Why is my CK so different from my training partner's after the same session?
People respond very differently, and the peak can come one to five days after. One value marks the load and the person, not readiness. Narrative review.
Baird, Graham, Baker and Bickerstaff ask whether a rise in serum creatine kinase after exercise means muscle damage, and whether high values are a health concern. CK is the enzyme of the phosphocreatine shuttle and makes up a large share of the soluble protein in muscle; its appearance in blood has long been read as damage, but how well it reflects exercise damage is disputed.
Resting CK in general populations is 35-175 U/L, with a reported range of 20 to 16,000 U/L, reflecting subclinical disorders, minor injury, genetics, activity and medication; regular high-volume trainers have raised resting values.
Clinically, more than 5,000 U/L without heart attack, stroke, trauma or disease is generally taken to mean serious disturbance to muscle, and rhabdomyolysis runs from 10,000 to 200,000 U/L; proposals to raise the upper limit of normal by 1.5 times have not produced an agreed standard.
The authors find no established link between habitual or acute high-intensity eccentric exercise and kidney dysfunction or muscle disorder in healthy individuals even above 20,000 U/L, with genetic disposition, environmental conditions (such as prolonged exercise in hot climates) or disease adding the risk of exertional rhabdomyolysis.
The rise depends on the exercise and the person: eccentric resistance work peaks at 72 to 120 h; full-body eccentric training peaked at 24 h in resistance-trained men but at 72 h in the untrained; plyometric jumps at 48 h; three days of 90-min cycling peaked straight after the third day.
Some people show far higher CK than comparable others on the same protocol; women showed a large rise after stepping exercise that men did not; candidate genes are named. In one cited study CK varied between comparable men whose biopsies showed similar damage.
In their conclusion the authors speculate that CK leaving the cell after mild to moderate exercise is a regulated energy-control event — driven by the energy sensor AMPK — rather than structural damage, and say themselves that the literature did not allow more definitive evidence.
Our reading: useful for the numbers that frame a CK value (ranges, thresholds, peak timing, spread) and for the statement that CK alone does not size damage; the AMPK idea is a hypothesis without data and should not be cited as a mechanism.
Protocol
Key Results
The authors' synthesis, with the numbers as printed and the sources they cite:
| Topic | Statement | Cited |
|---|---|---|
| Resting CK, general populations | 35-175 U/L; ranges from 20 to 16,000 U/L | refs [16], [17] |
| Trained people | regular high-volume, intense exercisers have significantly raised baseline CK; also regularly exercising premenopausal women | [14], [15] |
| Serious disturbance | above 5,000 U/L, in the absence of infarction, trauma or disease | [10] |
| Rhabdomyolysis | 10,000-200,000 U/L, reported as high as 3 × 10^6 U/L | [18] |
| Upper limit of normal | proposals: raise by 1.5 times; biopsy only at ≥3 times the limit without an exercise explanation; no universally agreed standard | [19] |
| Kidney risk in healthy people | no established link with habitual or acute high-intensity eccentric exercise, even above 20,000 U/L; genetics, environment or disease may add the risk of exertional rhabdomyolysis and acute renal failure (Table 1 lists heat, drugs, alcohol, infections, electrolyte imbalance among causes) | [13], [10] |
| Clearance | after isolated mild to moderate damage in healthy people, the body clears released muscle components back to baseline within 7-9 days | [4], [6] |
| Peak timing | eccentric resistance work 72 h, 96 h to 120 h; full-body eccentric: resistance-trained men peaked at 24 h, untrained still rising to 72 h; untrained men's plyometric jumps 48 h; 90-min cycling on three days: peak straight after the third day, 72 h from the start; one biphasic curve with a peak at 95 h | [31], [45], [67], [4], [68], [69], [6] |
| Seven daily eccentric bouts | repeating the elbow-flexion protocol on days 2-7 did not raise damage indices beyond one bout; CK peaked 4 days after the first bout in both groups | [27] |
| Sex | 30 min of stepping: 15 women from 191 ± 103 to 7239 ± 2403 U/L at day 3, no significant rise in 18 men; reviews judge sex differences small | [12], [40], [41] |
| CK vs biopsy | CK varied with prior hydration and between comparable men while biopsies showed similar Z-band damage; soreness did not differ | [24] |
| Water vs land | elbow-flexion sets raised CK at 48 h on land but not in water (fewer eccentric contractions) | [70] |
| Hypothesis | AMPK may phosphorylate CK and trigger its expulsion as a brake on ATP use — "speculated" by the authors | none (the authors' own) |
Mechanisms
Practical Application
Limitations
How this entry was checked and changed
a narrative review without a systematic method, from a university group (University of the West of Scotland) that is not one of the field's leading CK or muscle-damage laboratories; its numbers are cited from other papers, and its central hypothesis (AMPK-driven expulsion of CK from the cell) is offered without data