Creatine kinase (CK)

What creatine kinase measures

A biochemical review describes creatine kinase (CK) as an enzyme of the phosphocreatine system. CK reversibly catalyses the reaction between creatine and ATP to form phosphocreatine and ADP. Phosphocreatine serves as a readily available reservoir of high-energy phosphates, from which ATP can be regenerated during periods of high, intermittent energy demand.

The same review classifies CK clinically as a marker for diseases of the heart and skeletal muscle. However, the value is not automatically tissue-specific and, in the case of an isolated increase, does not indicate either the site of origin or the cause.

This is crucial for strength athletes: a high CK level can occur following an unusual level of exertion. The specific exercise protocol and its limits are described in the next section.

How CK changes following eccentric or concentric exercise

A controlled human study involved 16 young female university students. Eight performed an eccentric exercise and eight a concentric exercise. The protocol consisted of three sets, each comprising 35 isokinetic contractions of the knee flexors and extensors at 80 per cent of peak concentric torque. CK was measured before the exercise and 1, 24, 48 and 72 hours afterwards.

After 72 hours, the mean CK level in the eccentric group was 14,856 U/l, whilst in the concentric group it was 360 U/l. Delayed onset muscle soreness was also more pronounced following the eccentric exercise. This demonstrates a clear, time-dependent response in this specific protocol. It is not a general reference value for all strength training programmes, nor is it a direct study involving trained bodybuilders.

Despite the rise in CK, the same study found no significant increase in C-reactive protein (CRP) in any group. An elevated CK level should therefore not automatically be interpreted as evidence of a systemic inflammatory response in this experimental design. However, the negative result for CRP does not answer every other question regarding muscle damage, inflammation or recovery.

How CK can help with elevated liver enzyme levels

A narrative clinical review on bodybuilding and aminotransferases concludes that the pattern of elevated aminotransferases alone does not reliably distinguish between liver and muscle damage. Other muscle enzymes may be useful in making this distinction. However, the review does not report any pooled diagnostic accuracy that would allow a muscular origin to be reliably confirmed on the basis of individual laboratory results.

In this situation, elevated CK levels may provide further evidence of muscle involvement. However, such evidence does not automatically prove the cause of the elevated aminotransferases. There is no direct analysis of diagnostic accuracy for the specific combination of normal GGT and elevated CK.

Why a training value does not provide a general threshold

The human study provides two group mean values 72 hours after a specific single exercise session, but no reference range for trained strength athletes. The eccentric group’s value was 14,856 U/l, whilst the concentric group’s was 360 U/l. These are the results of this protocol and do not constitute general reference values.

The measurement taken after 72 hours shows that the timing and type of exercise can influence the results. It is not possible to determine a universally applicable training rest period prior to a CK or liver function test from this individual record; this question remains open.

When to consider a pay rise

A GRADE-based guideline recommends that, in adults with persistent oligo- or asymptomatic hyperCKaemia, further investigation should be carried out if CK levels exceed 1.5 times the upper limit of normal. Neurogenic and non-neuromuscular causes should first be considered or ruled out. Following normal genetic testing, a muscle biopsy may be considered in cases where, amongst other things, CK levels are three times or more the upper limit of normal, there are abnormal findings on muscle MRI, there is a positive family history, or other clinical criteria are present.

This recommendation relates to a persistent increase with no clear acute training-related cause. It is not intended as an automatic diagnostic threshold for a single CK value immediately following unusual training. Furthermore, the guideline is not sport-specific, and several recommendations are based on consensus or expert opinion.

What can be deduced from a CK level

In strength training, CK is primarily a contextual marker. The controlled human study measured an acute response up to 72 hours after a single training session, but did not assess muscle growth, strength gains or overtraining. It is therefore not possible to reliably infer any of these long-term outcomes from a single CK reading; robust, controlled long-term validation is lacking.

A single reading therefore does not replace the classification of the type of stress and the time of measurement, nor does it replace a clinical assessment in the event of a persistent increase.

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