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

Abstract and Introduction

Abstract

Purpose: To determine the joint loading during elliptical exercise (EE) by a detailed three-dimensional dynamic analysis, and to compare the results with those during level walking.
Methods: Fifteen male adults performed level walking and EE while 3D kinematic data, right pedal reaction forces (PRF), and ground reaction forces (GRF) were measured. Pedal rate (cadence) and step length during EE without workload were set according to those measured during level walking for each subject. The motion of the body's center of mass, lower-limb-joint angles and moments were obtained.
Results: Pedal rates and step lengths were 52.20 rpm (SD = 2.34) and 50.56 cm (SD = 2.14), respectively. During early stance the vertical PRF was smaller than the GRF, and the medial and posterior shear components were greater. PRF also occurred during swing. Loading rates around heelstrike during EE were all smaller than those during walking. During EE, the peak flexion angles of the hip, knee and ankle were greater. Peak hip flexor and knee extensor moments were also greater, whereas peak ankle plantarflexor moments and all abductor moments were smaller.
Conclusions: Different lower-limb kinematics and kinetics were found between EE and level walking. Smaller vertical PRF and loading rates during EE were achieved at the expense of greater hip flexor and knee extensor moments. Use of the elliptical trainer for athletic and rehabilitative training would have to consider users' joint function and muscle strength, especially at the knee, to avoid injuries.

Introduction

Exercise with an elliptical trainer (ET) has been shown to be beneficial for the development and maintenance of cardiorespiratory fitness by providing an adequate metabolic challenge.[11,16] Physiological improvements after training using ET were also found to be similar to those using a treadmill and stair-climber.[11] These findings suggest that elliptical exercise (EE) can be developed as a low-impact aerobic exercise modality. In fact, EE training has increased in popularity over the last decade. According to an analysis by the Sporting Goods Manufacturers Association (SGMA) of the United States published in 2006, the population of ET users grew by 170% from 2000 to 2005, an increase of nearly 10 million.[23] Health clubs and individuals have purchased these machines to provide an option of fitness training that can be incorporated into an exercise regimen.[3] Recently, EE has also been included for fitness training in the rehabilitation programs of patients who have undergone hip arthroscopy,[24] ACL revision surgery,[25] or patellar tendonectomy,[21] or who suffer from patellofemoral pain syndrome[12] or diabetes.[10] Despite the benefit in cardiorespiratory fitness, the rapid growth in the number of ET and their wide clinical applications, little is known regarding the loading applied to the lower-limb joints during EE. Considering the lower impact nature of ET, the loading to the lower limbs may be different from walking. Green et al.[14] found that for similar intensity, the ratings of perceived exertion (RPE) of the legs during EE were significantly greater than those during treadmill exercise, although the overall RPE were similar. Increased loading at a joint and of the surrounding muscles or muscle groups may lead to early fatigue, which could in turn limit the duration of the exercise. Sufficient and appropriate duration of training is important for cardiorespiratory training[2] as exercise duration and intensity interact to achieve a fitness goal. On the other hand, the forces transmitted in the lower limbs are also closely related to potential musculoskeletal overuse injuries. Therefore, knowledge of the loading in the lower limbs during EE is essential to ensure an efficient and safe exercise environment, especially for patients.

ET provide walking-like exercise with reduced impact loading. Walking has been shown to have positive effects on health.[22] However, the common daily activity of walking is not as smooth and harmless as has previously been believed.[9] It has generally been accepted that the motion of the human body during walking is controlled in such a way so as to minimize the amount of mechanical energy required.[4] With this goal, each joint of the leg is close to the line of action of the ground reaction force (GRF) to minimize the net external joint moments and thus reduce the muscle forces necessary to counteract these moments.[7] The GRF that identifies the mechanical energy fluctuations displays an "M"-shaped curve in the vertical component. While smooth during most of the stance phase, large repetitive impact forces occur at heelstrike. Repetitive impact loading is a likely source of deleterious stress.[9] Therefore, a major disadvantage of level walking is that repetitive vertical impact forces at heelstrike may induce musculoskeletal injuries such as knee osteoarthritis.[9] These impact forces also increase with increasing speed.[28] EE improves these negative effects (impact forces) by maintaining both feet in constant contact with the pedal platforms to form a closed kinetic chain during the exercise. This reduces not only impact forces around heelstride at the stance limb, but also the magnitude of the GRF owing to the weight sharing from the swing leg. This is potentially an advantage over ground walking, during which the body is supported solely by the stance limb for about 66% of the stance phase. However, reduced GRF may not necessarily reduce the net external joint moments, as the posture of the lower limb may affect the lever arms available to the GRF at the joints. Detailed mechanical analysis is needed for a better understanding of the loading in the lower limbs during EE.

The motion of EE resembles that of standing cycling[14] in that the user applies force with alternate legs to pedals while the body is supported mainly by the lower limbs. Both exercises are closed kinetic chain exercises with each leg following elliptical-shaped trajectories of the pedals during EE and circular-shaped ones during standing cycling. Even with impact forces much lower than walking and running, cyclists suffer from a considerable number of injuries, the most common of which seem to be at the knee.[6] This indicates that these injuries are not related to impact forces. Studies of cycling have shown that inappropriate pedal design and ankle positioning increase stresses at the knee.[5,6] During EE, the feet are constrained by pedals to follow an elliptical trajectory, with the possibility of producing disadvantageous joint loads.

Previous studies on EE have been limited and have focused mostly on its effects on the cardiorespiratory function.[3,11,14] To the best knowledge of the authors, no study has ever quantified the forces transmitted in the lower limbs during EE, and compared them with those during level walking for a better understanding of the benefits of EE and its potential risk of injuries to the musculoskeletal system. The purpose of the study was to determine the joint loading during EE through a detailed 3D dynamic analysis of the lower extremities and a comparison of the results with those during level walking. Complete knowledge of the biomechanics of lower-limb joints will be helpful for the improvement of the design and the establishment of guidelines for safe use of the ET in the training of healthy users, as well as of patients during the course of rehabilitation.

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