Recovery After Exercise-Induced Muscle Damage

Regardless of whether an individual is in rehabilitation or exercise for general health or athletic performance, resistance exercise is an essential form of exercise when the goal is to increase muscle mass, strength and function. Although, resistance exercise primarily is associated with positive effects it may also result in muscle damage when the exercise is of high intensity and/or unaccustomed. This is known as exercise-induced muscle damage (EIMD) and is reflected by a substantial decrease in force-generating capacity and often accompanied by intracellular swelling and delayed onset muscle soreness. On a cellular level, EIMD include myofibrillar disruption, inflammatory response and in severe cases of EIMD; myofibre necrosis. While EIMD with its symptoms clearly is evident, its underlying mechanisms are still to be fully elaborated.

One interesting hypothesis regarding the molecular basis of decreased muscle strength as a result of EIMD, is related to the strain of this exercise mode causing "popped" sarcomeres. When sarcomeres are stretched beyond actin-myosin overlap, some sarcomeres may over-stretch. This results in overload of membranes, leading to opening of stretch-activated channels, and subsequently influx of Ca2+. High levels of cytoplasmic Ca2+ may cause degradation of contractile proteins or Excitation-Contraction coupling proteins mediated through increased calpain activity. However, a recent study by Cully and colleagues (2017) suggest a protective mechanism post heavy-load strength training related to Ca2+-handling. Cully et al. observed formation of vacuoles in longitudinally connecting tubules post exercise when exposing fibers to 1.3 μM [Ca2+] in the cytoplasma. These vacuoles provide an enclosed compartment where Ca2+ can be accumulated, preventing Ca2+ from initiating damage to the muscle. The role of Ca2+-regulation in recovery of muscle function warrants further investigation and clarification.

To the best of the investigators knowledge, the most valid method for estimating EIMD is by investigating myofibrillar disruption, and in some cases necrosis, in muscle biopsies. This requires many resources and is rather expensive. Currently, the best non-invasive marker of muscle damage is the force deficit observed at 48 hours post exercise. However, a measurement estimating muscle damage immediately post exercise is warranted because force deficit immediately post exercise will be confounded by muscle fatigue.

A novel study performed by Lacourpaille et al. (2017) showed a strong negative correlation (-0.80) between stiffness of the muscle tissue, shear modulus, measured 30 minutes post exercise and peak isometric force measured at 48 hours post exercise and therefore a strong relationship between the decline in force production capacity and increased stiffness post exercise, suggesting a possible method to predict EIMD immediately after exercise. However, direct evidence of this association is warranted, with measurements of shear modulus and EIMD biomarkers, such as the proportion of disrupted fibers and sarcoplasmic Ca2+ regulation.

The ability to predict EIMD after training is of great interest to athletes, but also patients suffering from e.g. muscular dystrophies. Being able to predict EIMD quickly and non-invasively after exercise will help employ optimal recovery.