I appreciate the comments by Dr. Schikler concerning our extensive clinical study investigating lower extremity quantitative muscle function in patients with EDS-HT. We found severely reduced quantitative muscle function and a substantial impairment in physical function in patients with EDS-HT compared to age- and sex-matched controls. The patient group clearly showed lower extremity muscle weakness, as evidenced by reduced muscle strength, reduced muscle strength endurance, and diminished functional performance.
Muscle atrophy, in relation to reduced physical activity and pain, can be one possible explanation for the observed muscle weakness. However, in our study, lower extremity muscle mass was similar in the EDS-HT patient group compared to the control group and therefore was unlikely to have affected the muscle strength results. We used a total body dual x-ray absorptiometry (DXA) measurement to obtain the lower extremity muscle mass for each subject. This method has been validated against measurements with magnetic resonance imaging , which was not possible because several of the patients with EDS-HT could not lie still in the supine position on a hard surface for at least 10 minutes or some patients had osteosynthetic material in their body because of surgical procedures. A limitation of DXA measurement is that it is not possible to measure the muscle masses of individual muscles; however, this was beyond the scope of our research.
Nonetheless, I understand and agree with Dr. Schikler's interest in the individual muscle masses of the quadriceps muscles and, specifically, in the muscle mass of the VMO in relation to the muscle mass of the other quadriceps muscles. However, I do not agree with the rationale given by Dr. Schikler that because of genu recurvatum in patients with EDS-HT, the VMO is less frequently exercised because it is only engaged in the last 10–20° of knee extension. In fact, many authors have shown that this traditional interpretation of VMO function is out of date. These same authors have demonstrated that the VMO is active throughout the range of motion from full flexion to extension and has no specific responsibility for terminal extension [2-4]. The only selective function attributable to the VMO is patellar alignment [5, 6].
In spite of this, it could be interesting to look at muscle imbalance of the quadriceps in patients with EDS-HT within the scope of functional joint stability. All EDS-HT patients have important joint hypermobility leading to problems such as dislocations (or subdislocations) and distortions. With regard to this discussion, many patients regularly indicate having knee joint instability and patellar dislocations .
The main reason for the development of these symptoms is likely to be a lack of control of the hypermobile joint movements, which is an issue of joint instability. The capability to maintain joint stability during bodily movements is called functional joint stability and is accomplished through the close collaboration of passive, active, and control subsystems . The passive system refers to the congruity of bones and articular surfaces and the passive mechanical properties of ligaments, joint capsules, and muscle–tendon structures. The active system refers to the dynamic contribution of muscles and tendons. The control subsystem refers to a concerted action between afferent input (mainly proprioception) and efferent motor output responses (muscle activation) in order to restore or maintain a proper alignment of the joint . In 3 recent publications, we have shown deficits in all 3 subsystems (a decrease in passive muscle tension and tendon stiffness , a reduction in quantitative muscle strength and muscle strength endurance [as shown in the article], and an impairment in joint proprioception ), which led to the insight that only ligamentous laxity is responsible for the observed joint instability in EDS-HT.
With regard to the discussion about ligamentous laxity and joint instability in EDS-HT, the importance of the active system (of the muscles) should be viewed in further detail. It is known that coactivation of muscles, in this case the quadriceps and hamstrings, provides active stabilization of the joint when stabilization by the passive restraint system is lacking . In our study, compared to the normal hamstrings:quadriceps ratio of 60%, muscle imbalance in the EDS-HT group appeared to emerge. Furthermore, it could be possible that a muscle imbalance between the individual muscles of the quadriceps also was present and may have contributed to the knee joint instability, in particular to patellar instability and patellofemoral tracking disorder. Therefore, as Dr. Schikler suggests, it could be interesting to perform an individualized assessment of the mass of individual muscles in relation to each other. In addition, the amount of activity and the timing of muscle recruitment may also play a role; it would be interesting to assess this within the scope of knee joint problems in patients with EDS-HT.
At the moment, only a very fragmented picture of the various mechanisms underlying joint instability and other important symptoms in EDS/joint hypermobility syndrome has emerged. In general, there is still a large gap in our knowledge regarding EDS. Consequently, future research is highly needed.