| dc.description.abstract | A tight match schedule in elite football makes it challenging to balance training and match load with recovery and rest. Being able to reliably measure the external load and understand how a given amount of load affects the experience of internal load, neuromuscular fatigue, recovery time and physical performance for the individual player may be the key to balancing this as best as possible. In this thesis we explore the association between external load, measured with player tracking devices and internal load (perceived effort throughout the training session) and individual differences in this context (Paper I). Furthermore, we investigate how external load in a football match affects the subsequent changes in blood markers for muscle damage, and the recovery of neuromuscular function and physical performance (Paper II). Finally, we explore to which extent a football match leads to ultrastructural damage to the muscle fibers, via Heat Shock Proteins (HSP) as proxy markers, and how this relates to fatigue and recovery of muscle function (Paper III).
A total of 99 football players participated in two different studies. In study I we followed the same players over several training sessions in a 32-week period and measured external load with player tracking devices and internal load with the session rating of perceived exertion-derived training load method (sRPE-TL; Paper I). In study II, we measured external load in three matches, one match per player, and followed the subsequent recovery process with measurements of creatine kinase (CK), myoglobin, countermovement jump (CMJ), 30m sprint and YO-YO intermittent recovery test (Paper II), and the stress response in muscle fibers with analyses of HSP in muscle biopsies from m. vastus lateralis (Paper III).
The results from study I showed that the difference between training sessions with typical low and high external load (2 standard deviations of the variable PlayerLoadTM), led to a 106% (90% confidence interval; CI; 83–133%, effect size; ES; of 2.52–2.68) increase in sRPE-TL (within-player effect), with an individual response of ±24% (CI; 10–33, ES = 0.76). Furthermore, we found a difference of 19% (CI; 3–38, ES = 0.64) between players with low versus high average PlayerLoadTM (between-player effect). Finally, we observed that the variation in sRPE-TL from session to session was 21% (CI; 13–27, ES = 0.68) after adjustment for PlayerLoadTM and individual differences in sRPE-TL.
The results from study II showed a reduction in CMJ-performance, and increases in CK and myoglobin with effect sizes of −0.75, 0.92 and 3.80 respectively 1 h after the match. Of the external load variables, high speed running distance, had a consistent effect on changes in CK, 1–72 h after the match (ES = 0.60–1.08). Total distance had a small effect (ES = 0.56) on the 30m sprint 72 h after the match. The effect of the investigated external load variables on CMJ performance were either trivial or unclear, even though CMJ height was the performance indicator with the most consistent reductions in the recovery period.
In the subgroup of players that donated muscle biopsies, we observed a decrease in soluble HSPs (the cytosol fraction) of 15–17% (p < 0.01) 1 h after the match. Concurrently, HSPs bound in the cytoskeletal fraction increased 3.6 and 1.8 times the baseline levels (p < 0.01). For αB-Crystallin, which is a small HSP rapidly binding to denatured proteins, the increased levels bound in the cytoskeletal fraction returned to baseline levels after 72 h, whereas HSP70, which is a larger HSP involved in the repair process, remained elevated. With immunohistochemistry methods on frozen muscle cross section, we found a 20–27% increase in staining intensity for the two HSPs in myofibrillar structures (p < 0.01) 1 h after the match in both Type I and Type II fibers. Staining intensity did not return to baseline level within 72 h. In addition, there was a 2.2-fold increase in the proportions of fibers that showed granular staining patterns of αB-Crystallin, indicating sarcomere disruption, 1 hour after match (p < 0.01).
In summary, in study I there was a close relationship between external load, measured by player tracking devices, and internal load measured with the sRPE-TL method, where external load variables with a low or no intensitythreshold showed the strongest relationship with sRPE-TL. This confirms that measurements of external load with player tracking devices is a valid method monitoring training load, but also that some external load variables are better than others. Nonetheless, we observed large individual differences in the effect of external load on internal load, which emphasizes the necessity of individual follow-up as a fixed quantity of external load leads to different perceived exertion for a set of players. Lastly, we observed large variation in internal load between sessions that could not be explained by the external load variables or the individual response to them. This suggest that there are loading patterns that are not captured by the external load variables. Based on these results, we recommended to use both measurements of the internal and external load, where the importance of individual follow-up is emphasized.
Furthermore, the results in study II showed that the amount of high speed running distance in a match was positively associated with increased levels of the blood markers of muscle damage both immediately and 72 h after match. Total distance and PlayerLoadTM had a negative effect on 30m sprint performance 72 h after match, whereas surprisingly no relationship was found between the measured external load variables and CMJ, even though CMJ performance was strongly reduced after the match. The results suggest that several different external load variables should be chosen for the evaluation of match load as these can provide different information about the recovery period. Although external load variables showed an effect on time to recovery at the group level, there was not enough statistical power to predict the recovery outcome of the individual player.
Finally, we found that the HSP stress response in muscle fibers, increased levels of blood markers for muscle damage, decreased neuromuscular function and increased perceived muscle soreness indicates mild muscle damage after football match. Such ultrastructural muscle damage likely plays a role in the prolonged recovery time after match. Compared to studies where the load on the muscle is unfamiliar or extreme, football matches resulted in considerably lower HSP response. This means that the players are generally well-adapted to the match load, but there are still loading patterns that exceeds the tolerability threshold and results in muscle damage, hence a subsequent slow recovery of muscle function. | en_US |
