Effect of caffeine on neuromuscular function following eccentric-based exercise

This study investigated the effect of caffeine on neuromuscular function, power and sprint performance during the days following an eccentric-based exercise. Using a randomly counterbalanced, crossover and double-blinded design, eleven male jumpers and sprinters (age: 18.7 ± 2.7 years) performed a half-squat exercise (4 x 12 repetitions at 70% of 1 RM), with eccentric action emphasized by using a flexible strip attached to their knees (Tirante Musculador®). They ingested either a capsule of placebo or caffeine (5 mg.kg-1 body mass) 24, 48 and 72 h after. Neuromuscular function and muscle power (vertical countermovement-jump test) were assessed before and after the half-squat exercise and 50 min after the placebo or caffeine ingestion at each time-point post-exercise. Sprint performance was measured at pre-test and 75 min after the placebo or caffeine ingestion at each time-point post-exercise. Maximal voluntary contraction (overall fatigue) and twitch torque (peripheral fatigue) reduced after the half-squat exercise (-11 and -28%, respectively, P < 0.05) but returned to baseline 24 h post-exercise (P > 0.05) and were not affected by caffeine ingestion (P > 0.05). The voluntary activation (central fatigue) and sprint performance were not altered throughout the experiment and were not different between caffeine and placebo. However, caffeine increased height and power during the vertical countermovement-jump test at 48 and 72 h post half-squat exercise, when compared to the placebo (P < 0.05). In conclusion, caffeine improves muscle power 48 and 72 h after an eccentric-based exercise, but it has no effect on neuromuscular function and sprint performance.

showed that eccentric action is considerably increased by using this device (EDIR DA SILVA et al., 2005; http://www.redalyc.org/articulo.oa?id=551656963007). We have added this information in the manuscript on Page 8, Line 179 as follow: "This flexible strip has been demonstrated suitable to increase eccentric action during a half-squat exercise (17). Participants were familiar with this accessory as they had used it in their training routine as a form to increase eccentric action without using an excessive external load, which may reduce the risk of injuries associated with eccentric-based training, mainly in a preparatory phase of an annual training periodization (17)".
Regarding the sprint training performance, we agree that pre-test cannot be directly compared to 24h, 48h and 72h recovery. We performed the pre-test sprints to check whether athletes would be fully recovered at the transition from the first experimental block (using placebo or caffeine) to the second experimental block (when supplement was inverted). Only one repetition of each distance was performed to avoid the development of fatigue and to not influence the following days tests. This procedure guaranteed that all blocks (placebo and caffeine) started with participants having fully recovered from previous tests and training and with the same recovery level in both conditions.  Minor: Page 2, line 42: Before "and", "," should not be used. On the other hand, between "72" and "h" the "-" it is not necessary and the authors due write in the same form along the manuscript because in some case don't use hyphen.
Response: Thank you for this recommendation. We have modified accordingly throughout the manuscript.
differences in the numbers of sprints between pre and post sprints (please, see our answer for your first comment). Thus, we think this is now not properly a limitation. The limitation paragraph reads (Page 20, line 452-458) "There are some limitations in the present study that should be mentioned. Our resistance training protocol generated mild to moderate muscle pain, resulting in no impairment of subsequent training capacity. Thus, whether caffeine would be useful when greater muscle pain is present deserves further investigation.
Another potential limitation is that experimental conditions were performed only once each (one for placebo and another one to caffeine). Repetitions of the experimental blocks may have provided additional information regarding the reproducibility of our findings".

Figures 3 and 4: Unify the levels of values in the Y axis of all figures. MVC has
nine levels, Qtwpost has seven levels or Qtw100 has eight levels.

Response: We have modified Y-axis in figures 3 and 4 and now all Y-axis
have nine levels.

Reviewer #2:
This experiment was designed to assess the neuromuscular effects of caffeine intake during the recovery phase of an eccentric-based exercise protocol that induces muscle damage. Overall, the experiment seems well designed, it contains several measurements to fulfill the objectives of the investigation and it is conveniently described in a well-written manuscript. However, the experiment has a serious flaw: it is designed to assess caffeine's effects in the recovery phase of a muscle-damaging exercise when most, if not all, performance variables returned to basal -non-fatigued-values within 24 hours after the end of exercise. This fact is likely due to the use of a "light" eccentric exercise protocol that produced very low levels of muscle damagea pilot study would have been helpful to detect the inefficacy of this protocol to produce moderate muscle damage". Thus, although I found merit in the experiment and I congratulate authors for the hard work developed for this investigation, I would suggest that authors reconsider the use of "recovery" through the manuscript because this is not what they were investigating -the recovery lasted < 24 h and they did not perform any measurement in this time. For example, the conclusion of the abstract should indicate that: "Caffeine improves muscle power 48 and 72 hours after an eccentric-based exercise…". A limitation paragraph including this aspect is highly recommended.

Response:
We are happy that you have appreciated our experiment design and the quality of writing of the paper. We also thank you for your suggestions to improve our paper. We agree that our eccentric exercise protocol was not intense enough to provoke long impairment in our main outcomes. A pilot study would be helpful, but as we recruited athletes involved in national and international competitions and they were engaged in experiments taking 3 weeks, we were afraid to propose a longer experimental design and impair their preparation and carry to decline participation. In addition, we were afraid to propose a more intense protocol to produce larger muscle damage because they were in the preparatory phase of an annual training periodization; therefore, more susceptive to muscle injuries if using intense muscle damage protocol.
To not miss the opportunity to test performance in athletes of this level, we ascertain this light protocol with their coach. However, we recognize this is a limitation and the term "recovery" should not be used. Based on your suggestion, we have removed the use of recovery throughout the manuscript and replaced to "after a half-squat exercise with eccentric emphasis" or something similar. Conclusion in the abstract was also altered as suggested (Page 2, Line 54) to "In conclusion, caffeine improves muscle power 48 and 72 h after an eccentric-based exercise, but it has no effect on neuromuscular function and sprint performance". We have also added a paragraph in the discussion recognized this point as a limitation, as suggested (Page 20, line 452-458) "There are some limitations in the present study that should be mentioned. Our resistance training protocol generated mild to moderate muscle pain, resulting in no impairment of subsequent training capacity. Thus, whether caffeine would be useful when greater muscle pain is present deserves further investigation. Another potential limitation is that experimental conditions were performed only once each (one for placebo and another one to caffeine).

Repetitions of the experimental blocks may have provided additional
information regarding the reproducibility of our findings".
Based on the data presented, I have the following comments/questions. 2. Indicate why you selected this protocol of eccentric exercise. And discuss the lack of a relevant increase in CK (for instance, you could compare these values with the ones found after a marathon).

Response: We have included an explanation about this choice on Page 8, Line
200-204 "We ascertain this protocol with athetes's coach to simulate their training routine. We chose a protocol able to provoke moderate rather than a more intense DOMS because athletes were in a preparatory phase of an annual training periodization; therefore, protocol to induced more intense DOMS would make athletes more susceptive to muscle injuries".
We have also included in the discussion section the lack of a relevant increase in CK and the comparison with values after a marathon, as suggested (Page 18, Line 406-412) "We also found an increase in CK levels 48 h after exercise, regardless of the supplement ingested. This finding is in agreement with a study showing that CK peaks within 48 h after a marathon (24). However, our CK level 48 h after the half-squat exercise was much lower than those reported 24 h after a marathon (434-844 U/L) (24). This is expected because CK levels after prolonged exercise such as marathon can reach up to 50 times the rest values due to the greater muscle damage caused by this kind of activity (24)".
3. Indicate the habituation to caffeine of these individuals.
4. The CK concentrations before exercise seems quite high (especially one participant with > 500 U/L). Please, discuss this in the light of previous investigations. Did they have previous muscle damage due to training?
Response: Thank you for this question. The baseline CK upper limit for general population is 174 U/L. We found an average baseline value of 330 U/L, which suggests a slightly already elevated CK. Unfortunately, daily training results in persistent CK elevation in athletes, which makes hard to find a "true" baseline.
We tried to control this by providing two days of low-intensity training before blood sample, as it would be difficult to maintain high performance athletes without any kind of training. Although we have not got values below 174 U/L, the values were quite close (~ 330 U/L) and not different between supplements, which suggests that any effect of previous training on CK as similar across experimental conditions. We have added a discussion about this on Page 17, Line 397-406 "The CK after an eccentric exercise peaks 1-4 days and can remain elevated for several days (3,24,25). An important issue is that daily training results in persistent CK elevation in athletes, with resting values being higher than in non-athletes (25). In the present study, even providing two days of low-intensity training before taking blood sample, we found baseline CK values ranging from 199 to 514 U/L, which are slightly desired power of 0.80, the total sample size necessary to achieve statistical significance was estimated to be 10 participants. However, the starting sample size was increased to 11 participants, assuming that 10% might drop out during the data collection. The sample size calculation was performed using G*Power software (version 3. considerable practical relevance as they are indicating that caffeine can optimize jumping performance in well-trained athletes even when a certain degree of muscle damage and DOMS are present. Thus, ingestion of anhydrous caffeine in some jump training sessions may be an interesting strategy to improve training quality and performance of these athletes".
Response: This typo error has been fixed accordingly. The text now reads "(sprinters and jumpers)". (Page 20, line 464).