Introduction Long tendons such as the human Achilles vary considerably in cross-sectional area (CSA) along their length.1 If a constant force is assumed to act along the tendon, regions of smaller CSA will be subject to greater stresses and may undergo larger deformations. This study aimed to determine whether regional variations in tendon CSA cause changes in tendon deformation.
Methods We studied nine males (mean age 25 years) free from tendon injury or pathology. Magnetic resonance imaging of the lower leg was performed at rest and during isometric plantarflexions at 10, 20 and 30% of the maximum voluntary contraction (MVC) torque. The Achilles and gastrocnemius tendon CSAs were measured along their length (21 scans; Figure 1) during the four contraction levels. Tendon CSA was compared at each corresponding anatomical level between four contraction levels (rest, 10, 20 and 30% of MVC). Data were analysed using a repeated measures analysis of variance and post-hoc testing.
*denotes significantly (p < 0.01) different from rest. SOL MTJ = soleus myo-tendinous junction.
Results At 10% of MVC, CSAs were significantly smaller throughout the gastrocnemius tendon compared to those at rest, but there were no significant differences in the Achilles tendon CSAs (Figure 1A). At 20% and 30% of MVC, the gastrocnemius tendon CSAs remained significantly smaller throughout its length (Figure 1B and C). At 20% of MVC only the most proximal CSA of the Achilles tendon was significantly smaller compared to that at rest (Figure 1B). At 30% of MVC, CSAs in the proximal region (5 scans) of the Achilles tendon were significantly smaller compared to those at rest (Figure 1C).
Discussion Reductions in tendon CSA as a result of tensile loading applied by muscle contraction were assumed to represent regional-specific longitudinal elongations, i.e. as the tendon is stretched it becomes thinner. Contrary to our initial hypothesis, the largest deformations did not occur in the region of smallest tendon CSA, but were instead tendon-specific with greater deformations in the gastrocnemius compared to the Achilles tendon. This occurred despite presumably lower forces acting on the gastrocnemius tendon, which suggests a reduced stiffness and modulus in the gastrocnemius tendon compared to the Achilles.
Acknowledgements EPSRCs Bridging the Gaps funding.
Reference 1 Magnusson, Kjaer. Eur J Appl Physiol. 2003;90:549–553
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