Joint hypermobility syndrome is a condition in which a joint can move effortlessly beyond the normal limit of motion expected for that joint. This syndrome is affected by some factors including gender and tends to be inherited. It may cause some symptoms such as pain in an individual’s synovial joints. The objective of the current study was to compare the energy loss of connective tissues between joints with and without hypermobility. A differential equation model, namely the Kelvin-Voigt model, was used for the energy loss analysis. The results show the difference in energy loss for the tissues attached to joints with and without joint hypermobility. As the stiffness of the connective tissue decreases, the energy loss increases. Muscle activity about the ankle was measured via electromyography during simple functional tasks, and the recorded data were used to correlate with the theoretical analysis of the energy loss. The result would shed light on the pathology analysis of the symptoms such as the cause of pain.
Joint hypermobility is a condition in which a joint can move effortlessly beyond the normal limit of motion expected for that joint. It has been reported that hypermobile joints can be inherited or obtained by stretching or exercising over a period of time [
Joint hypermobility is quite prevalent. It has been reported that incidence is 5% in the USA [
The objective of the current work is to study the relationship between the soft connective tissue stiffness and the energy loss of the tissue, and investigate the effects of hypermobility on muscle activity experimentally. A differential equation model, Kelvin-Voigt model, was used for the theoretical analysis. Muscle activity about the ankle joints of women with and without joint hypermobility was measured via electromyography (EMG) during simple functional tasks, and the recorded data were used to correlate with the theoretical analysis. It has been found that as the stiffness of the connective tissues decreases, the energy loss of the tissues increases significantly. The EMG results also showed increased co- contraction of antagonistic muscles, which correlates to increased joint loading. While the mechanical properties of tissues in ankle joint were used to illustrate the effect of stiffness, the result would shed light on the pathology analysis of JHS on any synovial joint.
With the soft connective tissue such as ligaments and tendons being viscoelastic in nature, their behavior is dependent not only on the stress applied but also on time [
In Equation (1), E, the modulus of elasticity, indicates the stiffness of the spring, or in this case the connective tissue stiffness. The damping coefficient η is responsible for the time dependent aspect of the tissue’s viscoelastic behavior.
Also in the equation, t is time and
Biological tissue properties vary greatly depending on sex, age, and many other factors [
The median value of stiffness was assumed to be 500 MPa. The damping coefficient η = 200 was chosen initially. Regarding the chosen stress, based on the fact that an Achilles tendon with a cross sectional area of 1 cm2 can bear between 500 kg and1000 kg of mass, a maximum stress of 60 MPa was chosen to be applied to the model [
A MATLAB code for the simulation was developed to study the stress and strain relationships. Stress and strain curves for stiffnesses of 1000 MPa, 500 Pa, and 25 MPa are shown in Figures 2-4, respectively. In each plot, a loading cycle of
stress was plotted. That is, stress increases from zero and reach a maximum of 60 MPa, then the stress decreases to zero. Because the strain is limited to less than 10%, there is no permanent deformation. When the stress returns to zero, so does the strain. It is clear that the relationship between the stress and strain was not linear, which was expected because the tendon is not purely elastic but has a viscous component as well. The hysteresis loop can be seen very clearly in the plots. Since the area under a curve on a stress-strain diagram yields the toughness (or energy absorbed), the area inside the hysteresis loop yields the energy lost due to tendon viscosity [
The relationship between energy loss (percentage) and the modulus of elasticity is shown in
To correlate with the theoretical model above, an experiment was performed to look at muscle function of the ankle joint during functional tasks. Several factors have been suggested as the cause of chronic ankle instability, including anatomic instability, muscle weakness, and proprioceptive deficits. Those with joint hypermobility syndrome experience joint movements that extend beyond the normal range of motion, a similar symptom to those who have experienced ankle sprains. In many cases of instability, weakness of the peroneal muscle was reported. The peroneus longus muscle is responsible for both eversion and plantar flexion. Other muscles of the ankle responsible for plantar flexion include the gastrocnemius, soleus, plantaris, and the posterior tibialis. The anterior tibialis, extensor hallicus longus, and extensor digitorum longus are the muscles of the ankle responsible for dorsiflexion. Together, these muscles function to
stabilize the ankle joint and also enable the motion of the ankle [
Hypermobile ankles exhibit a high degree of instability due to the overall laxity of the ankle ligaments. In order for a person with hypermobile ankles to complete the same task as an individual without hypermobile ankles, it is expected to find a larger number of muscles contracted. This increased contraction correlates to an increased load on the ankle [
For this study, the ankle function of two subjects was compared: one with hyper- mobility, and one without. EMG electrodes were placed over the tibialis anterior (TA), extensor digitorum longus (EDL), peroneus longus (PL), lateral gastrocnemius (LG) and soleus (SOL) of the left leg (Trigno; Delsys Inc., Natick MA, USA). EMG data were acquired while the subjects performed repetitive heel raises (ankle plantarflexion with eversion) for one minute while seated with a 20 kg weight over their knees. The repetitive motion was paced to reduce timing variability between subjects. Subjects then performed repetitive toe raises (ankle dorsiflexion with inversion) following the same procedures.
The raw EMG data were normalized by maximum voluntary contraction (MVC) and RMS filtered with a 125 ms window in order to better compare the muscle activation amplitudes between subjects. MVC normalization allows amplitudes to be reported in terms of percent maximum contraction for each respective muscle, otherwise signals from different sensors, muscles, and subjects would not be comparable [
The muscles that contribute directly to the motion of the ankle plantarflexion with eversion task are the PL, LG, and SOL. The hypermobile subject showed significantly less LG activation (
is warranted to flush out these details, but this preliminary work shows potential evidence of work lost by the abnormal elasticity of the Achilles tendon.
The TA and EDL muscles serve as antagonists during the plantarflexion with eversion task. Increased activity would mean that these muscles are co-con- tracting, which effectively increases the stiffness of the ankle during the motion as well as the forces experienced by the ankle joint. The hypermobile subject showed slightly increased TA activity and significantly higher EDL activity during this task (
The muscles that contribute directly to the motion of the ankle dorsiflexion with inversion task are the TA and EDL. For this task, all muscle activations were similar between subjects, except for the PL, which was significantly higher for the normal subject (
To summarize, this preliminary experiment showed potential evidence that certain muscles require more activity to make up for the loss of work when hypermobility is present. The results also showed evidence of co-contraction to stabilize the hypermobile joint during certain functional activities. These results were evident in the plantarflexion with eversion task, but not in the dorsiflexion with inversion task. Further work should be completed with a larger subject population to rule out differences between these two particular subjects. Further investigation should also be made into different functional activities that might elicit more detrimental effects of joint hypermobility.
In this paper, the Kelvin-Voigt model has been adopted to study the relationship between energy loss and the stiffness of soft connective tissues in synovial joints. The results showed that as the stiffness of the connective tissue decreases, the energy loss increases. While connective tissues of hypermobile joint have relatively low stiffness, more energy loss of connective tissues was predicted. To correlate with the theoretical analysis, preliminary experiments have been per-
formed, which showed potential evidence that certain muscles require more activity to make up for the loss of work when hypermobility is present. The results also showed evidence of co-contraction to stabilize the hypermobile joint during certain functional activities. Further work should be completed with a larger subject population with different functional activities.
Tatarkov, S.A., Tesny, A.C., Lauck, B.M., DiBerardino III, L.A. and Shen, H. (2017) Connective Tissue Energy Loss Comparison between Joints with and without Hypermobility. J. Biomedical Science and Engineering, 10, 18-27. https://doi.org/10.4236/jbise.2017.105B003