A study of the reproducibility and reliability of the musculo-articular stiffness of the ankle joint.

The objective of this work was to evaluate the reproducibility, reliability and usefulness of the musculo-articular stiffness (MAS) of the ankle joint, measuring it by the free vibration technique. Seventeen (nine males and eight females) healthy university students were included in the study. Force (f), MAS (k) and unitary MAS (ku) (defined as the ratio between the value of stiffness k obtained in the test (absolute terms) and the value of force (f)) were obtained. A test-retest protocol was designed and performed on the same day to determine the short-term reproducibility of f, k and ku. Short-term reproducibility of k and ku on 1 day in absolute terms (< 7% Coefficient of Variation (CV)) and relative reproducibility (Intraclass Correlation Coefficient (ICC) and Pearson ≥ 0.97) for both feet were obtained. The reliability of k and ku in absolute terms (< 9% CV) and in relative terms (ICC and Pearson ≥0.93) based on repeating the protocol for 1 week was analysed for both feet. To analyse the usefulness, the Effect Size (ES) ratio = "Trivial" for all variables (for 1 day and 1 week) and the Smallest Worthwhile Change (SWC) ratio (Typical Error (TE)< SWC) = "GOOD" for k and ku (1 day and 1 week) were considered. The Minimal Difference needed to be considered "real" (MD) for ku ≅ 3.5% (1 day); ku≅ 8.5% (1 week) (p < 0.05) was obtained. The statistical analysis carried out displayed the high reproducibility, reliability and usefulness of the MAS test, which was more consistent with ku than k. Therefore, the unitary stiffness (ku) proven to be representative of the mechanical response of the ankle joint obtained by free vibration techniques, which allows comparison between different subjects.

Evaluation of the stiffnesses of the Achilles tendon and soleus from the apparent stiffness of the triceps surae.

The triceps surae plays an important role in the performance of many sports. Although the apparent average mechanical properties of the triceps surae may be a satisfactory parameter for estimating the training level of an athlete, a knowledge of the mechanical properties of the individual constituents of the triceps surae (in particular the Achilles tendon and soleus) permits a more detailed and in-depth control of the effects of training from more physically based parameters. The objective of this work is therefore the estimation of the individual viscoelastic properties (stiffness and viscosity) of soleus and Achilles tendon from the apparent properties of the triceps surae obtained by free vibration techniques. Different procedures have been developed and discussed, showing a high degree of robustness in the predictions. The results obtained for a non-oriented set of subjects present a high level of variability, depending on the training conditions and anthropometric features, although the corresponding average values compare well with data previously reported in the literature, particularly those associated with the tendon stiffness.

A critical study on the experimental determination of stiffness and viscosity of the human triceps surae by free vibration methods.

Muscles and tendons play an important role in human performance. Their mechanical behaviour can be described by analytical/numerical models including springs and dampers. Free vibration techniques are a widely used approach to the in vivo determination of stiffness and viscosity of muscle-tendon complexes involved in sport movements. By considering the data reported in the literature, it appears that the visco-elastic properties of the triceps surae muscle-tendon complexes are independent of the modality in which free vibration is induced as well as they do not depend on the composition of the population of subjects submitted to the experiments. This research will critically discuss this important aspect focussing in particular on two studies documented in the literature. For this purpose, two equipments will be developed to reproduce literature experiments under the assumption that the oscillating part of the body behaves as a single-degree-of-freedom system: The governing degree of freedom is associated with the vertical displacement of the lower leg or with the rotation of the foot around the ankle articulation. Unlike literature, measurements are now conducted on the same population of subjects in order to draw more general conclusions on the real equivalence of results and validity of the mechanical properties determined experimentally. Free vibration tests are accurately simulated by analytical models describing the response of each vibrating system. It is found that if the two measurement protocols are applied to the same population of individuals as it is done in this study, values of visco-elastic properties of muscle-tendon complexes extracted from experimental data are significantly different, the differences presenting a convincing consistency. This result is in contrast with the literature and confirms the need to evaluate results of free vibration techniques by taking homogeneous bases of comparison.