and all subjects performed extensive familiarization trials be- fore baseline testing. METHODS Subjects Sixteen healthy individuals volunteered to take part in the experi- ment (Table 1). Eight subjects (2 women) were assigned to a training group and performed exercise tests before and after a 2-wk sprint training intervention. Eight other men served as a control group and performed the exercise performance tests 2 wk apart with no training intervention. We also obtained needle biopsy samples from the training group to examine potential training-induced adaptations in resting skeletal muscle. We did not obtain biopsies from the control group for ethical reasons, because other studies have shown no change in resting muscle metabolite concentrations or the maximal activities of mitochondrial enzymes when control subjects are tested several weeks apart with no sprint training intervention (1, 28). All subjects were recreationally active individuals from the McMaster University student population who participated in some form of exercise two to three times per week (e.g., jogging, cycling, aerobics), but none was engaged in any sort of structured training program. After routine medical screening, the subjects were informed of the procedures to be employed in the study and associated risks, and all provided written, informed consent. The experimental protocol was approved by the McMaster University and Hamilton Health Sciences Research Ethics Board. Preexperimental Procedures Before taking part in any experimental trial (i.e., before baseline measurements), all subjects performed familiarization trials to become oriented with all testing procedures and training devices. Specifically, all subjects performed 1) a VO2 �� peak test 2) a ���practice ride��� to establish a workload that elicited 80% of VO2 �� peak and 3) an endurance capacity test that consisted of cycling to volitional fatigue at 80% of VO2 �� peak on at least two separate occasions. Details of Exercise Performance Tests VO2 �� peak test. Subjects performed an incremental test to exhaustion on an electronically braked cycle ergometer (Excalibur Sport V2.0, Lode, Groningen, The Netherlands) to determine VO2 �� peak using an online gas collection system (Moxus modular oxygen uptake system, AEI technologies, Pittsburgh, PA). The initial three stages of the test consisted of 2-min intervals at 50, 100, and 150 W, respectively, and the workload was then increased by 25 W every minute until voli- tional exhaustion. The value used for VO2 �� peak corresponded to the highest value achieved over a 30-s collection period. Cycle endurance capacity test. Subjects cycled to volitional ex- haustion on an electronically braked cycle ergometer (Lode) at a workload designed to elicit 80% of VO2 �� peak . All performance trials were conducted in the absence of temporal, verbal, or physiological feedback. The test was terminated when pedal cadence fell below 40 rpm (according to the manufacturer���s specifications, the power output displayed may not have been valid below this cadence), and exercise duration was recorded. Expired breath samples for the determination of ventilation rate, oxygen uptake, carbon dioxide production, and respiratory exchange ratio were collected and averaged over the 6- to 10-min period of exercise. Reproducibility of exercise performance tests. Ten individuals who were not subjects in the present study performed a VO2 �� peak test and cycle endurance capacity test on separate days at least 1 wk apart, and method error reproducibility was calculated as described by Sale (35). The coefficient of variation for the VO2 �� peak test and cycle endurance capacity test was 3.7 and 12.0%, respectively. Experimental Protocol The experimental protocol consisted of 1) baseline testing (i.e., after familiarization procedures described above) 2) a 2-wk sprint training intervention or similar period without sprint training (control group) and 3) posttesting, as described further below. Baseline testing. Baseline measurements for all subjects consisted of a VO2 �� peak test and a cycle endurance capacity test. Each baseline test was conducted on a separate day with 24 h between tests. Subjects in the training group also underwent a muscle biopsy procedure 3 days after the baseline cycle endurance capacity test and several days before the start of the training intervention. For the biopsy procedure, the area over the lateral portion of one thigh was anesthetized (2% lidocaine, AstraZeneca Canada, Ontario, Canada), and a small inci- sion was made through the skin and underlying fascia to permit a tissue sample (50���100 mg) to be obtained from the vastus lateralis muscle (1). Details regarding the experimental protocol are summa- rized in Fig. 1. Training. Training was initiated 3���5 days after the baseline muscle biopsy procedure and consisted of six sessions of sprint interval training spread over 14 days. Each training session consisted of repeated 30-s ���all-out��� efforts on an electronically braked cycle ergometer (Lode) against a resistance equivalent to 0.075 kg/kg body mass (i.e., a Wingate test). Subjects were instructed to begin pedaling as fast as possible against the ergometer���s inertial resistance, 2 s before the appropriate load was applied by a computer interfaced with the ergometer and loaded with appropriate software (Wingate soft- ware version 1.11, Lode). Subjects were verbally encouraged to continue pedaling as fast as possible throughout the 30-s test. Peak power, mean power and fatigue index were subsequently determined using an online data acquisition system. During the 4-min recovery period between tests, subjects remained on the bike and either rested or were permitted to cycle at a low cadence ( 50 rpm) against a light resistance ( 30 W) to reduce venous pooling in the lower extremities and minimize feelings of light-headedness or nausea. The training protocol consisted of exercise performed three times per week on alternate days (i.e., Monday, Wednesday, Friday) for 2 wk. The number of Wingate tests performed each day during training increased Table 1. Subject characteristics Training Group Control Group Age, yr 22 1 25 2 Weight, kg 83 5 79 2 Height, cm 180 4 180 2 VO2 �� peak, ml kg 1 min 1 44.6 3.2 46.4 1.4 Values are means SE for 8 subjects. VO2 �� peak, peak oxygen uptake. Fig. 1. Overview of experimental protocol. VO2 �� peak, peak oxygen uptake PRE, preexercise POST, postexercise SIT, sprint interval training. Numbers in boxes denote number of Wingate tests completed during each of 6 training sessions over a 2-wk period. 1986 ADAPTATIONS TO SHORT SPRINT INTERVAL TRAINING J Appl Physiol ��� VOL 98 ��� JUNE 2005 ��� www.jap.org on June 6, 2005 jap.physiology.org Downloaded from

from 4 to 7 over the first five training sessions, and on the final session subjects completed four intervals, as summarized in Fig. 1. Posttesting. A second muscle biopsy sample was obtained 3 days after the final training session to examine training-induced changes in resting muscle, and a second battery of performance tests was initiated 2 days after the biopsy procedure (Fig. 1). The control group per- formed a second set of tests 2 wk after the baseline tests. The nature of the posttesting exercise performance measurements were identical in all respects to the baseline tests. Dietary Controls In an attempt to minimize any potential diet-induced variability in exercise metabolism and the resting metabolic profile of skeletal muscle, subjects were instructed to consume the same types and quantities of food during the baseline and posttesting phases. The subjects in the training group were particularly encouraged to keep their diet as similar as possible during the 24 h before the pre- and posttraining biopsy procedures. Subjects were asked to record all food intake during these periods, and compliance was assessed by perform- ing dietary analyses on the individual food records maintained by the subjects. Pre- and posttraining food diaries were analyzed for total energy intake and proportion of energy derived from carbohydrates, fats, and protein (Nutritionist Five, First Data Bank, San Bruno, CA). These analyses confirmed that there was no difference between trials in the total amount of energy consumed or macronutrient proportions. Muscle Analyses On removal from the leg, each muscle biopsy sample was imme- diately frozen by plunging the biopsy needle into liquid nitrogen. The samples were subsequently divided into two pieces while still frozen, and one piece was kept in liquid nitrogen for the determination of muscle enzyme activities. The remainder of each sample was freeze- dried, powdered, dissected free of blood and connective tissue, and stored at 86��C before metabolite analyses. Citrate synthase. Frozen wet muscle samples were initially homog- enized using methods described by Henriksson and Reitman (17) to a 50-fold dilution. The maximal activity of citrate synthase was deter- mined on a spectrophotometer (Ultrospec 3000 pro UV/Vis) using a method described by Carter et al. (6). The intra-assay coefficient of variation for the citrate synthase assay, based on 10 repeats of the same sample, was 4.9%. Protein content of the homogenate was determined by the method of Bradford (5) using a commercial assay kit (Quick Start, Bio-Rad Laboratories, Hercules, CA), and enzyme data are expressed as moles per kilogram of protein per hour. Metabolites. An aliquot of freeze-dried muscle was extracted on ice using 0.5 M perchloric acid (containing 1 mM EDTA), neutralized with 2.2 M KHCO3, and the resulting supernatant was used for the determination of all metabolites except glycogen. ATP, phosphocre- atine and creatine were measured using enzymatic assays adapted for fluorometry (Hitachi F-2500, Hitachi Instruments, Tokyo, Japan) (15, 31). For glycogen analysis, an 2-mg aliquot of freeze-dried muscle was incubated in 2.0 N HCl and heated for 2 h at 100��C to hydrolyze the glycogen to glucosyl units. The solution was subsequently neu- tralized with an equal volume of 2.0 N NaOH and analyzed for glucose by using an enzymatic assay adapted for fluorometry (31). The intra-assay coefficient of variation for all muscle metabolite assays, based on 10 repeats of the same sample, ranged from 2 to 3%. All muscle metabolite measurements were corrected to the peak total creatine concentration for a given subject. Statistical Analyses All exercise performance data were analyzed by using a two-factor repeated-measures ANOVA. For the single Wingate test, endurance capacity test and VO2 �� peak test, the factors were trial (pretraining, posttraining) and condition (training, control). For the comparison of power output during the first vs. last sprint training session (training group only), the factors were trial (pretraining, posttraining) and sprint bout (1���4). All muscle data were analyzed using paired (2-tailed) t-tests. The level of significance for analyses was set at P 0.05, and significant interactions and main effects were subsequently analyzed using Tukey���s honestly significant difference post hoc test. All data are presented as means SE. RESULTS Cycle endurance capacity. After training, the individual improvements in cycle endurance capacity ranged from 81 to 169% compared with baseline, with the exception of one subject (16% decrease) who, on completion of the study, disclosed that he had sustained a minor ankle injury (unrelated to the experiment) on the day before his posttraining ride. Even with the inclusion of this subject���s data (Fig. 2), the mean increase in cycle endurance time to fatigue for the training group (n 8) was 100% compared with baseline (51 11 vs. 26 5 min P 0.05), and this was higher (P 0.05) compared with the control group, who showed no change in performance (Fig. 2). Oxygen uptake during exercise was not different between the first and second rides in either group however, expired ventilation (posttraining: 91 7 vs. pretrain- ing: 104 9 l/min) and respiratory exchange ratio (posttrain- ing: 1.18 vs. pretraining: 1.24) were lower (P 0.05) after training in the sprint-training group (P 0.05). VO2 �� peak did not change in either group over the course of the study. Anaerobic work capacity. Peak power output during each of the four consecutive Wingate tests performed during the last (sixth) training session was higher (P 0.05) compared with the first training session (Fig. 3). However, fatigue index was also higher (P 0.05) posttraining, and thus there were no differences in mean power output for each of the four Wingate tests during the first compared with the last training session. Citrate synthase activity and resting muscle metabolite con- centrations. The maximal activity of citrate synthase increased (P 0.05) by 38% after training (Fig. 4). Resting muscle glycogen concentration increased (P 0.05) by 26% after training (Fig. 5) however, there were no training-induced changes in the resting muscle concentrations of ATP, phos- phocreatine or creatine (Table 2). DISCUSSION The primary novel finding from the present study was that six bouts of sprint interval training performed over 14 days Fig. 2. Cycle endurance time to fatigue before and after a 2-wk sprint training protocol (training group SIT) or equivalent period without training (control Con). Values are means SE for 8 subjects. Individual data are also plotted for all subjects in each group. *P 0.05. 1987 ADAPTATIONS TO SHORT SPRINT INTERVAL TRAINING J Appl Physiol ��� VOL 98 ��� JUNE 2005 ��� www.jap.org on June 6, 2005 jap.physiology.org Downloaded from