Introduction

Inline speed skating at the World Championships is divided into two major disciplines; track and road racing. Official distances of track races range from 300 m to 15,000 m and road races range from 200 m to 42,195 m. Both track and road races require high intensity bouts of sprinting, interspersed with short periods of low-to-moderate intensity “rest” periods.1 Success in inline speed skating has been attributed to powerful starts and high rates of ATP re-phosphorylation via non-oxidative (“anaerobic”)2, 3 and oxidative metabolism.4, 5 As such, inline speed skaters require well-developed anaerobic and aerobic energy systems.2, 5, 6

Inline speed skaters are required to obtain peak performance several times over an annual training. Training programs capable of increasing aerobic metabolism are based mainly on periods of at least 6 weeks and such programs are often based on continuous endurance training,7 in these cases, low-volume HIIT may represent an alternative to endurance training to improve aerobic and anaerobic performance in a short time frame.13

High-intensity interval training (HIIT) is a potent training stimulus to improve anaerobic and aerobic energy systems8, 9, 10, 11 over a short time period (e.g., 6 sessions over 2 weeks12, 13). HIIT performed with various intensities (

) have been used to improve performance in athletes from a wide range of sports;10, 14 however, to date information regarding the effects of HIIT on performance in female inline speed skating athletes is limited. Furthermore, Because of the paucity of data that relate directly to our understanding of the physiological and performance adaptations that occur following high-intensity interval training in trained athletes,14 it is unclear which intensity of HIIT is more effective for improving required adaptations of female speed skaters in short period. Accordingly, the aim of the present study was to examine the effects of three different HIIT protocols with different intensities (100, 115, and 130% velocity at []) compared to traditional endurance training (TET) over 3 weeks on physiological, hematological and performance adaptations in trained female inline speed skaters. The hypothesis is that traditional endurance training will show less training effects on aerobic and anaerobic performances than HIIT. In addition, we hypothesize that HIIT with intensity equivalent to would induce greater physiological and performance adaptations compared to higher intensities.

Material and methods Participants

Twenty-eight female inline speed skaters (mean ± SD; age: 20 ± 4 years; height: 160.5 ± 13 cm; body mass: 59.5 ± 13 kg) volunteered for the study. All subjects had a minimum of 3 years of inline speed skating experience (years’ experience: 7 ± 3 years), competed at the national level, and currently trained at least 3 times per week. Prior to any participation, the experimental procedures and potential risks were explained fully to the subjects and all provided written informed consent. All the procedures were in accordance with ethical principles of declaration of Helsinki, approved by ethical committee of local University.

Experimental protocol and procedures

The experimental procedures consisted of a familiarization phase (including 3 sessions to familiarize the participants to the equipment and protocols and to reduce any learning effect), followed by pre-testing, then 3 weeks of treadmill training, and then post-testing (Figure 1). The training was conducted on a treadmill because during the winter month's weather conditions make sport specific training difficult,5 as such inline speed skaters often cross train on treadmills, which are highly accessible. Pretesting of aerobic and anaerobic performances, along with hematological parameters, was conducted before the beginning of the preseason phase of the athletes’ yearly training program. Before and after the training programs, participants reported to the laboratory on three occasions, each session was separated by 48 hours. The first day of testing included an incremental test to exhaustion to determine maximum oxygen uptake (

), anaerobic threshold (AT ()), and time to exhaustion at (Tmax). The second day included a 30-second lower-body Wingate test to determine peak power output (PPO) and mean power output (MPO) on an electrically braked ergometer. The third day consisted of a sport specific 300 m and 3000 m time trial on a 200 m skating track.

Figure 1. Overview of experimental protocol. PRE, pre-exercise; POST, post-exercise; HIIT, high-intensity interval Training; TET, traditional endurance training; wk, week; d, day; S/wk, sessions/week; B/S, bouts/session.

Following pre-testing participants were randomly assigned to one of four groups. Participants completed either a progressive HIIT program or TET program three sessions per week. These protocols were modified from previous research using similar training intensities and durations.7, 15 Two days after the completion of the last training session participants repeated the same battery of tests in the same order and under similar conditions as pre-testing. Dietary food records were collected two days before pre testing and this diet was replicated prior to post testing, to reduce any dietary influence on performance.

Procedures

Graded exercise test. In order to determination of

, , O2 pulse and AT, participants performed an incremental treadmill test consisting of a 3 min walking warm-up at 6 km h−1 with 0% incline, followed by 1 km h−1 increments each minute until volitional exhaustion.7, 16, 17 Metabolic variables were measured using a gas analyser (Cosmed K4B2, Italia) that was calibrated before, and verified after each test according to the manufacturer's instructions. was confirmed when 3 or more of the following criteria were met: (1) a plateau in despite an increase in running speed; (2) a respiratory exchange ratio (RER) higher than 1.1; (3) a heart rate within 10 bpm of its predicted maximum; and/or (4) volitional exhaustion. was recorded as the minimal velocity at which the athlete was running when occurred.7, 17, 18 O2 pulse was determined by dividing the absolute by the corresponding heart rate. Anaerobic threshold () was determined using the V-Slope method.7, 19

Determination of Tmax. For determination of Tmax, after a 10 min warm up on a treadmill (Techngym, DAK9EC, Italia), speed was increased to

and participants were verbally encouraged to run as long as possible. Time to exhaustion at (Tmax) was recorded.16 The post-HIIT Tmax test was completed at the same speed as the pre-HIIT Tmax test under the same conditions.9

Anaerobic power. Peak power output (PPO) and mean power output (MPO) were assessed by a 30-second all-out effort (Wingate test) on a cycle ergometer (894E, Monark, Sweden) against a resistance equivalent to 0.075 kg kg−1 body mass.20 The subjects’ feet were firmly strapped to the pedals, and the seat height was adjusted for optimal comfort and pedaling efficiency. Participants reached maximum pedaling velocity against the ergometer's inertial resistance over 2 seconds before the full load was applied and the electronic revolution counter was activated. Participants were verbally encouraged to continue pedaling as fast as possible throughout the 30-second test. The 5 second PPO, and 30 second MPO were subsequently determined using a data-acquisition system.7, 16, 17

Blood sampling. For hematological measurements, blood was drawn by venepuncture and dispensed into EDTA tubes. Red blood cell (BBC), hemoglobin (Hb), hematocrit (Hct), and mean corpuscular hemoglobin (MCH) counts were measured using an automated cell counter (Diatron, Abacus C, Hungary).

HIIT and TET protocols. The study consisted of four training interventions (Figure 1). The training sessions of each HIIT group (H100, H115, H130) consisted of 60 seconds of running performed at

(i.e., H100), (i.e., H115), and (i.e., H130). Training volume (bouts/session) increased each week (6 [1st week], 8 [2nd week], 10 [3rd week] bouts/session) in all three HIIT groups. In all HIIT groups, recovery between intervals was set at 3 min. The participants in the TET group performed 3 sessions per week consisting of 60 min at an intensity equivalent to .9 All subjects were instructed to not engage in strength training or change their diet for the duration of the study.21 All training sessions for all groups were directly supervised by a study investigator.

Statistical analysis

Results were expressed as means ± SD. The Shapiro–Wilk's test was used to test the normality of the distribution. A 2 (pre, post) by 4 (groups) repeated-measures ANOVA compared changes in the dependent measures over time and between groups. A Tukey's post hoc test compared differences between groups when a significant F-ratio was observed. Statistical analyses were completed using Statistica, version 8.0 (StatsSoft Inc., Tulsa, OK), with α set at ≤ 0.05.

Results

Changes in physiological variables are presented in Table 1. Following training

significantly increased in the H100 (p = 0.01) and H115 (p = 0.04), but not in the H130 and TET groups with no difference among groups. HIIT increased in H100 (p = 0.04), H115 (p = 0.05) and H130 (p = 0.03); however, there was no significant change over time following TET (p > 0.05). No differences were found among groups in . Also, no significant changes took place in O2 pulse and AT () to all training protocols.

PPO was not significantly different among groups, but significantly improved in H100 (p = 0.03) and H115 (p = 0.05); despite no changes following H115, H130, and TET. MPO was significantly improved in H100 (p = 0.01), H115 (p = 0.01) and H130 (p = 0.01) and remained unchanged following TET (p > 0.05). No significant differences were observed among groups in PPO and MPO.

The 3000-m time trial performance was significantly improved following H100 (p = 0.05), H115 (p = 0.04), and H130 (p = 0.004); while there was no significant change following TET (p > 0.05) (Table 2) with no difference among groups. There was no significant interaction, time or group effect for the 300 m time trial (Table 2). Tmax significantly increased following H100 (p = 0.01), while did not improve following H115, H130, and TET.

There was no significant interaction, time, or group effects for any of the hematological responses (RBC, Hb, Hct, and MCH) (Table 3).

Discussion

This is the first study to examine HIIT in trained female inline speed skaters. The main findings of the present study support our hypothesis that HIIT would provide higher improvements in aerobic and anaerobic performance changes compared to endurance training. Interestingly, HIIT performed at 100 and 115%

provided superior cardio-respiratory responses following 3 weeks of training.

In the present study, HIIT at 100 and 115% of

(i.e. H100 and H115) were able to statistically increase , which support previous findings.7, 10, 22 Interestingly there was no change in in the group training at 130% of and the TET group, suggesting that there may be an optimal training intensity. Improvement in may be attributed to an increase in the delivery of oxygen (i.e., increases in stroke volume) to exercising skeletal muscles and or an increase in oxygen utilization.9, 10, 16 O2 pulse is an indirect measure of stroke volume16 which was not altered in any of the groups in the present study. On the other hand, no significant change in blood volume was observed in the HIIT groups. RBC and Hb did not increase for any of the groups, indicating no change in oxygen-carrying capacity with training. Hence, blood volume and oxygen-carrying capacity of the blood do not seem to explain the changes in in this experiment. This supports the studies of Farzad et al.16 and Laursen et al.21 who reported no change in hematological variables and plasma volume in response to a short-term HIIT. These results suggest that HIIT may have increased oxygen extraction due to an increase in peripheral rather than central adaptations.10, 22 To support of this, Gibala and McGee23 found an increase in muscle oxidative capacity (assessed using the maximal activity or protein content of mitochondrial enzymes such as citrate synthase and cytochrome oxidase) ranging from approximately 15% to 35% following six sessions of HIIT over 2 weeks indicating peripheral adaptations are likely responsible for enhanced consumed oxygen following HIIT performed in short period of time. In this regard, MacPherson et al.22 demonstrated that post-training enhancements in stroke volume and maximal cardiac output (Qmax) were only observed following endurance training, and they concluded that improvement following HIIT are due to peripheral adaptations. Mechanistically, HIIT activates AMP-activated protein kinase (AMPK) and p38 mitogen-activated protein kinase (MAPK) which are both known to activate peroxisome proliferator-activated receptor co-activator (PGC-1α) and have been implicated in exercise-induced mitochondrial biogenesis.13 PGC-1α coactivates transcription factors to activate signaling pathways leading to mitochondrial and metabolic adaptations.13 A limitation of the present study was no muscle biopsies were taken to directly determine muscle oxidative capacity. Future research examining oxidative capacity in trained female athletes is warranted.

Velocity at

significantly increased following 3 weeks of training in the present study. These findings are in accordance with previous research reporting an improvement in (3 to 10%) after HIIT in subjects of varying aerobic capacities.9, 10, 11 Beside enhanced , improvements in anaerobic capability,24 running economy9 muscle power,25 and neuromuscular skill,25 may be responsible for the improvement in .

In the present study 3000 m skating performance was significantly improved after only 3 weeks of HIIT. This finding was in line with those of previous researches found improvements in 3000 m running time trial,9, 11 and 2000 m rowing time trial8 following HIIT. It has been shown that over 92% of the variance in performance is related to the

at AT and muscle capillary density.27 However, the present study found no significant change in AT in any training group when expressed as . These results suggest that the precise mechanisms that enhanced 3000 m skating performance after different types of HIIT used in our experiment were multifactorial. Data from other studies suggest HIIT can stimulate a range of adaptations that might improve performance aside from changes in AT. For example, enhanced muscle buffering capacity19 as well as increases in Na+/K+ pump capacity,9 and/or motor unit activation26 could be other potential adaptations that may contribute to an improvement in endurance performance following HIIT as recorded in our study. However, these markers were not directly assessed in the present study.

Tmax significantly improved pre- to post-training in H100 group. In line with our findings, Esfarjani and Laursen9 demonstrated that in moderately trained runners, a running-based HIIT program increased time to exhaustion at

by 32%. Smith et al.11 reported a significant increase (33%) in Tmax after 4 weeks of HIIT. A decrease in the rate of glycogen depletion and thus improved exercise tolerance,16 as well as greater muscle buffering capacity12, 28 are potential contributing factors for the improved Tmax.

The training in the present study resulted in a significant increase in PPO and MPO. These findings support other investigations7, 16, 20 which have reported increases in peak and mean anaerobic power after a short period of HIIT. Sheykhlouvand et al.7 demonstrated that PPO and MPO increased after 3 weeks of paddling HIIT (6,7,8,9,9,9,8,7,6 repetitions/session from 1st to 9th session respectively) × 60-second at 100%

, 1:3 work to recovery ratio). In another study, Farzad et al.16 demonstrated that peak and mean anaerobic power increased after 4 weeks of running HIIT (6 × 35-meter all-out running with 10-second recovery between each sprint). Enhanced muscle phosphocreatine concentration,16 anaerobic enzyme activities20 and a significant increase in fast oxidative glycolytic fibers, along with a decrease in percentage of slow twitch fibres16 may contribute to the present findings.

A limitation of the present study was the non-specific training. Stangier and colleagues,5 examined the influence an 8 week running or cycling program on inline speed skating performance. Both running and cycling were sufficient to improve sport specific performance over time despite possible decrements in skating technique. Furthermore, it is important to note that inline speed skaters often perform non-specific training during the winter months, since weather conditions make regular sport specific training difficult5 more accessible modalities (e.g., treadmills) are often used.

Conclusions

The present study found that HIIT induced significant changes in aerobic, anaerobic, and performance adaptations in female inline speed skaters. Given the markedly lower training volume in HIIT groups, our result suggest that running-based HIIT is indeed a time-efficient strategy to induce rapid physiological and performance adaptations comparable to TET. Interestingly, HIIT training at 100 and 115%

resulted in superior cardio-respiratory improvements compared to training at 130% . Despite differences in cardio-respiratory improvements all HIIT groups were able to significantly improve 3000 m time trial performance. Future research is required to determine the ideal training intensity and volume as well as how to incorporate HIIT into a periodized annual training plan to optimally improve female inline speed skating performance. In addition, future research examining sport specific training is warranted.

Considering that such training protocols have a very low volume, inline speed skaters and their coaches can use this type of training prescriptions when speed skaters have to reach several peaks over an annual cycle, particularly when the aim is to increase performance in limited period.

Conflict of interest

The authors declare that they have no conflict of interest.

Acknowledgements

The authors thank Professor Philip Bishop for his precious scientific assistance and our participants for their time and effort.

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Received 7 February 2017;

accepted 12 June 2017

Available online 14 October 2017

∗ Corresponding author.

E-mail address: m.sheykhlouvand@gmail.com (M. Sheykhlouvand).

http://dx.doi.org/10.1016/j.apunts.2017.06.003