Limb Load Asymmetries In Static And Dynamic Postural Tasks: Athletic Training Influences

Introduction
Limb load asymmetries during postural and dynamic activities such as standing up or gait are considered normal in
healthy, able-bodied individuals [1]. These are attributed to several factors such as specific task constraints [2], limb
dominance [3], aging, absence of vision [4] or simply individual differences [3]. On the other hand, a growing concern
is the fact that sport-specific training over the years results in the development of specialized muscular synergies that
generate an imbalance of the forces acting statically or dynamically at the joints leading to alterations of posture and
predisposing athletes to injury [5]. The purpose of the present study was to examine whether long term specialized
training of jumpers and long distance runners enhances limb load asymmetries during quiet standing and performance
of a sit to stand task.

Methods
A group of 16 jumpers and long distance runners (mean age: 25±8 years, height: 178±16 cm, weight: 71.5±28.5 kg,
years of training: 6±3.1) and 14 age-matched controls (mean age: 24.5±5.5 years, height: 176±14 cm, weight: 74±19
kg) were asked to stand on two adjacent force platforms (ERBE Balance System, sampling rate: 1000 Hz) and perform
the following tasks: a) 20s of quiet standing, b) a sit to stand task (subjects were sitting upright with thighs supported
half length by the seat and the shanks flexed 10o over the vertical). Both tasks were performed with eyes open (EO) and
closed (EC). Performance was quantified by the mean vertical ground reaction force (Fz) depicted on each foot during
the 20s of quiet standing and the force integral calculated over the push-off phase of the sit to stand task and normalized
with respect to body weight. An asymmetry index (AI) in limb load distribution was then calculated based on the
following relationship: (FzR-FzL /FzR+FzL)*100 [6]. Differences between the groups and task conditions (static vs.
dynamic, eyes closed vs. eyes open) were analyzed by means of a 2x2x2 repeated measures ANOVA.

Results
Asymmetry in limb load distribution increased significantly [F(1,28) = 50.5, p<0.001] during performance of the
dynamic task (36% mean difference) compared to quiet standing (6% mean difference). In addition, a significant task x
group interaction [F(1,28) = 4.59, p<0.05] suggests that limb load asymmetry was significantly different between the
two groups only in the dynamic task. Specifically, non-athletes exhibited a 13% greater asymmetry in weight
distribution than athletes were rising from a chair (Fig.1). This difference was even greater (20%) with EC, although
overall differences in limb load asymmetries due to the absence of vision were not confirmed by the statistical analysis.

Discussion
The results of the present study do not seem to confirm our hypothesis that sport-specific training of jumpers and long
distance runners results in the development of postural imbalances between the two sides of the body that may
predispose athletes to injury. Instead, it was shown that athletes distribute their body weight more symmetrically when
compared to untrained individuals who preferentially increase loading of their dominant limb when standing up from a
chair. This is in accordance with knee isokinetic strength measurements revealing greater interlimb asymmetries in nonathletes
when compared to runners and jumpers [5].

References
[1] Sadeghi et al. (2000). Gait and Posture, 12: 34-45
[2] Balasubramanian R. and Turvey MT (2000). Hum. Mov. Sci. 19:667-684
[3] Hesse S., Schauer M, Jahnke M. (1996). Gait and Posture 4: 287-292
[4] Blaszczyk et al. (2000). J. Biomech. 33: 1243-1248

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