What do Creatine Kinase (CK) and Vertical Jump (CMJ) tell us about fatigue in elite soccer?
Observational Study in Professional Soccer Players
This observational study in professional soccer players analyzes how elevated levels of creatine kinase (CK) and a decrease in countermovement jump (CMJ) performance directly influence physical performance during training sessions. Both markers reflect the state of muscle fatigue after matches or intense training.
CK is an enzyme released into the bloodstream when there is muscle damage. It has been shown that after a match, CK levels increase significantly, peaking between 24 and 48 hours afterward. This indicates that players are still in a recovery phase up to 72 hours after competing.
The Countermovement Jump (CMJ), in turn, is a jump test used to evaluate the neuromuscular capacity of the legs. A decrease in jump height reflects functional fatigue, and it was also observed that after a match, CMJ values dropped significantly and took several days to recover.
In total, more than 7,000 training sessions were analyzed across 30 players from a high-level European team. Before each session, CK levels were measured and the CMJ test was performed. GPS technology was then used to record physical variables such as total distance, sprints, accelerations, decelerations, and top speed.
What did the data reveal?
An increase of +1 in CK Z-score (i.e., significantly higher levels compared to the individual average) was associated with:
- −25.5% in high-speed distance.
- −24.3% in accelerations.
- −24.6% in top speed.
- Similar reductions in other key training variables.
A decrease of −1 in CMJ Z-score was also associated with:
- −23.5% in high-speed distance.
- −25.6% in top speed.
- −25.3% in accelerations.
- Lower ability to perform explosive actions.
What does this mean for coaches?
Both indicators, CK and CMJ, are highly useful to predict a player’s physical performance before a training session begins. If a player shows elevated CK or low CMJ performance, it is likely that they will not reach their optimal level in high-intensity activities.
This allows coaching staff to adjust training loads, modify tasks, or even shorten the session to avoid overload. In doing so, they can help prevent the risk of injury and improve individualized recovery between matches.
Key Takeaways
- CK reflects structural muscle damage, while CMJ measures functional neuromuscular fatigue.
- Both are correlated, but provide complementary information.
- Performance reductions due to fatigue can be detected before training, enabling better planning.
- Coaches can design safer and more effective sessions, tailored to the real condition of the players.
Practical Application
This study shows that with a simple jump test and a capillary blood sample, it is possible to anticipate a player’s performance and fatigue status. This information, combined with GPS technology, is a powerful tool to optimize performance, protect muscle health, and prevent injuries.
