An increased antioxidant status leads to the reduction of oxidative stress caused by the production of reactive oxygen species during the inflammatory process (Myburgh, 2014). Foods with high antioxidant or anti-inflammatory properties have been studied with conflicting results on recovery kinetics (Connolly et al., 2006; Howatson et al., 2009; Goldfarb et al., 2011). While some studies show a positive effect of the consumption of cherries and berries with a high content in polyphenols (Connolly et al., 2006; Bowtell et al., 2011; McLeay et al., 2012), others have failed to find significant effects of antioxidant supplementation on recovery kinetics (Bryer and Goldfarb, 2006; Bailey et al., 2011). These differences in results could be linked to the different polyphenol content of the supplementation used. A recent study showed that two supplements with the same antioxidant capacity, but with different polyphenol content, can exert contrasting effects following exercise-induced muscle damage (McLeay et al., 2012). Curcumin is rich in polyphenols and shows high anti-inflammatory and antioxidant abilities (Basnet and Shalko-Basnet, 2011) through a decrease in the expression of Nuclear Factor-Kb (NF-Kb) which is linked to the expression of pro-inflammatory genes (Singh and Aggarwal, 1995). Curcumin leads to a decrease in the activity of Cyclooxygenase-2 (COX-2) and 5-Lipoxygenase, which are pro-inflammatory enzymes (Menon and Sudheer, 2007). It also has an effect on the synthesis and the activity of the pro-inflammatory cytokine Tumor Necrosis Factor-Alpha TNF-a), and reduces the activity of m-prostaglandin E2 synthase-1 (Koeberle, 2009). Some studies have analyzed the effect of curcumin on the delayed onset of muscle soreness (Tanabe et al., 2015) or the effect of a highly bioavailable form of curcumin on the muscle function recovery kinetics (Drobnic et al., 2014) following exercise-induced muscle damage. The curcumin alone shows a very low bioavailability, however, piperine, the active component of black pepper, has been shown to enhance the bioavailability of curcumin by 2000% in humans (Shoba et al., 1998) and also presents antioxidant properties, with a radical oxygen species scavenging ability and through inhibition of lipid peroxidation (Srinivasan, 2014).
To our knowledge, no study has examined the effects of a mix of curcumin and piperine on the recovery kinetics following exercise-induced muscle damage. The aim of this study was to analyze the effects of combined curcumin and piperine supplementation on the muscle function recovery kinetics after exercise-induced muscle damage.
Sixteen elite level rugby players (age: 20.7 [+ or -] 1.4 y; height: 1.82 [+ or -] 0.06 m; mass: 89.4 [+ or -] 14.8 kg) with a weekly training volume based on rugby and gym training of 13 [+ or -] 4 h took part in this study. All participants were fully informed of the purpose, benefits and risks involved with participation before giving written informed consent. This study was conducted in accordance with the local ethical committee on biomedical research (N[degrees] 5915052012) and the standards of the declaration of Helsinki. Prior to the experimentation, players completed an assessment to verify the following inclusion criteria : (1) had not been injured during the previous 2 months, (2) were not taking any medication/drug, (3) were not participating in any session during the 2 days prior to and during the experiment, (4) were not using any recovery strategy, (5) respected the nutritional recommendations established by the investigators with the help of a dietician. According to these criteria, 6 players among the 16 initial were excluded, as they did not follow the inclusion criteria.
A randomized, balanced cross-over design was used, which comprised two phases of 4 days; these were separated by 15 days. The participants were divided into 4 groups: 1) dominant leg and curcumin + piperine supplementation, 2) non-dominant leg + curcumin + pipeline supplementation, 3) dominant leg + placebo and 4) non-dominant leg + placebo in a randomized and balanced way (Table 1). Supplementation was blinded: placebo versus pills containing curcumin and piperine (Table 1).
In the experimental condition, the participants consumed 2g of curcumin and 20mg of pipeline, 3 times a day (MGD Nature, Branderion, France), starting 48 h pre-exercise and continuing until 48 h post-exercise. This ratio enables the bioavailability of the curcumin to be increased (Shoba et al., 1998). In the control condition, the subjects were asked to take a placebo (glucose pills) at the same time of the day. The daily supplementation was divided into 3 intakes, every 6h between 8 am and 10 pm except on the exercise day. On the exercise day, the first intake was taken 45 min before the exercise, as it has been shown that with pipeline, the highest curcumin availability is reached 45 min following consumption (Shoba et al., 1998) The second intake was taken immediately after the exercise, and the last intake was consumed 6h after the second one. Fifteen days following the first session, the participants were evaluated on the other leg and in the other condition, at the same time as during the first session.
Before each session, the participants performed a cycling warm up lasting 6 minutes, with an increasing intensity standardized using the Borg perceived intensity scale (Borg, 1982): 2 minutes at a perceived intensity of "9" (very light), 2 minutes at a perceived intensity of "11" (light) and 2 minutes at a perceived intensity of "13" (somewhat hard). On the first day of each period, the participants performed baseline testing, and then the exercise task, and were evaluated on the different tests immediately after, then 24h, 48h and 72h post-exercise.
The exercise inducing muscle damage task comprised 25 repetitions over 25m of one leg jumps on an 8% downhill slope. Each repetition was separated by 90 s. The participants were asked to cover the 25m as fast as possible and to stop in a pre defined zone of 3.5m at the end of the 25m slope.
The timing of the supplementation, the tests and the exercise task are summarized in Figure 1.
For all tests, equipment calibration, participant posture, starting position and use of verbal encouragement was standardized.
Muscle function was assessed during the pre test familiarization session, just before exercise, then immediately, 24 h, 48 h and 72 h following exercise. The concentric peak torque of the knee extensors at 60[degrees] [s.sup.-1] (N x [m.sup.-1]) (CV=3.2%; ICC=0.98) and the isometric peak torque at 60[degrees] (N) (CV=5.5%; ICC=0.97) (Maffiuleti et al., 2007) were assessed on an isokinetic dynamometer (ConTrex, Duebendorf, Switzerland). The position on the isokinetic dynamometer was standardized, the athltetes were seated with a hip flexion set at 75[degrees]. Full extension of the leg was considered as 0[degrees] and the range of motion for the test was set at 90[degrees] (0-90[degrees]). The position of the distal shin pad was standardized and set at 4 cm from the external malleolus. The isometric peak torque was performed at 60[degrees] of knee flexion, following the protocol of Maffiuletti et al. (2007). The mean power developed on a one leg 6 seconds sprint (W) on a calibrated nonmotorized treadmill (Woodway Force 3.0, Waukesha, USA) was also measured (ICC = 0.94; CV = 7.9%). For this test, the subjects were asked to put the foot of the non tested leg on the side of the treadmill and to push rearward on the treadmill as fast as possible with the other...