Cardiometabolic response to high intensity functional training versus rowing-based high intensity interval training

Abstract

This study compared the cardiometabolic response between rowing high intensity interval training (HIIT) and high intensity functional training (HIFT). Twenty two adults (age and $\dot{\mathrm{V}}{\mathrm{O}}_2\max$ ​= ​[25 ​± ​7] yr and [40 ​± ​9] mL/kg/min) underwent incremental exercise on a rowing ergometer to assess $\dot{\mathrm{V}}{\mathrm{O}}_2\max$ and peak power output (PPO). Subsequently, they underwent rowing HIIT (six 1 minutes [min] efforts at 85% PPO with 75 seconds [s] recovery) or HIFT (six 1 ​min efforts including pushups, jump squats, mountain climbers, and air squats with 75 ​s recovery). Gas exchange data, heart rate (HR), and blood lactate concentration (BLa) were acquired during exercise and post-exercise. These regimens elicited 92%–96% HRmax, with higher peak HR ([174 ​± ​14] b/min vs. [167 ​± ​15] b/min, p ​= ​0.002) and time spent ≥ 85% HRmax ([6.1 ​± ​2.2] min vs. [5.0 ​± ​2.8] min, p ​= ​0.03) for HIFT versus rowing HIIT. Results showed similar peak $\dot{\mathrm{V}}{\mathrm{O}}_2$ between HIFT and HIIT ([81 ​± ​9] vs. [82 ​± ​8] $\dot{\mathrm{V}}{\mathrm{O}}_2\max$, p ​= ​0.72), yet lower mean $\dot{\mathrm{V}}{\mathrm{O}}_2$ ([1.73 ​± ​0.38] L/min vs. [1.93 ​± ​0.50] L/min, p ​< ​0.001). Mean and peak ventilation (${\dot{\mathrm{V}}}_{\mathrm{E}}$) was significantly higher (p ​< ​0.05) with HIFT ([62 ​± ​13] L/min and [84 ​± ​18] L/min vs. [53 ​± ​15] L/min and [73 ​± ​19] L/min). BLa was significantly higher with HIFT versus rowing HIIT (p ​< ​0.001) and was elevated 15 ​min post-exercise ([7.0 ​± ​3.0] mM vs. [3.5 ​± ​1.1] mM). In healthy, active adults, HIFT elicits vigorous intensities similar to traditional HIIT.

List of abbreviations

b/min

beats per minute

BLa

blood lactate concentration

d

days

HIFT

high intensity functional training

HIIT

high intensity interval training

h

hours

HR

heart rate

MICT

moderate intensity continuous training

min

minute

PACES

physical activity enjoyment scale

PPO

peak power output

RER

respiratory exchange ratio

RPE

rating of perceived exertion

s

seconds

SD

standard deviation

SIT

sprint interval training

$\dot{\mathrm{V}}{\text{CO}}_2$

volume of carbon dioxide produced

${\dot{\mathrm{V}}}_{\mathrm{E}}$

ventilation

$\dot{\mathrm{V}}{\mathrm{O}}_2$

volume of oxygen consumed

$\dot{\mathrm{V}}{\mathrm{O}}_{2\max }$

maximal oxygen uptake

wk

weeks

yr

years

1. Introduction

High intensity interval training (HIIT) is defined as “near-maximal” efforts performed at an intensity that elicits > 80% (but often 85%–95%) of maximal heart rate (HR).¹ Despite a lower volume of work than a typical session of moderate intensity continuous training (MICT), HIIT induces significant increases in maximal oxygen uptake ($\dot{\mathrm{V}}{\mathrm{O}}_{2\max }$),² fat oxidation,³ and reductions in insulin resistance,⁴ blood pressure,⁵ and body fat.⁶ In addition, superior increases in $\dot{\mathrm{V}}{\mathrm{O}}_{2\max }$ are reported in response to HIIT compared to MICT,⁷ further supporting its efficacy to enhance health and fitness in various adult populations.

Nevertheless, a few criticisms of HIIT have been raised. First, most studies implement unimodal aerobic modalities including walking, running, or cycling which require a fixed stimulus and equipment that may be inaccessible to some adults. Moreover, although unimodal HIIT consistently increases $\dot{\mathrm{V}}{\mathrm{O}}_{2\max }$,^2,7 it does not typically increase muscular strength.⁸ This inability to augment strength is important, since muscular strength is related to lower mortality risk in adults.⁹ Therefore, a minimum of 2 days (d) per week of resistance training are recommended to improve strength,¹⁰ yet this requirement may be burdensome for some adults due to a perceived lack of time.

One iteration of HIIT that has potential to simultaneously enhance $\dot{\mathrm{V}}{\mathrm{O}}_{2\max }$ and muscular strength is high intensity functional training (HIFT). Although a standardized definition of this regimen does not exist, HIFT typically involves repeated and consecutive bouts of dynamic, multimodal exercise such as burpees, mountain climbers, lunges, squats, etc. using body weight or dumbbells as resistance. In active women, McRae et al.¹¹ showed that 16 sessions of HIFT consisting of burpees, jumping jacks, mountain climbers, and squat thrusts led to a significant increase in $\dot{\mathrm{V}}{\mathrm{O}}_{2\max }$ similar to MICT, but it also improved whole-body muscular endurance. More recently, Scott et al.¹² demonstrated that 12 ​weeks (wk) of home-based HIFT consisting of various body weight exercises significantly increased $\dot{\mathrm{V}}{\mathrm{O}}_{2\max }$, reduced body fat, and enhanced muscle capillarization similar to lab-based cycling HIIT in adults with increased risk of cardiovascular disease. In a meta-analysis of 17 studies, Wilke and Mohr¹³ demonstrated small to moderate increases in muscular strength in response to HIFT versus no exercise. In addition, another meta-analysis¹⁴ showed that compared to MICT, chronic HIFT elicits significantly lower improvements in $\dot{\mathrm{V}}{\mathrm{O}}_{2\max }$, yet similar changes in fat mass and fat free mass. Together, these data show the potential of HIFT to elicit adaptations typically associated with HIIT using aerobic modalities, while also increasing muscle mass and strength.

The long-term increase in $\dot{\mathrm{V}}{\mathrm{O}}_{2\max }$ observed in response to HIIT may be related to the ability of a single session of HIIT to elicit substantial duration at or near maximal heart rate (HR) or oxygen uptake.¹⁵ In addition, Egan and Zierath¹⁶ denoted that the metabolic stress of acute exercise is related to the long-term endurance and muscular response. Bellissimo et al.¹⁷ reported significantly higher peak HR and $\dot{\mathrm{V}}{\mathrm{O}}_2$ in response to treadmill-based HIIT at 100% of the velocity attained at $\dot{\mathrm{V}}{\mathrm{O}}_{2\max }$ versus HIFT (high knees, squat jumps, scissor jacks, jumping lunges, and modified burpees), although HIFT revealed higher blood lactate concentration (BLa). Similarly, 7 ​minutes (min) of cycling-based HIIT at 70 percent of peak power output (%PPO) led to significantly higher HR and $\dot{\mathrm{V}}{\mathrm{O}}_2$ compared to a 7 ​min HIFT protocol consisting of 12 different body weight exercises.¹⁸ Nevertheless, Gist et al.¹⁹ showed no difference in peak HR/ $\dot{\mathrm{V}}{\mathrm{O}}_2$ between completion of four Wingate tests and 4 ​× ​30 ​seconds (s) “all-out” burpees in active young adults. Differences in the structure of HIIT and HIFT regimens and specific exercises completed during HIFT likely explain disparate results across studies and merit further characterization of the change in HR, $\dot{\mathrm{V}}{\mathrm{O}}_2$, and BLa, as these changes seem to be associated with the long-term response to training.

The aim of the present study was to compare the cardiometabolic response to two whole-body interval protocols, rowing-based HIIT and HIFT. Rowing-HIIT as previously implemented²⁰ was used in lieu of cycling or running, as cycling induces leg pain²¹ which is unpleasant for many adults. Also, running may not be feasible for many adults due to onset of lower-extremity strains, injuries, or inability to run for prolonged periods. In addition, we acquired gas exchange data, HR, and BLa post-exercise to examine post-exercise metabolism, which is elevated after HIIT and its more intense form, sprint interval training (SIT), typically performed at intensities above that associated with PPO or velocity from a $\dot{\mathrm{V}}{\mathrm{O}}_{2\max }$ test.²² It was hypothesized that $\dot{\mathrm{V}}{\mathrm{O}}_2$ would be significantly higher in response to rowing-HIIT, yet HIFT would elicit significantly higher BLa. It was also predicted that post-exercise $\dot{\mathrm{V}}{\mathrm{O}}_2$ and BLa would be significantly higher in response to HIFT versus rowing-HIIT.

2. Material and methods

2.1. Ethical approval

All participants were informed of the procedures attendant with the protocol and provided written informed consent, with the study approved by the California State University—San Marcos Institutional Review Board (protocol 2028365-1). The study was implemented in accordance with the Declaration of Helsinki.

2.2. Experimental design and participants

The study was a randomized, crossover design in which participants completed three sessions at the same time of day within subjects (08:00 a.m.-12:00 p.m.). Data collection began in October 2022 and was completed in May 2023. Each session was preceded by 36 ​h abstention from physical exercise, no food for 3 ​h prior, and no caffeine that morning, which were verified with a written log. During each session of exercise, gas exchange data, HR, and BLa were measured as well as perceptual responses. These measures were acquired for 15 ​min post-exercise while participants sat quietly by themselves. The trial protocol was registered with OSF.

Habitually active men and women ages 18–55 ​years (yr) who complete a minimum of 150 ​min/wk of moderate exercise or 75 ​min/wk of vigorous exercise (determined with the health-history questionnaire) were recruited by word-of-mouth. Participants were non-obese, did not smoke, were weight stable, and did not have any condition including pregnancy or menopausal status which may preclude their participation. They initially completed a standard health-history questionnaire as well as the PAR-Q to confirm their eligibility. Twenty four men and women initially provided consent, yet only 22 completed all requirements of the study, as one male participant did not complete HIFT, and one female participant learned that she was pregnant after completing baseline testing. Their demographic traits are listed in Table 1, and participant flow through the study is shown in Fig. 1.

Table 1.

Participants demographic characteristics.

Parameter	Mean ​± ​SD	Range
Age (yr)	25 ​± ​7	20–50
Sex (M:F)	17:5	NA
Body mass (kg)	74 ​± ​14	57–102
Body mass index (kg/m2)	24.1 ​± ​3.1	18.6–29.7
Body fat (%)	16 ​± ​7	8–31
Physical activity (h/wk)	7 ​± ​2	3–14

yr ​= ​years; M ​= ​male; F ​= ​female; kg ​= ​kilograms; m ​= ​meters; h/wk ​= ​hours per week.

Fig. 1.

Participant flow diagram; $\dot{\mathrm{V}}{\mathrm{O}}_{2\max }$ ​= ​maximal oxygen uptake; HIFT ​= ​high intensity functional training; HIIT ​= ​high intensity interval training.

2.3. Baseline session

Body mass and height were measured with a balance beam scale and stadiometer, and body composition was determined using a sum of three skinfolds following the methods of Jackson and Pollock.^23,24 Subsequently, participants were familiarized with exercise on the rowing ergometer (Concept 2 Model E, Morrisville, VT) by requiring them to row for approximately 1 ​min at power outputs equal to 40 ​W, 50 ​W, 60 ​W, and 70 ​W. Participants were instructed to use proper form, including driving through the legs, engaging the hips, pulling the handle with the arms to the bottom of the chest, and then reversing the movement by allowing the arms to straighten and bending the legs to return to the starting position. After a brief passive recovery, incremental exercise ensued to determine $\dot{\mathrm{V}}{\mathrm{O}}_2\max$ and PPO. Power output was individualized for the sex, body size, and exercise capacity of each participant and began at 30 ​W, 40 ​W, 50 ​W, or 60 ​W for the first 2 ​min followed by 15 ​W/min– 30 ​W/min increases in power output until volitional fatigue, which was represented by three consecutive strokes not within 5 ​W of the desired intensity. This protocol was used in prior studies in adults unfamiliar with rowing,^20,25 and pilot testing showed that this protocol leads to valid and reliable estimates of $\dot{\mathrm{V}}{\mathrm{O}}_2\max$. In addition, it is apparent that rowing-based $\dot{\mathrm{V}}{\mathrm{O}}_2\max$ is not different from cycling-based $\dot{\mathrm{V}}{\mathrm{O}}_2\max$ in individuals unfamiliar with either modality.²⁵ Peak power output (in W) was identified as the intensity attained at volitional fatigue if the stage was performed for at least 30 ​s; whereas, if exercise termination occurred less than 30 ​s into the stage, the prior stage's power output represented PPO. During the bout, $\dot{\mathrm{V}}{\mathrm{O}}_2$, carbon dioxide production ($\dot{\mathrm{V}}{\text{CO}}_2$), ventilation (${\dot{\mathrm{V}}}_{\mathrm{E}}$), and respiratory exchange ratio (RER) were recorded every 15 ​s using a metabolic cart (ParvoMedics True One, Sandy, UT), which was calibrated pre-exercise. Heart rate was assessed using telemetry every second (Polar electro, Woodbury, NY), and BLa was measured 3 ​min after incremental exercise from a dry fingertip using a lancet (Owen Mumford, Marietta, GA) and portable monitor (Lactate Plus, Nova Biomedical, Waltham, MA).

Participants completed a 2 ​min recovery at 40 ​W–60 ​W, rested for 5 ​min, then initiated rowing at 20% PPO for 2 ​min, 40% PPO for 1 ​min, and then constant load rowing at 85% PPO until volitional fatigue to verify $\dot{\mathrm{V}}{\mathrm{O}}_{2\max }$ attainment, which was identified as the mean value from the average of two highest 15 ​s data values in the last 45 ​s of exercise. Maximal values of HR, RER, and ${\dot{\mathrm{V}}}_{\mathrm{E}}$ were also calculated using this approach. Lastly, participants were familiarized with the HIFT session as they completed one set of this protocol.

2.4. Sessions of rowing-HIIT and HIFT

Upon completion of baseline testing, order of these sessions was allocated by the Primary Investigator using a coin flip. Sessions were preceded by a 3 ​h fast, no physical activity for 36 ​h, and no caffeine that day. In addition, participants recorded dietary intake on a written log for 48 ​h before the initial session, and this pattern was repeated prior to the subsequent session. Initially, participants sat quietly for 5 ​min to enable assessment of BLa. Subsequently, participants completed a 2.5 ​min warmup at 20% PPO on the rowing ergometer, followed by completion of one of two time-matched protocols, whose order was randomized across sessions. Rowing-HIIT consisted of six 1 ​min intervals at 85% PPO separated by 75 ​s recovery at 20% PPO, which elicits peak HR equal to 90% HRmax.²⁰ Our prior work^20,25 shows that adults unfamiliar with rowing can attain target intensities as required in the present study. During each interval, researchers repeatedly reminded each participant to maintain the target intensity. Stroke rate ranged from 30 to 40 strokes per minute across participants and was self-selected. High intensity functional training consisted of six sets of the following four exercises, completed as fast as possible, in this order: 10 pushups (women were allowed to have their knees on the ground if appropriate), 10 jump squats, 10 mountain climbers per leg, and 20 air squats. Each interval was separated by 75 ​s of walking in place. Recent work revealed that this circuit required approximately 1 ​min duration and elicited peak HR equal to 90% HRmax.²⁶ Gas exchange data and HR were acquired during and post-exercise, and BLa was measured immediately after set 3 ​min and then 5 ​min, 10 ​min, and 15 ​min into recovery.

Outcomes acquired from the sessions included mean and peak $\dot{\mathrm{V}}{\mathrm{O}}_2$, HR, RER, and ${\dot{\mathrm{V}}}_{\mathrm{E}}$. Mean values were calculated as the average of all 15 ​s values acquired in each session, not including rest, warm-up, or post-exercise; whereas, the peak value represented the highest value recorded from any of the six intervals. Values from each interval were calculated from the average of the last two 15 ​s exercise values and the first value in recovery.27, 28, 29 Post-exercise values of gas exchange data and HR were acquired as the average of three consecutive 15 ​s values prior to minute 5, 10, and 15. In addition, total $\dot{\mathrm{V}}{\mathrm{O}}_2$ (in L) was calculated as the sum of all 15 ​s $\dot{\mathrm{V}}{\mathrm{O}}_2$ values derived during and post-exercise, divided by 4. Time spent (in min) at ​≥ ​85% HRmax was also determined for each session.

2.5. Assessment of perceptual responses

During $\dot{\mathrm{V}}{\mathrm{O}}_2\max$ testing, participants were familiarized to the Borg³⁰ 6–20 rating of perceived exertion (RPE) scale as well as affective valence (The Feeling scale, +5 to −5),³¹ which is used to measure the pleasure: displeasure experienced during exercise. To assess RPE, investigators instructed participants to report their exertion based on their level of fatigue, breathing, and HR. To describe affective valence,³¹ we read the participants the following script: While participating in exercise, it is common to experience changes in mood. Some individuals find exercise pleasurable; whereas, others find it to be unpleasant. Additionally, feeling may fluctuate across time. That is, one might feel good and bad a number of times during exercise. These values were recorded every other stage during incremental exercise and immediately upon exercise cessation. During the subsequent two sessions, these values were recorded pre-exercise, during the warm-up, immediately after bout 3 and 6, and then at 5 ​min, 10 ​min, and 15 ​min post-exercise.

At 15 ​min post-exercise, the 3-way valve was removed, and participants reported their enjoyment using a validated scale (Physical Activity Enjoyment scale, PACES)³² and rated their intention to complete each regimen 3 ​d/wk and 5 ​d/wk, as previously used.³³ This measure contains two items which state: Please rate the extent to which you agree with the following statements 1) I intend to engage in the type of exercise I performed today at least 3 times per week during the next month’’ and 2) I intend to engage in the type of exercise I performed today at least 5 times per week during the next month’. Participant scores were evaluated on a 7-point scale with anchors ranging from ‘‘Very unlikely’’ (1) to “Very likely” (7).

2.6. Statistical analyses

Data are reported as mean ​± ​standard deviation (SD) and were analyzed using SPSS Version 27 (IBM, Armonk, NY). The Shapiro-Wilks test was used to verify that all outcomes had a normal distribution. Two-way repeated measures ANOVA was used to examine differences in the timecourse of changes in gas exchange data, HR, perceptual responses, and BLa between HIFT and rowing HIIT. The Greenhouse-Geisser correction was used if the sphericity assumption was violated. Paired t-test was used to examine differences in mean and peak $\dot{\mathrm{V}}{\mathrm{O}}_2$, ${\dot{\mathrm{V}}}_{\mathrm{E}}$, HR, time ≥ 85% HRmax, as well as total O₂ and calorie expenditure between sessions. Tukey's post hoc test was used to identify differences between means with a significant F ratio. Partial eta-squared was used to assess the effect size from ANOVA, and Cohen's d was used to identify the effect size between means, with 0.01, 0.06, and 0.14 representing a small, medium, and large effect size, and d values equal to 0.20, 0.50, and 0.80 indicating a small, medium, and large effect, respectively. G Power³⁴ was used to confirm that a sample size of 12 per condition is adequate to detect a difference in $\dot{\mathrm{V}}{\mathrm{O}}_2$ equal to 0.20 ​L/min across conditions. Statistical significance was established as p ​< ​0.05.

3. Results

Maximal exercise data: Values from incremental and verification testing were equal to (40 ​± ​9) mL/kg/min (95% CI ​= ​34–46 ​mL/kg/min), (114 ​± ​28) L/min, (1.14 ​± ​0.07), and (181 ​± ​11) b/min for $\dot{\mathrm{V}}{\mathrm{O}}_{2\max }$, ${\dot{\mathrm{V}}}_{\text{Emax}}$, RERmax, and HRmax, respectively. PPO was equal to (227 ​± ​70) W. Maximal BLa, RPE, and affective valence were equal to (9.9 ​± ​2.1) mM, (16.2 ​± ​2.1), and (−0.22 ​± ​2.5).

Changes in gas exchange data and HR in response to HIFT and rowing HIIT: Changes in gas exchange data are demonstrated in Fig. 2. Oxygen uptake significantly increased from rest (p ​< ​0.001, η² ​= ​0.93) and there was a significant time ​× ​session interaction (p ​< ​0.001, η² ​= ​0.15). Post hoc analyses showed that all values were significantly higher versus rest (p ​< ​0.05) with exception of those acquired 10 and 15 ​min post-exercise. There was also no difference in $\dot{\mathrm{V}}{\mathrm{O}}_2$ between intervals 3 and 4, 4 and 5, and 5 and 6. During exercise, rowing HIIT revealed 4%–6% higher $\dot{\mathrm{V}}{\mathrm{O}}_2$ compared to HIFT, with post hoc analyses showing significant differences (p ​< ​0.05) between sessions only for interval 5 (d ​= ​1.17). At 5 (d ​= ​0.54) and 10 ​min (d ​= ​0.29) post-exercise, a significantly higher $\dot{\mathrm{V}}{\mathrm{O}}_2$ was shown in response to HIFT. Peak $\dot{\mathrm{V}}{\mathrm{O}}_2$ of rowing HIIT and HIFT represents 82% and 81% $\dot{\mathrm{V}}{\mathrm{O}}_{2\max }$, respectively.

Fig. 2.

Differences in a) oxygen consumption, b) ventilation, c) respiratory exchange ratio (RER), and d) heart rate in response to high intensity functional training (HIFT) and rowing high intensity interval training (HIIT). Values were acquired at rest, during each interval, and 5, 10, and 15 ​min post-exercise (mean ​± ​SD). ∗ ​= ​p < 0.05 between regimens.

Results showed an approximately seven- and six-fold increase in ${\dot{\mathrm{V}}}_{\mathrm{E}}$ from rest in response to HIFT and rowing HIIT (p ​< ​0.001, η² ​= ​0.93). There was also a significant time ​× ​session interaction (p ​< ​0.001, η² ​= ​0.22). Post hoc analyses showed that all values were significantly different (p ​< ​0.001) from each other, with exception of no difference in ${\dot{\mathrm{V}}}_{\mathrm{E}}$ between the resting value and that acquired 15 ​min post-exercise (p ​= ​0.08) as well as ${\dot{\mathrm{V}}}_{\mathrm{E}}$ between intervals 3 and 4 (p ​= ​0.23). Compared to rowing HIIT, post hoc analyses showed significantly higher ${\dot{\mathrm{V}}}_{\mathrm{E}}$ for intervals 4 through 6 in response to HIFT (p ​= ​0.02 to p ​= ​0.004, d ​= ​1.6), and at 5 ​min post-exercise (p ​= ​0.001, d ​= ​0.88). RER increased from rest to exercise (p < 0.001, η² ​= ​0.79) and there was a significant time ​× ​session interaction (p < 0.001, η² ​= ​0.15). Post hoc analyses showed that all exercise values were different than pre-exercise (p = 0.1 to p < 0.001), with the value acquired at 5 ​min post-exercise significantly higher (p < 0.05) than all exercise values. RER was significantly higher (p = 0.03 to p < 0.001) in response to HIFT versus rowing HIIT at all timepoints (d ​= ​0.6–1.1) with exception of 15 ​min post-exercise (p = 0.51).

The HR response is depicted in Fig. 2d. Heart rate significantly increased in response to exercise (p < 0.001, η² ​= ​0.94) and there was a significant time ​× ​session interaction (p = 0.004, η² ​= ​0.11). Post hoc analyses showed that HR from interval 5 (p = 0.004) and 6 (p = 0.002) and all post-exercise values (p = 0.01 to p < 0.001) were significantly higher in response to HIFT versus rowing HIIT (d ​= ​0.9–1.0). HR peaked at (174 ​± ​15) b/min and (167 ​± ​15) b/min in response to HIFT and rowing HIIT, which is equivalent to 96% and 92% HRmax.

Mean and peak responses to HIFT and rowing HIIT: Table 2 demonstrates mean and peak gas exchange data, HR, and calorie expenditure between HIFT and rowing HIIT. Mean $\dot{\mathrm{V}}{\mathrm{O}}_2$ and total $\dot{\mathrm{V}}{\mathrm{O}}_2$ were significantly (10%) higher in response to rowing HIIT versus HIFT, yet no differences were shown in peak $\dot{\mathrm{V}}{\mathrm{O}}_2$. In contrast, HIFT displayed significantly higher mean/peak ${\dot{\mathrm{V}}}_{\mathrm{E}}$ and lower energy expenditure compared to rowing HIIE. No significant differences were evident in mean HR, although HIFT revealed higher peak HR versus rowing HIIT. Time spent ≥ 85% HRmax was significantly higher for HIFT versus rowing HIIT ([6.1 ​± ​2.2] min vs. [5.0 ​± ​2.8] min, p = 0.03, d ​= ​0.54).

Table 2.

Differences in mean and peak gas exchange data, heart rate, and calorie expenditure in response to HIFT and rowing HIIT (mean ​± ​SD).

Parameter	HIFT	Rowing HIIT	p value	Cohen's d
Mean $\dot{\mathrm{V}}{\mathrm{O}}_2$ (L/min)	1.73 ​± ​0.38	1.93 ​± ​0.50	< 0.001	0.45
Mean $\dot{\mathrm{V}}{\mathrm{O}}_2$ (% $\dot{\mathrm{V}}{\mathrm{O}}_{2\max }$)	60 ​± ​8	66 ​± ​7	< 0.001	0.82
Peak $\dot{\mathrm{V}}{\mathrm{O}}_2$ (L/min)	2.34 ​± ​0.52	2.44 ​± ​0.71	0.12	0.16
Peak $\dot{\mathrm{V}}{\mathrm{O}}_2$ (% $\dot{\mathrm{V}}{\mathrm{O}}_{2\text{peak}}$)	81 ​± ​9	82 ​± ​8	0.72	0.12
Total $\dot{\mathrm{V}}{\mathrm{O}}_2$ (L)	29 ​± ​6	32 ​± ​8	0.017	0.44
Mean ${\dot{\mathrm{V}}}_{\mathrm{E}}$ (L/min)	62.6 ​± ​12.7	57.3 ​± ​15.4	0.05	0.39
Mean ${\dot{\mathrm{V}}}_{\mathrm{E}}$ (% ${\dot{\mathrm{V}}}_{\text{Emax}}$)	57 ​± ​10	51 ​± ​9	0.02	0.65
Peak ${\dot{\mathrm{V}}}_{\mathrm{E}}$ (L/min)	84.4 ​± ​18.2	72.9 ​± ​19.3	0.004	0.66
Peak ${\dot{\mathrm{V}}}_{\mathrm{E}}$ (% ${\dot{\mathrm{V}}}_{\text{Emax}}$)	77 ​± ​13	65 ​± ​12	0.003	0.98
Mean HR (b/min)	147.5 ​± ​17.6	145.0 ​±17.0	0.25	0.12
Mean HR (%HRmax)	81 ​± ​8	80 ​± ​7	0.30	0.14
Peak HR (b/min)	174.0 ​± ​13.8	167.3 ​± ​14.6	0.002	0.51
Peak HR (%HRmax)	96 ​± ​4	92 ​± ​5	0.002	0.90
Time > 85% HRmax (min)	6.1 ​± ​2.2	5.0 ​± ​2.8	0.03	0.54
EEexercise (kcal)	108 ​± ​23	123 ​± ​33∗	0.02	0.54
EEpost-exercise (kcal)	39 ​± ​9	36 ​± ​11	0.12	0.31

HIFT ​= ​high intensity functional training; HIIT ​= ​high intensity interval training; $\dot{\mathrm{V}}{\mathrm{O}}_2$ ​= ​oxygen uptake; ${\dot{\mathrm{V}}}_{\mathrm{E}}$ ​= ​ventilation; HR ​= ​heart rate; EE ​= ​energy expenditure. Mean values were calculated using data from the entire exercise session including recovery between intervals; whereas, peak values were calculated as the highest value from any interval.

Changes in blood lactate concentration in response to HIFT and rowing HIIT: Fig. 3 shows the change in BLa in response to rowing HIIT and HIFT. Compared to rest, BLa acquired 5 ​min post-exercise increased six and eight–fold in response to rowing HIIT and HIFT (p < 0.001, η² ​= ​0.87) and there was a significant time ​× ​session interaction (p < 0.001, η² ​= ​0.44). Post hoc analyses showed that all values were significantly different (p < 0.001 to p = 0.17) from each other with exception of BLa acquired after interval 3 and BLa obtained 10 ​min post-exercise. BLa was significantly higher (p < 0.001) during (d ​= ​2.2) and post exercise (d ​= ​3.1–3.6) in response to HIFT versus rowing HIIT.

Fig. 3.

Differences in a) blood lactate concentration, b), rating of perceived exertion, and c) affective valence in response to high intensity functional training (HIFT) and rowing high intensity interval training (HIIT). Outcomes were acquired at rest, after interval 3, and 5, 10, and 15 ​min post-exercise. (mean ​± ​SD). ∗ ​= ​p < 0.05 between regimens.

Changes in perceptual responses in response to rowing HIIT and HIFT: There was no difference in PACES between regimens ([95 ​± ​16] and [97 ​± ​16] for rowing HIIT and HIFT, respectively, p = 0.68). Changes in RPE and affective valence are displayed in Fig. 3. Data showed a significant increase in RPE during exercise (p < 0.001, η² ​= ​0.84) and a significant time ​× ​session interaction (p < 0.001, η² ​= ​0.25). All RPE values were significantly different from each other (p < 0.001 to p = 0.01) other than RPE recorded pre-exercise versus 15 ​min post-exercise, which were not different (p = 0.33). Post hoc analyses showed that RPE in response to HIFT was significantly higher (p < 0.05) versus rowing HIIT after intervals 3 (p = 0.01, d ​= ​1.1) and 6 (p = 0.003, d ​= ​2.2) but not post-exercise. As for affective valence, it significantly declined from pre-to post-exercise (p < 0.001, η² ​= ​0.53) by approximately 4.0 and 2.5 units in response to HIFT and rowing HIIT, and there was a time ​× ​session interaction (p = 0.003, η² ​= ​0.16). Affective valence was consistently more positive in response to rowing HIIT versus HIFT, and post hoc analyses demonstrated significantly more aversive values immediately post-exercise with HIFT versus rowing HIIT ([0.7 ​± ​3.1] vs. [2.2 ​± ​2.2], p = 0.004, d ​= ​3.6). Intention to participate did not differ between regimens and was equal to (2.9 ​± ​1.4) and (3.0 ​± ​1.2) for 3 ​d/wk (p = 0.65, “likely”) and (1.9 ​± ​1.1) and (1.7 ​± ​0.8) for 5 ​d/wk (p = 0.36, “somewhat unlikely”) for HIFT and rowing HIIT, respectively.

4. Discussion

This study explored the cardiometabolic response to a brief bout of HIFT versus rowing HIIT in active adults. Results showed a higher peak HR, BLa, and ${\dot{\mathrm{V}}}_{\mathrm{E}}$ in response to HIFT compared to rowing HIIT, although there was no difference in peak $\dot{\mathrm{V}}{\mathrm{O}}_2$ between regimens. Despite similar enjoyment and intention, HIFT revealed significantly higher RPE and lower affective valence than rowing HIIT, which is likely related to the significantly higher BLa accumulation and “all-out” nature of this regimen.

Previous studies reveal that long-term HIFT leads to significant increases in $\dot{\mathrm{V}}{\mathrm{O}}_{2\max }$.^12,14,35,36 Two results provide potential explanations for this increase in aerobic fitness. First, HIFT as performed in the present study elicits near-maximal HR, high mean HR (81%HRmax), and high peak $\dot{\mathrm{V}}{\mathrm{O}}_2$ equal to 81% $\dot{\mathrm{V}}{\mathrm{O}}_{2\max }$ which are similar to values reported in a recent acute HIFT study.¹⁷ In fact, these relative intensities satisfy the ACSM guidelines¹⁰ for “vigorous” and “near-maximal to maximal” for HR and “moderate” to “vigorous” for $\dot{\mathrm{V}}{\mathrm{O}}_2$. The “all-out” effort attendant with our protocol and use of dynamic movements engaging a large muscle mass likely induce the high HR and rates of oxygen consumption which seem to be requisite to increase $\dot{\mathrm{V}}{\mathrm{O}}_{2\max }$.¹⁵ Second, the HIFT protocol reveals high BLa which peaked 5 ​min after exercise (9.3 ​± ​2.7) mM. This value is similar to that acquired from our $\dot{\mathrm{V}}{\mathrm{O}}_2\max$ test and values reported from cycling HIIT at 85% PPO.^27,29 It is apparent that BLa acts as an energy regulator³⁷ which like high $\dot{\mathrm{V}}{\mathrm{O}}_2$, is associated with the adaptive response to exercise. In fact, Preobrazenski et al.³⁸ showed that BLa in response to aerobic exercise at 65% PPO was positively associated with the long-term increase in $\dot{\mathrm{V}}{\mathrm{O}}_{2\max }$ in adults. At 15 ​min post-exercise, BLa was equal to (7.0 ​± ​3.0) mM, which suggests maintenance of high BLa for extended periods after HIFT, as has been shown after SIT.³⁹

Our data show a significantly higher BLa, RER, and ${\dot{\mathrm{V}}}_{\mathrm{E}}$ in response to HIFT compared to rowing HIIT, yet lower mean $\dot{\mathrm{V}}{\mathrm{O}}_2$. The substantial effort attendant with HIFT likely requires greater contractile force which in turn requires greater overall recruitment of motor units, especially type II motor units, thereby causing higher BLa. The metabolic acidosis associated with this phenomenon activates a respiratory compensation, leading to elevations in ${\dot{\mathrm{V}}}_{\mathrm{E}}$. The lower ${\dot{\mathrm{V}}}_{\mathrm{E}}$ observed in rowing HIIT may also be related to mechanical limitations of exercise performed in a seated position, as previously demonstrated.⁴⁰ Subsequently, the excess production of CO₂ inflates RER both during exercise and in recovery. In fact, many participants revealed RER values above 1.5 in the immediate and post-exercise recovery to HIFT, demonstrating the substantial metabolic perturbation induced by this exercise modality. Lower total O₂ and mean $\dot{\mathrm{V}}{\mathrm{O}}_2$ in response to HIFT is also attributed to the eccentric contraction attendant with squats and squat jumps, as this lowers oxygen consumption versus activities involving concentric contractions such as running.^17,41

To our knowledge, only one other study has explored the post-exercise metabolic response to HIFT. Fidalgo et al.⁴² required trained men ($\dot{\mathrm{V}}{\mathrm{O}}_{2\max }$ ​= ​57 ​mL/kg/min) experienced with HIFT to perform four “all-out” 20 ​s sets of dumbbell thrusters, mountain climbers, kettlebell swing, and dumbbell snatch. Oxygen consumption (∼0.4–1.0 ​L/min) and energy expenditure (∼2–5 ​kcal/min) remained elevated 30 ​min post-exercise versus resting values equal to 0.25–0.30 ​L/min and 1.0–1.5 ​kcal/min, which led to an EPOC equal to approximately 3 ​L. In response to HIFT, our results (Fig. 2) show that $\dot{\mathrm{V}}{\mathrm{O}}_2$ at 5 (0.44 ​± ​0.11) L/min and 10 (0.35 ​± ​0.08) L/min min post-exercise was significantly higher than rowing HIIT, with the 5 ​min value markedly higher than resting $\dot{\mathrm{V}}{\mathrm{O}}_2$ recorded in young, active men and women equal to (0.31 ​± ​0.08) L/min.²⁷ Similar data were shown for post-exercise ${\dot{\mathrm{V}}}_{\mathrm{E}}$, HR, RER, and BLa in response to HIFT, with perhaps the most striking result being the sustained elevation in HR and BLa. For example, these values recorded 15 ​min post-exercise, equal to (101 ​± ​15) b/min and (7.0 ​± ​3.5) mM, are 30 ​b/min and 6 ​mM higher than typical resting values. These data suggest that HIFT, and to a lesser degree, rowing HIIT, induce a potent metabolic stimulus that prolongs the post-exercise recovery period leading to elevations in $\dot{\mathrm{V}}{\mathrm{O}}_2$ and overall energy expenditure.

Our perceptual results show that HIFT elicits a significantly more aversive response and higher RPE than rowing HIIT, despite equivalent mean HR and lower $\dot{\mathrm{V}}{\mathrm{O}}_2$. The “all-out” nature of HIFT, which elicits significantly higher peak HR, ${\dot{\mathrm{V}}}_{\mathrm{E}}$, and BLa, likely explains the 2 unit greater RPE compared to rowing HIIT. The peak RPE value equal to 15 is similar to that reported in response to HIFT¹⁷ and higher than values reported from HIIT cycling at 85% PPO.²⁹ The markedly higher BLa observed in response to HIFT likely is related to the lower affective valence, as well as large muscle mass activated and higher contractile forces and velocity required of this “all-out” regimen. The end-exercise values of affective valence (0.7 ​± ​3.1) in response to HIFT represent lower pleasure than “fairly good”; whereas, affective valence for rowing HIIT was “fairly good to good” (2.2 ​± ​2.2). Nevertheless, a closer examination of our data reveals marked variability in the affective valence response to HIFT, as previously shown with HIIT.⁴³ Participants with low $\dot{\mathrm{V}}{\mathrm{O}}_{2\max }$ tended to exhibit the most aversive affective valence at the end of HIFT (values ​= ​−4 or −5 at end-exercise). It is possible that outcomes including muscular strength, endurance, power, or various personality characteristics explain more of the variability in affective valence to these modalities. Despite these differences in affective valence between sessions, there was no difference in PACES or intent to participate, which suggests that a less pleasant in-task experience to HIFT may not induce different responses recorded post-exercise.

This study has a few limitations. First, data cannot be applied to other HIIT or HIFT regimens characterized by different modalities, intensity, work: recovery ratio, or duration. Participants were mostly unfamiliar with rowing, so it is possible that this may alter our data, although intensities attendant with interval exercise were attained. Second, although many studies have explored the acute response to HIIT in inactive adults,⁴⁴ our findings are not generalizable to less fit adults or those with obesity or chronic disease. Third, although our sessions had identical interval and recovery duration, the recovery modality was different, which was chosen to best mirror daily practice in health and fitness settings. Completing low intensity rowing after HIFT eliminates one of the great advantages of this regimen, which is the absence of expensive equipment. This disparity may elicit a small impact on our data. Fourth, the pace of our regimens was different, as we instructed participants to perform HIFT repetitions as fast as they could; whereas, rowing HIIT was not performed “all-out.” This was done to match exercise duration which is critical to effectively compare responses between regimens. Nevertheless, if rowing HIIT was performed “all-out,” this would represent SIT requiring brief efforts (5–30 ​s) eliciting intensities greater than that associated with $\dot{\mathrm{V}}{\mathrm{O}}_{2\max }$ or PPO, which would in turn elicit different exercise volume and overall bout duration between regimens. Fifth, our acute results cannot shed light on the chronic response to these modalities. Lastly, we only measured the post-exercise response for 15 ​min, so the complete post-exercise metabolic response to these regimens cannot be identified from our results and is an area meriting additional study.

Overall, a duration-matched bout of whole-body HIFT consisting of “all-out” body weight exercise elicits similar peak $\dot{\mathrm{V}}{\mathrm{O}}_2$ and significantly higher BLa, peak HR, and ${\dot{\mathrm{V}}}_{\mathrm{E}}$ compared to rowing HIIT. The HIFT protocol elicited peak exercise intensity equal to 96% HRmax, greater time spent above 85% HRmax, and sustained elevations in BLa which represent a potent cardiovascular and metabolic stimulus that if performed long-term, should increase aerobic fitness and muscular function. Due to its substantial stress on the cardiometabolic and muscular systems, HIFT should be considered as an alternative to HIIT using traditional aerobic modalities.

Practical Applications.

• 1)Low-volume, whole body high intensity functional training leads to higher peak HR, blood lactate concentration, and ventilation compared to rowing based interval training, but calorie expenditure is lower.
• 2)High intensity functional training elicits higher perceived exertion and lower pleasure, which enhances perceptual strain.
• 3)Similar to traditional high intensity interval training, high intensity functional training produces a substantial cardiometabolic stimulus while also challenging the muscular system. However, HIFT could also be less accessible due it being perceived as more unpleasant by active adults.

CRediT authorship contribution statement

Todd A. Astorino: Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Madison Mower: Writing – review & editing, Writing – original draft, Supervision, Methodology, Investigation, Data curation, Conceptualization. Armando Flores: Writing – review & editing, Writing – original draft, Supervision, Methodology, Investigation, Data curation, Conceptualization. Marissa Flannery: Writing – review & editing, Writing – original draft, Supervision, Methodology, Investigation, Data curation, Conceptualization.

Ethical approval

All participants were informed of the procedures attendant with the protocol and provided written informed consent, with the study approved by the California State University—San Marcos Institutional Review Board (protocol 2028365-1). The study was implemented in accordance with the Declaration of Helsinki.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

The following is the Supplementary data to this article: