Serum creatine kinase activity and its changes after a muscle biopsy

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Abstract
Summary. The significance of the absolute elevations of serum creatine kinase (CK) levels after intense exercise and injuries was studied by measuring CK activities from seven healthy active males during a 2‐week period, with a muscle biopsy taken between the first and second week. Most of the subjects (three lifters and two runners) carried on their normal exercise activities, while two lifters stopped training during the 2 weeks. The weight of the biopsy, number of fibres, percentage of fibre types, and cross‐sectional areas of the muscle fibres were measured. The CK levels of the nonactive subjects and runners remained consistently low during the control week, whereas those of the lifters were usually 500% greater than those of the other two groups, and fluctuated with the intensity of their workouts. A muscle biopsy, having a mean weight of 71.3 mg and containing 1800 fibres, increased the CK values by approximately 100 units litre‐1 (U1‐1) in most of the subjects. One runner injured his right hamstring muscles 2 days prior to the biopsy, and his CK values rose from 50 to 4400 U I‐1. The increases in CK after the biopsy were not related to fibre type, activity, weight of the biopsy, or number or size of fibres removed. These results indicate that: CK values are consistently lower in normal subjects and runners than in lifters. Weight training results in chronic elevations of CK. Compared to a muscle biopsy, muscular injury dramatically increases CK levels. Elevation of serum CK is observed as early as 1 h after an intense weight‐lifting session. The elevation of serum CK by 100 U 1‐1 is associated with damage to approximately 2000 fibres.