The main finding of the present study was that caffeine increased maximal vertical jumping height by ~ 2 cm. However, caffeine did not increase jumping height after three sets of heavy leg press strength training. Of note, blood lactate was higher in the recovery period after intake of caffeine indicating more severe effort during the strength training. The ~ 2 cm increase in maximal jumping height will have large impact in sport competitions.
Caffeine improves performance in a number of different experimental situations. We have shown that caffeine increases double-poling performance in exercise tests lasting from 4 min until 30 min [30,31,32]. Caffeine lowers perceived exertion at submaximal loads during endurance exercise [11, 16, 30,31,32] and increases anaerobic capacity . In the present study, the effect of caffeine on jumping height was consistent and caffeine increased CMJ on the two first and mean CMJ height during the three maximal jumps. Moreover, caffeine increased mean jumping heights at the 5-jump series with 7 s between jumps prior to the strength training.
In many sports, like soccer, handball and volleyball, jumping height is a valuable physical quality, and maximal strength contributes to development of power and speed . In line with this, many athletes conduct strength training to improve jumping height . Importantly, the participants had experience with both strength and jump training, which allow reliable tests. In the present study, coefficient of variation (CV) for maximal jumping height between the two first maximal jumps was 1.1%, which allows detecting small effects. The effect size for maximal jumping height was moderate (0.68) and the fact that caffeine increased jumping performance has practical implications in sports like high jump, where ~ 2 cm increase during competitions may have dramatic effects at the final result. Recent studies support the finding that caffeine increases maximal jumping height [3, 24, 41]. Importantly, Perez-Lopez et al. found that caffeine increased performance at a number of test evaluating explosive strength including counter movement jumping height . Increased jumping height has also been found in volleyball players  and basketball players  after intake of caffeine. The present study supports that caffeine improves jumping height, which requires fast and high force development.
Caffeine has several mechanisms of action, which may mediate the improved performance. Caffeine binds to and inhibits adenosine receptors at low physiological concentrations . Adenosine receptors are expressed on most cell types, and caffeine has effects on both the nervous system and peripheral tissue. Caffeine has many effects on the nervous system, and caffeine increases spinal excitability , but caffeine affects several systems in the brain, which can explain increased jumping heights . In the present study, jumping heights were calculated with a force platform and the speed leaving the ground determines the heights. We were unable to detect any significant effects of caffeine on maximal force production, although the statistical analyses indicated a higher maximal force production (ANOVA; P = 0.059) and faster press time in jumps (ANOVA; P = 0.11). Higher force production in the jump phase can result from recruitment of more motor units or higher firing rate . In the present study, EMG was not recorded, but there are several mechanisms that could explain the elevated jumping height.
Caffeine has direct effect on skeletal muscles , and it is well-documented that caffeine increases Ca2+ release via binding to ryanodine receptor . In humans, physiological concentrations of caffeine increases force production during nerve stimulation, indicating that caffeine has a direct effect on skeletal . Caffeine has been reported to preserve force production longer during isometric contraction without increasing motor neuron firing rate , which indicates direct effects on skeletal muscles. In particular, the fact that caffeine stimulates Ca2+ release  could contribute to elevated force and jumping height.
In the present study, the effect of caffeine was observed before the heavy strength training session in leg press, but no significant effect was observed immediately after and during the recovery period. The strength training session decreased maximal CMJ by ~ 8 cm immediately after the strength training in both PLA and CAF, but jumping height was not significantly higher after caffeine intake. These data show that jumping height decreases substantial after maximal effort strength training, and the jumping height remained significantly reduced during the 15 min recovery despite jumping height increased significantly during the first 5 min recovery. Caffeine intake increases performance, which results in more severe fatigue and elevated physiological markers of stress . It has been reported that twitch-force was lower after a 4-km time trial with intake of caffeine compared to placebo . In the present study, blood lactate was higher in the recovery period after caffeine ingestion compared to placebo, indicating that more work was done during the strength training session.
The mechanisms for the large decrease in jumping height after the strength training are not clear. Blood lactate is normally higher after caffeine ingestion when endurance exercise performance tests are conducted [30,31,32]. In the present study, the strength training session was though and increased lactate concentration to ~ 8 mmol/L, and blood lactate was higher in the recovery period after intake of caffeine, supporting that larger effort during the strength training session after caffeine ingestion, which might be the reason why caffeine no longer improved jumping height. However, blood lactate decreased during the recovery period and lactate was only slightly elevated after 15 min recovery, whereas jumping height did not increase between 5 and 15 min recovery.
To investigate the mechanism for the reduction in maximal jumping height after the short maximal effort strength training session, maximal force production and the duration of force production during jumping were investigated. Fatigue reduces maximal force, maximal contraction velocity and curvature of the force–velocity relationship, which will all contribute to reduced power production [18, 20]. In the present study, maximal force production decreased immediately after the strength training session, which have contributed to the reduction in vertical jumping height. Immediately after the strength training, the duration of the force production was also longer, indicating that lower force was produces at higher velocities during the jump. Maximal force production had recovered after five recovery, which is supported by other studies . However, the duration of jump remained longer, suggesting that the maximal effort strength training primarily affects force development at higher speed. This idea agrees with the fact that fatigue increases the curvature of the force–velocity curve . After 15 min recovery, no difference in maximal force or duration of force production was observed, agreeing with increased curvature of the force–velocity curve, and lower force production at higher contraction speeds , resulting in lower jumping height. Caffeine did not influence the vertical jumping height during the recovery period, although duration of force production time tended to be lower after caffeine intake.
Interestingly, Glaister et al. reported that caffeine improved power during the first three sprints, whereas no effect was observed during the following seven sprints . This suggests that one has to “pay the price” for the better performance during the first exercise tasks after caffeine intake later on. It has also been reported that caffeine increases 1RM in back squat , and the higher performance requires higher energy production and metabolic stress. The higher effort during physical activity after intake of caffeine, compared to placebo, will cause exhaustion and muscles damage, supported by what we previously reported that higher plasma creatine kinase (CK) was observed the day after caffeine intake prior to the performance test . Therefore, it is not surprising that caffeine did not increase performance after a hard strength training session.
There are some limitations and strengths to keep in mind when the data in the present study are interpreted. It is a limitation that the number of participants are low, since low number of participants is associated with risk for type II errors. In the present study, the investigations of force production in the jumps did not show significant effect of caffeine. This lack of significant effect could be a type II error and these data need to be interpreted with caution and this question need to investigated more carefully. However, the effect of caffeine prior to the strength training session was consistent. Moreover, it is a strength that the participants had experience from both jump and strength training.
In conclusion, caffeine improved maximal vertical jump height and jump height during a 5-jump series. Jumping height decreased by ~ 8 cm immediately after three series of heavy leg press strength training and no significant effect of caffeine was observed. After 5 and 15 min recovery of recovery, jumping height was still reduced and no significant effect of caffeine on recovery was observed. The data suggest that caffeine can improve performance in competitions like high jumping.