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Time-to-Fatigue During Incline Treadmill Running

Implications for Individualized Training Prescription

Ferley, Derek D.1; Vukovich, Matthew D.2

The Journal of Strength & Conditioning Research: July 2015 - Volume 29 - Issue 7 - p 1855–1862
doi: 10.1519/JSC.0000000000000834
Original Research
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Ferley, DD and Vukovich, MD. Time-to-fatigue during incline treadmill running: Implications for individualized training prescription. J Strength Cond Res 29(7): 1855–1862, 2015—Uphill running has been touted as a key interval training tactic for distance runners despite few scientifically derived recommendations for individualized training prescription. To date, a majority of uphill training research has focused on shorter, faster training bouts; however, longer, slower bouts based on an individual's velocity at maximum oxygen consumption (Vmax) may prove more effective. One potential longer bout length may be associated with the time Vmax can be maintained (Tmax), an approach proven effective in level-grade interval training. Therefore, the purpose of this investigation included examining the Tmax, heart rate, and test-retest reliability of incline treadmill running (INC) on a 10% grade at 65, 70, 75, 80, and 85%Vmax compared with level-grade running at Vmax. Twelve moderately trained distance runners (age, 26.4 ± 4.8; body mass, 64.3 ± 12.9 kg; height, 171.2 ± 9.3 cm; and V[Combining Dot Above]O2max, 56.6 ± 7.6 ml·min−1·kg−1) completed 2 Tmax INC trials at each submaximal Vmax and a level-grade Tmax at Vmax. The dependent variables were Tmax, heart rate plateau (HRPlateau), and half-time to heart rate plateau (½HRPlateau) of each condition. Statistical significance was set to p ≤ 0.05. Student's t-test revealed no significant differences in Tmax, HRPlateau, and ½HRPlateau between trials 1 and 2 at any INC condition. One-way analysis of variance revealed significant differences in (a) Tmax during INC at 75, 80, and 85%Vmax and level-grade at Vmax and (b) ½HRPlateau during INC at 80 and 85%Vmax and all other conditions. In conclusion, Tmax and heart rate dynamics during INC proved reliable, and simple regression analysis revealed ∼68%Vmax during INC yields the same level-grade Tmax at Vmax.

1Department of Physical Medicine and Rehabilitation, Avera McKennan Hospital and University Health Center, Sioux Falls, South Dakota; and

2Department of Health and Nutritional Sciences, South Dakota State University, Brookings, South Dakota

Address correspondence to Derek D. Ferley, derek.ferley@avera.org.

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Introduction

Competitive distance runners often incorporate uphill running as part of a comprehensive training program. For example, Kurz et al. (17) reported in a survey of teams competing in the 1996 National Collegiate Athletic Association (NCAA) Men's Division I National Cross-Country meet that uphill training was widely used and correlated with faster team times. Moreover, scholarly reviews suggest that uphill running may represent a sport-specific high-velocity resistance-to-movement exercise (2,18). Although the few studies of this topic that have been completed indicate uphill training seems to have merit as a training tactic, at present, very few proven recommendations exist for prescribing individualized training velocities or bout durations when performing either over-ground uphill (HILL) or incline treadmill interval running (INC) training. In fact, a review of the literature produced just 3 investigations controlling for individualized running velocity when performing INC (1,9,10). These investigations used individually determined training intensities ranging 80–120% of the velocity corresponding to V[Combining Dot Above]O2max (Vmax) while incorporating a variety of bout durations lasting 8 seconds to 25 minutes and treadmill grades between 4 and 18% grade. Based on these investigations, twice-weekly INC training for 6 weeks has been shown to significantly improve time-to-fatigue (Tmax) while running at Vmax (9) and measures of running economy (RE) (1,10).

Despite these findings, many questions remain for individualizing the training prescriptors when performing HILL or INC. For example, whereas previous investigations have focused primarily on faster, shorter INC bouts (1,9,10,14), perhaps slower, longer INC training would yield even greater performance gains. Based on previous level-grade research, one possible approach to determine a longer (and individualized) bout length would be to use a percentage of Tmax, a duration reported to range 2–10 minutes (2,18). In level-grade interval training, the individually prescribed running intensity and bout duration of Vmax and 60%Tmax, respectively, have been shown to represent training prescriptors that result in achieving V[Combining Dot Above]O2max during a bout and the completion of a greater amount of work at a high intensity during training sessions involving repeated interval bouts (13,22)—2 key considerations when performing interval training (19). As such, previous studies into level-grade interval training using 60%Tmax while running at Vmax have reported a variety of significant improvements in both laboratory- and field-based indices (7,22,23). In contrast, based on our previous experiences with INC, well-trained individuals using Vmax and 10% grade cannot run much, if at all, beyond 30 seconds. Hence, if seeking to complete a longer, slower interval bout during INC equal to 60%Tmax the running intensity must be reduced. At present, however, no investigations have taken place to provide proven individual recommendations for prescribing a reduced running intensity when seeking to perform longer-duration intervals at any slope.

Because coaches and distance runners perform a variety of uphill training tactics as part of a comprehensive distance running training routine despite a lack of individualization, we sought to conduct an investigation to establish a scientifically derived rationale for prescribing an individually based training velocity to perform INC bouts equal to level-grade 60%Tmax. Therefore, the purpose of this investigation included documenting and comparing the Tmax responses and test-retest reliability of INC running on a 10% grade at 65, 70, 75, 80, and 85%Vmax to the Tmax achieved during level-grade running at Vmax. Furthermore, as previous research (6) has investigated the kinetics of heart rate response to exercise of various intensities, we also aimed to report on the heart rate dynamics—specifically, heart rate plateau (HRPlateau) and half-time to heart rate plateau (½HRPlateau)—associated with all INC trials. We hypothesized that (a) the test-retest of each INC training velocity would prove reliable and (b) the Tmax of the slower submaximal running velocities would most closely match that of the level-grade test. We chose to use treadmill training for the present investigation as previous research has suggested that adequately powered and noncompliant treadmills can replicate the electromyographic, kinematic, and kinetic patterns associated with over-ground running conditions while also allowing for greater training precision and control over environmental factors (15,16,20).

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Methods

Experimental Approach to the Problem

A randomized repeated measures experimental design was used to assess Tmax and heart rate during treadmill running on a 10% grade at 65, 70, 75, 80, and 85%Vmax. Additionally, the same measures were assessed during level-grade running at Vmax. All participants had previously run on an inclined treadmill slope as part of their normal run training. Treadmill velocity and surface grade represented the independent variables, whereas dependent variables consisted of the Tmax and heart rate response of each condition. Within-subject reliability and comparisons were made between the dependent variables to determine which INC condition resulted most closely resembled the level-grade condition.

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Subjects

In addition to contacting members of a local running club, we also recruited potential participants through social media and word-of-mouth. Twelve participants (6 men and 6 women) who consistently engaged in weekly running sessions voluntarily enrolled and gave their written consent to participate in this study after being fully informed of the risks and discomforts associated with the experimental and training procedures. Age range of subjects was 21–34 years. Inclusion criteria for the participants required being 18–40 years of age and having a willingness to report to the testing center 8 times over 21 days for testing purposes. We excluded those individuals who had experienced a lower-body injury in the previous 3 months or evidence of other medical or health issues that precluded running on a treadmill. The Avera McKennan Hospital and University Health Center's Institutional Review Board approved this study, and it conformed to the recommendations of the Declaration of Helsinki. Table 1 highlights the participants' characteristics.

Table 1

Table 1

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Procedures

During an initial visit to determine whether the participant met the investigation's inclusion criteria, we asked each person his or her willingness to report to the testing center on 8 separate occasions spaced a minimum 48 hours apart. Those meeting the investigation's criteria and willing to participate in the study's procedures first performed a V[Combining Dot Above]O2max test, and then 48–72 hours later completed a level-grade Tmax at Vmax. During each of the next 5 visits, participants completed 2 randomly selected Tmax trials on a 10% grade spaced 30 minutes apart at 65, 70, 75, 80, or 85%Vmax. The study took place July–October, 2013 and consisted of (a) familiarization training and (b) performance testing.

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Familiarization Testing

In the week before the start of the testing, participants reported to the training center to become familiarized with a warm-up routine and V[Combining Dot Above]O2max test. During this visit, each participant performed a 10-minute run on a treadmill at a self-selected speed, followed by an additional 10- to 15-minute dynamic warm-up routine. After a 5-minute rest, each participant then performed a familiarization V[Combining Dot Above]O2max test that involved an incremental protocol consisting of completing four to six 2-minute stages interspersed with 30-second rests. The investigator modified the initial treadmill speed for each individual to determine a comfortable starting speed. At the completion of the V[Combining Dot Above]O2max familiarization test, the investigator explained the concepts and significance of Vmax and Tmax for developing individually based interval training programs. The participants completed the same warm-up routine before every testing session throughout the investigation. For all performances, testing participants were instructed to arrive in a rested and hydrated state, and to avoid caffeine, alcohol, and strenuous exercise in 2 days preceding a test session. Additionally, attempts were made to ensure all participants completed testing procedures at approximately the same time of day. All testing days were separated by 2–3 days.

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Performance Testing

Within 7 days of completing the familiarization trial, participants returned to the testing facility to complete their day 2 V[Combining Dot Above]O2max test. This performance test was an incremental running test and involved using a breath-by-breath gas analyzing system (Physiodyne MAX-II; AEI Technologies, Inc., Bastrop, TX, USA), which the investigator calibrated before each test, and having the participants complete 2-minute stages on a treadmill (Super Treadmill; Standard Industries, Fargo, ND, USA) set to a 1% grade (5). At the completion of each 2-minute stage, a 30-second pause occurred at which time the participant stopped running, straddled the treadmill belt, and the investigator increased the treadmill speed by 0.8 km·h−1. The investigator used the following criteria to determine the participant's V[Combining Dot Above]O2max: (a) a respiratory exchange ratio >1.10; (b) an ending heart rate within ±10 b·min−1 of age-predicated HRmax (220 years) (11); (c) no further increase in O2 consumption despite an increased work rate; and (d) volitional exhaustion. In determining Vmax, the investigator required the participant to complete at least 90 seconds of the 2-minute stage; if the participant completed less than 90 seconds of the 2-minute stage, then Vmax was defined as the treadmill speed from the previous stage.

Day 3 performance testing took place 48–72 hours after baseline V[Combining Dot Above]O2max testing and consisted of completing a level-grade Tmax test at Vmax and a 1% treadmill grade. Day 4–8 performance testing involved performing 2 randomly selected Tmax tests on a 10% treadmill grade at 65, 70, 75, 80, or 85%Vmax spaced 30 minutes apart. Participants completed each of the 5 trials before being allowed to complete trial 2. For both the V[Combining Dot Above]O2max and Tmax tests, the investigator had the participants wear a heart rate monitor (Polar RS400 heart rate monitor; Polar Electro Oy, Kempele, Finland). Heart rate data were collected in 5-second increments and downloaded to a personal computer after each testing session.

All testing sessions used the Super Treadmill, which has a running belt area measuring 183 × 51 cm and speed and elevation capacities 0–48 km·h−1 and 10–40%, respectively. Each week the investigator calibrated the Super Treadmill for speed and incline. Additionally, when running on the Super Treadmill participants gathered visual feedback on running form by a 183 × 91 cm wall-mounted mirror in front of the Super Treadmill. The 8-day performance testing protocol appears in Table 2.

Table 2

Table 2

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Statistical Analyses

For all data analysis, the investigator used the statistical analysis program JMP (v.10.0, SAS Institute, Cary, NC, USA). Descriptive statistics of each outcome variable, including mean values, SDs, and SEs were determined. A paired Student's t-test was used to test for the reliability between trials 1 and 2 of all INC conditions for Tmax, HRPlateau, and ½HRPlateau, whereas a one-way analysis of variance (ANOVA) was used to assess Tmax, HRPlateau, and ½HRPlateau of all trial 1 INC and level-grade conditions. A significance level of p ≤ 0.05 was set for all statistical analyses; and where significance was found, a Tukey-Kramer post hoc test was performed. Finally, a simple regression analysis was performed to predict the %Vmax while performing INC that matches the Tmax of level-grade running at Vmax.

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Results

Tmax, HRPlateau, and ½HRPlateau Responses of Trials 1 and 2 During INC Running

No significant difference was found in mean Tmax, HRPlateau, and ½HRPlateau of trials 1 and 2 at any of the tested INC conditions, a finding indicating intracondition reliability. Table 3 highlights the Tmax, HRPlateau, and ½HRPlateau response of trials 1 and 2.

Table 3

Table 3

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Tmax Analysis by Intensity

A main purpose of this investigation was to determine which INC condition yielded a Tmax most closely replicating level-grade Tmax. A 1-way ANOVA revealed a statistically significant shorter INC Tmax at 75, 80, and 85%Vmax compared with level-grade Tmax, whereas INC Tmax at 65 and 70%Vmax were not significantly different from level-grade Tmax, suggesting the 2 slowest INC conditions most closely replicated the level-grade condition. Figure 1 highlights the Tmax response between the conditions.

Figure 1

Figure 1

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Half-Time to HRPlateau Analysis by Intensity

The one-way ANOVA revealed no statistically significant difference in ½HRPlateau between INC at 65% and 70%Vmax and level-grade Tmax, indicating that the heart rate kinetics of these 3 conditions were similar. In contrast, a statistically significant shorter ½HRPlateau was observed during INC at 75, 80, and 85%Vmax and all other conditions. Figure 2 highlights the comparison of the ½HRPlateau between the conditions.

Figure 2

Figure 2

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Prediction of the %Vmax at INC That Matches Level-Grade Tmax While Running at Vmax

Simple regression analysis revealed a running intensity equal to ∼68%Vmax at INC results in the same Tmax as running at level-grade at Vmax. The correlation coefficient and SE about the regression line were determined as 0.9542 and 84.38, respectively, with a 95% confidence interval of −0.99 to −0.69. Figure 3 highlights the simple regression analysis showing the submaximal running intensity associated with running on a 10% grade for the same length of time as running on a level-grade at Vmax.

Figure 3

Figure 3

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Discussion

Uphill training has long been used by distance runners as part of a comprehensive training program. Reporting on its use in well-trained runners, Kurz et al. (17) showed that those NCAA Division I teams incorporating uphill training during the 3–6 months before the Men's National Championships out-performed those teams not using HILL. However, at present, few proven recommendations exist for prescribing individualized uphill running workouts. To date, only 3 investigations have examined INC while controlling for running velocity, bout duration, and treadmill grade; and whereas all 3 integrated individually prescribed training intensities, none involved individually prescribed bout durations. Two of these investigations were performed in our laboratory and revealed a training intensity, bout duration, and treadmill grade equal to Vmax, 30 seconds and 10%, respectively, led to significant improvements in level-grade Tmax performed at Vmax (9), as well as submaximal measures of RE, including oxygen consumption (V[Combining Dot Above]O2) and blood lactate, and the percent of maximum oxygen consumption at lactate threshold (%V[Combining Dot Above]O2max at VLT) (10). Additionally, Barnes et al. (1) reported twice-weekly INC for 6 weeks using training intensities, bout durations, and treadmill grades ranging 90–120%Vmax, 8 seconds to 5 minutes, and 7–18%, respectively, was strongly associated with increased 5-km time trial performance, measures of maximal and submaximal aerobic characteristics, and a variety of biomechanical indices.

Because uphill running has long been used by distance runners and coaches as part of a comprehensive training routine despite a lack of proven individualized training prescriptor recommendations, we sought to build on the paucity of evidence into this mode of training in an attempt to provide insights for prescribing more individually based workouts. Whereas the majority of previous investigations have focused on the physiological effects associated with shorter and faster INC, we sought to establish a rationale for prescribing individualized longer and slower INC based on Tmax in a manner similar to previous investigations involving level-grade interval training (2,3,18). Therefore, the purpose of this investigation included examining the Tmax and heart rate dynamics of a variety of submaximal running intensities during running on a 10% grade and comparing the outcomes to maximum intensity running performed on a level-grade. Moreover, because we were not aware of any previous investigations that have established the test-retest outcomes of Tmax associated with INC, we also sought to report on the test-retest reliability associated with this form of training.

In the current investigation, the main findings show significant test-retest reliability of Tmax, HRPlateau, and ½HRPlateau at all INC conditions. Furthermore, comparisons of Tmax revealed significant differences in Tmax between level-grade and INC at 75, 80, and 85%Vmax. Finally, simple regression analyses revealed a running intensity equal to ∼68%Vmax during INC resulted in the same Tmax as level-grade running at Vmax. Based on the present investigation's results, we accept our initial hypotheses of test-retest reliability between each condition, and no significant difference in the Tmax and heart rate dynamics of the slowest INC conditions and that of the level-grade test performed at Vmax.

To the best of our knowledge, this investigation represents the first attempt to determine an individualized approach to performing HILL and INC based on similarly established training methods for level-grade running. Previous research into level-grade running using individualized training intensities and bout durations has proven effective for improving a variety of laboratory- and field-based indices. In particular, a training intensity and interval duration equal to Vmax and 60%Tmax, respectively, have been shown effective at enhancing middle- and long-distance running performance, as well as the 3 main physiological determinants associated with high-level running, V[Combining Dot Above]O2max, lactate threshold, and RE (7,22–23). Specifically, Smith et al. (22,23) and Denadai et al. (7) reported well-trained runners and athletes performing 2 weekly interval sessions for 4 weeks using these training prescriptors experienced significant improvements in 3- and 5-km time trial performance, ventilatory threshold, submaximal V[Combining Dot Above]O2, and Tmax (7,22). To this end, Hill and Rowell (13) examined various interval lengths while running at Vmax and determined 60%Tmax to be the minimum bout length for eliciting V[Combining Dot Above]O2max, whereas bout durations less or more than 60%Tmax, respectively, either did not result in achieving V[Combining Dot Above]O2max (13) or proved too difficult for completing repeated efforts (13,22). In support of the findings by Hill and Rowell (13), Smith et al. (22) determined bout lengths of 70%Tmax in well-trained athletes proved too difficult—and thus less total work able to be performed during a training session—compared with a group using 60%Tmax bout lengths in a 4-week investigation of twice-weekly interval training.

When distance runners perform interval training, it has been suggested that near-maximum efforts eliciting ≥90% of age-predicated maximum heart rate (HRmax) should be used (12,24). Helgerud et al. (12) had 4 groups of well-trained runners performing an equal amount of total work at varying intensities based on HRmax and determined only the 2 groups incorporating 90–95%HRmax experienced significant improvements in V[Combining Dot Above]O2max. Smith et al. (22) tracked groups using Vmax and either 60 or 70%Tmax bout lengths of 131 and 152 seconds, respectively, and reported postinterval heart rates during each training session exceeding 90%HRmax in both groups. Based on the heart rate response in the present investigation, each INC condition resulted in an effort ≥93% of the age-predicated HRmax. Furthermore, Noakes (19) suggested that 2 key considerations for performing interval training include maximizing the time spent running at V[Combining Dot Above]O2max as well as the total work completed at a high intensity. Although in this investigation we did not measure the V[Combining Dot Above]O2 response during INC, the ½HRPlateau revealed by our regression formula suggests a maximum effort as indicated by heart rate was achieved after ∼48 seconds and maintained for 211 seconds, a Tmax response similar to what has been reported in previous investigations (4,7,9,10,22,23).

A main pursuit of this investigation was to compare the submaximal running intensity Tmax responses during INC with the maximum intensity Tmax responses during level-grade running and determine the submaximal intensity at which one should run when performing INC on a 10% grade if seeking to match the Tmax of level-grade running performed at Vmax. As we reported previously (9,10), our experience with INC suggests that participants running on a 10% grade at Vmax cannot run much, if at all, beyond 30 seconds. Interestingly, one group in an investigation by Barnes et al. (1) completed 2- to 2.5-minute bouts while running on a 10% grade at Vmax. In the present investigation, we found that running intensity had to be reduced to ∼80%Vmax before participants could achieve a similar effort. However, several explanations may exist to describe this difference. For example, (a) Barnes et al. (1) did not identify their definition of Vmax, and perhaps they used a protocol that resulted in a lower assessment of Vmax; (b) they did not report Tmax, and perhaps the prescribed 2-minute bout length was well-below each participant's 60%Tmax; (c) they appear to have used very well-trained participants given the average personal best 5-km time of 16.5 ± 1.2 minutes, and perhaps this training status results in metabolic, muscular, or neuromuscular features, which allows for Vmax efforts to be performed for a longer period of time while performing HILL or INC; and (d) the authors did not conclusively report whether their participants performed HILL or INC training, and perhaps training surface may impact the Tmax and repeat-effort outcomes associated with training and testing.

In conclusion, we determined through simple regression analysis that a submaximal running intensity equal to ∼68%Vmax allows one to achieve a Tmax while performing INC on a 10% grade similar to that achieved during level-grade running at Vmax. Our high test-test reliability for Tmax and heart rate dynamics associated with a variety of submaximal running velocities while performing INC at 10% suggests that our approach for determining individualized training prescriptors for running intensity and bout duration has merit.

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Practical Applications

The results of this investigation provide a logically determined rationale for prescribing INC with distance runners. Previous investigations have shown that INC training performed at or above Vmax for relatively short bout durations can positively impact distance running performance; however, the possibility exists that longer, albeit slower, INC bouts may yield even greater results. Therefore, it seems prudent to model an INC training program on the experimental protocols of previous investigations into level-grade interval training using 60%Tmax bout durations. For example, previous level-grade research has shown that completing 4–6 bouts of 60%Tmax intervals at Vmax twice per week for 4–6 weeks can enhance a variety of field- and laboratory-based indices including 1,500- and 3,000-m running performance and the 3 primary determinants associated with distance running performance, respectively. Using the results of this investigation, a similar approach to INC may be developed in which individuals perform 4–6 bouts on a 10% grade at a running intensity of ∼68%Vmax 2 times per week for 4–6 weeks while using rest periods between bouts—in addition to performing days of continuous-run training—in a manner similarly reported in previous investigations (8–10,21,22). As with many laboratory-based investigations, however, extrapolating the results of an incline treadmill training tactic to an over-ground setting represents a limitation to this study, and we recommend further in situ research be conducted to verify our results. Furthermore, we also recommend completing additional research into a variety of running speeds, hill/treadmill grades, and bout durations in an attempt to determine the most effective hill training tactic(s) and further individualize the training prescriptions for distance runners when using this mode of training.

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Acknowledgments

The authors wish to thank the participants for their effort, willingness, and enthusiasm for participating in this study. They also greatly appreciate the insights and recommendations of Roy Osborn, the assistance of the staff of Avera Sports, including Jason Askew, Cameron Ditlevson, David Hopper, and Whitnee Fester, and the support of Avera McKennan Hospital and University Health Center. The authors of this study received no funding for this investigation and have no conflicts of interest.

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Keywords:

interval training; hill training; spring training

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