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Tears to the anterior cruciate ligament (ACL) are fairly common in most popular sports. While surgery can mechanically repair the injury, athletes returning from injury can often experience decreases in muscle strength around the insult. Impaired muscle regeneration has repeatedly been described after anterior cruciate ligament reconstruction (ACL-R).
A team of researchers from the University of Heidelberg investigated the adaptation to concentric/eccentric quadriceps strength training with eccentric overload CON/ECC+ in comparison to CON/ECC strength training. In order to shed further light on the impaired muscle recovery after ACL-R, the researchers measured the steady state expression of selected marker mRNAs in the vastus lateralis biopsies obtained from recreational athletes in our previous investigation.
Muscle regeneration after anterior cruciate ligament reconstruction (ACL-R) is impaired. Even with guided rehabilitation, atrophy and strength deficits of the quadriceps femoris muscle were reported up to one year after ACL surgery and even longer in subjects with chronically insufficient ACL. Prolonged quadriceps weakness could only partially be explained by rather small reductions in quadriceps muscle mass indicating a discrepancy between the regain in quadriceps muscle mass and quadriceps strength. This discrepancy might partly be due to impaired neuromuscular function, which is observed after ACL injury. However, it might also be explained by altered muscle quality or phenotype.
Friedmann-Bette et al hypothesized that differences in the adaptation to the two different training regimes would emerge in the steady state expression of genes involved in skeletal muscle myogenesis and hypertrophy as well as in skeletal muscle contractility. In addition, the 12-wk strength training would also affect the expression of some genes involved in muscle wasting and muscle atrophy in the still dysfunctional muscle after ACL-R.
During the study, they observed a significant increase in quadriceps muscle mass after 12 weeks of supervised progressive strength training performed subsequently to the early rehabilitation period after ACL-R in recreational athletes. The increase in quadriceps muscle mass was significantly enhanced, when the strength training was conducted as concentric/eccentric strength training with eccentric overload (CON/ECC+) compared with conventional concentric/eccentric strength training (CON/ECC). Besides the discrepancy between quadriceps muscle mass and strength, a significant increase in the proportion of type 1 fibers after CON/ECC+ was a surprising result.
So far, CON/ECC+ had been found to be especially beneficial in enhancing strength, power and speed performance, which are the prevailing characteristics of a fast muscle phenotype. Differences between the two different modes of the 12-wk strength training, CON/ECC+ and CON/ECC, emerged in a significantly different regulation of the MSTN-mRNA expression with a trend towards a decrease after CON/ECC+ and in some slight differences in the expression changes of further 15 mRNAs involved in skeletal myogenesis/hypertrophy and contractility.
The significantly increased expression of 6 marker mRNAs involved in muscle wasting after both types of strength training added further evidence to the observation of a declined muscle quality after ACL-injury. Furthermore, effects of different types of autografts were reflected in the expression changes of 6 mRNAs involved in skeletal myogenesis/hypertrophy and contractility.