Two days after a hilly marathon or a heavy leg session, the stairs feel steep. That soreness is a familiar part of training, and it reflects real changes inside the muscle. Some athletes and coaches like to put numbers on those changes, and three simple blood tests come up again and again: creatine kinase (CK), C-reactive protein (CRP) and the erythrocyte sedimentation rate (ESR).
All three are in the BPR catalogue and are processed by London Medical Laboratory (LML). This guide explains what each muscle recovery blood test measures, why hard training moves the numbers, and how to read them without over-reading them.
Creatine kinase: an enzyme that leaks from working muscle
Creatine kinase is an enzyme found in skeletal muscle, heart muscle and the brain. The LML explains that the test measures CK levels in the blood, and that raised levels can point to muscle damage or injury, among other causes, including some heart-related conditions. The LML also notes that CK is not specific and that further tests are usually needed to understand a high result.
In sport, CK has long been used as an indirect marker of muscle damage after exercise. A review on CK and exercise describes, however, that its value as a measure of how much damage a session caused is debated (Baird et al., 2012). The same review points out that factors you cannot change, such as ethnicity, age and sex, also affect CK levels, and suggests that the usual upper limits may not fit very active people well. Two practical consequences follow:
- a high CK the day after a hard session is common and is not, on its own, a cause for alarm;
- your own previous results, taken under similar conditions, are a better comparison than someone else's.
When a high CK needs prompt attention
There is one situation where CK matters urgently. Exertional rhabdomyolysis is a breakdown of muscle cells after intense exercise in otherwise healthy people. A systematic review found it most often after running events, marathons included, and weightlifting, and highlighted muscle pain or cramps and dark urine after heavy endurance events as signs that need medical assessment (Bäcker et al., 2023). If you notice severe muscle pain, marked weakness, swelling or dark urine after exercise, seek medical care straight away rather than waiting for a routine test.
CRP: a general marker of inflammation
C-reactive protein is made by the body when there is inflammation. The LML describes the high sensitivity CRP test as a way to assess the presence and severity of inflammation, and notes that raised levels can relate to infections, autoimmune conditions, cardiovascular disease and other causes. That breadth is important: CRP says that inflammation is present, not where it comes from.
A hard session can raise CRP for a short time. Over months, though, regular training points the other way: a meta-analysis of 83 controlled trials found that exercise training was associated with a fall in CRP, regardless of age or sex, with larger falls when body fat also went down (Fedewa et al., 2017). A single CRP reading taken just after a race is therefore hard to interpret, while a series taken on easy days tells a clearer story.
ESR: an older, slower inflammation test
The erythrocyte sedimentation rate measures how quickly red blood cells settle in a vertical tube. The LML calls it a non-specific test that helps show inflammation in the body. A clinical review on ESR and CRP explains that neither test is sensitive or specific enough to be used alone, that both should be read together with the clinical history and examination, and that the two often disagree, partly because they rise and settle at different speeds (Bray et al., 2016).
For this reason ESR is rarely a stand-alone recovery marker in sport. It is more often requested alongside CRP when a health professional wants a broader view of inflammation, for example with persistent joint pain or symptoms that do not settle with rest.
Putting the three together
- CK reflects muscle stress and is very sensitive to recent training.
- CRP reflects inflammation from any source and responds within days.
- ESR also reflects inflammation, changes more slowly and is read alongside CRP.
None of them measures "recovery" directly. They are pieces of information to set beside how you feel, how you sleep and how you perform. Blood panels such as the Sports Fitness Profile and the athlete panels in the BPR catalogue already include CK and high sensitivity CRP, so it is worth checking before ordering single tests.
