Elsevier

Gait & Posture

Volume 31, Issue 2, February 2010, Pages 218-222
Gait & Posture

Sagittal plane bending moments acting on the lower leg during running

https://doi.org/10.1016/j.gaitpost.2009.10.009Get rights and content

Abstract

Sagittal bending moments acting on the lower leg during running may play a role in tibial stress fracture development. The purpose of this study was to evaluate these moments at nine equidistant points along the length of the lower leg (10% point–90% point) during running. Kinematic and ground reaction force data were collected for 20 male runners, who each performed 10 running trials. Inverse dynamics and musculoskeletal modelling techniques were used to estimate sagittal bending moments due to reaction forces and muscle contraction. The muscle moment was typically positive during stance, except at the most proximal location (10% point) on the lower leg. The reaction moment was predominantly negative throughout stance and greater in magnitude than the muscle moment. Hence, the net sagittal bending moment acting on the lower leg was principally negative (indicating tensile loads on the posterior tibia). Peak moments typically occurred around mid-stance, and were greater in magnitude at the distal, compared with proximal, lower leg. For example, the peak reaction moment at the most distal point was −9.61 ± 2.07%Bw.Ht., and −2.73 ± 1.18%Bw.Ht. at the most proximal point. These data suggest that tensile loads on the posterior tibia are likely to be higher toward the distal end of the bone. This finding may explain the higher incidence of stress fracture in the distal aspect of the tibia, observed by some authors. Stress fracture susceptibility will also be influenced by bone strength and this should also be accounted for in future studies.

Introduction

Stress fractures frequently occur in distance runners [1] and military recruits [2]. The tibia is the most common stress fracture site, with incidence rates of up to 20% [1], [2]. Greater understanding of the pathomechanics underpinning tibial stress fracture development may improve rehabilitation and prevention strategies for such injuries.

Biomechanical models of the lower leg can provide a valid surrogate measure of tibial load. During running, the tibia is subjected to axial and shear forces, which arise from a combination of joint reaction (from the ground reaction force (GRF)) and muscle forces. Forces acting along the neutral longitudinal axis of the lower leg cause compression only, however, forces acting at an angle to this axis generate bending and torsional moments. Of these moments, the sagittal plane bending moments are likely to be most appreciable [3]. Two opposing sagittal bending moments act on the lower leg. A ‘reaction’ moment is generated from the shear component of the joint reaction force (Fig. 1). Due to the orientation of the GRF with respect to the lower leg during running this moment predominantly bends the lower leg convex posteriorly. A ‘muscle’ moment is generated from the compressive component of the plantarflexor muscle force (e.g. gastrocnemius). The plantarflexor line-of-action is offset posterior to the longitudinal axis of the lower leg and predominantly bends it concave posteriorly. The ‘resultant’ sagittal plane bending moment experienced by the lower leg is the summation of the reaction and muscle moments.

The importance of lower leg sagittal plane bending moments during running to tibial stress fracture development can be gleaned from circumstantial evidence. Firstly, the resultant sagittal plane bending moment is characterised by a predominantly negative profile [4], i.e. bending the tibia convex posteriorly. This results in tensile forces imparted on the posterior aspect of the tibia. Since cortical bone is relatively weak under tensile loading [5] these findings might explain why tibial stress fractures occur frequently in the posterior aspect of the tibia [6]. Secondly, indices of the tibia's ability to resist sagittal plane bending moments, such as anteroposterior tibial width, are reportedly lower in military recruits who develop tibial stress fractures during training [7], [8]. Thirdly, as described previously, the plantarflexor muscles have the capacity to attenuate the reaction moment during running. Reduced calf girth in athletes and military recruits has been identified as a risk factor for stress fractures [9], [10]. It is possible that reduced calf girth reflects a compromised capacity of the plantarflexor muscles to attenuate sagittal bending. While further research is clearly required, the available evidence provides support for a potential relationship between sagittal plane bending moments acting on the lower leg during running and tibial stress fracture development. Given these findings, a thorough understanding of these moments is required.

To date, only one preliminary study has evaluated the sagittal plane bending moments acting on the lower leg during running. Scott and Winter [4] captured GRF and two-dimensional kinematic data from three healthy adults running at 3.5–5.3 m/s. They only reported peak data at a single location on the lower leg (distal one-third point). As the longitudinal location of tibial stress fractures has been reported to vary [2], [11], [12], further knowledge is required regarding the sagittal plane bending moments acting along the length of the lower leg in a larger cohort of runners. Moreover, measurement of total loading throughout stance, i.e. angular impulse, may be important. Indeed, angular impulse measures have been associated with tibial stress fractures [13], and other overuse running-related pathologies [14].

This study aimed to investigate the sagittal plane bending moments acting at nine equidistant points (10% increments) along the lower leg during running. The hypotheses were:

H1

The lower leg resultant sagittal plane bending moment during the stance phase of running will be primarily negative, i.e. attempting to bend the lower leg convex posteriorly.

H2

The magnitude of the resultant sagittal plane bending moment will depend upon the longitudinal location on the lower leg.

Section snippets

Subjects

Twenty healthy male, heel-strike runners were recruited. The average (SD) height, mass and age were 1.77 (0.06) m, 73.3 (8.4) kg and 35.0 (9.9) years, respectively. All subjects ran ≥20 km per week and were injury free for ≥12 months prior to testing. Ethical approval was obtained from The University of Melbourne Human Research Ethics Committee. Subjects provided written informed consent. Subjects wore a Straprunner IV running sandal (Nike, Beaverton, US) throughout testing.

Instrumentation and procedures

Kinematic and kinetic

Results

The group mean sagittal plane reaction and muscle bending moments acting along the length of the lower leg are depicted in Fig. 3. Except for a small positive peak at ∼10% of stance, the reaction bending moment was found to be predominantly negative, reaching a peak around mid-stance. This was the case for all modelled points on the lower leg. A large peak occurred in the muscle moment around mid-stance. It was positive in magnitude for all points modelled, except the most proximal (10%) point.

Discussion

This study evaluated the sagittal plane bending moments acting on the lower leg during running to gain further insight into tibial loading and stress fracture pathomechanics. The resultant moment was characterised by a large negative component during mid-stance, which increased in magnitude in a linear fashion to the most distal measured point on the lower leg. These results confirm earlier preliminary findings that the posterior aspect of the tibia is likely to be subjected to tensile strain

Conflict of interest statement

The authors confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.

Acknowledgements

The authors acknowledge the contribution of Dr. Sharon J. Dixon in the writing of the mathematical model code. We also thank Ms. Sally Child, Mr. James Pope, and Ms. Narelle Wyndow for their assistance with subject recruitment and data collection.

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