An in vivo rodent model of contraction-induced injury in the quadriceps muscle
Introduction
Muscle strains are one of the most common complaints treated by physicians and account for the majority of all sports-related injuries.1, 2 It is difficult to study muscle injuries in humans, as the incidence is a random event that is difficult to predict, and the clinical presentation varies greatly. Much of the data regarding muscle injuries has therefore been ascertained from studies on animals, which provide control over many variables and the ability to study mechanisms of injury and recovery. An in vivo injury provides a method for assessing contractile function without dissecting the muscle, and thus without the need to euthanise the animal under study. In the laboratory setting, investigators have used toxins,3, 4 lacerations,5, 6 freeze damage, and contusions7, 8 to study muscle damage, but by far the majority of muscle injuries in humans are attributable to excessive strain of an activated muscle (e.g., forceful lengthening, or “eccentric”, contractions). Submaximal lengthening contractions are used in everyday activities, but it is well known that high force lengthening contractions are associated with muscle damage and pain.9
One problem with many of the biological markers used to assess muscle injury, including those used in most animal studies, is that they usually do not correlate with loss of force. Since full contractile function can persist despite the presence of injury markers, loss of force may be the most valid measure of injury,10 and probably the most relevant. Most animal studies of in vivo or in situ muscle contraction utilise the dorsiflexor or plantarflexor muscles (the crural muscles), which offer the advantage of accessibility to the nerves innervating them, and the ability to compare results to previous literature.11, 12, 13, 14, 15 A disadvantage of using the crural muscles is their small size (especially in mice), which not only makes the contractility assay and injury more challenging, but also provides limited amounts of harvested tissue for examination. Variation from muscle to muscle with regards to architecture, fibre type, regeneration properties, etc., also underlies the importance of studying different muscle groups. Despite a common nerve supply and insertion, the quadriceps muscle is often described as four muscles (rectus femoris, vastus lateralis, vastus intermedius and vastus medialis) due to the different points of origin. The quadriceps femoris was chosen as an alternative to crural muscles due to its large mass and multiple fibre types. For example, approximately half the fibres in the vastus intermedius are slow (Type I), but the other “heads” of the quadriceps contain primarily fast (Type II) fibres.16 An injury model using the quadriceps allows the study of a larger muscle that has a significant role in gait, even in the quadruped.
The purpose of this study was to describe a novel muscle injury model in the quadriceps that results in a significant and reproducible loss of force. Force production and force loss are compared in both mouse and rat quadriceps. To the best of our knowledge, this is the first study to utilise a controlled lengthening contraction-induced injury model in the quadriceps femoris.
Section snippets
Injury
All protocols were approved by the University of Maryland Institutional Animal Care & Use Committee (IACUC). We have previously described a model to injure the tibialis anterior muscle (TA) of rats and mice, which results in a significant and reproducible injury.17, 18, 19, 20 Here we tested a modified version of this apparatus on the quadriceps muscles. We used five age-matched male mice (Jackson Laboratory, Bar Harbor, ME C57BL/10ScSn, N = 5, 28.5 ± 1.4 g) and five age-matched male rats (Charles
Results
We modified a previously described tibialis anterior muscle (TA) injury apparatus to induce injury to the quadriceps muscles. The new custom designed lever arm is described in Materials and methods and in Fig. 1. Table 1 lists the body weights and quadriceps muscle masses of animals used in this study. The mass of the TA from the same animals is also provided for reference.
The uninjured rat quadriceps generated a mean peak torque of 58.3 ± 8 N mm (Fig. 2B). This is approximately an 8-fold increase
Discussion
To date, most animal studies using contraction-induced injury do so using muscle groups that move the ankle. Some studies have described injury in the quadriceps; however, they use methods such as downhill running and treadmill exercise.22, 23, 24 Such methods may cause muscle damage, but also increase the possibility of unwanted variables including partial fibre recruitment or variable injury. We describe an animal injury model where variables including muscles recruited, amount of
Conflict of interest statement
The authors have no conflicts of interest to report.
Acknowledgements
This work was supported by grants from the National Institutes of Health (K01AR053235 and 1R01AR059179 to RML; K08 AR059750, L40 AR057721 and P50 NS040828 to MWL).
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