REF-305 · Totsuka M, Nakaji S, Suzuki K, Sugawara K & Sato K, 2002 · Journal of Applied Physiology, · Paper · PubMed
Break Point of Serum Creatine Kinase Release After Endurance Exercise
15 healthy young men, 90 min cycling on 3 consecutive days: CK rose from 3 h after the first session; peak CK split into high (>500 IU/l) and low (<300 IU/l) responders, so the "break point" of CK release is 300-500 IU/l (2-3× resting) and tracks quadriceps size and strength, not fitness
Participants
n=15 healthy young men
Protocol
90 min of bicycle exercise on 3 consecutive days
Before the test: body composition, properties of the quadriceps femoris muscle (QFM: cross-sectional area, volume), aerobic and anaerobic capacities, knee-extensor strength
Blood sampled 22 times over the experimental period
Key Results
Serum CK was significantly elevated from 3 h after the first session (P < 0.05) and increased gradually thereafter across the 3 days
Subjects split by peak CK into high responders (HR, > 500 IU/l) and low responders (LR, < 300 IU/l)
Peak CK correlated (P < 0.01) with workload / QFM cross-sectional area (r = 0.658), workload / QFM volume (r = 0.648) and NEGATIVELY with knee-extensor strength / body mass (r = −0.634); the HR and LR groups were separated on each of these variables
Authors' conclusion: the break point of CK release after endurance exercise under these conditions is 300-500 IU/l — two to three times the resting value — and is associated with properties of the quadriceps
Mechanism
The "break point" in this paper is a CK LEVEL (300-500 IU/l) that separates two kinds of responders, not an exercise-intensity threshold. Individuals doing the same 90-min ride with a higher workload per unit of quadriceps size, and with less knee-extensor strength per kg body mass, leaked more CK. Inference, not tested: a smaller/weaker quadriceps working at the same absolute load is loaded closer to its ceiling, which is where membrane leakage rises.
Practical Application
In 15 healthy young men cycling 90 min on 3 consecutive days, CK response was bimodal — high responders peaked above 500 IU/l and low responders stayed below 300 IU/l — and the split was explained by quadriceps size and strength relative to the workload (r = 0.658 and 0.648 for workload per muscle area and volume; r = −0.634 for strength per kg), not by aerobic fitness
A post-ride CK of two to three times resting (300-500 IU/l) is the boundary the authors identify between "normal leakage" and a high-responder pattern
Inference, not tested: two riders on the same watts can be on opposite sides of this line if their leg muscle mass differs — CK has to be interpreted per athlete, against that athlete's own baseline
Not from this paper; the base's own practice: the "golden zone / inflection point" idea (train just below the intensity where recovery cost jumps) is a coaching heuristic; this paper does not test intensity domains — for intensity-domain physiology see Black 2017 REF-306
Limitations
Non-athletes (healthy young men, not endurance trained)
Only cycling modality; a single absolute protocol — intensity was not manipulated
CK is an indirect marker — does not directly measure contractile damage
How this entry was checked and changed
Why this level
regraded 2026-09-27 from B+: uncontrolled pre-post, n=15
Content correction 2026-09-27
"Nonlinear CK response: below a certain intensity minimal elevation, above it disproportionate release" -> the abstract's break point is a CK concentration (300-500 IU/l, 2-3× resting) separating high and low responders, not an intensity threshold; intensity was not varied.
"CK break point correlates with fitness — fitter individuals have a higher break point" -> peak CK correlated with workload per quadriceps cross-sectional area (r = 0.658) and volume (r = 0.648) and negatively with knee-extensor strength per body mass (r = −0.634); fitness is not mentioned.
"Supports the golden-zone concept / Bakken's inflection point" relabelled as house practice, not a finding of this paper.
Level B+ -> B (uncontrolled pre-post, n=15); Tier 2-3 -> Tier 1 (healthy young men, no training status).