Abstract
In 11 untrained subjects (mean age = 22.5 yr) serum creatine kinase (CK) was measured
before and after two types of knee extension exercise regimens. One regimen consisted
of 35 10-s maximal isometric contractions separated by a 20-s rest (10:20). This regimen
provided adequate recovery time between trials so that strength loss amounted to only
23.4% over the 35 contractions. The other exercise regimen consisted of the same 35
maximal isometric contractions but separated by 5 s between trials (10:5), resulting
in a 54.6% strength loss. Muscle fiber type was also determined to assess the relationship
of fiber type with serum CK and strength loss. The fast-twitch (FT)/slow-twitch (ST)
area ratio correlated with resting CK. Baseline strength correlated with strength
loss over the 10:5 condition but not with strength loss over the 10:20 condition suggesting
different mechanisms responsible for strength decline for the two exercise conditions.
Although a greater strength loss was found for the 10:5 condition, serum CK levels
were significantly higher following the 10:20 exercise. An analysis of covariance
showed that the mean strength level over the 35 contractions was significantly related
to CK increase in the two exercise conditions. Results suggest that mechanical factors
associated with high tension levels may be responsible for CK efflux following isometric
exercise.
Key words
creatine kinase - fiber type - isometric fatigue
