Joint Loading in the Lower Extremities During Elliptical Exercise

Tung-Wu Lu; Hui-Lien Chien; Hao-Ling Chen

Disclosures

Med Sci Sports Exerc. 2007;39(9):1651-1658. 

In This Article

Materials and Methods

Subjects

Fifteen male adult volunteers (age: 25.5 ± 2.5 yr; height: 173.7 ± 4.9 cm; mass: 72.7 ± 7.9 kg) participated in the current study. They were free from any musculoskeletal, cardiovascular, or neurological pathology, or any other disorders that might have affected the activity. Each subject gave written informed consent following the guidelines of the institutional human research ethics committee.

Instruments

A commercially available ET (Cycling & Health Tech Industry R&D Center, Taiwan) was used in this study (Figure 1). For the measurement of the pedal reaction force (PRF) and its point of application, the right pedal of the ET was instrumented with a six-component force transducer (Bertec Corporation) (Figure 1). For level walking, two forceplates (Advanced Mechanical Technology Inc.) located in the center of an 8-m level walkway were used to measure the GRF. Three-dimensional kinematic data of the body segments of the subjects were measured using a seven-camera motion-analysis system (Vicon512, Oxford Metrics Group, UK) at a sampling rate of 120 Hz. All the measurements were performed synchronously.

Figure 1.

Experimental set-up showing the elliptical trainer and a six-component force transducer instrumented under the right pedal.

Data Collection and Analysis

For the measurement of the 3D kinematic data, 39 retroreflective markers were placed on the body segments. For the pelvis-leg apparatus, 28 markers were placed over the bilateral ASIS and PSIS, greater trochanter, midthigh, medial and lateral epicondyles, head of the fibula, tibial tuberosity, medial and lateral malleoli, navicular tuberosity, second metatarsal head, fifth metatarsal base, and heel. Another 11 markers were attached on the acromial processes, lateral and medial humeral epicondyles, the ulnar styloid, the ear canal, and the C7-T1 junction. Four additional markers were placed on the transducer of the right pedal to track the motion of the transducer. To remove the inertial effects of the transducer on the measured PRF during motion, the instrumented pedal was unloaded and driven by the noninstrumented pedal to move at a series of pedal rates while the transducer outputs were collected and represented in terms of pedal rate. These values were then subtracted from the measured PRF during tests for subsequent dynamic analysis. Coordinates of the markers gathered during a static calibration trial were used to define the anatomical coordinate systems of each of the segments and the transducer with the positive x-axis directed anteriorly, the positive y-axis superiorly, and the positive z-axis to the right.

With the measured GRF, PRF, and kinematic data, inverse dynamics using Newton-Euler equations of motion were used to calculate the intersegmental internal forces and moments at the joints of the lower limbs. A cardanic rotation sequence (Z-X-Y) was used to describe the rotational movements of each joint.[8] Inertial properties for each body segment-namely, segment mass, center of mass, and moment of inertia-were obtained using Dempster's coefficients.[29] Whole-body COM position data were calculated as the weighted sum of those of all the body segments, including head and neck, trunk, pelvis, arms, forearms, thighs, shanks, and feet. All the calculated joint moments were normalized to body weight and leg length (distance between ipsilateral ASIS and medial malleolus). The movement was assumed symmetrical, so joint angles and moments were analyzed for the right leg only. The cycle of an EE could be divided into two phases marked by two transitional points: T1, the pedal reached the most anterior position; and T2, the pedal reached the most posterior position. For comparisons with level walking, the period from T1 to T2 was defined as the stance phase, and from T2 to T1 as the swing phase.

Protocol

Each subject first performed level walking at a self-selected speed in a gait laboratory. Mean step length and cadence for each subject were then calculated from the walking trials. The parameters of the ET were then set to enable an EE similar to the measured level walking, with an average pedal rate of 50 rpm, an average stride length of 110% leg length, and no workload. The subjects were allowed to familiarize themselves with the walkway and ET before experimental data were recorded. A metronome was used to assist the subject to perform EE at the prescribed pedal rate. Data collection started when the subject reached the correct pedal rate and maintained that cadence for 5 s. Eight cycles were then collected, and the middle three were used for subsequent dynamic analysis. GRF, PRF, COM displacements, and the angles and moments at each joint of the lower limbs during both activities were calculated, and their peak values were extracted for subsequent statistical analysis. The maximum loading rates around heelstrike of the PRF/GRF were calculated as the maximum gradient of a cubic spline fitted to the corresponding measured data. Stick figures of a typical subject with the measured PRF/GRF vectors superimposed were generated, to demonstrate the effects of the PRF/GRF and the lower-limb posture on the joint moments for both walking and EE.

Statistical Analysis

The calculated variables between the two activities were analyzed using a paired t-test with a significance level of 0.05. SPSS version 10.0 (SPSS Inc., Chicago, IL) was used for all statistical analyses.

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