), pp. 271-276, 2005.

[3] Will W. Peveler, Effects of Saddle Height on Economy in Cycling, Journal of Strength and Conditioning Research 22(4), pp. 1355-1359, 2008.

[4] Lu, T.-W. and O'Connor, J. J., Bone position estimation from skin marker co-ordinates using global optimisation with joint constraints, Journal of biomechanics 32(2), pp. 129-34, 1999.

[5] Wu, Ge and Cavanagh, Peter R., ISB recommendations for standardization in the reporting of kinematic data, Journal of Biomechanics 28(10), pp. 1257-1261, 1995.

[6] Wu, G., Siegler, S., Allard, P., Kirtley, C.. et al., ISB recommendation on definitions of joint coordinate system of various joints for the reporting of human joint motion--part I: ankle, hip, and spine. Journal of Biomechanics 35(4), pp. 543-8, 2002.

[7] S.-C. Chen, H.-J. Hsieh, T.-W. Lu and C.-H. Tseng, A method for estimating subject-specific body segment inertial parameters in human movement analysis. Gait & Posture 33(4), pp. 695-700, 2011.

[8] David A. Winter, Biomechanics 3rd Ed. WILEY, pp 63-64, 2005.

[9] Woltring, H. J. A Fortran package for generalized, cross-validatory spline smoothing and differentiation. Advances in Engineering Software (1978) 8(2), pp. 104-113, 1986.

[10] Faria, I. E. and Cavanagh, P. R., The Physiology and Biomechanics of Cycling, Wiley, New York, 1978.

Fig. 2. Averaged joint angle curves in a crank cycle of the hip, knee and ankle when the subjects rode with high, middle and low seat positions.

Fig. 3. Ensemble-averaged maximum and minimum joint angles over a crank cycle when the subjects rode with high, middle and low seat positions. Vertical bars represent one standard deviation. An arrow represents a significant linear increasing trend.

Fig. 4. Averaged joint angle curves in a crank cycle of hip, knee and ankle when the subjects rode with anterior, middle and posterior seat positions.

Fig. 5. Ensemble-averaged maximum and minimum joint angles over a crank cycle while subjects rode in anterior, middle and posterior seat position. Vertical bar represent one standard deviation range. An arrow to the right represents a significant linear increasing trend while an arrow to the left represents a decreasing trend.

Fig. 6. Averaged joint moment curves in a crank cycle of hip, knee and ankle while subjects rode in high, middle and low seat position.

Fig. 7. Ensemble-averaged maximum and minimum joint moment values over a crank cycle when the subjects rode with high, middle and low seat positions. Vertical bars represent one standard deviation. An arrow to the right represents a significant linear increasing trend while an arrow to the left represents a decreasing trend.

Fig. 8. Averaged joint moment curves in a crank cycle of hip, knee and ankle when the subjects rode with anterior, middle and posterior seat positions.

Fig. 9. Ensemble-averaged maximum and minimum joint moments over a crank cycle when the subjects rode with anterior, middle and posterior seat positions. Vertical bars represent one standard deviation. An arrow to the right represents a significant linear increasing trend while an arrow to the left represents a decreasing trend.