Does Power Output at Critical Power Intensity Interchange between Cycling and Running?

Abstract

The differences in power meters and gestures between cycling and running can have an impact on determining Critical Power (CP) intensity in each sport. CP is a concept that has been extensively researched in cycling, but with the advent of power measurement in running, it can now be examined in that discipline as well. The purpose of the present study was to determine whether power output at CP intensity is interchangeable between cycling and running segments measured with their respective discipline-specific power meters. A group of 18 trained triathletes (age 33.0 ± 11.1 years, height 1.75 ± 0.06 m, body mass 71.2 ± 7.1 kg) performed a CP test in cycling (3-min All-Out Test) and running (9/3-min Stryd CP Test). The main results of the present study showed significant differences (p < 0.001) between CP in cycling and running. The running CP (301.8 W ± 41.5 W) was 20.2% overestimated compared with the cycling CP (251.1 W ± 37.0 W). Cycling power only explained 26.7% of the running power (R² = 0.267; p = 0.284). Therefore, power would not be interchangeable between the cycling and running disciplines at CP intensity. In conclusion, it would be necessary to carry out a specific test for each discipline to be able to make a correct determination of CP.

1. Introduction

Endurance competitions, which combine two or more sports or disciplines, such as triathlon, are a relatively recent phenomenon that thousands of people participate in, and they are governed by rules and regulations [1]. In particular, the transfer of training effects between the swimming, cycling, and running disciplines has been studied in depth [2], making triathlon a complex sport. Rapid development in recent years has prompted industries to develop novel paradigms that allow coaches to accurately quantify the internal and external loads performed by athletes and help protect them from injury [3].

In brief, drafting is permitted in the swimming segment of the competition, as it is crucial for conserving energy in the following segments [4]. The term drafting is used to describe the tactic of performing a mode of activity in a protected position. In swimming, it has been shown that the metabolic cost of exercise is reduced by 5–10% during swimming in a drafting position, or the maximum speed is increased by 3.2–6% due to a reduction in frontal resistance or drag of 10–26% [5,6]. Considering that triathlon regulations allow any swimming technique to be used to cover the distance of the segment if the feet are not placed on the ground, the crawl technique is the most commonly used, as it is the fastest and most economical.

The second and third segments of the triathlon require special attention [7]. Several authors consider the running segment to be the most decisive in achieving victory, given the greater variability of the time of this segment compared to the other two [8,9]. The cycling segment should also be analyzed because power output is the main measure of external load to measure intensity in this discipline [10], as well as being studied and compared in this investigation with the running segment. This is essential to correctly understand the concept of power in the running segment. It is also important to note that performance in the cycling segment shows the best concordance with overall performance for both elite men and elite women at both sprint distances [11], as performance in the cycling segment shows the best concordance with overall performance as well as at Olympic distances [12].

In cycling, the increasing availability of power meters has led some authors [13] to associate a certain watt value to Functional Threshold Power (FTP) and other concepts such as Critical Power (CP). Currently, with the capability to calculate running power, these concepts have expanded within the sport. Allen and Coggan [13] defined FTP as the highest power that can be maintained for 1 h. The Running Functional Threshold Power (rFTPw), measured in watts (W), is a measure of power associated with a certain intensity that can be calculated theoretically through a complex calculation model [14]. When using off-the-shelf power meters such as the Stryd Summit Power Meter (Boulder, CO, USA), data can be uploaded from various tests [15] to calculate the rFTPw or CP in the “Power Center” (https://www.stryd.com/powercenter, accessed 10 February 2023), which can be used to predict race times for different distances. However, a study of comparison between different tests to determine rFTPw concluded that it varies depending on the test performed and its duration [16].

The literature has explored the concept of CP from both a physiological and a mathematical point of view. Mathematically, CP is defined as the power asymptote of the hyperbolic relationship between power output and time to exhaustion [17]. For longer endurance events, it has been suggested that CP is most relevant to continuous activities lasting around 2 to 30 min [17]. This concept has also been applied to other sports, such as running [18], where the treadmill velocity–endurance time relationship for runs of 2 to 12 min duration conform to a similar hyperbolic function as that described for cycle ergometry [19]. From a physiological standpoint, CP represents the boundary between the steady state and a nonsteady state. As a result, it may offer a more significant performance indicator compared to other established benchmarks of aerobic fitness, such as maximal O₂ uptake and the lactate threshold [17]. The CP concept has been used to distinguish the boundary between the heavy and severe exercise domains, which can be sustained for a duration of 15–40 min before exhaustion sets in or before the power output needs to be lowered to continue exercising [18]. Regardless of the approach taken, the ability to analyze mechanical power during running introduces a new paradigm in the scientific literature of this field. Currently, researchers have determined CP intensity in relation to ventilatory thresholds and maximum oxygen uptake [20].

The main purpose of this study was to investigate whether power output at CP intensity could be used interchangeably between cycling and running segments that were measured using their respective discipline-specific power meters and trained triathletes. The previous hypothesis established was that the power output between cycling and running would not be interchangeable at CP intensity because both the sport-specific gestures (pedaling and stride) and the power meters are different between the two disciplines.

2. Materials and Methods

2.1. Participants

A group of 18 trained triathletes participated in this study, all of whom were members of a local triathlon club. All triathletes fulfilled the following criteria to be selected as trained triathletes [21]: (1) local-level representation, (2) regularly training more than 3 times per week, (3) identifying with the sport of triathlon specifically, and (4) training with a purpose to compete. All participants read and signed an informed consent document in which they were informed of the characteristics of this study and the strictly scientific use of the data obtained, as specified in the Declaration of Helsinki of the World Medical Association (WMA); Ethical Principles for Medical Research Involving Human Subjects of 1975 (revised in Fortaleza, Brazil in 2013). This study has also been approved by the ethics committee of the University of Alicante (UA-2023-02-04). Descriptive data for the participants are shown in

Table 1. Anthropometric characteristics of the triathletes. Mean ± SD.

	Mean (M)	Standard Deviation (SD)
Age (years)	33.0	11.1
Body Mass (kg)	71.2	7.1
Body Height (m)	1.75	0.06
∑ 8 skinfolds (mm)	71.9	22.9
Muscle Mass (kg) [22]	32.2	2.8
Fat Mass (kg) [23]	7.56	3.16
Fat Mass (%) [23]	10.5	3.4

.

2.2. Procedure

An observational study design was employed to assess the possibility of exchanging power output at CP intensity between the cycling and running disciplines. This study was conducted in three test sessions. The initial session involved the anthropometric characteristics of the participants. In the second test session, the 9/3-minute Stryd CP test was performed. Following a rest period of at least 24 h, the 3-min All-Out test was carried out. Both the running and cycling tests were performed outdoors, in an identical weather environment and under the same weather conditions. The experimental procedure can be seen in Figure 1.

2.3. Anthropometry

The body compositions of the triathletes were estimated using an anthropometric method, with all measurements taken by the same anthropometrist, who had achieved Level 1 of the International Society for the Advancement of Kinanthropometry (ISAK). The measurements were taken three times for each participant, following the Ross and Marfell-Jones [24] protocol. A Holtain skinfold caliper (Holtain Ltd., Crymych, UK), a Holtain bone-breadth caliper (Holtain Ltd., Crymych, UK), scales, a stadiometer, and anthropometric tape (SECA Ltd., Hamburg, Germany) were used as equipment. The physical characteristics of age, body mass, and height were measured in the following order: the biepycondilar-humerus, bistyloid, and biepicondylar-femur breadths; the relaxed-, flexed-, and tensed-arm, waist, hip, and calf girths; and the tricep, subscapular, bicep, iliac-crest, supraspinal, abdominal, thigh, and calf skinfolds. Muscle mass was estimated using the Lee equation [22] and fat mass using the Withers equation [23].

2.4. Running: Stryd CP Test

To determine CP in running, the 9/3-min Stryd CP test proposed by the Stryd group was performed following protocol applied in previous investigations [20]. In this case, the test was conducted in the field instead of with a treadmill in the laboratory. The test was carried out on an approved 400 m athletics track. All tests were conducted under normal weather conditions of temperature, wind, and no rain. For the warm-up, the triathletes performed for 10 min at low moderate intensity and then 2 to 3 high-intensity 1-min short bouts with 2 min of active rest. Following that, the running-power meter (Stryd Summit Power Meter) was fastened onto the laces of the right shoe. The main part of the exercise involved two maximum efforts of 9 and 3 min, respectively, with a 30-min active recovery break between the two efforts [20]. A researcher marked the starts and ends of the two maximum efforts by blowing a Fox 40 Classic whistle (Fox 40 International, Hamilton, Ontario, Canada) [25]. Athletes had immediate feedback on their wristwatch, of both the running pace (min/km) and the time remaining for each of the maximal efforts.

The power output in absolute values (W) and relative to body mass (W/kg) in the 9- and 3-minute trials was entered into the Stryd CP calculator (https://www.stryd.com/powercenter, accessed 10 February 2023).

2.5. Cycling: 3-min All-Out Test

To determine CP in cycling, the 3-min All-Out test was performed following the protocol applied in previous investigations [26,27]. The cycling test was conducted outdoors, with identical weather conditions to the running test. All participants used their own bicycles. The rear wheel of each bicycle was removed and attached to a direct drive Tacx Flow Ergotrainer (Technische Industrie Tacx BV, Oegstgeest, Netherlands) with 11 speeds (11–28 tooth). Triathletes first performed a warm-up at 100 W, followed by 5 min of rest. The test started with 3 min of unloaded baseline pedaling at each subject’s preferred cadence, followed by an all-out 3-min effort. Subjects were asked to increase their cadence to approximately 110 rpm during the last 5 s of the baseline period. Strong verbal encouragement was provided throughout the test, although the subjects were not informed of the elapsed time, to prevent pacing. To ensure all-out effort, subjects were instructed to maintain their cadence to be as high as possible at all times throughout the test. CP was calculated as the average power output during the final 30 s [26,27]. For the cycling test to be valid, the maximum peak power had to be reflected within the first 5 s of the start of the test. This was checked using the publicly available software Golden Cheetah (v 3.5, Cycling Power Analysis Software) and Microsoft Excel 2022 (Redmond, WA, USA).

2.6. Statistical Analysis

All data are presented as mean ± SD. The Shapiro–Wilk test was used to study the normality of the data. Student’s t-test for independent samples was applied to determine the possible significant differences between the power outputs of the cycling and running tests both in absolute values and relative to body mass. Significance was established at p < 0.05. Levene’s test for homogeneity of variance indicated that there was an equal distribution of variance. The magnitudes of differences and effect sizes (ESs) were calculated according to Cohen’s d [28] and interpreted as trivial, (ES < 0.2), small (0.2 ≤ ES < 0.4), moderate (0.4 ≤ ES < 0.8), or large (ES ≥ 0.8). The linear regression model and the coefficient of determination (R²) [29] were used to determine the degree to which cycling power output explained variations in running power output. All data were analyzed using the SPSS 28.0 statistical package (SPSS Inc., Chicago, IL, USA).

3. Results

Table 2. Critical Power in running, determined with the 9/3 Stryd Test. Mean ± SD.

	9-Min Effort		3-Min Effort
Distance (km)	2.52 ± 0.21		0.93 ± 0.08
Absolute Power (W)	313.9 ± 35.5		332.3 ± 40.3
Relative Power (W/kg)	4.40 ± 0.40		4.66 ± 0.52
% Critical Power	104.01 ± 11.75		110.11 ± 13.36
Critical Power (W)		301.8 ± 41.5
Critical Power (W/kg)		4.23 ± 0.51

shows the values of both absolute power (W) and relative power to body mass (W/kg) of the triathletes during the 9/3 Stryd CP test of running. The absolute and relative power values were higher in the 3-min effort than in the 9-min effort, these efforts being performed at 110.11 ± 13.36% CP and 104.01% CP, respectively. The determination of CP in the triathletes was carried out at 301.8 ± 41.5 W and 4.23 ± 0.51 W/kg.

The absolute (W) and relative to body mass (W/kg) power values of the triathletes during the 9/3 Stryd CP test in running are shown in Figure 2. In the 3-min effort, the triathletes achieved the highest absolute (332.3 ± 40.3) and relative power (4.66 ± 0.52) records. In the 9-min effort, the absolute (313.9 ± 35.5) and relative power (4.40 ± 0.40) records were lower. The CP values obtained from the Stryd CP calculator were lower than both the 3-min and 9-min values.

Figure 3 shows a comparison of the CP values between the cycling and running disciplines. There are significant differences and large ESs between the cycling and running CP in both absolute watts (W) (p < 0.001; ES = 1.29) and watts relative to body mass (W/kg) (p < 0.001; ES = 1.25). The running CP (301.8 W ± 41.5 W) was 20.2% overestimated compared with the cycling CP (251.1 W ± 37.0 W). Linear regression analysis showed that cycling power output explains only 26.7% of the running power output (R² = 0.267; p = 0.284).

4. Discussion

The main purpose of the present study was to determine whether power output at CP intensity could be interchangeable between cycling and running segments. Although both the power meters and the sport-specific gestures in each of these disciplines are different, triathletes could be ideal participants for this type of study due to their previous familiarity with both technical gestures, pedaling and stride, respectively. To the best of our knowledge, this study provides the first comparison of CP values between cycling and running. While a theoretical basis for power application exists in cycling [10,13,30,31,32,33,34], a more detailed examination of running power is necessary.

Firstly, it is important to note that the available information on running power is limited [35]. However, the concept of power in running has been systematically reviewed [36,37]. The validity and reliability of one of the main devices for measuring power output [38], the Stryd running-power meter (Stryd Summit Power Meter, Boulder, CO, USA) has been studied at different running speeds [39], as well as in other modalities such as walking [40] and trail walking [41]. Additionally, data training metrics have been analyzed with this device [42,43,44]. The CP concept has been investigated from a physiological perspective, including its relationship to ventilatory thresholds and maximum oxygen uptake [20] as well as its association with physiological variables such as oxygen consumption [45] and changes in power as functions of different intervals [46]. One of the main advantages of the Stryd running-power meter is its practical applications for data collection in both training and competition settings, in contrast to laboratory-based devices [47].

Secondly, in relation to running power, it is worth highlighting a study by Ruiz-Alias et al. [20], in which CP was determined with respect to ventilatory thresholds and maximum oxygen uptake using the same 9/3-minute Stryd CP test proposed by the Stryd group. In that study, conducted on 15 high-caliber athletes, higher values of CP were observed (4.67 ± 0.42 W/kg) compared with those found in our trained triathletes (4.23 ± 0.51 W/kg). Similarly, higher values were observed for the 15 high-caliber athletes in the two efforts performed in the running test to determine the CP, in the maximum efforts of both 9 min (4.91 ± 0.42 W/kg vs. 4.40 ± 0.40 W/kg) and 3 min (5.39 ± 0.44 W/kg vs. 4.66 ± 0.52 W/kg) [20]. It is important to consider the protocol used for determination of CP in running, and attention should also be given to the study of Olaya et al. [16], in which the CP or rFTPw values of nine recreational triathletes were lower (3.7 ± 0.5 W/kg) than those found in our study of trained triathletes (4.23 ± 0.51 W/kg). Therefore, the CP values of the present study (4.23 ± 0.51 W/kg) are justified by the levels of the 18 trained male triathletes [21], as they are lower than those determined for a group of 15 high-caliber athletes (4.67 ± 0.42 W/kg) by Ruiz-Alias et al. [20] and higher than those found in recreational triathletes (3.7 ± 0.5 W/Kg) by Olaya et al. [16].

Finally, an in-depth analysis is necessary to extrapolate on power output between cycling and running, and vice versa. Two main considerations need to be taken into account, namely, the differences between the power meters and the specific gestures involved in each sport. Running involves a semi-open-chain cyclic gesture, while cycling is a closed-chain cyclic movement [48]. Therefore, the CP in cycling determined with the 3-min All-Out test [26,27] was considered to be 100% (251.1 W ± 37.0 W) due to its importance as the primary indicator of external load in this sport [10], as well as its consistent theoretical basis for use and application of power [10,13,30,31,32,33,34]. In contrast, the watts corresponding to the running CP determined with the 9/3-minute Stryd CP were overestimated by 20.2% (301.78 W ± 41.55 W) compared with the cycling CP. On one hand, the results of the present study could be partially explained by transferability of running and cycling training zones in triathletes [1]. This study suggests that triathletes should perform sport-specific testing to assess training zones for cycling and running. Consistently with this recommendation, our results showed that power output in cycling explained only 26.7% of the running power output (R² = 0.267; p = 0.284). In addition, significant differences and large ESs were found between cycling and running CP in both absolute power output (p < 0.05; ES = 1.29) and power output relative to body mass (p < 0.05; ES = 1.25). Therefore, it is necessary to carry out a specific test for each discipline to make a correct determination of CP. The hypothesis that power output is not interchangeable between the cycling and running disciplines at CP intensity would be acceptable. One of the main reasons for this is the differences in specific gestures between each sport, as well as differences in power meters.

On the other hand, previous research has shown that there is a relationship between variables such as heart rate and VO_2max obtained during cycloergometer and running tests, which suggests that athletes can use only one mode of testing to determine their training guidelines [49]. However, the present study data indicates that this statement applies only to certain physiological variables of internal-load variables, not to external-load variables such as power.

From a practical point of view, in multisport activities such as triathlon, coaches and athletes should focus on improving their cycling and running segments due to their significant impact on overall performance [11,12]. Nevertheless, it is important to consider the limitations of this study, such as the variability of CP protocols [50,51], the varying levels of performance for each segment of the triathletes, the technical difficulties of running compared to cycling, and the differences in the power meters used for each sport. To minimize the effect of these limitations, the most commonly used tests for determination of CP in cycling and running have been carried out [20,26,27], this study was performed on a population of triathletes familiar with both sports, and a single model of power meter was used for CP determination in each sport. Future studies should investigate the possibility of extrapolating power output at CP intensity between the cycling and running segments in triathletes of different levels (highly trained, elite, and world-class) and sexes (female). These studies should also compare different protocols for CP determination in both cycling and running to provide information on the best power records for different levels and sexes, which would allow ranking of athletes according to their Record Power Profiles. The data from our study can serve as a foundation for comparing cycling and running power data in trained triathletes.

5. Conclusions

The main results of this study suggest that the power outputs of CP in cycling and running cannot be considered interchangeable in trained triathletes. Our findings demonstrate that running CP is overestimated by 20.2% compared to cycling CP. Thus, a discipline-specific test is necessary to determine the CP as well as the corresponding training zones for each discipline. This is due to the significance difference in the specific gestures, the sensors used to measure power, and the protocols of the most-used tests to determine CP in both disciplines. In addition, it is important to consider the performance level of each triathlete in each discipline, as this can also affect CP values.