Twenty-one untrained healthy male college students volunteered as subjects for this study. The inclusion criteria included: 1) in good health, with no chronic diseases such as hypertension, diabetes, cardiovascular disease, or elevated cerebrovascular risk; no lower limb injury; no other diagnosed diseases; 2) no regular and organized exercise training over the past 6 months; 3) no more than 2 exercise sessions per week for 30-min in the past 6 months; 4) no exercise-relevant nutrient supplements were consumed within 1 month of the study initiation; 5) no regular participation in massage or water bath to relieve fatigue and/or enhance relaxation. The study protocol was approved by the Internal Review Board of Beijing Sport University (BSU IRB). All subjects were informed of the aims of the study and were asked to sign an informed consent.
Participating subjects were asked to complete the Physical Activity Readiness Questionnaire and were measured for height and weight. They were then taken to the indoor track for the first plyometric exercise session that aimed to purposefully induce DOMS in the lower limbs. Since muscle soreness is a characteristic of DOMS, subjects were matched according to BMI and muscle soreness after exercise, and were randomly assigned to either the D-ribose (DRIB, n = 11) or the placebo (PLAC, n = 10) groups to assure group equivalence. No significant differences in BMI and muscle soreness were found between groups at baseline (Table 1). A single-blind protocol was used in this study, with subjects unaware if they were assigned to the experimental or control group.
Two weeks after the initial evaluations, all subjects were taken to the indoor track again for the same plyometric exercise session as in the first round. DRIB was supplemented with D-ribose 1-h before, 1-h, 12-h, 24-h and 36-h after exercise; PLAC was supplemented with the placebo (sorbitol, β-cyclodextrin), which was provided at the same times as DRIB. Measurement of muscle soreness, isokinetic muscle strength and venous blood-related markers were obtained 1-h before, 24-h and 48-h after exercise. Subjects were instructed to avoid vigorous exercise within 1-week before the study and to avoid consuming carbonated drinks, alcohol, caffeine and other substances that may affect the results within 2-h before the tests. Subjects were asked to avoid other training and nutritional supplementation during the study. Fig. 1 provides a summary of the experimental design.
DOMS induction protocol
Subjects were taken to the indoor track, performed a 5-min warmup jog, rested for a moment and then performed the DOMS initiating exercise. The plyometric exercise protocol used in this study to induce DOMS in the lower limbs consisted of 7 sets of 20 consecutive frog hops, with 90-s of rest between each set. The action essentials of the frog hop include: The starting position involves standing with two hands behind the head and feet shoulder-width apart, squat down, keep the torso upright and the head and chest up. Once in this position, subjects were asked to jump forward, avoiding unnecessary high jump. When the feet touch the ground, the legs are used to cushion the impact of landing (Fig. 2). A total distance of 30-m was required for each set. During the interval, subjects walked slowly back to the starting line and were not allowed to stretch. All subjects were monitored under the supervision of two experimenters. Anyone who could not continue to exercise after being encouraged was classified as fatigued and his exercise was terminated. All subjects completed two full plyometric exercise sessions according to instructions.
Subject muscle soreness was assessed by the same experimenter 1-h before, 24-h and 48-h after exercise using the Visual Analogue Scale (VAS), which is a Likert-based scale identifying the severity of soreness [35,36,37]. The scale has a total of 10 points, with ‘0’ indicating no soreness and ‘10’ indicating extreme soreness. To familiarize with the VAS scale, subjects were provided with an unmarked VAS prior to evaluation. Then they walked a distance at a regular speed. Before they stopped walking, subjects indicated the point on the scale that corresponded to the soreness they felt while walking.
Isokinetic muscle strength
The test was measured 1-h before exercise, and again 24-h and 48-h after two plyometric exercise sessions. The knee flexor-extensor muscles of the dominant leg were measured at an angular velocity 60°/s with an IsoMed 2000 dynamometer (D&R Ferstl GmbH, Hemau, Germany). All measures were obtained by the same researcher and were in accordance with the operation manual . To assess range of motion, subjects sat on a seat with 75° hip flexion, with range of motion set to reach from 10 to 90°. The knee articulation axis was the same as the dynamometer mechanic arm lever axis. A Velcro strap was fixed to the distal portion of the tibia and the length of the dynamometer arm varied according to the length of the subject’s leg. After the subject was fixed on the training seat, the weight of the tested leg in a relaxed state at terminal extension was measured and a gravity correction was made. Subjects exercised 5 times with submaximal intensity to familiarize themselves with the test process. Then they were asked to perform 5 reciprocal maximum flexion extension repetitions. Strong verbal encouragement and visual online feedback were provided during testing to assure maximum effort. Flexor peak torque (FPT), extensor peak torque (EPT), flexor total work (FTW) and extensor total work (ETW) were recorded.
At the 5 time points of 1-h before, 1-h, 12-h, 24-h, 36-h after exercise, DRIB was supplied with 15 g of D-ribose (Cheng Zhi Life Science Co., Ltd., Beijing, China) and PLAC with supplied with the same dose and taste of a calorically equivalent sorbitol and β-cyclodextrin containing beverage. Supplements were powders, which were mixed in 200 ml of drinking water for oral administration.
Blood samples (5 ml) were obtain from the ulnar vein 1-h before, 24-h and 48-h after exercise. The samples were collected in gel serum tubes (Vacuette, Frickenhausen, Germany) and centrifuged at 3500 rpm in an Eppendorf Centrifuge 5417R (Eppendorf AG, Hamburg, Germany) for 10-min at 4 °C to obtain serum, which was stored at − 80 °C for future analysis.
Blood samples were analyzed for creatine kinase (CK), lactate dehydrogenase (LDH), myoglobin (MB), superoxide dismutase (SOD), total antioxidant capacity (T-AOC) and malondialdehyde (MDA). CK, LDH, SOD, T-AOC and MDA were assessed in duplicate using commercially enzymatic kinetic kits (Nanjing Jiancheng Bioengineering Institute, Jiangsu, China) with a visible 721 spectrophotometer (Shanghai optical instrument factory, Shanghai, China). Coefficients of variation were 1.5, 1.5, 5.1, 3.6, 2.3%, respectively. MB was determined in duplicate using commercially available ELISA kits (Nanjing Jiancheng Bioengineering Institute, Jiangsu, China). Coefficients of variation was 9.9%. Each completed assessment was performed according to manufacturer instructions.
Data were analyzed using SPSS for windows, version 19.0 (SPSS Inc. Chicago, IL, USA), and presented as means and standard deviations. One-way ANOVA was performed on VAS score between exercise sessions. Two-way repeated measures ANOVA was performed on all markers before, 24 h and 48 h after exercise between groups and within groups. Data were analyzed using absolute changes from before exercise (pre) ± 95% confidence intervals (CIs); If 95% CI didn’t cross 0 (i.e., 0 change), it was considered significant.