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High Intensity Interval Training: A Potential Method for Treating Sarcopenia

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Sarcopenia, an age-related disease characterized by loss of muscle strength and muscle mass, has attracted the attention of medical experts due to its severe morbidity, low living quality, high expenditure of health care, and mortality. Traditionally, persistent aerobic exercise (PAE) is considered as a valid way to attenuate muscular atrophy. However, nowadays, high intensity interval training (HIIT) has emerged as a more effective and time-efficient method to replace traditional exercise modes. HIIT displays comprehensive effects on exercise capacity and skeletal muscle metabolism, and it provides a time-out for the recovery of cardiopulmonary and muscular functions without causing severe adverse effects. Studies demonstrated that compared with PAE, HIIT showed similar or even higher effects in improving muscle strength, enhancing physical performances and increasing muscle mass of elder people. Therefore, HIIT might become a promising way to cope with the age-related loss of muscle mass and muscle function. However, it is worth mentioning that no study of HIIT was conducted directly on sarcopenia patients, which is attributed to the suspicious of safety and validity. In this review, we will assess the effects of different training parameters on muscle and sarcopenia, summarize previous papers which compared the effects of HIIT and PAE in improving muscle quality and function, and evaluate the potential of HIIT to replace the status of PAE in treating old people with muscle atrophy and low modality; and point out drawbacks of temporary experiments. Our aim is to discuss the feasibility of HIIT to treat sarcopenia and provide a reference for clinical scientists who want to utilize HIIT as a new way to cope with sarcopenia.
Molecular mechanism of HIIT on treating and preventing sarcopenia. (A) Promotion of muscle protein synthesis. HIIT and enough nutrient supplementation upregulate PI3K/Akt pathway. Akt inhibits tuberous sclerosis complex (TSC) through phosphorylation. TSC facilitates the conversion of RHEB-GTP to RHEB-GDP, thereby inhibiting the function of RHEB-GTP to activate mTORC1. Thus, mTORC1 pathway is relieved from the inhibition of TSC. And with the existence of amino acids, Rag guanine triphosphatases (Rag GTPases) promote the translocation of mTORC1 to lysosome where RHEB-GTP activates mTORC1. Thus, muscle protein synthesis is triggered. (B) Inhibition of muscle protein breakdown. Akt-mediated phosphorylation inhibits FOXO and the expression of the atrophy-related ubiquitin ligases Atrogin 1 and MURF1, and thus suppresses muscle protein breakdown caused by Proteasomal proteolysis. (C) Enhancement of mitochondrial biogenesis. Concentration of Calcium ion and expression of AMPK in muscle cells are unregulated by HIIT. Calcium ion and Akt promote PGC-1α pathway through CREB, and AMPK activates PGC-1α through direct phosphorylation and SIRT1-dependent deacetylation. PGC-1α enhances the expression of NRF1 and NRF2, which stimulate mitochondrial biogenesis and increase of mitochondrial respiratory capacity. (D) Promotion of the GLUT4 expression. AMPK, Ca 2+ and p38 MAPK can induce the expression of GLUT4 gene. Abbreviations: Akt, protein kinase B; AMPK, AMP activated kinase; eIF, eukaryotic initiation factor; mTORC1, mammalian target of rapamycin complex 1; 4E-BP1, eukaryotic initiation factor 4E (eIF4E)-binding protein-1; Rag, Ras-related GTPase; rpS6, ribosomal protein S6; S6K1, p70 ribosomal S6 kinase 1; CaMK, calcium/calmodulindependent protein kinase; CREB, cAMP response element-binding protein; ETC, electron transport chain; NRF, nuclear respiratory factor; PGC-1α, proliferator-activated receptor γ coactivator 1α; SIRT1, sirtuin 1; Tfam, mitochondrial transcription factor A; Mt, mitochondria; GLUT4, glucose transporter 4; MAPK, mitogen-activated protein kinase.
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REVIEW
High Intensity Interval Training: A Potential
Method for Treating Sarcopenia
Qian-Qi Liu
1,2,
*, Wen-Qing Xie
1,3,
*, Yu-Xuan Luo
1,2
, Yi-Dan Li
1,2
, Wei-Hong Huang
4
, Yu-Xiang Wu
5
,
Yu-Sheng Li
1,3
1
Department of Orthopedics, Xiangya Hospital, Central South University, Changsha, Hunan, 410008, People’s Republic of China;
2
Xiangya School of
Medicine, Central South University, Changsha, Hunan, 410083, People’s Republic of China;
3
National Clinical Research Center for Geriatric
Disorders, Xiangya Hospital, Central South University, Changsha, Hunan, 410008, People’s Republic of China;
4
Mobile Health Ministry of Education -
China Mobile Joint Laboratory, Xiangya Hospital Central South University, Changsha, Hunan, 410008, People’s Republic of China;
5
Department of
Health and Kinesiology, School of Physical Education, Jianghan University, Wuhan, Hubei, 430056, People’s Republic of China
*These authors contributed equally to this work
Correspondence: Yu-Sheng Li, Department of Orthopedics, Xiangya Hospital, Central South University, 87 Xiangya Road, Changsha, Hunan, 410008,
People’s Republic of China, Tel +86-13975889696, Email liyusheng@csu.edu.cn; Yu-Xiang Wu, Department of Health and Kinesiology, School of
Physical Education, Jianghan University, No. 8, Sanjiaohu Road, Wuhan, Hubei, 430056, People’s Republic of China, Tel +86 27 8422 6921,
Email yxwu@jhun.edu.cn
Abstract: Sarcopenia, an age-related disease characterized by loss of muscle strength and muscle mass, has attracted the attention of
medical experts due to its severe morbidity, low living quality, high expenditure of health care, and mortality. Traditionally, persistent
aerobic exercise (PAE) is considered as a valid way to attenuate muscular atrophy. However, nowadays, high intensity interval training
(HIIT) has emerged as a more effective and time-efcient method to replace traditional exercise modes. HIIT displays comprehensive
effects on exercise capacity and skeletal muscle metabolism, and it provides a time-out for the recovery of cardiopulmonary and
muscular functions without causing severe adverse effects. Studies demonstrated that compared with PAE, HIIT showed similar or
even higher effects in improving muscle strength, enhancing physical performances and increasing muscle mass of elder people.
Therefore, HIIT might become a promising way to cope with the age-related loss of muscle mass and muscle function. However, it is
worth mentioning that no study of HIIT was conducted directly on sarcopenia patients, which is attributed to the suspicious of safety
and validity. In this review, we will assess the effects of different training parameters on muscle and sarcopenia, summarize previous
papers which compared the effects of HIIT and PAE in improving muscle quality and function, and evaluate the potential of HIIT to
replace the status of PAE in treating old people with muscle atrophy and low modality; and point out drawbacks of temporary
experiments. Our aim is to discuss the feasibility of HIIT to treat sarcopenia and provide a reference for clinical scientists who want to
utilize HIIT as a new way to cope with sarcopenia.
Keywords: Sarcopenia, high intensity interval training, persistent aerobic exercise, aging
Introduction of Sarcopenia
Denition, Prevalence, and Consequences
Sarcopenia is a disease closely related to aging.
1
The rst meeting of the European Working Group on Sarcopenia in
Older People (EWGSOP) dened sarcopenia as “a syndrome characterized by progressive and generalized loss of
skeletal muscle mass and strength with a risk of adverse outcomes such as physical disability, poor quality of life and
death” performance.
2
The diagnostic criteria include three elements: low muscle mass, low muscle strength and low
physical performance.
2
In 2018, EWGSOP2 prioritized decreased muscle strength as the most signicant diagnostic
parameter for sarcopenia,
3
and conrmed that this disease begins at an early age.
3
An updated consensus of Asia, the
Asian Working Group for Sarcopenia (AWGS) 2019, dened sarcopenia as “age-related loss of skeletal muscle mass plus
loss of muscle strength and/or reduced physical performance”, which suggested the decline of muscle strength and
physical performance are attributed to the loss of muscle mass.
4
The Foundation for the National Institutes of Health
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(FNIH) also diagnoses this disease by assessing muscle mass and muscle strength, but there are disparities in cut-off
values and measurement modes between FNIH and EWGSOP2.
5
Through assessing the data from eight epidemiological
cohorts with large scale, the 2020 Sarcopenia Denitions and Outcomes Consortium (SDOC) strongly emphasized the
prognostic value of low grip strength and low gait speed on falls, low mobility, disability in daily life, and mortality, and
SDOC suggested that weakness and slowness dened by low grip strength and low gait speed, respectively, should be
included into the denition of sarcopenia. However, unlike other criteria, SDOC excluded the lean mass from the
denition of sarcopenia.
6
The prevalence of sarcopenia differs in ages, ethnicities, and sexes. In Europe, dened by EWGSOP standard, 5%–
13% in people aged 60 to 70 and 11%–50% in people aged over 80 were suffered from sarcopenia, and the prevalence of
sarcopenia was 1–29% (up to 30% in women) for older adults living in the community, 14–33% (up to 68% in men) for
those living in long-term care institutions and 10% for those in acute hospital care.
7,8
In Asia, according to AWGS, the
prevalence of sarcopenia was 5.5–25.7% in elderly people,
4
and in an elderly Chinese suburb dwelling population, 6.4%
in men and 11.5% in women were diagnosed with sarcopenia.
9
Another report using AWGS criteria found that the
prevalence of sarcopenia differed with ethnic groups in China, as 22.3% in Han ethnic, 18.2% in Tibetan, 11.8% in
Qiang, 34.7% in Yi and 26.7% in Hui.
10
In the USA, the prevalence of sarcopenia ranges from 2.5% to 27.2% in women
and ranges from 3.1% to 20.4% in men.
11
A meta-analysis synthesized the results of 41 studies which evaluated the
prevalence of sarcopenia through different criteria (including the standard of EWGSOP, AWGS and so on).
12
On the
whole, 14% (95% CI: 11–17%) men and 12% (95% CI:10–15%) women were suffered from sarcopenia.
12
In commu-
nity-dwelling individuals, the prevalence were 11% (95% CI: 8–13%) in men and 9% (95% CI: 7–11%) in women. 51%
(95% CI: 37–66%) men and 31% (95% CI: 22–42%) women in nursing-home and 23% (95%, CI: 15–30%) men and
24% (95% CI: 14–35%) women in hospitalized patients were attacked by sarcopenia.
12
Women are more likely to get
sarcopenia than men.
9,13,14
Sarcopenia causes serious loss of muscle quality and function, elderly people with sarcopenia have a higher risk of
falling and fracture, difculties in standing and walking, and they are prone to losing the ability to take care of
themselves, causing a heavy burden on families and the society.
1,15
According to data from the UK and the USA,
sarcopenia is associated with heavy cost of medical service.
16
And sarcopenia exacerbates heart failure and respiratory
diseases, leading to low physical performance.
17,18
Because of inadequate movement, sarcopenia patients are easy to
become obese, which further damages their health.
19
Sarcopenia is also associated with impaired cognitive function,
20
osteoporosis
21
and poorer prognosis in patients with surgery.
7
Risk Factors and Molecular Mechanisms of Sarcopenia
Aging is the primary risk factor for sarcopenia. Generally, muscle mass is maintained during early life, but then declines
at a rate of 1% or 0.5% per year in men or women, respectively.
22
From 20- to 80- year-olds, about 30% of our muscle
mass and 20% of our cross-sectional area (CSA) will be lost.
22
Muscle waste in the elderly is mainly due to an imbalance
between muscle protein synthesis (MPS) and muscle protein breakdown (MPB),
23
which are balanced in younger
individuals.
24
Reduction of type II muscle ber size accounts for the majority of the muscle waste during aging.
25
During skeletal muscle aging, the mammalian target of rapamycin complex 1 (mTORC1) pathway plays an important
role. MTORC1 pathway is activated by phosphoinositide 3-kinase (PI3K)-Akt pathway, which is upregulated by food
(especially by protein rich in leucine) or exercise.
26,27
Two downstream targets of mTORC1 pathway, the 70-kDa
ribosomal protein S6 kinase 1 (S6K1) and eukaryotic initiation factor 4E (eIF4E)-binding protein-1 (4E-BP1), promote
the initiation and elongation of translation. After activation, mTOR can phosphorylate 4E-BP1 and S6K1.
Phosphorylation of 4E-BP1 will remove the inhibition of eIF4E by 4E-BP1. As a result, eIF4E can directly bind to
the 5’ end of mRNA and recruit eIF4G and eIF4A to form the translation preinitiation (eIF4F) complex.
28
Once
phosphorylated, S6K1 will phosphorylate lots of translation-related factors, including the 40S ribosomal protein S6
(rpS6), eIF4B and eukaryotic elongation factor 2 (eEF2) kinase (phosphorylation of eEF2 kinase relieves the suppression
of eEF2).
28
These events lead to hypertrophy of muscle cells.
29
In young adult muscles, anabolic stimuli such as exercise
and feeding stimulate MPS and suppress MPB through mTORC1 signaling. Conversely, with aging, muscle becomes
resistant or insensitive to anabolic stimuli, leading to impaired MPS and suppressed inhibition of MPB.
24,30
Twice as
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much leucine is required in aged rats than in young rats to stimulate the MPS to a predened level.
31
Genes related to
mitochondria, insulin signaling, and muscle growth were downregulated by aging, which may trigger anabolic stimuli-
resistance.
32
Scientists hypothesized this phenomenon as the primary cause of muscle mass wasting in the elderly.
23
Atrophy of aged muscle may also be associated with the denervation: a reduction of motor neuron population has been
observed in old animals due to impairment of normal cycling of denervation-reinnervation, but the mechanism of age-
related denervation remains unclear.
33,34
Due to dysfunction of antioxidant enzymatic (such as peroxiredoxin 6) and increased oxidative stress in aging,
telomere attrition and DNA damage appear.
35,36
Thus, abilities of muscle satellite cells to proliferate and regenerate are
impaired.
37
In addition, reports showed that oxidative stress suppressed the phosphorylation of eIF4E and 4E-BP1, and
thus inhibited the mTORC1 pathway.
38,39
Simultaneously, oxidative stress damages protein homeostasis and induces
proteolysis,
40
and it is reported that protein abundance decreased in the elderly, especially mitochondrial proteins.
32
Moreover, aging downregulates hormones which are crucial for the maintenance of muscle mass, strength and prolifera-
tion of satellite cells, including growth hormone (GH), testosterone, thyroid hormone (TH) and insulin-like growth
factor-1 (IGF-1).
41–46
Other risk factors associated with sarcopenia include malnutrition, inactivity, obesity, diseases, and early environment
for growth. Dietary protein intake is pivotal in maintaining muscle mass of old people, as amino acids like leucine
activate the mTORC1 pathway via the Rag guanosine triphosphatase (Rag GTPase) mechanism.
26,47
Malnutrition
aggravates imbalance between MPS and MPB and signicantly elevates the morbidity of sarcopenia in people aged
over 65,
48,49
which could be prevented through high intake of protein and vitamin D;
50–52
exercise is a key stimulus for
mTORC1 pathway,
53–56
but inactivity increases the risk of getting sarcopenia, an inactive period even as short as 2 days
can signicantly reduce muscle volume,
57
and one-hour increase in sedentary behavior per day led to 1.06 (95% CI =
1.04–1.10) times higher possibility for getting sarcopenia;
58
obesity promotes the inltration of lipid into muscle, and
thereby causing oxidative stress and impairment of mitochondria and leading to lipotoxicity;
59
diseases such as cancer
often coincide with sarcopenia:
60
they raise abnormalities in glucose metabolisms,
61
upregulate pro-inammatory
cytokines, myostatin, and proteolysis-inducing factor (PIF), which activates forkhead box O (FOXO) (activation of
FOXO causes autophagy and expression of the atrophy-related ubiquitin ligases Atrogin 1 and muscle RING nger-
containing protein 1 (MURF1)).
62,63
Feasibility of High Intensity Interval Training (HIIT) to Deal with
Sarcopenia
While exercise has been observed to play a pivotal role in health, various physical activity guidelines have persuaded
people to take part in exercise. The American College of Sports Medicine (ACSM) proposed every adult to strengthen
and maintain the functions of cardiopulmonary through moderate exercise 30–60 min per day (≥5d per week), or
vigorous exercise 20–60 min per day (≥3d per week), or a combination of moderate and vigorous exercise per day (≥3-5d
per week).
64
At present, no specic drug has been approved for the treatment of sarcopenia, and hence exercise remains
the most effective strategy to deal with sarcopenia.
1
Traditionally, moderate intensity continuous exercise (MICT) with
high exercise volume was recommended by most guidelines.
65
MICT is a modality of exercise at approximately 64%–
76% of their HR
max
, or exercised with prescribed intensity as a percentage of VO
2max
, VO
2
R, HRR, or RPE equivalent to
64–76% of HR
max
with long duration (more than 30 min).
66,67
However, high intensity interval training (HIIT), which is
characterized by repeated short to long bouts of relatively high-intensity exercise (≥90%VO2max or >90–95% HRmax
for 6 s to 4 min) alternate with recovery periods of either low-intensity exercise or rest (ranging from 20% to 40%
VO2max for 10 s to 5 min), emerged as an alternative for traditional continuous training.
68,69
Firstly, HIIT displays comprehensive effects on exercise capacity and skeletal muscle metabolism. HIIT induces great
growth of muscle, prevents skeletal muscle atrophy, and improves the motor function via promoting great phosphoryla-
tion of mTOR and rps6 and inducing the expression of transcriptional coactivator peroxisome proliferator-activated
receptor γ coactivator 1α (PGC-1α), which is crucial for mitochondrial biogenesis.
70,71
It is also of importance to the
vascularization of muscle.
72
Animal studies have already proved that HIIT signicantly enhances physical performance
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and muscle mass in frail aged mice.
73,74
Some studies showed that elder people received HIIT protocol were observed to
have signicantly increased muscle mass, muscle quality, physical performance and muscle strength, compared with
MICT or control groups.
75–79
In a scoping review, Hayes and colleagues summarized 32 articles related to HIIT. In this
review, there were 20 papers tested the effect of HIIT on muscle function, and most of them reported that HIIT could
enhance muscle strength and power; 12 studies focused on the effect of HIIT on physical function, all of which showed
the improvement of physical performance after HIIT; nevertheless, 22 studies which analyzed the effect of HIIT on
muscle quantity had contradictory conclusions, and generally speaking, the effect of HIIT on muscle quality and quantity
is unclear.
80
Secondly, HIIT provides a time-out for the recovery of cardiopulmonary and muscular functions without causing
severe adverse effects.
81–83
A pilot study conrmed that HIIT was feasible and safe for hospitalized patients over 65 who
were recovering from acute medical condition.
84
Rognmo Ø et al assessed the risk of HIIT and MICT among 4846
coronary heart disease patients (mean age = 57.8), and the results reected that only one fatal case occurred during MICT
(129 456 exercise hours) and two non-fatal cases occurred during HIIT (23,182 hours).
85
Many other studies also showed
that HIIT would not cause severe adverse effects on patients with coronary artery disease.
86–88
Interval exercise may be
safer than continuous exercise for patients with cardiovascular disease (CVD).
89
Furthermore, numerous studies have demonstrated that HIIT could improve the cardiopulmonary functions of patients
with CVD and the outcomes of diabetes mellitus. CVD patients have been found to acquire higher quality of life and
greater heart rate (HR) response to exercise after receiving HIIT.
90
HIIT also reduces insulin resistance and enhances
skeletal muscle sugar intake.
91
Effects of Different Exercise Parameters on Sarcopenia
When evaluating the physiological responses raised by a specic type of exercise, multiple variables should be taken into
consideration. The intensity, volume, rest interval between sets, order of exercises, movement velocity, load lifted and
training frequency are the main methodological variables of prescription.
92
Exercise-induced physiological strain, also
called training load, results from the combination of exercise intensity, volume, and frequency.
93
To demonstrate the
rationality of HIIT in the treatment of sarcopenia, we will discuss the effects of different intensities and rest intervals
between sets and volumes of exercise on sarcopenia and the elderly, and try to nd an optimal combination of these
parameters for sarcopenia treatment.
Intensity
Exercise intensity is an important determinant of the physiological responses to exercise training. Methods measuring
exercise intensity include percentage heart rate maximum (%HR
max
), percentage heart rate reserve (%HRR), percentage
peak oxygen uptake (%VO
2max
), percentage VO
2
reserve (%VO
2
R), rating of perceived exertion (RPE), metabolic
equivalent (MET), or competition pace.
94
VO
2max
(equation 1) is a physiological characteristic which presents the
maximal rates of oxygen utilization in skeletal muscle. And it is determined by the ability of the heart and muscle to
deliver and accommodate oxygen, respectively.
95
Eq.1 VO2max= (left ventricular (LV) end-diastolic volume —LV end-systolic volume) × HR ×arterio-venous oxygen
difference
VO
2max
reects the exercise ability of a person, which is closely associated with endurance performance.
95
HR
exhibits a linear relationship with VO2, particularly between HR of 110–150 beats per minute.
96,97
Because different
people respond diversely to the same modality of exercise, scientists prefer to use %HR
max
and %VO
2max
for determin-
ing the optimal exercise intensity during HIIT.
98
As resting HR and HR
max
changes with age and tness level, and
therefore %HRR (equation 2) was recommended as a more accurate way to quantify and prescribe exercise intensity.
99
Eq:2%heart rate reserve ¼HRex—HRrestð Þ � 100
HRmax—HRrest
HR
ex
: average heart rate of the exercise session; HR
rest
: resting heart rate.
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Metabolic Equivalent (MET), Borg’s Rating of Perceived Exertion (RPE) Scale and Repetition maximum (RM) are
also being widely used to assess subjective perception of effort during exercise.
100
One MET is dened as the resting
metabolic rate, which is the amount of oxygen consumed at rest, approximately 3.5 mL O2/kg/min (1.2 kcal/min for
a 70-kg person);
101
the RPE (6–20) scale begins with “no exertion at all” (RPE = 6), and ends with “very, very hard,”
(RPE = 20);
102
the 1-RM is dened as the greatest load that one can mobilize during the concentric phase of a movement
in a single contraction, 40%–50% 1RM (very light to light intensity) was recommended by ACSM for older persons
beginning exercise to improve strength.
64
The range of exercise intensity calculated by% HRmax, % VO2max, % 1-RM,
RPE scale and MET level is shown in Table 1.
High intensity exercise has already been studied to cope with sarcopenia. One research compared the effects of high
intensity resistance training (HI-RT) and inactive state on sarcopenia patients. Participants in HI-RT group were observed
to get an increase in skeletal muscle mass index (SMI) and gait velocity, and hand grip strength was maintained, while
both SMI and hand grip strength were reduced in the inactive control group (CG).
103
Other studies also showed HI-RT
was of benet in keeping bone mineral density.
81,104
Multiple groups investigated the effects of different intensities of resistance training (RT) on muscle quality and
physical performance. Lasevicius et al designed a within-subject experiment, in which one leg and arm trained at 20%
1RM (G20) and the contralateral limb was randomly distributed to three groups: 40% (G40); 60% (G60), and 80% 1RM
(G80). After 12 weeks, elbow exion 1RM and muscle CSA increase in G80 condition was signicantly higher than
those in G20, G40, and G60 conditions.
105
There was no signicant difference in the increase in unilateral leg press
strength between G60 and G80, but they both displayed more obvious effect than G20 and G40.
105
Seynnes et al found
that while both high intensity (HI) (80% 1RM) and low intensity (LI) training (40% 1RM) signicantly improved muscle
strength and endurance of frail elders compared with the control, high intensity training group elicited signicantly better
outcomes than low intensity training group.
106
A similar phenomenon was observed by other scientists.
107,108
Results
from Sahin and colleagues showed that, though improvement of muscle strength in the HI group was not superior to the
LI group, frail elders in the HI group displayed a better physical performance, which was analyzed by walking speed,
balance while standing and standing up from a chair.
109
However, when it comes to the gains in muscle mass, low to
moderate intensities (30–50% 1RM) have a similar or even greater effect compared with high intensities.
105
Above all, all the low to high intensities training enhances muscle strength and muscle mass of the elderly, but high
intensities displayed greater effect than low-intensity to moderate intensity on increasing strength without causing higher
risks.
Rest Interval Between Sets
The staple characters that differentiate HIIT from continuous training are the duration and ratio of high-intensity and low-
intensity intervals, which play a pivotal role in the physiological response caused by HIIT.
110
Compared with continuous
training, interval training induces greater health benets when training volumes are equal or similar.
111
The acute physiological requirements of different interval-training protocols are determined by VO
2max
, as the
improvement of VO
2max
is linked with the duration of a high level of VO
2
. From the perspective of athletic training,
three categories of interval training are usually described: long intervals (3–15 minutes, intensity 85–90% VO
2max
),
moderate intervals (1–3 minutes, intensity 95–100% VO
2max
), short intervals (10 seconds to 1 minute, 100–120%
VO
2max
).
112
From the perspective of the training for old people with CVD, there are also three categories of interval
Table 1 Ranges of Exercise Intensity Calculated Through %hr
max
, %VO
2max
, %1-RM, RPE Scale and MET level
64,101,102
Intensity %HR
max
%VO
2max
% HRR % 1-RM RPE Scale MET Level
Light 57 to <64 37 to <45 30 to <40 40–50 9–11 <3
Moderate 64 to <76 46 to <64 40 to <60 60–70 12–13 3 to <6
High 76 to <96 64 to <91 60 to <90 >80 14–17 6 to <8.8
Near maximal ≥96 ≥91 ≥90 ≥18 ≥8.8
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training, which may be more appropriate for patients with sarcopenia: long intervals (high/low intensity interval: 3–4/3-4
minutes, intensity 85–95% VO
2max
), moderate intervals (high/low intensity interval: 1–2/1-4 minutes, intensity 85–95%
VO
2max
), short intervals (high/low intensity interval: 15–60/15-120 seconds, 85–95% VO
2max
).
90
Multiple groups have studied the exercise performance for different rest interval lengths. Schoenfeld BJ and
colleagues separated twenty-one young resistance-trained men to either a group that performed an RT program with
1-minute rest intervals (SHORT) or with 3-minute rest intervals (LONG), and they found that maximal strength and
muscle thickness were signicantly greater for LONG compared with SHORT;
113
results from another group suggested
that 1-minute rest might be detrimental, which signicantly elevates the blood lactate from baseline, compared with
resting 3 or 5 minutes.
114
Traditionally, when training with the intention to enhance muscle strength, 3–5 minutes’ rest
between sets can produce greater effect, because longer rest intervals ensure higher intensities and volumes;
92
moderate-
intensity sets combined with short rest intervals of 30–60 seconds might be the best choice for muscle hypertrophy,
which induces a greater level of GH.
92
Many studies compared the effects of HIIT protocols with different rest intervals. Edge and colleagues allocated 12
young women to two groups: subjects performed HIIT regimes with the same training intensity and volume, but either
a short (1 min; HIT-1) or a long (3 min; HIT-3) rest intervals. There were no signicant differences in the enhancement of
physical performance and muscle adaptation (like Na(+), K(+)-ATPase content) between HIT-1 and HIT-3.
115
In the trial
of Tucker et al, 14 recreationally active males participated in either a 4 × 4 (four 4-minute intervals at 90–95% HRpeak,
separated by a 3-minute recovery at 50 W) or 16 × 1 (sixteen 1-minute intervals at 90–95% HRpeak, separated by
a 1-minute recovery at 50 W) protocol on a cycle ergometer, and physiological responses elicited by these two protocols
were similar.
116
Schoenmakers and colleagues showed that the total physiological strain endured during training was not
greatly affected by the length of recovery durations.
117
However, these trials, as well as other similar studies,
118–120
were
all focused on the athletic abilities of young people, which may not be practical for sarcopenia patients.
Efforts have been spent to nd out whether interval exercise was suitable for old people. Previous study indicated that
interval exercise could accelerate cerebral blood ow as effectively as continuous exercise, without leading to a large
increase in blood pressure, and therefore interval exercise may be safer than continuous exercise for the elderly,
especially for those suffering from CVD.
89
One paper pointed out that a 30-second rest interval was enough for older
women to recover between sets of a knee exor exercise, but younger women needed more time, which indicated that
when prescribing rest interval between sets, practitioners should consider age differences.
121
Villanueva MG and
colleagues conducted a study on elderly men to assess the different effects of short rest intervals (RI) in between sets
(SS, 60 s) and extended RI (SL, 4 min) on body composition and performance. Outcomes showed that after 8 weeks of
low-volume and high-intensity strength training, SS group presented a greater increase in lean body mass (LBM).
122
This
result was consistent with previous ndings which showed that short rest intervals were more benecial in inducing
hypertrophy.
92
However, SS group also showed greater improvement in strength and muscular performance,
122
while
previous research suggested that long rest intervals (3–5 minutes) are required for optimizing strength improvement,
92
So far, previous studies of interval training were mostly focused on young people, and experiments implemented on
the elderly or sarcopenia patients are scarce. Additionally, no studies had compared the effects of HIIT with different rest
intervals on sarcopenia patients or elderly people. Data from Villanueva et al indicated that a rest interval as short as 60
seconds was more benecial than a long rest interval for elderly people in high intensity training, but this result was
limited by a small sample size (only 22 participants) and short-term intervention (only 8 weeks).
122
Therefore, future
research should test the effects of different lengths of rest intervals between sets on sarcopenia patients, and try to nd
a precise rest interval length in HIIT which can play the optimal role in treating sarcopenia. Furthermore, future research
should be carried out with a larger sample size and longer training intervention to determine if the effects of different rest
interval lengths led to chronic changes in body composition, muscular performance adaptations, and functional capacity
of elderly people.
Volume
Training volume is a measure of the total amount of work (joules) performed in a given time period. The number of sets,
the total number of repetitions, the total duration of work and the total work are used to estimate the amount of training in
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previous studies.
123
The amount of training is commonly described as the product of the number of repetitions × number
of sets × intensity load.
124
Schoenfeld et al studied the increase in strength and muscle mass after different volumes of training.
125
They
classied training volume by total number of sets: low-volume group (1SET), moderate-volume group (3SET), high-
volume group (5SET) performing one, three or ve sets per exercise per training session, respectively. Each group trained
three sessions per week. The training times of each session of different groups are 13 min for 1SET, 40 min and 68 min
for 3SET and 5SET. Afterwards, 1SET had equal elevation of strength and muscular endurance compared with 3SET and
5SET. Therefore, low volume training can be used as a time-efcient way for strength training.
125
A study conducted on
athletes showed that moderate volume exercise contributes more to strength gains in high intensity exercise than low
volume and high volume, but this result needed to be veried if it can be applied to old people with sarcopenia.
126
Nevertheless, higher volume was more effective than lower volume training on inducing muscle hypertrophy.
125,127
Meta-analysis depicted the dose–response relationship between training volume and muscle hypertrophy, as a higher
increase in muscle mass was induced by higher weekly training volumes.
128
Physiological responses resulting from low volume HIIT have been studied. Low-volume HIIT (total volume:
approximately 225 kJ week (−1)) was as effective as endurance training (total volume: approximately 2250 kJ week
(−1)) in increasing skeletal muscle oxidative capacity and inducing specic metabolic adaptations.
129
Low volume HIIT
and MICT exhibited similar effects on improvements of functional capacity in elderly women, while mean energy
consumption of HIIT was only 45% of energy consumed by MICT.
130
Besides, low volume high intensity training may
be even more enjoyable; as a result, prescriptions for low volume training may attract more sarcopenia patients to
follow.
131
To sum up, low volume HIIT might become a time-sufcient and pleasant way to treat old patients with sarcopenia.
High volume training is more effective in improving muscle size, which seems to be more suitable for bodybuilders than
sarcopenia patients. The time spent on training is negatively associated with the risk of CVD and type 2 diabetes, and
therefore high-volume training is also benecial to sarcopenia patients.
124
Furthermore, high training volume may
complement the disadvantages of low intensity training compared with high intensity training, so when patients are
reluctant to increase the intensity of load in their training, high volume and low intensity training is also effective.
124
However, no study has compared the effects of high-volume HIIT and low-volume HIIT on muscle, so above experi-
ments can only be considered as indirect evidence, and efforts are needed to make up this problem.
Comparison Between HIIT and Persistent Aerobic Exercise (PAE)
PAE is a traditional exercise method which has already been applied in various elds including management of
obesity, maintenance of physical performance, treatment of hypertension and improvement of cardiopulmonary
functions.
132–135
Aerobic exercises are cardiorespiratory endurance exercises, such as jogging, running, treadmill
walking, stationary cycling, stair climbing and cycling.
67,136
MICT is the foundation of aerobic-based exercise
prescription.
90
Most of the PAE training methods were carried out in the modality of MICT.
137–139
However, the status of PAE is being challenged by HIIT. For instance, studies reported that, both in young and old
people, HIIT was more effective than MssssICT in enhancing vascular function
133
and improving cardiorespiratory
capacity;
140–143
oxidative stress of the myocardium after myocardial infarction can be better attenuated by HIIT than
MICT;
144
and compared with MICT, HIIT displayed similar or greater impacts on reduction of adiposity,
65,145
increase in
insulin sensitivity in obese people,
146
reduction of blood triglycerides (TGs) and glucose levels in older individuals,
138,147
and enhancement of immune system with signicant reduction of the time commitment.
148
At the same time, both HIIT
and MICT displayed similarly high rate of completion and attendance, and low rate of adverse events in patients with
CVD.
66,85
Studies comparing the effects of HIIT and MICT on aged skeletal muscles and sarcopenia are emerging. An animal
experiment showed that both HIIT and MICT increased running time to exhaustion and maximum running speed of aged
rats similarly, but HIIT improved grip power performance greater than MICT.
74
This phenomenon was also observed by
Li and colleagues, and meanwhile they found that rats in HIIT group showed a larger increase in muscle weight
compared with MICT group, and HIIT was more powerful than MICT in ameliorating oxidative stress and inammation
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aggravated by aging.
149,150
Multiple studies compared the effects of HIIT and MICT on muscle strength of the elderly.
A study on older women (age = 67.8 ± 6.2 years) showed that HIIT improved upper limb strength better than MICT, but
there were no statistical differences between their effects on cardiorespiratory function, strength of lower limb and gait/
dynamic balance.
76
Nemoto K and colleagues designed an experiment to test the effects of high-intensity interval
walking training (3-minute low-intensity walking at 40% of peak aerobic capacity and 3-minute high-intensity walking
above 70% of peak aerobic capacity) and moderate-intensity continuous walking training, and results showed that the
high-intensity interval walking training was signicantly better than MICT in increasing thigh muscle strength (examined
by isometric knee extension and exion), along with the peak aerobic capacity for cycling and walking.
151
Some studies were concentrated on physical performance and muscle mass. A group of elderly people were randomly
divided into three conditions: no walking training, moderate-intensity continuous walking training, or high-intensity
interval walking training. Outcomes showed that in HIIT group, all of the isometric knee extension, isometric knee
exion, peak aerobic capacity for cycling, and peak aerobic capacity for walking got better augmentation than those in
MICT group.
151
Keogh and colleagues tested the change in physical function (measured by Timed Up and Go (TUG), Sit
to Stand (STS) and preferred gait speed) of osteoarthritis patients after receiving home-based HIIT or MICT. HIIT
displayed superiority in improving TUG, but there were no statistical differences between the changes in STS, gait speed
and muscle mass in HIIT and MICT groups.
152
However, for overweight/obese postmenopausal women, only the
combination of HIIT and RT could signicantly enhance muscle mass, while HIIT or MICT alone did not have this
function.
153
Although in some cases, compared with MICT, HIIT did not present superior capacity in enhancing muscle
function and physical performance, HIIT and MICT displayed similar effects, and HIIT seemed to be more acceptable:
participants in HIIT group tended to complete more sessions than those in MICT group.
84
Expressions of certain genes are inuenced by HIIT and MICT, which might explain the phenomena happening in
skeletal muscles after such training. Animal experiment revealed that while phosphorylated mTOR protein levels of
sarcopenic rats were similarly elevated by HIIT and MICT, HIIT group exhibited higher level of PGC1-α.
145
Both MICT
and HIIT can strengthen the antioxidative system through inducing the expression of succinate dehydrogenase (SDH) and
superoxide dismutase 2 (SOD2), promote the function of mitochondria through upregulation of oxidative phosphoryla-
tion (OXPHOS) proteins, and sustain the calcium homeostasis, but only HIIT can signicantly upregulate levels of
autophagy-related gene (Atg)-3, microtubule-associated protein 1 light-chain 3-II (LC3-II), B-cell lymphoma 2 (Bcl-2),
the Bcl-2/Bcl-2-associated X protein (Bax) ratio, AMP-activated protein kinase (AMPK), p-AMPK, and ADP receptor
1.
74
Elevated expression of Bcl-2 and Bcl-2/Bax ratio prevent age-related apoptosis of skeletal muscle cells, and elevated
expression of Atg-3, LC3-II, AMPK and ADP receptor 1 indicated that HIIT had a greater potential than MICT to
improve autophagy damaged by aging.
74
Overall, compared with PAE, HIIT is more effective at improving the physical performance and attenuating the
process of sarcopenia. Simultaneously, HIIT is superior to PAE in ameliorating patients’ physical tness and motor
function in various aspects. Furthermore, the time expenditure of HIIT is much less than MICT and HIIT is more
enjoyable for people to perform.
154,155
Therefore, HIIT may be a valid and time-efcient way to slow down the
progression of sarcopenia. However, the study performed directly on sarcopenia patients is devoid. Therefore, the
above results still need to be further studied.
Metabolic Changes During and After HIIT
HIIT will raise remarkable metabolic changes in sarcopenia patients. Firstly, acute skeletal muscle responses will occur. HIIT
upregulates 22 mitochondrial genes in older people, including genes participating in translational regulation and mitochondrial
tRNA transferase, thereby resulting in a signicant increase in protein abundance.
32
HIIT’s impact on mRNA expression and
MPS is predominant in both young and older people, and even greater among the elderly, and thus HIIT may overcome the
anabolic stimuli-resistance associated with aging.
32
Phosphorylation of AMPK and the p38 mitogen-activated protein kinase
(MAPK) will increase after HIIT. During contraction, p38 MAPK, which might be activated by growing level of reactive
oxygen species, stimulates upstream transcription factors of PGC-1a gene.
156,157
AMPK directly phosphorylates PGC-1α and
activates Sirtuin 1 (Sirt1) by increasing the level of NAD
+
, and thereby Sirt1 can promote the deacetylation of PGC-1α.
158
PGC-1α coactivates two key nuclear respiratory factors (NRFs), NRF-1 and NRF-2, which activate mitochondrial
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transcription factor A (Tfam) and bind to promoter regions of nuclear genes encoding subunits of complexes in mitochondrial
electron transport chain (ETC), thereby reinforcing respiratory capacity of mitochondria.
159,160
PGC-1α protects muscle mass
and retards the atrophy induced by denervation, and inhibits the ability of FOXO to bind to the promoter of atrogin-1.
161,162
Akt also can suppress FOXO pathway.
161
HIIT can enhance glucose transportation, insulin sensitivity and C reuptake of
sarcoplasmic reticulum (SR), so as to improve energy supply and working ability.
163
Akt/PKB, which is required for the
translocation of glucose transporter 4 (GLUT4), is upregulated by HIIT, and simultaneously, upregulation of AMPK, Ca
2+
and
p38 MAPK during HIIT induces the expression of GLUT4 gene.
164,165
Then, GLUT4 can increase insulin sensitivity.
156
Secondly, cardiac adaptation will be observed. Like skeletal muscles, the mTORC1 pathway in myocardium will also
be activated, resulting in physiological hypertrophy.
163
Intermittent training can restore the contractile function damaged
by sedentary, which reinforces synchronicity of SR Ca
2+
release, density of T-tubule, and activity of SR Ca
2+
ATPase.
166
Furthermore, cardiac structure will be protected and improved by HIIT: HIIT could ameliorate end-diastolic pressure and
systolic pressure, reduce left ventricular hypertrophy, and left ventricular end-diastolic diameter.
167–169
HRR, MET
capacity and VO
2max
can all be improved by HIIT.
167,170,171
Furthermore, if patients suffer from obesity, which potentiates the progress of sarcopenia,
59
HIIT is capable of
enhancing skeletal muscle and reducing adiposity at the same time. High-intensity exercise signicantly increases
catecholamine responses, which promotes fat oxidation (Fox) by β-adrenergic receptors. Plasma catecholamine levels
and sympathetic neural activity grow exponentially over intensity and time, especially at work intensity above 70%
VO2max.
172,173
HIIT can also elevate β-adrenergic receptor sensitivity in adipose tissue.
171
Furthermore, the rise of β-
Hydroxyacyl acyl-CoA dehydrogenase, citrate synthase and fatty acid-binding protein during HIIT promote the con-
sumption and transportation of free fatty acid.
174
Collectively, HIIT signicantly reduces fat in the blood and liver.
163
Nevertheless, HIIT may not reduce body weight while reducing body fat because of muscle hypertrophy.
171
The
molecular mechanisms of HIIT on treating and preventing sarcopenia are shown in Figure 1.
Conclusion
A growing number of scientists have realized the clinical importance of sarcopenia. Studies have revealed multiple risk
factors for this disease, including genes, malnutrition, inactive lifestyle, obesity, hormone imbalance, evolutionary basis
and so on. Despite spectacular progress in science and technology, effective treatments of sarcopenia are still devoid, and
the specic pathophysiology of the age-related loss of muscle remains unclear.
HIIT is a potential method to treat sarcopenia, and its feasibility has been demonstrated in various aspects. High
intensity training is more effective than low to moderate intensity in increasing strength without causing higher risks;
compared with the continuous training, the interval training raises more acute physiological responses, and provides
a rest time to restore muscle strength and cardiorespiratory functions, making it easier for body to adapt; low volume
training elicits similar increase in strength compared with moderate-to-high volume training with less expenditure on
time. In addition, relative to PAE, HIIT is more effective on improving the physical performance, and it also has greater
impacts on reinforcing cardiorespiratory function for long-term training, reducing body fat and improving the effective-
ness of energy use through managing diabetes, but the time expenditure of HIIT is much less than MICT, so HIIT is
a potent alternative for PAE; moreover, HIIT elicits great level of mRNA expression and protein abundance in both
young and old people, and therefore HIIT might overcome the anabolic-stimuli resistance in elderly. Above all, low HIIT
is a time-efcient exercise strategy for attenuating the progress of sarcopenia and improving the living quality of patients.
However, multiple questions remain to be gured out. Short rest interval is demonstrated to be the best choice for
inducing muscle hypertrophy, but there are controversial results of the effects of short or long rest interval between sets
on the improvement of muscle strength, which is required to be elucidated; low volume HIIT is time-efcient and
benecial to muscle strength, but the effects of low to moderate intensity and high-volume training on promoting muscle
hypertrophy and reducing the risks of CVD and type 2 diabetes should not be ignored; and experiment comparing the
effects of HIIT protocols with different volumes or different rest intervals on the elderly was still scarce. Besides,
previous studies were limited by short-term intervention and small sample size, and many results were achieved in
animal experiments. Furthermore, it is worth mentioning that no study was conducted directly on sarcopenia patients.
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Hence, it is necessary to implement clinical trials on elderly people with long-term intervention and large sample size in
the future.
Funding
This work was supported by National Key R&D Program of China (2019YFA0111900), National Natural Science
Foundation of China (82071970, 81874030, 82072506), Provincial Natural Science Foundation of Hunan (2020JJ3060),
Provincial Clinical Medical Technology Innovation Project of Hunan (2020SK53709), the Administration of Traditional
Chinese Medicine of Hunan Province (2021075), Innovation-Driven Project of Central South University (2020CX045),
Science and Technology Innovation Project of Jianghan University (2021kjzx008), Hunan Yong Talents of Science and
Figure 1 Molecular mechanism of HIIT on treating and preventing sarcopenia. (A) Promotion of muscle protein synthesis. HIIT and enough nutrient supplementation
upregulate PI3K/Akt pathway. Akt inhibits tuberous sclerosis complex (TSC) through phosphorylation. TSC facilitates the conversion of RHEB-GTP to RHEB-GDP, thereby
inhibiting the function of RHEB-GTP to activate mTORC1. Thus, mTORC1 pathway is relieved from the inhibition of TSC. And with the existence of amino acids, Rag
guanine triphosphatases (Rag GTPases) promote the translocation of mTORC1 to lysosome where RHEB-GTP activates mTORC1. Thus, muscle protein synthesis is
triggered. (B) Inhibition of muscle protein breakdown. Akt-mediated phosphorylation inhibits FOXO and the expression of the atrophy-related ubiquitin ligases Atrogin 1
and MURF1, and thus suppresses muscle protein breakdown caused by Proteasomal proteolysis. (C) Enhancement of mitochondrial biogenesis. Concentration of Calcium
ion and expression of AMPK in muscle cells are unregulated by HIIT. Calcium ion and Akt promote PGC-1αpathway through CREB, and AMPK activates PGC-1αthrough
direct phosphorylation and SIRT1-dependent deacetylation. PGC-1αenhances the expression of NRF1 and NRF2, which stimulate mitochondrial biogenesis and increase of
mitochondrial respiratory capacity. (D) Promotion of the GLUT4 expression. AMPK, Ca
2+
and p38 MAPK can induce the expression of GLUT4 gene.
Abbreviations: Akt, protein kinase B; AMPK, AMP activated kinase; eIF, eukaryotic initiation factor; mTORC1, mammalian target of rapamycin complex 1; 4E-BP1,
eukaryotic initiation factor 4E (eIF4E)-binding protein-1; Rag, Ras-related GTPase; rpS6, ribosomal protein S6; S6K1, p70 ribosomal S6 kinase 1; CaMK, calcium/calmodulin-
dependent protein kinase; CREB, cAMP response element-binding protein; ETC, electron transport chain; NRF, nuclear respiratory factor; PGC-1α, proliferator-activated
receptor γcoactivator 1α; SIRT1, sirtuin 1; Tfam, mitochondrial transcription factor A; Mt, mitochondria; GLUT4, glucose transporter 4; MAPK, mitogen-activated protein
kinase.
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Technology (2021RC3025), Wu Jieping Medical Foundation (320.6750.2020-03-14), and the Independent Exploration
and Innovation Project for Postgraduate Students of Central South University (2021zzts1024).
Disclosure
The authors declare that they have no competing interests.
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High-intensity interval training (HIIT) can effectively increase peak oxygen consumption, body composition, physical fitness, and health-related characteristics of adults; however, its impact in the older population remains highly debated. This review and meta-analysis aimed to evaluate the effects of high-intensity interval training on cardiorespiratory fitness, body composition, physical fitness, and health-related outcomes in older adults. Four electronic databases (PubMed, Scopus, Medline, and Web of Science) were searched (until July 2020) for randomized trials comparing the effect of HIIT on physical fitness, metabolic parameters, and cardiorespiratory fitness in older adults. The Cochrane risk of bias assessment tool was used to evaluate the methodological quality of the included studies; Stata 14.0 software was used for statistical analysis. HIIT significantly improved the maximum rate of oxygen consumption (VO2peak) as compared to a moderate-intensity continuous training (MICT) protocol (HIIT vs. MICT: weighted mean difference = 1.74, 95% confidence interval: 0.80–2.69, p < 0.001). Additional subgroup analyses determined that training periods >12 weeks, training frequencies of 2 sessions/week, session lengths of 40 min, 6 sets and repetitions, training times per repetition of >60 s, and rest times of <90 s were more effective for VO2peak. This systematic review and meta-analysis showed that HIIT induces favorable adaptions in cardiorespiratory fitness, physical fitness, muscle power, cardiac contractile function, mitochondrial citrate synthase activity, and reduced blood triglyceride and glucose levels in older individuals, which may help to maintain aerobic fitness and slow down the process of sarcopenia.
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Importance: High-intensity interval training (HIIT) is recognized as a potent stimulus for improving cardiorespiratory fitness (volume of oxygen consumption [VO2] peak) in patients with coronary artery disease (CAD). However, the feasibility, safety, and long-term effects of HIIT in this population are unclear. Objective: To compare HIIT with moderate-intensity continuous training (MICT) for feasibility, safety, adherence, and efficacy of improving VO2 peak in patients with CAD. Design, setting, and participants: In this single-center randomized clinical trial, participants underwent 4 weeks of supervised training in a private hospital cardiac rehabilitation program, with subsequent home-based training and follow-up over 12 months. A total of 96 participants with angiographically proven CAD aged 18 to 80 years were enrolled, and 93 participants were medically cleared for participation following a cardiopulmonary exercise test. Data were collected from May 2016 to December 2018, and data were analyzed from December 2018 to August 2019. Interventions: A 4 × 4-minute HIIT program or a 40-minute MICT program (usual care). Patients completed 3 sessions per week (2 supervised and 1 home-based session) for 4 weeks and 3 home-based sessions per week thereafter for 48 weeks. Main outcomes and measures: The primary outcome was change in VO2 peak during the cardiopulmonary exercise test from baseline to 4 weeks. Further testing occurred at 3, 6, and 12 months. Secondary outcomes were feasibility, safety, adherence, cardiovascular risk factors, and quality of life. Results: Of 93 randomized participants, 78 (84%) were male, the mean (SD) age was 65 (8) years, and 46 were randomized to HIIT and 47 to MICT. A total of 86 participants completed testing at 4 weeks for the primary outcome, including 43 in the HIIT group and 43 in the MICT group; 69 completed testing at 12 months for VO2 peak, including 32 in the HIIT group and 37 in the MICT group. After 4 weeks, HIIT improved VO2 peak by 10% compared with 4% in the MICT group (mean [SD] oxygen uptake: HIIT, 2.9 [3.4] mL/kg/min; MICT, 1.2 [3.4] mL/kg/min; P = .02). After 12 months, there were similar improvements from baseline between groups, with a 10% improvement in the HIIT group and a 7% improvement in the MICT group (mean [SD] oxygen uptake: HIIT, 2.9 [4.5] mL/kg/min; MICT, 1.8 [4.3] mL/kg/min; P = .30). Both groups had high feasibility scores and low rates of withdrawal due to serious adverse events (3 participants in the HIIT group and 1 participant in the MICT group). One event occurred following exercise (hypotension) in the HIIT group. Over 12 months, both home-based HIIT and MICT had low rates of adherence (HIIT, 18 of 34 [53%]; MICT, 15 of 37 [41%]; P = .35) compared with the supervised stage (HIIT, 39 of 44 [91%]; MICT, 39 of 43 [91%]; P > .99). Conclusions and relevance: In this randomized clinical trial, a 4-week HIIT program improved VO2 peak compared with MICT in patients with CAD attending cardiac rehabilitation. However, improvements in VO2 peak at 12 months were similar for both groups. HIIT was feasible and safe, with similar adherence to MICT over 12-month follow-up. These findings support inclusion of HIIT in cardiac rehabilitation programs as an adjunct or alternative modality to moderate-intensity exercise. Trial registration: Australian New Zealand Clinical Trials Registry Identifier: ACTRN12615001292561.