Lesson 8: An Evidence-Based Approach to Aerobic Training, Concurrent Training, and Exercise Safety

Introduction

In Lesson 3 I discussed general health guidelines regarding endurance training and in Lesson 7 I discussed relevant terminology and concepts. In this lesson I’ll present the research in more detail to help determine:

how you should specifically incorporate both continuous aerobic training and interval training
how you can structure this with resistance training in a concurrent training design to help minimize the interference effect between the two
if there are any significant safety concerns when beginning these exercise programs

Aerobic exercise structure: interval and continuous training

The guidelines discussed in Lesson 3 basically advise ≥150 minutes of moderate-intensity physical activity or ≥75 minutes of vigorous-intensity physical activity weekly. They state any duration of activity counts but do not suggest if interval & continuous exercise have distinct benefits. Therefore, let’s first look at the research evaluating interval training and then look at the research comparing interval training to continuous training.

Research evaluating interval training

I have included several review articles discussing interval training in the box below. You can read through them or skip to the below summary if you would like.

Click here for an overview of research evaluating interval training.

There have been several reviews evaluating interval training in populations who have previously partaken in endurance training or general athletics:

A 2017 systematic review (“SR”) evaluating studies in endurance runners found incorporating high-intensity interval training (“HIIT”) improved lactate metabolism, athletic performance, running economy, and oxidative capacity, among other attributes.^{(García-Pinillos, 2017)}The evidence suggested optimal results from:
- incorporating 2-3 HIIT sessions per week
- accumulating >10 minutes per session at close to maximum intensity
- utilizing <1 minute high-intensity intervals
- incorporating a work-to-rest ratio between 1:1 & 1:2
  - 1:1 example – 30 seconds at a fast pace and 30 seconds at a slow pace
  - 1:2 example – 30 seconds at a fast pace and 60 seconds at a slow pace
A 2018 SR of studies involving people with athletic backgrounds found HIIT led to a variety of improvements in different sports regardless of interval duration.^{(Girard, 2018)}
A 2020 systematic review & meta-analysis (“SR/MA”) of studies including at least moderately endurance-trained individuals comparing HIIT to sprint-interval training (“SIT”) found no difference between the two in time trial performance.^{(Rosenblat, 2020)} However, when only looking at longer duration HIIT protocols these did yield a 2% greater improvement. The authors proposed longer interval (4-6 minutes) with 2-4 minutes rest would lead to the greatest improvements in time trial performance.

Thus, in previously trained populations both long & short HIIT intervals improve fitness attributes.

There have also been several reviews in youth populations:

A 2017 SR/MA found HIIT performed 2-3 times weekly improved various metabolic biomarkers in children and adolescents.^{(Eddolls, 2017)}
A 2018 SR/MA of studies with youth athletes found HIIT & SIT performed similarly to alternative training protocols but generally took less time.^{(Engel, 2018)}
A 2020 SR/MA evaluating different HIIT protocols on indicators of adiposity in children and adolescents with elevated BMI found a work-to-rest ratio of 1:1 was more effective than 1:2 or 2:1. This resulted in a shorter total session time (~11 minutes compared to ~24 minutes with the other protocols).^{(Menezes Junior, 2020)}

This indicates interval training in short sessions yields significant benefits in a range of youth populations.

Regarding untrained adult populations:

A 2017 SR/MA of studies evaluating SIT protocols found VO₂ max increased on average ~8%.^{(Vollaard, 2017)}Including only 2 sprint repetitions yielded the most beneficial impact. Additional sprint repetitions led to less improvement. Interval lengths between 10-30 seconds were equivalent.
A 2021 SR/MA evaluating the impact of HIIT in adults with overweight and obesity found that this yielded a large improvement in cardiorespiratory fitness (5.65 mL/kg/min, 1.6 METs), particularly seen in interventions lasting ≥4 weeks, sessions with ≥16 minutes duration (not including warm-up and cool-down periods), and a training intensity of ≥7 METs.^{(Wang, 2021)}
- Refer back to Lesson 3 for an overview of METs if curious.
A 2022 SR/MA found that both HIIT and SIT lead to several physiologic improvements regarding mitochondrial and cardiovascular parameters, while more research is needed to better determine long-term differences between HIIT and SIT.^{(Rosenblat, 2022)}

Additionally, a 2022 SR/MA evaluating physically active adults and athletes found that short SIT (where repeated bouts of <10 second episodes are used) was just as effective as longer SIT, HIIT, and other baseline training at improving aerobic performance, anaerobic performance, and VO₂ max.^{(Boullosa, 2022)}

Summary of research evaluating interval training

The prior reviews clearly show interval training in a variety of formats improves cardiorespiratory fitness in multiple different populations regardless of baseline fitness status. As few as two sprint intervals and HIIT sessions <20 minutes can be effective. Thus, the various types of interval training are viable options for improving overall health and fitness.

Note: Just because small amounts of activity can be effective does not mean you should limit yourself to this forever. The last review cited above showed benefits with training programs including 2 sprint repetitions in untrained adult populations. After an initial phase of adaptation further progress may require greater training volume. Additionally, meeting the physical activity guidelines discussed in Lesson 3 requires more activity than 2 sprints provides.

Nonetheless, it does seem a small amount of activity goes a long way. This is encouraging for beginners to exercise as a small time commitment yields significant results. Over time, though, working towards the guidelines with some combination of interval and continuous training will yield greater health benefits.

Tip: While this course is not geared towards maximizing athletic performance, a 2021 SR/MA did analyze the literature to determine what interval training parameters optimize time- trial performances in trained individuals.^{(Rosenblat, 2021)} The authors ultimately concluded:

HIIT: Perform 5 reps of 5 minute work bouts within the severe domain (domains were discussed in Lesson 7). Include 2.5 minutes recovery (active or passive) between the work bouts. Repeat this twice a week for at least 4 weeks.
SIT: Within commonly studied ranges there is no dose-response seen with various SIT parameters. Trained individuals should perform 4 reps of 30 second work bouts at maximum effort. Include 4 minutes of passive recovery between the work bouts and repeat this twice weekly for 2 weeks.

Just because these are the optimal parameters seen overall does not mean they will be optimal for each individual and they do not consider additional non-interval training done throughout the week. Additionally, the total distance of the time-trial will likely dictate to some degree what percentage of one’s training should be done at faster or slower paces.

Comparison of interval and continuous training

I have included several review articles discussing comparisons of interval and continuous training in the box below. You can read through them or skip to the below summary if you would like.

Click here for an overview of research comparing interval and continuous training.

There have been several recent reviews evaluating HIIT vs moderate-intensity continuous training (“MICT”) in various adult populations:

Thus, a large body of evidence shows similar results or at times superior results of HIIT vs MICT with respect to cardiovascular fitness, anthropometric measurements, and health markers. Potential physiologic underpinnings of this were discussed in a recent review.^{(Gibala, 2021)} The health benefits seem to also extend to low-volume HIIT (<15 minutes of total time at a high-intensity per session).^{(Sabag, 2021b)}

There have also been reviews specifically evaluating HIIT vs MICT in youth populations:

A 2019 SR/MA of studies in children and adolescents found HIIT improved cardiorespiratory fitness more than MICT.^{(Cao, 2019)}
A 2020 SR/MA of studies specifically evaluating adolescents also found HIIT improved cardiorespiratory fitness relative to MICT regardless of initial BMI category or baseline fitness level.^{(Martin-Smith, 2020)} Protocols as short as 6 minutes in duration and lasting as little as 4 weeks were effective.
A 2021 SR/MA evaluating children aged 6-18 years with overweight or obesity and comparing HIIT vs MICT found that HIIT was more beneficial for cardiorespiratory fitness and systolic blood pressure, while there were no differences in diastolic blood pressure, anthropometric measurements, lipid profiles, or measures of glycemia.^{(Cao, 2021)} HIIT protocols were particularly more helpful when they were >8 weeks in duration and consisting of running, while other aspects such as the work-to-rest ratio and training time per session had variable impacts on cardiorespiratory fitness and systolic blood pressure.

Therefore, in addition to the improvement in health markers noted above, HIIT seems to improve cardiorespiratory fitness more than MICT in children and adolescent populations.

Regarding older populations:

A 2020 SR specifically evaluated HIIT vs MICT in studies including people with a median age >60.^{(Keating, 2020)} Of 15 included studies, 7 found a benefit of HIIT relative to MICT in at least 1 measured outcome. None had worse outcomes with HIIT relative to MICT. There were no strong benefits of any one specific HIIT protocol compared to another but the most common utilized was a 4×4 method entailing:
- initially perform a 5 minute warm-up
- then perform four 4-minute intervals at 85-95% of your HRmax with each followed by a 3-minute lower-intensity interval at 60-70% of your HRmax
- lastly perform a 5 minute cool-down

Summary of research comparing continuous and interval training

It is clear that HIIT produces similar and at times superior health benefits relative to MICT. Additionally, this holds true across a wide variety of populations and health metrics. Both short and long intervals seem effective and several analyses found benefits with well under 30 minutes per session.

However, just because HIIT provides benefits relative to MICT does not imply MICT is worthless. In most of the research above the benefits of HIIT relative to MICT were relatively small and trials were of relatively short duration (ie, 8-12 weeks). Additionally, there are physiologic distinctions between HIIT and MICT that may lead to different health outcomes.^{(Langan, 2021)} The figure below highlights some of these differences.

Reproduced from: Langan SP, Grosicki GJ. Exerciss is Medicine…and the Dose Matters. Frontiers in Physiology. 2021 May;12:1-5. doi: 10.3389/fphys.2021.660818

Furthermore, the above reviews do not generally compared HIIT + MICT vs HIIT alone. It is very possible that combining both modalities over the long term will yield superior health benefits compared to performing either in isolation.

Note: Regarding athletic endeavors, a 2020 SR/MA of marathon runners found factors correlating strongly with marathon performance were^{(Doherty, 2020)}:

weekly running distance – this is mostly MICT
length of the longest weekly run – this is also MICT
number of runs >32 kilometers – this is also MICT

It is actually commonly described that most training programs for endurance athletes have ~80% of the total training done at a low-to-moderate intensity pace.^{(Barrie, 2020)} In world-class athletes the total time spent performing MICT will vary by sporting discipline but can be several hours daily.^{(Sandbakk, 2021)} Thus, for anyone looking to perform endurance events, sufficient MICT is likely the most important component of a well-designed training protocol.

Tip: As stated in the last lesson HIIT is generally considered vigorous physical activity where your heart rate is >85% of your maximum heart rate. So if you are ever unsure if you are performing HIIT or not, simply calculate your maximum heart rate as described in the last lesson and see if your own heart rate is greater than 85% of this.

Example: If you are 40 years old and calculate your maximum heart rate at 220-40 = 180, then 180 * 0.85 = 153. Thus, if your heart rate is at least 153 beats per minute you are performing HIIT. While I did go over the fact that there are issues with maximum heart rate calculations in the last lesson, most studies on HIIT do not account for this, and thus this approximation should be sufficient.

Example: Let’s calculate the amount of weekly moderate & vigorous physical activity you would perform with 3 HIIT sessions using the 4×4 method described above:

4 intervals of 4 minutes duration at vigorous intensity = 16 minutes per session
4 intervals of 3 minutes duration + 10 minutes of warm-up & cool-down at moderate intensity = 22 minutes per session

Thus, with a 38 minute session done 3 times weekly this yields:

48 minutes of vigorous physical activity
66 minutes of moderate physical activity

As described in Lesson 3, one minute of vigorous physical activity counts as 2 minutes of moderate physical activity. Thus this yields:

2*48 + 66 = 162 minutes that count toward the guidelines

Therefore this meets the physical activity guidelines.

Now let’s calculate for a different HIIT protocol. Here you will perform 1 minute at vigorous intensity, 2 minutes at moderate intensity, and you will complete this for 9 cycles with 11 minutes of warm-up + cool-down to also equal a 38 minute session.

9 intervals of 1 minute duration at vigorous intensity = 9 minutes per session
9 intervals of 2 minute duration at moderate intensity + 11 minutes for warm-up & cool-down = 29 minutes per session

This equates to 27 minutes of vigorous intensity and 87 minutes of moderate intensity exercise weekly. Thus this yields:

2*27 + 87 = 141 minutes that count toward the guidelines

This does not completely meet the guidelines. However, the minutes total is within 15% of the 4×4 protocol, which is pretty close. Additionally, if you were performing resistance training concurrently and your heart rate was elevated while doing so that would also count towards moderate-intensity physical activity.

Overall, the main point is that HIIT protocols alone can go a long way towards meeting the guidelines.

Concurrent Training and the Interference effect

When performing both resistance training and aerobic training there are different ways to combine these exercise modalities.. This is termed “concurrent training”, where you train multiple attributes simultaneously. If the various training styles do not impact each other in any way then there are no additional considerations. However, if there is an “interference effect” between them you can structure your training program and choose exercise modalities to minimize this. Therefore, I will now summarize recent literature regarding how resistance and aerobic training sessions impact each other on an acute and chronic basis.

Research evaluating the acute impact of the interference effect on concurrent training

I have included several review articles evaluating the acute impact of the interference effect on concurrent training in the box below. You can read through them or skip to the below summary if you would like.

Click here for research evaluating the acute impact of the interference effect on concurrent training.

Several reviews have evaluated how incorporating resistance and aerobic training in close proximity results in an interference effect.

Pre-2020 reviews:

A 2017 review found that a resistance training session can impair running economy, alter gait mechanics, and decrease performance in running, cycling, and rowing for several days after the session is completed.^{(Doma, 2017)}
A 2018 SR evaluated the impact of strength training on distance running. The authors recommended^{(Blagrove, 2018)}:
- rest at least 3 hours after high-intensity running prior to resistance training
- rest at least 24 hours after resistance training prior to intense running
A 2018 SR/MA evaluated studies comparing resistance training alone or with HIIT on muscular strength and hypertrophy.^{(Sabag, 2018)} Authors found a small negative effect size (-0.248) favoring resistance training alone for lower body strength. However, when including >24 hours between resistance training and HIIT sessions strength gains were not compromised.
A second 2018 SR/MA evaluating studies of mostly untrained subjects compared the impact of exercise sequence in a single session.^{(Eddens, 2018)} Performing resistance training prior to aerobic training yielded a 6.9% greater improvement in lower body dynamic strength development compared to performing aerobic training first. However, exercise order made no difference for lower body static strength, lower body hypertrophy, body fat percentage, or maximal aerobic capacity.
A third 2018 SR/MA evaluating the impact of exercise sequence found performing strength training first was beneficial for strength gains.^{(Murlasits, 2018)} On the other hand, exercise order had no impact on aerobic capacity development.

More recent reviews:

A 2021 SR/MA evaluating studies of individuals with different training experience levels compared the impact of resistance training alone or with endurance training on lower body strength.^{(Petré 2021)} There was no interference effect for untrained individuals. Trained individuals had a negative interference effect when both training modalities were done in the same session but not if they were separated. However, this negative effect was primarily seen only in 2 study groups from the same study.
A 2021 review of the acute impact of performing aerobic training prior to strength training found^{(Panissa, 2021)}:
- a greater negative effect of high-intensity interval exercise on strength-endurance rather than maximal strength
- resting 4-8 hours between sessions largely minimized the negative effect
- generally longer distances will have a greater impact than shorter distances (ie, running 5 kilometers vs. 3 kilometers)
A 2021 SR/MA evaluating 15 studies in trained males found that performing aerobic training prior to strength training led to a performance decrease in strength but not power.^{(Markov, 2021)} A greater decrease was seen when the aerobic activity was cycling as opposed to running. The authors noted that all of the studies used tests of knee extensor strength after the aerobic activities and it is possible that cycling fatigued these muscles to a greater extent than running.

Summary of the acute impact of the interference effect on concurrent training

Resistance and aerobic training can negatively impact each other on an acute basis. It may be best to rest at least 24 hours after resistance training and at least several hours after aerobic training prior to performing the other modality. When incorporating both resistance training and aerobic training in the same session, performing resistance training first seems beneficial.

Tip: The overall impact of the interference effect is rather small. Therefore, this is likely more relevant for people looking to compete than individuals training to improve their health. Additionally, most of the research demonstrating an interference effect does so in the lower body. Thus, if feasible, you can separate resistance and aerobic training sessions by >24 hours. If this is not feasible then separating HIIT and heavier lower body strength training is preferable. When this is not possible there is no need to panic, you can still make tremendous progress doing these simultaneously. If you do plan to compete in an endurance event consider taking several days off from resistance training prior to the competition.

Research evaluating the chronic impact of concurrent training on strength and aerobic outcomes

I have included several review articles evaluating the chronic impact of concurrent training on strength and aerobic outcomes in the box below. You can read through them or skip to the below summary if you would like.

Click here for an overview of research evaluating the chronic impact of concurrent training on strength and aerobic outcomes.

Several reviews have examined whether concurrent training proves beneficial or detrimental to strength and aerobic training adaptations longitudinally.

Pre-2020 reviews:

A 2017 SR/MA evaluated concurrent training in endurance runners.^{(Denadai, 2017)}Incorporating strength training 2-3 times weekly improved running economy ~4% regardless of baseline endurance experience.^.
A 2018 SR evaluating the impact of strength training on distance running parameters found 2-3 sessions per week improved running economy without changing VO₂ max or lactate threshold.^{(Blagrove, 2018)} Sprint speed and time trial performance also improved. Heavy resistance training seemed to work better than lighter training. Decreasing strength training frequency to 1 time weekly after following a more frequent training program was sufficient to maintain previously developed benefits.
A 2018 SR/MA found that adding a strength training mesocycle to a sport-specific aerobic training program improved distance running and running economy.^{(Berryman, 2018)} Heavier training led to greater improvements than submaximal training. These improvements occurred across a range of sports as well as in high-level endurance athletes. Force production improved more than power production, potentially due to the interference effect negatively impacting power development to a greater extent.
In a 2018 review summarizing the concurrent training literature the authors suggested that when resistance training is the priority this should be performed prior to aerobic training and there should be a 2:1 – 3:1 ratio of resistance training to aerobic training sessions.^{(Methenitis, 2018)} When aerobic training is the priority then one should consider a 1:1 – 1:2 ratio.
A 2019 SR of studies evaluating training intensity found that higher resistance training intensity paired with low-to-moderate aerobic training intensity led to better strength and neuromuscular gains. Higher aerobic training intensities diminished strength and neuromuscular gains but led to greater increases in cardiorespiratory fitness.^{(Sousa, 2019)}

More recent reviews:

Summary of the chronic impact of concurrent training on strength and aerobic outcomes

Overall it is clear that incorporating strength training can improve endurance performance. Heavy strength training 2-3 times per week is most beneficial. Once benefits are obtained it seems possible to maintain them when strength training only once per week.

On the other hand, it does seem that significant amounts of aerobic training, particularly at higher intensities, interferes with strength gains. However, per the fourth-to-last review cited in the box above this does not seem to extend to SIT and RST. Additionally, similar to what was seen in the prior section, separating resistance training from HIIT mitigates the interference effect.

Tip: With the above knowledge you can now more confidently design a concurrent training program while minimizing the impact of the interference effect.

For aerobic training performance:

Overall, the negative impact of the interference effect on aerobic performance can be minimized by avoiding strength training several days prior to an endurance competition.
Otherwise, heavy strength training is beneficial for endurance performance regardless of your underlying fitness level (ie, from beginner to advanced) and thus should be included 2-3 days weekly even for individuals who want to primarily focus on endurance training. Additionally, this will meet the health guidelines discussed in Lesson 3 as well as the evidence-based resistance training program suggestions discussed in Lesson 5.
In the weeks leading up to a major endurance competition you can decrease strength training to one session weekly while maintaining the prior benefits.
If performing resistance training and aerobic training sessions separately but within the same day, the aerobic training session should be performed first.

For strength training performance:

HIIT done in close proximity to strength training sessions seems to have a negative impact. Therefore, SIT and RST seem preferable.
Power production seems to be impacted more than maximum force production. Thus, the interference effect seems most critical for athletes who need to maximize power output.
Resistance training should be performed prior to aerobic training if they are being conducted in the same session.

Safety considerations when starting exercise programs

In 2015 the American College of Sports Medicine published updated guidance regarding when medical screening prior to beginning an exercise program is needed.^{Riebe, 2015} Their full recommendations are shown in Figure 2 of the linked document. They basically suggest that individuals without cardiovascular, metabolic, or renal disease who have no signs or symptoms suggestive of cardiovascular, metabolic, or renal disease do not need medical clearance prior to starting an exercise program. The caption of Figure 2 states what counts regarding the diseases, signs, and symptoms.

A couple of recent reviews have evaluated the safety of overall exercise interventions:

A 2020 SR evaluated 773 studies comparing individuals undergoing exercise therapy to non-exercising controls for their rate of adverse events.^{(Niemeijer, 2020)} 378 reported serious adverse events and 375 reported non-serious adverse events. There was no increase in relative risk of a serious adverse event in the intervention groups compared to the control groups. However, the relative risk of non-serious adverse events was 1.29, meaning for every 6 people in an exercise intervention 1 would obtain a non-serious adverse event relative to the control group.
A 2020 SR/MA evaluated studies of exercise interventions in elderly participants.^{(García-Hermoso, 2020)} The interventions were found to be safe as there were similar dropout rates between intervention and control groups. Additionally, the exercise interventions decreased the risk of falls & fall-associated injuries in all elderly populations, while also decreasing the risk of mortality in clinical populations.
A 2021 SR/MA evaluated studies of exercise interventions in individuals >80 years old, finding in total only 1 serious adverse event (a compression fracture) out of >200 individuals, with 6 nonserious adverse events reported (mild muscle pain, mild hematoma, and dizziness).^{(Nicolson, 2021)} Of note, while there was a decrease in overall mortality seen (due to a large decreased risk in 1 study), other measures of physical performance did not improve, implying the intensity of the interventions in most studies may have been suboptimal for performance benefit.
A 2022 review notes that many of the cardiac events that do occur with exercise are likely a result of an inadequate warm-up and cool-down phase, so these should be included.^{(Franklin, 2022b)}

The exercise interventions included in the above reviews were frequently multi-component and included various forms of aerobic and strengthening activity. Additionally, the subject populations included a wide age range and had a multitude of different clinical conditions. Therefore, it seems most forms of exercise can be done safely by most people when appropriate programs are followed.

Outside of regular exercise interventions, safety concerns are particularly relevant to HIIT as HIIT may strain the heart more than other forms of cardiovascular exercise.

As a result, several reviews have specifically evaluated the safety of HIIT:

A 2018 SR evaluating the safety of HIIT vs MICT in people with cardiovascular disease included 23 studies.^{(Wewege, 2018)} All studies included pre-intervention peak stress tests with an electrocardiogram (“ECG”) and included some degree of patient supervision during exercise sessions. Altogether, there were only a total of 7 adverse events; 5 of these occurred in the HIIT interventions and 2 in the MICT interventions. Only 2 of the adverse events in the HIIT intervention were related to the cardiovascular system; one of these was syncope and that participant continued the intervention. The other was cardiac arrest in an individual who had previously refused cardioverter-defibrillator implantation. Five of the HIIT studies had mean age ≥65.
A 2020 meta-review of SRs found 14 SRs that reported adverse events of HIIT interventions.^{(Martland, 2020)} Very few significant adverse events were reported. Additionally, adherence rates were typically >80%.
A 2020 SR evaluating HIIT vs MICT in elderly individuals found that there was only 1 major adverse event (the same as the one mentioned above).^{(Keating, 2020)}

Thus, HIIT seems to be a safe intervention not only in the general population but also in those with cardiovascular disease, though it is important to note that many of the studies utilized a pre-intervention stress test with an ECG. Additionally, there are some less common cardiovascular conditions that would warrant additional considerations; you can look through the cited reference for additional information if curious.^{(Mannakkara, 2023)}

Note: For further guidance regarding individuals with cardiometabolic conditions who are considering beginning HIIT, please look through this guideline.^{(Taylor, 2019)} The authors provide a list of the various clinical considerations in Figure 2. In Figure 1 they actually provide a guideline for HIIT prescription and monitoring. For any health care providers working with individuals with cardiometabolic conditions who wish to begin HIIT, this is an excellent read.

Tip: The importance of a baseline stress test and ECG should not be overlooked for individuals with cardiovascular conditions. As noted in Lesson 3 it is the onset of vigorous-intensity physical activity that seems to provoke the greatest risk of adverse events; it is most vital to be evaluated prior to engaging in vigorous activity.

For generally healthy individuals, to minimize the risk of adverse events I recommend:

Start with simple walking to ensure you can tolerate this without triggering any aches, pains, or soreness. Gradually increase the speed you are walking until you feel comfortable that you are ready to begin MICT.
Start MICT and progress the duration (up to at least 10 minutes) to ensure you can tolerate this without triggering any aches, pains, or excessive soreness.
When you can tolerate MICT without issues then you can begin to incorporate longer intervals at a faster pace.
When you can tolerate these longer intervals you can decrease the interval duration and quicken the pace.
Continue increasing the pace gradually until you are performing HIIT.

For people with concerning health conditions I recommend that you ask your healthcare provider about the safety of including high-intensity intervals. For example, you can ask if it would be worth undergoing a baseline exercises stress test and ECG.

Oftentimes people start exercising and things go smoothly until problems arise if they begin to increase their overall activity too quickly. Regarding aerobic training activity, many people cite a “10% rule” which suggests your training load should not increase more than 10% weekly to help minimize the risk of injury.

Several reviews have evaluated the impact of increasing training load on injury risk:

A SR published in 2017 evaluated the impact of changes in training load on running-related injuries.^{(Damsted, 2018)} However, this included only 4 studies, all with novice or amateur runners. These studies did not support the 10% rule, and in some studies increasing the training load ≥20% weekly did not lead to increased injury risk. Training loads were low (<10 kilometers per session). Thus, this would apply to people who are beginners.
Looking at more experienced individuals, a 2018 SR of endurance sport populations evaluated 12 studies.^{(Johnston, 2018)}Roughly half found associations between external training load metrics and injury risk, indicating significant variability. The most frequently identified non-modifiable risk factor was prior injury. Increased age was associated with injury but it is possible this was mediated by prior injuries (older individuals are more likely to have experienced an injury at some point in their past). None of the studies examined internal training load metrics (ie, RPE for specific sessions); it is possible that incorporating this would help decrease some of the observed variability.

At this point there does not seem to be a direct quantifiable relationship between training load and injury risk. There are many other variables that can impact how your acute or chronic load, as well as the acute-to-chronic workload ratio, can influence the risk of injury.^{(Gabbett, 2020)} Additionally, the same training load may have very different effects on your body if you are fresh or fatigued.^{(Paquette, 2020)} Thus, some advocate quantifying training load as (duration * session RPE), where:

Duration = the length of your training session (you can omit the warm-up time).
Session RPE = a score on a 0-10 scale describing how difficult the session felt.

Tip: Guidance documents that acknowledge the lack of evidence for the 10% rule still advocate following it as anecdotally it proves useful.^{(Hamilton, 2018)} For individuals who are gradually increasing activity to improve their health and overall fitness it makes sense to increase things slowly to help minimize injury risk and ensure the body adapts accordingly. For example, you can track your duration * session RPE over time; if it rapidly increases this may indicate an increased risk of an overuse injury.

For individuals who are training for a specific event (ie, a 5 kilometer race or half-marathon), things can get trickier if you have not been training regularly. In this situation it is best to set a competition date far enough in the future to allow yourself to get acclimated to regular aerobic training and then work towards the competition gradually. If you set the competition date too soon and you need to increase your training load rapidly in preparation then your risk of injury will increase significantly. It is important to set intelligent and realistic goals to try to avoid this type of situation.

Summary of safety considerations when beginning to exercise

Overall, the above data suggest exercise (both resistance and aerobic training) can be performed safely by almost everyone with little risk of significant health-related adverse events if:

individuals with cardiovascular and other higher risk medical conditions obtain appropriate clearance from their healthcare provider (which may include a baseline exercise stress test & ECG)
people follow an appropriate exercise program (which generally entails starting with low-intensity exercise and building up gradually over time)
good technique is utilized for the various movements that are performed (this is trickier without the supervision that is performed in many of the studies; I discuss exercise technique in Lessons 9-12).

I have included an example of how you can gradually build up your aerobic training performance below:

Example: Let’s go through how an adult with a sedentary lifestyle can begin to incorporate aerobic exercise. I am using walking/jogging as an example but this can be replaced by other exercise modalities. People who experience significant joint pain in the hips/knees/ankles when walking/jogging/running from osteoarthritis or other medical issues may find it better to use a bicycle, elliptical machine, swimming, or some other modality that does not incorporate repetitive foot strikes on surfaces.

Training days 1-6: begin walking 5 minutes on day 1 and then increase by 5 minutes each subsequent day until you are walking for 30 minutes total. This can be consecutive time or split up throughout the day. This can be on successive days (ie, all in 1 week) if no soreness/aches/pains (“SAP”) are experienced. If these are experienced you should include rest days until they subsides and then repeat the last training day’s duration. When you are able to complete 30 minutes in 1 day without any SAP, move to the next step.

Training days 7-9: continue walking 30 minutes total each day, including rest days when needed if there is SAP. Continue doing this until you can walk for 30 minutes in a day for 3 days in a row without any SAP, then move to the next step. Thus, this may be as early as day 9, but it could be considerably later if several rest days are needed.

Training days 10-20: begin walking at a faster pace while maintaining 30 minutes walking time daily. This will be easiest to do if you include a step counter of some sort, but you can also track the total distance traveled in a certain amount of time. Each day try to walk slightly faster and incorporate a rest day if there is any SAP. After needing a rest day repeat the last training day’s pace. By training day 20 you should be able to walk briskly for 30 minutes daily.

Training days 21-23: continue brisk walking 30 minutes each day, including rest days when needed if there is any SAP. Once this can be done for 3 days in a row without SAP proceed to the next step.

Training days 24-29: start with a slow jog for 5 minutes a day and include 25 minutes of brisk walking. Each day without SAP increase the jogging time by 5 minutes and decrease the brisk walking time by 5 minutes, until you can jog for 30 minutes daily.

As indicated in Lesson 3, brisk walking is generally in the moderate-intensity physical activity category while jogging at a 15-minute mile pace is on the border between moderate and vigorous physical activity. Thus, if you are able to work up to a combination of brisk walking and jogging for 30 minutes daily you will meet the recommended guidelines and be in good position to progress further. You can then gradually increase the intensity of the jogging in the same manner as described above. When you can no longer increase the pace while running for the same length of time, then you are in a position to start doing intervals.

Tip: A recent 2-part guidance document suggested that when previously sedentary individuals start exercising they should start at a light-to-moderate intensity or 2-3 METs and gradually increase over 2-3 months as long as the RPE remains “fairly light” to “somewhat hard” and they remain asymptomatic.^{(Franklin, 2022a; Franklin, 2022b)} Thus, it is reasonably to slowly build up overall conditioning for 2 months prior to really pushing yourself with more intense intervals.

Conclusion

Both MICT and interval training have merit and can be included in an effective aerobic training program:

For general health purposes MICT alone will provide great benefits if you meet the guidelines. Working up to including interval training can lead to additional positive health outcomes.
For endurance athletes the majority of your training will likely be MICT but incorporating faster intervals is helpful.

Concurrent training is valuable not just for the health benefits but also because strength training directly benefits endurance performance. To help minimize the interference effect it is ideal to:

perform resistance training prior to aerobic training if done in the same session
perform aerobic training prior to resistance training if done in different sessions on the same day
separate HIIT from lower body strength training by at least 24 hours if possible
perform SIT or RST instead of HIIT if it is not possible to separate the interval training from lower body strength training
take several days off from strength training prior to an endurance competition

Exercise can be started by the vast majority of people in a safe manner if an appropriate program is followed with good exercise technique. For people with underlying health conditions, specifically cardiac in nature, it is always safest to ask your healthcare provider if there are any limitations or if any additional testing (such as an exercise stress test and ECG) should be performed prior to starting.

At this point in the course I have discussed the physiology of exercise, the general guidelines, and what the evidence suggests regarding resistance and aerobic training program design. With this knowledge you are close to being able to create a full exercise program. The next step is to discuss specific exercises and how to perform them. I will do this over the next 4 lessons.

Click here to proceed to Lesson 9

References

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