Mechanical and metabolic responses during high-intensity training in elite 800-m runners

1 MECHANICAL AND METABOLIC RESPONSES DURING HIGH INTENSITY TRAINING IN ELITE 800-M RUNNERS 2 ABSTRACT The purpose of this study was to describe the mechanical and metabolic responses of a typical high-intensity training session in high-level 800-m athletes. Nine male highlevel 800-m athletes (personal best 1:43-1:56 min:ss) performed a typical high-intensity interval training session consisting in 5x200-m, with 4 min rest. Countermovement jump and blood lactate were measured at rest and after each running bout. Running times, ground contact times and stride length were also measured. Running times and lactate (p<0.01) progressively increased from the first to the last running bout. Jump height (p<0.01) and stride length (p<0.05) progressively decreased from the first running bout to the last. A significant negative relationship (p<0.001; r =-0.83) was found between the individual values of jumping height and blood lactate concentration; and a significant positive relationship (p<0.01; r=0.67) was observed between the time in 200-m and the contact times. In conclusion, the results demonstrated that the typical training session performed by 800-m athletes produced a high level of fatigue showed up by significant alterations on the mechanical and metabolic response. The impairments observed on the mechanical and metabolic parameters may indirectly reflect a state of energy deficit of the muscle contractile machinery and a reduction on the force-generating capacity. Keywords: interval-training; performance; lactate; countermovement jump; middledistance running. 3 INTRODUCTION The 800-m training entails a big challenge to the middle-distance coaches, since it is an extremely demanding event that requires substantial contributions from both aerobic and anaerobic systems due to high relative values of oxygen uptake (VO2max) (64-72 mL·kg-1·min-1) [1,2] and high blood lactate concentrations (above 15 mmol·L-1) attained after the run [1,3,4]. In this line, other measures of anaerobic work capacity and muscle strength, such as vertical jump, leg strength and sprint, have also been shown as being related to 800-m event [5,6]. Although middle-distance races are run at a lower speed than sprint races, the ability to maintain force and power production in high levels following a sustained period of high-intensity exercise is of great importance and might be a differentiating factor for elite athletes’ performance [7]. For 800-m races, near maximal running velocities were registered during the first 200-m of the race [8]. Also, in the last decade a changing on the run strategy has been observed during the men’s championship 800-m event, showing predominantly a ‘gun-to-tape’ type race tactic in the finals, which means running at a high pace from the start of the race to the end [9]. This brings important questions with respect to the physiological and mechanical requirements for this event, and so for the training methods employed. It is known that high-intensity interval training (HIIT) represents a high percentage of total training content in 800-m runners [10]. This type of training allows: 1) improving VO2max, 2) improving anaerobic function, and 3) targeting and developing responses and adaptive comfort at race-specific speed, with specific emphasis on achieving adaptations at the neuromuscular level [10]. In terms of energy status, two main exercise modes of intermittent exercise can be differentiated [11,12]: (a) those characterized by blood lactate levels not exceeding 8–12 mmol·L-1, while energy status 4 and maximal running velocity are maintained; and (b) those characterized by a continuous increase in blood lactate that may reach values up to 13–30 mmol·L-1 at the end, leading to a significant decrease in maximal running velocity or muscle-force generating capacity. High-intensity training with short intervals (60–300m) of heavy intensity exercise (110–120% of the individual best 800-m record time) separated by recovery periods of varying length (2–10 min) are habitually used by elite 800-m athletes during training sessions. The metabolic responses of an exercise are usually monitored by measuring blood lactate and ammonia, since they are considered to indirectly reflect the degree of anaerobic glycolysis activation [13], and muscle ATP depletion [14], respectively. On the other hand, the mechanical response is usually monitored by vertical jump tests [15,16] or kinematic parameters such as: stride length, stride frequency, contact times [17-19]. In this line, loss of running velocity and countermovement jump (CMJ) height are considered as good markers of fatigue during sprint training [15,16]. Previous research has described metabolic and neuromuscular performance characteristics of several typical training sessions in different groups of track & field athletes: sprinters [15,16], 400-m runners [20], and 800-m runners [21]. However, limited scientific-based information is available concerning the metabolic and mechanical responses to different typical training sessions performed by 800-m athletes. A scientific-based approach to begin building optimal and effective training methods for 800-m running performance is to describe and to analyze the mechanical and metabolic responses of currently accepted training sessions, as used by elite athletes. The purpose of this study was, therefore, to describe the acute effects of a typical high-intensity 5 running training session on lactate concentration, vertical jump height, and kinematic stride changes in high-level 800-m athletes. A secondary purpose of this study was to investigate the relationships between changes in jump and sprint parameters and blood lactate concentration observed during the training session. METHODS Subjects Nine male athletes (n=9), who were specialists in 800-m at national and international level (with personal best ranging from 1:43 to 1:56 min:ss), participated in this study (age: 22.0±5.5 years; height: 178.7±3.9 cm; body mass: 64.8±5.5 kg). All participants were injury free and were familiarized with the testing exercises. The subjects were highly trained (2–4 h/day, 4–6 days/week, for 5–10 years). At time of the present study, 2 of the athletes had participated in the Olympic Games and other 2 in other international competitions. Before participating, the athletes and coaches were fully informed about procedures, potential risks and benefits of the study and they all signed written informed consent prior to the tests. The study was conducted in accordance with the Declaration of Helsinki II and approved by the Local Ethics Committee. The study meets the ethical standards of the journal [22]. The distance and recovery time set in our study were devised in consultation with coaches to represent training sessions usually carried out by 800-m runners during the pre-competition or competition phase, in order to have practical applicability. Design A cross-sectional experimental design was used. Athletes performed a typical highintensity interval training session consisting in 5x200-m, with 4 min rest between bouts, 6 in an outdoor track. Table 1 shows the characteristics of the training session. For each athlete, the average relative intensity of each running bout was expressed as a percentage index of the average velocity sustained during his best 800-m running performance in the same period of the season (%V800). The average of the relative intensity performed during the 5x200-m was 114.6±2.0 %V800, which means that the 200-m bouts were not performed all-out. The study was conducted during the precompetition or competition phase of their yearly program, at a time when most of the athletes were at a high level of competitive fitness. At the time of these observations, the track athletes had completed between 3 and 4 months of training. (Table 1 about here) Testing Procedures Participants were asked to abstain from any strenuous physical activity for at least 1 day before the training session. The training session was performed at the same time of the day for all subjects and under similar environmental conditions (20ºC and 60% humidity, approximately) on an outdoor 400-m Tartan track certified by the Spanish Track and Field Federation. Before the 5x200-m training protocol, the subjects had a light 25 min warm-up protocol consisting of 10 min of jogging at a self-selected easy pace, 5 min of joint mobilization exercises, 4x50m running accelerations at 80%, 85%, 90%, 95% perceived effort, 1x50m sprint at 100% effort, and 1x100m at 80% effort with 2 min rest periods between them. After general warm-up, a specific warm up was carried for the CMJ measurement: 10 squats without an external load, 5 CMJ progressive in intensity, and 3 maximal CMJ. Running bouts were interposed with rest periods of 4 min, consisting of walking or standing in a resting position. The subjects performed 5 maximal voluntary CMJ separated by 30 s at rest and 2 CMJ separated by 7 10 s immediately after each running bout. Blood lactate was drawn at rest and 1.5 min after each running bout. Blood lactate concentration Five µL of whole blood from the finger were measured with a portable lactate analyzer (Lactate Pro 2, Arkray, Kyoto, Japan). Countermovement jump An infrared timing system (OptojumpNext, Microgate, Bolzano, Italy) was used for determining jump height. The CMJ was performed with both hands on the waist, while performing a downward movement until about 90º of knee flexion followed by a maximal vertical jump. All participants were instructed to land in an upright position and to bend the knees after landing. Test–retest reliability measured by the coefficient of variation (CV) was 2.1% and the intraclass correlation coefficient (ICC) values were 0.99 (95% confidence interval, CI: 0.98‒0.99). 200-m Sprint times Sprint times were measured using photocells (Witty, Microgate, Bolzano, Italy). Runs were performed from static biped start position with the start line located 1 m behind the start photocell. Athletes were asked to perform each running bout at their own pace according to a mean relative intensity adjusted individually to their best seasonal 800-m performance, which allowed them to complete all the bouts getting the best mean times. Sprint kinematics 8 Ground contact time and stride length were measured for several steps during the last meters of each running bout using an optical measurement system (Optojump-Next; Microgate). The measurement system was connected in series over 10m distance (from the distance of 175-m to 185-m of the total 200-m). This system is developed to measure with 1 kHz precision all flight and ground contact times while running. Ground contact time was defined as the time from when the foot contacts the ground to when the foot toes off the ground, and it was determined by the disruption of the infrared gates of the system. This system has been shown to determine accurately these variables before [23]. The ICC for averaged biomechanical variables measured ranged from 0.94 to 0.97, which shows a high reliability of the data. The CV was 1.9%. The average value of ground contact time and stride length of the 4-5 steps registered for each running bout of each athlete was calculated. These kinematic stride variables were measured in 6 of the 9 athletes. Statistical Analysis All data are reported as means ± standard deviation (SD). Test-retest absolute reliability was measured by the CV, whereas relative reliability was assessed by the ICC and CI at 95% was calculated using the one-way random effects model. The normal distribution of the data was verified with the Shapiro-Wilk test. Statistical analyses for changes throughout the session were assessed using one-way repeated measures analysis of variance (ANOVA). The main effects were compared using the post hoc Bonferroni method. Pearson’s coefficients (r) were used to calculate the respective correlations. Statistical significance was set at p≤0.05. Statistical analyses were performed using SPSS for Mac (version 20.0, SPSS Inc., Chicago, IL, USA). 9 RESULTS Description of the evolution of running times, lactate concentration, and height in vertical jump during the training session Fig. 1 shows the individual and mean values for running times (1a), lactate concentration (1b), and height in vertical jump (1c) during the training session. Running times progressively increased from the first to the last running bout. No significant differences between running bouts were observed, however, an almost significant difference between the 1st and the 3rd running bout (p=0.07), and between the 2nd and the 4th running bout (p=0.09) was observed. Blood lactate concentration presented a similar pattern in all the athletes, increasing progressively from the first running bout to the last, showing a less marked increase from the 4th to the 5th bout. Significant differences in lactate concentration were observed between all running bouts (p<0.01), except between the 3rd and 4th bout, and the 4th and 5th bout. Jump height progressively decreased from the first running bout to the last, observing a marked decrease from Bout 3. Significant differences in CMJ were observed between the rest value and all running bouts (p<0.01), and on the 4th bout with respect to the 2nd (p<0.01) and the 3rd bout (p<0.01). 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Significant differences between running bouts: ***: p<0.001 (respect to 1st running bout); ##: p<0.01; ###: p<0.001 (respect to 2nd running bout); g: p<0.05; gg: p<0.01 (respect to 3rd running bout); ww: p<0.01 (respect to rest). Fig. 2. Individual and mean values for contact times (2a) and stride length (2b) during the training session. Significant differences between running bouts: #: p<0.05 (respect to 2nd running bout); g: p<0.05 (respect to 3rd running bout). Fig. 3. The relationship between the individual values of jumping height (expressed as a percentage of the individual maximum jumping height values attained during the training session) and the individual values of blood lactate concentration. Fig. 4. The relationship between the time in 200-m (expressed as the % of the best running bout time) and the contact times (expressed as the % of the best contact time value). TABLES Table 1. Descriptive characteristics of the training protocol performed for each athlete (5x200 m with 4 min rest).