An Objective Measure of “Muscle Soreness”
Any type of physical activity that places an unaccustomed load on muscle may lead to delayed onset muscle soreness (DOMS)/exercise induced muscle damage (EIMD). This type of soreness is different to acute soreness, which is pain that develops during the actual activity. Delayed soreness typically begins to develop 12-24 hours after the exercise has been performed and may produce the greatest pain between 24-72 hours post exertion (Cheung, Hume & Maxwell, 2003; Twist & Eston, 2005).
The mechanical effects of unaccustomed exercise places significant stress on the muscle microstructure, resulting in muscle damage and soreness. By nature the soreness is delayed and measurement is subjective, which means that the coach may not be provided with the necessary insight into their athlete’s physical state and readiness to train. There is a need to provide measurable data of the early signs of DOMS/EIMD in order to provide a more objective assessment.
Within the sporting community, the most common scale for determining an athlete’s exertion is known as the Borg RPE scale (Rate of Perceived Exertion). The RPE method of assessment was first proposed way back in the 19780s (Borg, 1982) and has stood the test of time as a quick and effective measurement of a person’s exertion. The scale has since been adapted and applied to measure levels of soreness that are felt post-exercise. The Borg RPE scale certainly has its merits as a quick and easy method of measuring someone’s perception of how hard they are exercising. However, when it comes to measuring pain – despite the scale being suitably adapted and a measure of perceived feeling – it can still be considered unreliable. The reason being that adapted muscle soreness RPE measure is subjective, which may result in a distorted representation of soreness. Evidently, pressure to perform and a high natural tolerance to pain may result in an athlete under representing their feelings. Quite simply, the athlete may over-train and not perform the required rehab following unaccustomed exercise, sustaining a more serious injury later on.
The mechanical effects of unaccustomed exercise can cause signiﬁcant damage to the sarcomere, sarcoplasmic reticulum, t-tubules and sarcolemma, which are parts of the muscle that produce force from substrate utilisation. The damage to the muscle structure causes a release of Ca2+ and other exercise by-products and hormones. Structural damage leads to a shift in the optimum length of the length tension relationship, a fall in active tension due to disruption of the sarcomeres, rise in passive tension, delayed soreness and swelling (Marcora & Bosio, 2006; Proske & Allen, 2005). Proske and Allen (2005) have suggested that passive tension can be an early indicator of muscle damage post-exercise.
The use of a muscle contraction sensor (MC sensor) is a novel approach for measuring the tension of a muscle or tendon, this method has been found to highly correlate with other methods such as EMG (electromyography) (Đorđević, Tomažič, Narici, Pišot & Meglič, 2014). The lower cost and reduced invasiveness make the MC sensor a more viable option for sports organisations. The sensor could be used in the assessment of the tension of a muscle post-exercise to ascertain an early indication of muscle damage before the subjective indicator of delayed soreness manifests – therefore catching EIMD early on. However, further research is required to substantiate these claims.
Moving away from a biomechanical standpoint towards a biochemical view, the use of a salivary or blood biomarker would assist sporting organisations hoping to monitor the physical state of their athletes after exercise. Serum Creatine Kinase has been reported multiple times as being an indicator of muscle damage. McLellan, Lovell and Gass (2010) report that following muscle damaging exercise CK increased and remained elevated for 120 hours. This suggests that following the early indication of muscle damage post-exercise, an athlete needs at least 5 days of modified activity to achieve full recovery. Certain endocrine responses in the form of testosterone and cortisol collected from a saliva sample have been shown to become elevated after muscle damaging exercise and it was also indicated that a minimum period of 48 hours is required for endocrine homeostasis post-exercise (McLellan et al, 2010). Both the CK and endocrine responses would give early indication to coaches to modify training load, type and intensity.
While both methods mentioned require further substantiating investigation, it is clear that a more objective and pre-soreness measure of DOMS/EIMD is attainable. More objective measures could aid in the effective rehab and injury prevention of athletes. The biomechanical and biochemical methods incorporated into a wider battery of tests would contribute to a more objective measure of “Wellness”, which could quite seamlessly be captured within EDGE10, then tracked and combined with other performance data.