Differences between Males and Females in Determining Exercise Intensity

Abstract

Even though there are physiological differences between males and females, heart rate (HR), ratings of perceived exertion (RPE), power output (PO), oxygen consumption (VO₂), and blood lactate (BL) levels have been used as measures of exercise intensity independently of sex. The purpose of this study was to determine differences between sexes in different exercise intensity models. Thirty (15 females) young, healthy individuals were scheduled for two testing visits 48–72 hours apart. During the first testing visit, a graded exercise test (GXT), with BL obtained at the end of each exercise step, was administered on a stationary bicycle to determine peak PO and VO₂max. BL during the GXT was used to determine three 5-min steady-state workloads (low: 0–2 mmol/L; moderate: 2–4 mmol/L; and high: >4 mmol/L) for the second test. HR, %HRmax, RPE, PO, %POmax, VO₂, %VO₂max, and BL were obtained at the end of each steady-state workload. A two-way repeated measures ANOVA was performed to compare all exercise intensity variables obtained during the second test between males and females (α=0.05). Only RPE, %PO, and BL did not differ between sexes on all 3 exercise intensities. HR, %HR, and PO differ between sexes on at least 2 exercise intensities. Females have higher HR and %HR than males for similar %PO. VO₂ and %VO₂max differ between sexes on at least 1 exercise intensity. Based on the current results, traditional exercise intensity markers are different between males and females. BL and %PO appear to be markers that might be used independently of sex.

INTRODUCTION

Important components of exercise prescription include the frequency, duration, type, and intensity of physical activity (2). Among these components, the most important is the intensity of the exercise (25). In general, exercise intensity can be determined using appropriate target heart rates, ratings of perceived exertion, or blood lactate levels (23).

To achieve optimal performance, exercise prescription should be individualized for each person. However, there is not much difference when prescribing exercise intensities for males and females (1). Physiological differences under resting conditions between men and women, such as fat free mass and blood volume, are well established (7, 10, 32). The effect that these differences have on physiological responses to exercise has been tested but many controversial results have been reported (10, 16, 39). If men are larger, have more muscle mass, and less body fat than women, and women have shown greater cardiac work requirements to meet the same external workload demand as men (7), why should exercise intensity prescriptions be the same for both sexes?

Studies have shown controversial findings when comparing men and women during and after submaximal exercise tests (10, 16, 28, 39). For example, Garcin et al. (16) and Deschenes et al. (10) showed no differences between men and women in regard to heart rate (HR) during aerobic exercise. However, Wheatley et al. (39) and Laurent et al. (28) reported higher HR in females compared to males for similar workloads. Similarly, studies have shown conflicting results for oxygen uptake (VO₂) with results ranging from no gender differences during resting conditions to females having higher VO₂ than males for the same workload during moderate and vigorous intensities, suggesting females may be at a higher cardiopulmonary demand (28, 39).

In addition, exercise prescription following absolute or relative intensities (e.g., HR vs. %HR or VO₂ vs. %VO₂max) is still controversial (5, 31). For example, Berglund et al. (5) showed that peak HR during a traditional exercise test was different to real HRmax obtained in a specific HRmax test. These findings confirm that some of these exercise intensity markers have multiple factors that could affect their specificity and/or reliability.

Even though there are physiological differences between males and females, HR, rate of perceived exertion (RPE), power output (PO), VO₂, and blood lactate (BL) levels have been used as measures of exercise intensity independently of the sex. Moreover, absolute or relative exercise intensities are normally prescribed also independently of sex. Therefore, the present study is novel as it seeks to determine if traditional exercise intensity markers behave similarly between males and females. The purpose of this study was to examine differences in determining exercise intensity between young, healthy males and females during steady state aerobic exercise. We hypothesized that markers of exercise intensity will be different between males and females at different exercise intensities.

METHODS

Participants

Thirty young healthy participants (15 males and 15 females) were recruited to participate in the study. A priori power and sample size analyses, using G*Power 3.1 (13, 14), showed a minimum of 8 subjects per group. Participants were eligible if they were between 18 and 35 years old and if they were considered apparently healthy based on a preliminary health questionnaire. Exclusion criteria included known history of cardiovascular, cardiac, pulmonary, or metabolic disease, taking prescription medications, any extremity injury that could affect cycling or attaching a blood pressure cuff to the arm, and pregnant females. Participants that were taking over-the-counter painkillers or nutritional supplements containing antioxidants were required to abstain from their use 12 h prior to each lab visit. Female subjects were tested within 4 days before and after menses to standardize any hormonal influence on vascular reactivity (3). Oral contraceptive female users were tested during inactive-pill phase to be consistent with other female subject’s instructions, even though the use of contraceptive has shown no differences in exercise responses (33). The study was approved by the Institutional Review Board of The University of Texas at El Paso and all individual participants included in the study signed the informed consent before scheduling the first visit to the Clinical Applied Physiology (CAPh) Lab. This research was carried out fully in accordance to the ethical standards of the International Journal of Exercise Science (36).

Protocol

The current protocol has been described before (8, 35). Briefly, two exercise tests were performed on a cycle ergometer (Corival, Lode, The Netherlands) in the same timeframe 24–48 h apart. Before each exercise test, height (Seca Medical, Germany), and weight (WB-110A Class III, Tanita, Japan) were obtained. Seat height was adjusted to allow a 5° to 15° of knee flexion during cycling. Participants were instructed to sit quietly on the bike for 10 min to ensure that increased sympathetic activity due to nervousness did not alter blood pressure readings (27). During the resting period, three peripheral blood pressure values were recorded using an automated brachial blood pressure cuff (BP760, Omnron Healthcare, Inc., Lake Forest, IL).

The first exercise test was an 8–12-min maximal, incremental exercise test that followed the American Heart Association and American College of Sports Medicine guidelines for exercise testing (2, 15). The first exercise test was designed to obtain individual’s VO₂max, PO in watts, and lactate threshold, which were used to determine exercise intensities for the second test. Lactate levels have been employed as a biomarker for exercise intensity and training monitoring in athletes for several years (12) and currently they have been proposed as intensity markers for exercise prescription in clinical population (20, 21). Blood lactate levels, assessed via micro-sample from the earlobe and analyzed with an automated lactate analyzer (Lactate Plus, Nova Inc., Boston, MA), HR (Quinton Q-Stress Cardiac Stress System, Mortara Instrument, Milwaukee, WI), VO₂ (TrueOne 2400, Parvomedics Inc., Sandy, UT), and RPE (Borg scale) were measured at rest and at the end of each 2-min stage of the exercise test. Lactate threshold curves obtained during this first exercise test were used to determine the three steady-state workloads to be used for the second exercise test.

During the second exercise test, participants were asked to exercise 5 min at each different workload, according to the following lactate training zones: low intensity (BL levels of 0–2 mmol/L), moderate intensity (BL levels of 2–4 mmol/L), and high intensity (BL levels>4 mmol/L) maintaining a constant cadence between 60 and 70 rpm (20, 29). There was no pause between 5-min workloads and all subjects performed low intensity first, then moderate intensity, and finally high intensity. HR and VO₂ were continuously monitored throughout the 15-min exercise test and blood lactate and RPE were assessed at baseline and at minutes 2 and 4 of each three exercise intensities. If BL levels were over or under the target, PO was adjusted accordingly.

Statistical Analysis

Descriptive statistics, including mean, standard deviations (S.D.), standard error of the means (S.E.M), and minimum and maximum values were obtained. Normal distribution for all dependent variables was confirmed using Shapiro-Wilkins and Smirnov tests (at least one test p>0.05). Independent samples t-tests were performed for comparisons of group demographic variables. Two-way repeated measurements-ANOVAs comparing exercise intensities and sex (Intensity x Sex) were performed for all exercise intensity markers. Fisher’s Least Significant Difference was used as post-hoc analysis. Data is expressed as mean ± S.D. unless otherwise stated. All statistical analyses were performed using SPSS (version 24.0, IBM, Chicago, Il), and statistical significance was considered when p<0.05.

RESULTS

Data were confirmed normally distributed. Demographics and general characteristics of the sample are shown in Table 1. Males were taller (p=0.002) and heavier (p<0.001) than females; although no difference on BMI was observed. In addition, males had a larger VO₂max (p=0.008) than females. BL levels during each workload staid within all 3 specific ranges (0–2, 2–4, and >4 mmol/L) and no PO adjustments were needed.

Table 1.

Demographic characteristics of the sample.

	Overall n=30	Females n=15	Males n=15
Age (years)	21.9 ± 3.2	21.3 ± 2.3	22.7 ± 4.0
Height (cm)	172.1 ± 9.1	166.5 ± 8.4	177.6 ± 5.9*
Weight (kg)	72.9 ± 18.4	62.1 ± 9.4	83.7 ± 19.0*
BMI (kg/m2)	25.0 ±4.9	23.4 ± 3.5	27.0 ± 5.3
SBP (mmHg)	114.5 ± 7.2	113.1 ± 6.9	116.1 ± 7.5
DBP (mmHg)	77.6 ± 5.6	77.1 ± 5.8	78.2 ± 5.6
VO2max (ml/kg/min)	31.8 ± 7.4	28.3 ± 6.3	35.2 ± 6.9*
Resting BL (mmol/L)	0.9 ± 0.3	0.8 ± 0.3	0.9 ± 0.3

BMI= Body mass index; SBP=Systolic blood pressure; DBP=Diastolic blood pressure; VO₂max= Maximal oxygen uptake; BL= Blood lactate;

^*=p<0.05 males vs. females

Two-way repeated measurements ANOVA statistics are presented in Table 2. All exercise intensity markers showed a significant exercise intensity effect. Only PO, HR, and %HR showed a significant sex effect. Significant interaction was only observed in PO, %PO, VO₂, and %VO₂max.

Table 2.

Two-way repeated measurements ANOVA data.

Variable	Intensity Effect	Sex Effect	Interaction
PO (Watts)	F(2, 28)=470.4	F(1, 14)=7.868	F(2, 28)=30.88
	p<0.001	p=0.014	p<0.001
%PO (%)	F(2, 28)=2496	F(1, 14)=3.148	F(2, 28)=4.447
	p<0.001	p=0.098	p=0.021
VO2 (mL/kg/min)	F(2, 28)=203.3	F(1, 14)=2.977	F(2, 28)=8.890
	p<0.001	p=0.106	p=0.001
%VO2max (%)	F(2, 28)=292.7	F(1, 14)=4.458	F(2, 28)=3.931
	p<0.001	p=0.053	p=0.031
BL (mmol/L)	F(2, 28)=321.1	F(1, 14)=0.148	F(2, 28)=0.812
	p<0.001	p=0.706	p=0.454
RPE	F(2, 28)=253.3	F(1, 14)=0.073	F(2, 28)=0.558
	p<0.001	p=0.791	p=0.578
HR (bpm)	F(2, 28)=152.0	F(1, 14)=9.310	F(2, 28)=9.310
	p<0.001	p=0.009	p=0.705
%HR (%)	F(2, 28)=412.6	F(1, 14)=24.04	F(2, 28)=1.443
	p<0.001	p<0.001	p=0.253

PO= Power Output; VO₂= Oxygen uptake; BL= Blood Lactate; RPE= Ratings of Perceived Exertion, HR= Heart Rate.

Different exercise intensity markers for all 3 physiological workloads are presented in Table 3. Females worked at a higher percentage of the VO₂max during the lower physiological workload (p=0.010). However, at higher intensities males worked out at a higher VO₂ and PO than females (p=0,034 and p=0.001, respectively), but no differences were observed between males and females for percentage of the VO₂max and relative PO (%VO₂max and %PO, respectively). Throughout the stages of Low (0–2mmol/L), Moderate (2–4mmol/L), and High (>4mmol/L) workloads RPE, and BL did not differ between sexes.

Table 3.

Different exercise intensity markers at different physiological workloads.

Workload	Variable	Females (n=15)	Males (n=15)
Low (0–2 mmol/L)	PO (Watts)	61.3 ± 20.3	67.3 ± 15.3
	%PO (%)	44.1 ± 11.2	36.4 ± 9.7
	VO2 (mL/kg/min)	15.1 ± 3.7	15.7 ± 3.6
	%VO2max (%)	54.0 ± 9.8	45.6 ± 8.4*
	BL (mmol/L)	1.5 ± 0.4	1.3 ± 0.4
	RPE	7.7 ± 2.0	7.5 ± 1.6
Moderate (2–4 mmol/L)	PO (Watts)	93.3 ± 22.0	116.7 ± 19.5*
	%PO (%)	67.6 ± 10.3	62.1 ± 8.3
	VO2 (mL/kg/min)	20.1 ± 4.2	24.7 ± 5.1*
	%VO2max (%)	71.9 ± 10.0	71.3 ± 6.7
	BL (mmol/L)	2.6 ± 0.6	2.8 ± 0.7
	RPE	10.5 ± 2.4	10.8 ± 2.4
High (>4 mmol/L)	PO (Watts)	123.7 ± 23.0	166.0 ± 29.2*
	%PO (%)	89.9 ± 8.6	87.8 ± 8.2
	VO2 (mL/kg/min)	26.3 ± 5.5	31.0 ± 5.6
	%VO2max (%)	93.3 ± 7.4	89.6 ± 5.5
	BL (mmol/L)	4.7 ± 0.7	4.8 ± 1.1
	RPE	13.7 ± 2.4	14.1 ± 2.4

PO= Power Output; VO₂= Oxygen uptake; BL=Blood Lactate; RPE= Ratings of Perceived Exertion;

^*=p<0.05 males vs. females.

Females had higher HR than males at Low (116 ± 14 bpm vs. 97 ± 14 bpm, p=0.004) and High (164 ± 16 bpm vs. 149 ± 17 bpm, p=0.016) workloads (Figure 1). In addition, females had higher %HR than males in all three workloads (Low: 69 ± 8% vs. 59 ± 9%, p=0.002; Moderate: 85 ± 6% vs. 75 ± 7%, p<0.001; High: 97 ± 4% vs. 91 ± 7%, p=0.016) (Figure 2).

Figure 1.

Heart Rate Comparison between Females and Males. * = p<0.05

Figure 2.

Percent Heart Rate Comparison between Females and Males. * = p<0.05

DISCUSSION

The present study intended to find differences in determining exercise intensity between young, healthy males and females during steady state aerobic exercise. The present study found that some exercise intensity markers were different between males and females. The key findings of the present study were 3-fold; 1) VO₂ and %VO₂max differed between sexes only at Moderate and Low intensities, respectively; 2) variables which differed on at least 2 exercise intensities were HR, %HR, and PO; and 3) only RPE, %PO, and BL did not differ between sexes on all 3 exercise intensities.

Even though VO₂ and %VO₂max did not show a main sex effect (Table 2), the observed interaction can be attributed to differences between sexes on at least one intensity (Table 3). Males exercised at a larger VO2 than females during Moderate intensity (BL: 2–4 mmol/L), while females exercised at a larger %VO2 than males during Low intensity (BL: 0–2 mmol/L). Previous reports have shown that women have lower maximal oxygen uptake in comparison to men of similar age and fitness level (9, 10). These differences can be attributed to a decrease in cardiac output and stroke volume in females while exercising at equivalent relative submaximal intensities (%VO₂max) compared to males (41) and/or to a lower hemoglobin concentration observed in females (40–42). On the other hand, other studies have shown consistent findings expressing no significant differences between genders when accounting for a relative VO₂ (10, 37). The results on the present study show that VO₂ might be associated with PO in both males and females (Table 2 and 3); as PO increases, VO₂ increases. In addition, %VO₂max might also be associated with %PO and there were not significant differences between males and females, except at the lower exercise intensity, where females had to exercise at a higher percentage of the VO₂max (54.0%±9.8% vs. 45.6%±8.4%, p<0.05) to maintain the same BL levels (<2.0 mmol/L). This finding could be associated to a lower relative muscle mass observed in females who might need to recruit more muscle fibers at lower intensities, increasing peripheral oxygen extraction and %VO₂max (6, 39).

The present study showed that HR and %HR were higher on females than males during all 3 workloads (Figures 1 and 2). These results are consistent with previous reports (28, 34, 38, 39, 41). These differences can be attributed to females having smaller hearts and levels of blood volume than males, even when adjusted for body mass, which will elicit a higher HR to keep blood supply to exercising muscles (34). Considering that our sample was obtained from young, apparently healthy, non-trained individuals, we could attribute the observed differences to normal baseline differences such as heart size and lower blood volume reported by others (41). Deschenes et al. (10) performed a similar study conducted to compare the physiological responses of males and females to similar relative submaximal intensity where they found that HR and %HR behaved similarly between males and females during both exercise and recovery periods. However, they did not compare males and females during individual workloads what make difficult to compare with the present study (10). The present study also showed similar HR and %HR behaviors in both males and females. HR and %HR in both males and females increased with increased exercise intensity (Table 2 and Figures 1 and 2). However, in the present study females’ HR and %HR increased to a greater extent than males in at least 2 exercise intensity levels, in all 3 levels for %HR, showing that %HR and HR may not be good markers for exercise intensity to use interchangeably in males and females.

The present study showed no differences in RPE between males and females during all 3 exercise workloads. These results are consistent to other reports where RPE did not differ between males and females using different exercise intensity markers (10, 37, 39). Wheatley et al. (39) studied 31 males and 33 females during 2 exercise intensities (i.e. 40% and 75% PO) and showed no differences in RPE between males and females. Robertson et al. (37) studied 9 males and 10 females during different exercise intensities performing 3 types of weight bearing exercise. Even though the authors found differences on RPE between males and females at absolute VO₂, these differences disappeared when using %VO₂max. Consistently, Deschenes et al. (10) studied 10 males and 10 females during one 60–65 %VO₂max bout of exercise and found that there were no differences in RPE values reported by males and females throughout a 30-minute exercise session. Even though RPE might not represent the best marker for exercise intensity (11, 18, 19, 21, 26), the present and previous studies show that RPE works independently of biological sex, which could make it a consistent tool for prescribing exercise intensity in both males and females.

Interestingly, the current results showed an intensity x sex interaction for %PO, without any differences between sexes (Tables 2 and 3). This interaction might be associated with a smaller difference between sexes at high intensity (Δ=2.1%) when compared to low intensity (Δ=7.7%), which reflects a rather dynamic workload. Nevertheless, BL and %PO appear to be the exercise intensity models that showed more consistency and might be used independently of biological sex. These results are consistent with previous studies (9, 22, 39). On their study, Wheatley at al. (39) also showed no differences in BL between males and females at resting, moderate (40% PO), vigorous (75% PO), and after exercise. Demello et al. (9) studied 10 untrained males and females during several bouts of aerobic exercise at different exercise intensities determined by %VO₂max. Their findings showed that there were no differences in BL between males and females in any of the 5 exercise intensities (9). Blood lactate levels have also shown to be dependable across multiple studies finding that males and females achieve maximal lactate steady state at similar relative intensities (4, 6, 22, 24). In addition, Green at al. (17) did not find any difference in BL levels between males and females during 2 bouts of exercise, constant load cycling and interval cycling. Similarly, Zhang and Li (43) showed no differences in BL at resting, peak (after a VO₂max test), and at any time of 30-min recovery between untrained males and females. Finally, Hafen and Vehrs (22) and Hoffmann et al. (24) also observed similar results that the current study in well-trained males and females. Interestingly, Deschenes et al. (10) showed a significant difference in BL levels between males and females during a submaximal (i.e. 60–65 %VO₂max) exercise bout. These contrasting results might have several reasons. First, even though Deschenes et al. (10) stated that their subjects were untrained, their VO₂max (45.1 and 38.2 mL/kg/min, males and females respectively) are considered average (2, 7) and were higher than the observed in the current study (35.2 and 28.3 mL/kg/min, males and females respectively), which are considered ‘way below average’. Secondly, it is not clear the %PO their subjects performed to stay on target as they altered the workload (in watts) as needed. Finally, there is a notorious difference in the BL levels between both studies during a similar %VO₂max (~2.5 mmol/L @~71.5% in the current study vs. ~6.5 mmol/L in males and ~4.3 mmol/L in females @~63.0%), which might be due to plasma processing time, which did not include any glycolysis inhibitor such as Glyceraldehyde (30). Nevertheless, it appears that subjects in the current study workout at lower absolute PO than the previous study what could have driven the differences in BL levels between studies.

The present study was not without limitations. First, our sample size was composed of only young healthy individuals that may not allow us to generalize our findings. In addition, our subjects were categorized as ‘way below average’ by their VO2max (2, 7), which could have affected some of the results at the High intensity workload, especially after 15 minutes of continuous exercise. Moreover, measurements of body composition, to characterize fat mass and fat free mass, could help explaining some of the current results. A larger sample size, including subjects of different age groups and fitness levels, would serve as a better representation of the population. For further studies, we may include well-trained individuals and a randomization of the workloads in different days to see if these results can be replicated. Finally, the present study used cycle ergometer testing and results might be different when using other exercise modalities such as running, swimming, or rowing.

Although VO₂, %VO₂max, HR, %HR, PO, %PO, RPE, and BL have continuously been used interchangeable in males and females, the present study found different behaviors of exercise intensity markers in males and females. Based on the present and other studies, it appear that the use of %PO, supported by BL and RPE, might be a more consistent way to determine exercise intensities, regardless of biological sex. Traditional exercise intensity markers, such as HR, %HR, PO, VO₂, and %VO₂max, showed more variability when comparing males and females; therefore, should not be used interchangeably between males and females.