Abstract
his study examined the differences in physiological, metabolic and running dynamics responses between level and inclined treadmill protocols and their implications for accurately determining training intensities. Twenty-three healthy, active adults (18 male and 5 female) from 25 to 59 years old (age: 42.7years, height: 1.77 m, body mass: 71.9 kg, VO2max : 54.3mL·kg −1 ·min −1 ) completed both protocols. Physiological markers (gas exchange threshold (GET), respiratory compensation point (RCP),VO2max ), metabolic variables (HR,VO2,VCO2 , RER, VE, speed) and running dynamic variables (running economy (RE), stride length (SL), ground contact time (GCT), cadence) were measured and matched for the external work rate at each stage. The data were analyzed using one-way repeated measures ANOVA with Tukey’s post hoc procedure. No significant differences were observed in the physiological markers for the inclined and flat protocols across all the intensities. However, the metabolic variables showed significant differences (p= 0.0333 to <0.0001) between the inclined and flat protocols at higher intensities. The RE was consistently improved in the flat protocol compared with the inclined protocol, with significant differences observed at the high-intensity stages (p= 0.0232 to <0.0001). While the physiological markers remained unaffected, metabolic responses and running kinematics differed significantly between the protocols. These results highlight that training intensity zones derived
<0.0001) between the inclined and flat protocols at higher intensities. The RE was consistently improved in the flat protocol compared with the inclined protocol, with significant differences observed at the high-intensity stages (p= 0.0232 to <0.0001). While the physiological markers remained unaffected, metabolic responses and running kinematics differed significantly between the protocols. These results highlight that training intensity zones derived from inclined protocols may not be appropriate for flat terrain training, underlining the importance of testing specificity in athlete preparation. Keywords:testing; exercise; training;VO2max; inclined 1. Introduction Aerobic capacity is often assessed with a treadmill test of maximum oxygen uptake (VO2max ). An individual’sVO2max , along with other physiological variables measured during this test, is an important clinical and performance indicator [1,2]. For athletes, it is often used to prescribe training and performance intensities, as recommended by the Amer- ican College of Sports Medicine [3] and observed in many recentstudies [4,5] . Therefore, the results of treadmill tests play a crucial role in the preparation of an athlete’s competition. An early treadmill protocol was developed by Taylor et al. in 1955, which involved a discontinuous test that was conducted over multiple days while increasing the treadmill incline (grade) and keeping the speed constant. Subsequent protocols, such as the Balke, Bruce and the World Health Organization (WHO) protocols [6], also used an increasing treadmill grade and have been widely used with patients and athletes. Many concerns were raised about their application [7,8], as the increments in intensity and test duration Life2025,15, 569 https://doi.org/10.3390/life15040569
Life2025,15, 569 2 of 13 are not suitable for all groups and all require steep running. The current standard in treadmill exercise protocols is to increase the intensity through increases in grade, forcing the participant to run at higher inclines as the intensity reaches competitive intensity equivalents. However, training and competition occur on both level and inclined terrains, and testing conditions were shown to closely simulate the specific sport practiced by the athlete [9]. Thus, a single testing mode may not accurately determine the training and competition intensity zones. However, there is a lack of studies that match the external work rate between flat and inclined protocols, which is critical to isolate the effect of the incline on metabolic and physiological responses. Previous research often compared protocols without standardizing kinetic energy requirements, potentially conferring biased results for the training description. Despite the prevalence of inclined treadmill protocols, recent research explored more individualized and self-paced approaches to improve the ecological validity of testing. For example, a study found that self-paced intermittent protocols on non-motorized tread- mills can reliably mimic the fluctuating intensity of real-world athletic competitions [10]. Additionally, individualized treadmill protocols, where participants control the speed and incline based on their preferences, were shown to yield higherVO2max values compared with standardized tests, suggesting a closer reflection of true aerobic capacity [11]. Fur- thermore, a four-minute self-paced running time trial on a treadmill was validated as a reliable method for aVO2max assessment [12], offering a fixed-duration, athlete-centered approach. However, these approaches have not addressed whether the commonly used inclined testing protocols appropriately reflect the metabolic demands of flat terrain run- ning, particularly when the training zone prescription is based on data obtained under non-specific conditions. Despite the obvious problem, there were a few studies that attempted to optimize treadmill test protocols. A group of researchers suggested a newVO2max protocol that allows participants to self-pace within stages of the test while maintaining an incremen- tal procedure [13]. Others explored the effects of manipulating the frequency of data acquisition on the achievement of maximum oxygen consumption [14] and examined the implications of altering the
few studies that attempted to optimize treadmill test protocols. A group of researchers suggested a newVO2max protocol that allows participants to self-pace within stages of the test while maintaining an incremen- tal procedure [13]. Others explored the effects of manipulating the frequency of data acquisition on the achievement of maximum oxygen consumption [14] and examined the implications of altering the duration of stages on various physiological variables during incremental testing protocols [15]. However, little has been done to address the issue that in- cremental inclined tests do not provide an opportunity to assess high-intensity performance on a level surface. Running on flat terrain compared with inclined terrain involves differences in run- ning economy; running dynamics; and, ultimately,VO2max . Multiple studies found that increasing the grade resulted in an increased energetic cost of running, and therefore, a lower running economy [16–18]. A higher stride frequency and reduced stride length were observed in treadmill running on an incline [19], which has been known to increase the metabolic cost of running at given speeds [20]. An increased peak oxygen deficit was also recorded in incline running compared with flat running [21]. The authors attributed the increased peak oxygen deficit to the greater leg muscle activation seen in incline running, which may have resulted in a decreased running efficiency. Ground contact times during running may play a role in this, as many studies indicated that a reduced ground contact time relates to a decreased metabolic cost [22,23]. These studies that compared the biome- chanics behind the different running types indicate the potential for incline running to cause earlier and greater leg fatigue, which decreases the maximal oxygen uptake. There are contradictory observations about the effect of flat and inclined protocols onVO2max . Some studies indicated differences in theVO2max achieved between incline and flat pro- tocols [24–26], while other studies found none [27,28]. Alternative markers of intensity, such as heart rate, respiratory exchange ratio and maximum ventilation, may be higher
indicated differences in theVO2max achieved between incline and flat pro- tocols [24–26], while other studies found none [27,28]. Alternative markers of intensity, such as heart rate, respiratory exchange ratio and maximum ventilation, may be higher
Life2025,15, 569 3 of 13 with inclined running, while protocol effects on the gas exchange threshold have not been characterized [27]. However, none of these studies matched speeds between stages in inclined and flat protocols and did not compare the metabolic data at each stage. The purpose of this study was to compare physiological markers (gas exchange threshold, respiratory compensation point andVO2max ) and metabolic variables (VO2, VCO2 , VE, RER and HR) at a common external work rate between the flat and inclined treadmill protocols. By standardizing the external workload between protocols, this study sought to fill a gap in current research and provide evidence for the need to align testing modalities with athletes’ actual training environments. 2. Materials and Methods 2.1. Experimental Approach to the Problem Following familiarization with the study protocols and procedures, this study utilized a repeated-measures crossover design, where each participant completed both the flat and inclined treadmill protocols in a randomized order to minimize order effects. The participants attended one session in the laboratory to perform two ramp tests (randomized order), with either inclined or flat protocols for the determination of the aerobic parameters: VO2max , gas exchange threshold (GET) and respiratory compensation point (RCP), with a 30–45 min rest between the tests. This rest period was selected based on Hinckson and Hopkins (2005), who demonstrated that 30 min was sufficient to maintain test–retest reliability in time-to-exhaustion tests, with a coefficient of variation below 5% and no significant performance decrease observed in the second trial, indicating effective recovery between tests [29]. The design ensured that each participant served as their own control, which enabled a direct comparison between the testing conditions. All exercise tests were conducted using a calibrated treadmill (Woodway Path, Waukesha, WI, USA) that was capable of adjusting with 0.1% grade and0.045 m·s −1 increments. The running dynamics variables (ground contact time (GCT), cadence) were measured second by second with a Garmin Forerunner 620 (Olathe, KS, USA). Metabolic variables (RER, heart rate (HR), VE, VO2,VCO2 ) were measured breath-by-breath throughout the test from a metabolic analyzer (Metamax ® 3B, Leipzig, Germany), and
USA) that was capable of adjusting with 0.1% grade and0.045 m·s −1 increments. The running dynamics variables (ground contact time (GCT), cadence) were measured second by second with a Garmin Forerunner 620 (Olathe, KS, USA). Metabolic variables (RER, heart rate (HR), VE, VO2,VCO2 ) were measured breath-by-breath throughout the test from a metabolic analyzer (Metamax ® 3B, Leipzig, Germany), and theVO2max was determined as the highest average VO2over a 20 s period. The GET and RCP were estimated using the V-slope method [30] by identifying breakpoints in theVO2vs.VCO2 relationships. Given the variability in threshold identification, two independent researchers reviewed and confirmed the inflection points. In cases of ambiguity, a third reviewer was consulted. The running economy was calculated as the energetic cost of running in mL·O2·km −1 . 2.2. Subjects The participants who were members of local running clubs were recruited through postings and word of mouth to take part in this study. The candidate participants were informed about the study aims, procedures and risks associated with the tests, and all participants gave their written and informed consent. This study was approved by the University of Toronto Review Ethics Board and was conducted in accordance with the Declaration of Helsinki. The participants were asked to refrain from participating in strenuous physical activity in the 24 h prior to the test and avoid caffeine and alcohol consumption 3 h before reporting to the laboratory. Twenty-three healthy, athletic adults (18 male and 5 female) from 25 to 59 years old (Table) participated in this study.
Life2025,15, 569 4 of 13 Table 1.Subject characteristics. Male Female Age (years) 43.9 ±5.7 33.5 ±3.7 Height (m) 1.80 ±0.09 1.64 ±0.08 Body mass (kg) 75.0 ±6.6 60.9 ±4.2 VO2max(mL of O2·min −1 ·kg −1 ) 57.6±5.7 47.6 ±5.7 2.3. Procedures The participants were screened using the PAR-Q+ and an athlete consent form in person. The PAR-Q+ (Physical Activity Readiness Questionnaire) was used to determine the safety and possible risks for an individual to begin the exercise program [31]. The ques- tionnaire was used in several studies that involved maximum efforts [32–34]. Once written consent was obtained, subjects were familiarized with all the protocols and procedures. In addition, the participants were instructed on how to immediately signal to the tester whether they experienced discomfort, dizziness or any adverse symptoms that required the test to be stopped. A safety clip connected to the treadmill’s emergency stop mechanism was worn by each participant throughout testing. Trained personnel were present at all times to ensure an immediate response to any emergency situation. This study included twenty-three trained runners (18 males, 5 females), which ensured a homogeneous athletic population for assessing the aerobic capacity and running dynamics. Inclusion criteria: • • Engagement in regular aerobic activity (≥3 days per week for at least 30 min per session) for a minimum of six months; • Exclusion criterion: • A priori power analysis was conducted using G*Power (Version 3.1.9.6, Heinrich Heine University Dusseldorf,Dusseldorf, Germany) to determine the required sample size. The analysis was based on prior studies that examinedVO2max , metabolic variables and running kinematics across treadmill protocols. The following parameters were used: effect size (F) = 0.30,α= 0.05, power (1−β) = 0.80 and a within-subject correlation of 0.5 (based on similar treadmill-based studies). The analysis indicated that a minimum sample size of20 participantswas required to detect significant differences between the protocols. To account for potential dropouts and ensure statistical power, 23 participants were recruited. The participants warmed up for 5–10 min on a treadmill at a self-selected comfortable speed, which was subsequently used as their starting speed after 5–10 min of rest. Each treadmill test
indicated that a minimum sample size of20 participantswas required to detect significant differences between the protocols. To account for potential dropouts and ensure statistical power, 23 participants were recruited. The participants warmed up for 5–10 min on a treadmill at a self-selected comfortable speed, which was subsequently used as their starting speed after 5–10 min of rest. Each treadmill test began with 2 stages of 2 min duration at lower intensities (i.e., 1% and 3% grade or incline-matched speed), and then the intensity was increased (either 2% grade or incline-matched speed) every minute until volitional exhaustion or the participants signaled to the tester that they could not complete the next 30 s period. A 1% grade was chosen as the starting level in both protocols since it most accurately represents the energetic costs of outdoor running [35]. The treadmill speeds in the flat protocols were determined using energy-matching equations at each distinct stage (see below). Strong verbal encouragement was provided to elicit full effort. To ensure consistency across the tests, the same researcher provided standardized verbal encouragement to all the participants. Pre-determined phrases were used at consistent time points throughout the test to maintain uniform motivation. Since this study utilized a within-subjects design, each participant served as
Life2025,15, 569 5 of 13 their own control, which mitigated potential variability in encouragement between the trials. To further minimize the bias, the researcher followed a scripted encouragement protocol and avoided any unintentional cues that could influence the performance. The attainment ofVO2max was confirmed by the achievement of a plateau inVO2 (increase of <2 mL·kg −1 · min −1 despite an increase in workload) [36,37], RER values above 1.20 and a heart rate within 5 bpm of the age-predicted maximum. For all the participants, at least two of the three criteria were met. Expired breath-by-breath gas exchange data were collected continuously from a metabolic analyzer (Metamax ® 3B, Leipzig, Germany). The second test was performed on the same day after 30 min of rest. 2.4. Speed Calculations The speed for each stage in the flat protocol was calculated using kinetic equations to ensure the stages between the flat and inclined protocols were matched in kinetic energy. Kinetic energy considers the mass of the individual, as well as their speed, and is represented by the following equation: E= 1 2 mv 2 (1) whereErepresents the energy in Joules,mrepresents the mass in kilograms andvrepresents the speed in m·s −1 . To increase the kinetic energy expended, two methods were employed: (1) thegrade or incline of the treadmill was increased (which increased the effective mass due to gravity) while the speed remained constant, and (2) the speed was increased as the grade remained constant. These two methods were the premises of the inclined and flat protocols we designed for this study. To start, the kinetic energy on an incline is represented by the following equation: E= 1 2 Mv 2 inc (2) whereMrepresents the mass in kilograms (considering the incline) andv increpresents the speed on the incline in m·s −1 . Due to the fact the participant was running on an incline, the mass was affected by gravity and the angle of inclination. In the protocol, we used the grade to describe the level of the incline. The grade refers to the rise over run of the treadmill and is expressed as a
speed on the incline in m·s −1 . Due to the fact the participant was running on an incline, the mass was affected by gravity and the angle of inclination. In the protocol, we used the grade to describe the level of the incline. The grade refers to the rise over run of the treadmill and is expressed as a percentage (units of rise per 100 units of run). Taking into account the effect of gravity, we used the following equation to determine the effective massM: M=m+m·g·sin(x) (3) wheremrepresents the body mass in kilograms,grepresents the gravitational acceleration (9.81 m·s −2 ) andxrepresents the inclination in degrees. Substituting Equation (3) into Equation (2) results in E= 1 2 (m+m˙g·sin(x))·v 2 inc (4) The kinetic energy expended during the flat protocol is represented by a similar equation but without having to account for gravity and the inclination: E= 1 2 mv 2 f lat (5) wherev f latrepresents the speed during the flat protocol in m·s −1 . To determine the speeds for the flat protocol, the two energy Equations (4) and (5) were combined: 1 2 (m+m˙g·sin(x))·v 2 inc = 1 2 mv 2 f lat (6)
Life2025,15, 569 6 of 13 By isolating forv f latthe resulting equation is v f lat= q 1+g·sin(x)·v inc (7) Using this equation, we could determine the speeds for the participants at each stage of the flat protocol from the grade of the inclined protocol stage and initial speed the participants selected. 2.5. Statistical Analysis The individual stage oxygen uptake threshold, running dynamics and metabolic variables for the inclined and flat protocols were compared using a two-way repeated measures analysis of variance mixed model in R (version 4.4.2, Vienna, Austria), with Tukey’s HSD post hoc procedure used to control for type I errors in making multiple comparisons and determine the significant differences between the variables. Significance was set atp< 0.05, and thep-valuesare reported in the results. Prior to the analysis, data were tested for normality using the Shapiro–Wilk test, and the sphericity was assessed with Maulchly’s test. When the sphericity assumptions were violated, a Greenhouse–Geisser correction was applied. The residual plots were all inspected to confirm the homogeneity of variance to ensure that the presuppositions for using parametric analysis were satisfied. 3. Results There was no difference in the markersVO2, speed and HR between the grade and flat protocols at the threshold points GET, RCP andVO2max, as represented in Figure. Figure 1.(A)VO2 , (B) speed and (C) HR corresponding to the attained threshold markers GET, RCP andVO2max during the horizontal (flat) and inclined (graded) treadmill running by fit young adults (n = 23). NS represents non-significant; GET is the gas exchange threshold; RCP is the respiratory compensation point;VO2max is the maximum oxygen consumption (all threshold values expressed in L·min −1 ). There were significant differences between the inclined and flat protocols for the metabolic variable minute ventilation (VE). These differences were observed in the latter
Description
This study compares physiological responses between flat and inclined treadmill protocols.