It is no secret that exercise is very important for optimal health. But many people still do not see the connection and it seems to be a low priority for them when it comes to managing their health. However, I realize that many people truly do not understand how important it really is, even from the perspective of glycolytic pathway, so I want to take the time to summarize it here. Exercise is known to stimulate glycogenolysis, especially when it is conducted first thing in the morning after an overnight fast. In fact, I often tell my clients to do their exercise fasted when they want to optimize weight loss. But how does it work?
Before I review that, I want to do a quick summary of biochemistry.
In our muscles, glycogen supplies glucose-6-phosphate for ATP synthesis in the glycolytic pathway. Any enzyme known as glycogen phosphorylase in the muscle is stimulated during exercise by the increase of AMP and by phosphorylation. The phosphorylation is stimulated by calcium released during contraction and by epinephrine, the fight or flight hormone, as well as hypoglycemia during stressful situations or exercise where there is an immediate need for glucose. It is important to understand that liver glycogen stores are principally for the support of blood glucose during fasting or extreme need such as exercise, and the degradative and biosynthetic pathways are regulated principally by changes in the insulin/glucagon ratio and by blood glucose levels. The key point to remember is that muscle glycogenolysis is regulated principally by AMP, which signals a lack of ATP, and by Ca2+ released during contraction. Epinephrine, which is released in response to exercise and other stress situations, also activates skeletal muscle glycogenolysis.
Let’s take a practical look at what happens when someone begins to contract their muscles during exercise. If someone were to immediately begin running as fast as possible, the following cascade would take place.
- Within 3 seconds, muscle cells exhaust stored ATP.
- As exercise continues, this ATP must be regenerated, so the ATP–PCr system kicks in to shoulder most of the load. This lasts for about 10 seconds. And because time is required for ATP to be regenerated, you start to slow down a bit.
- As exercise continues and the ATP–PCr stores are depleted, the glycolytic system will begin to provide most of the energy transfer for ATP regeneration. This lasts for about 90 to 120 seconds or so, depending on the intensity of the exercise. Since the glycolytic system generates ATP more slowly than the ATP–PCr system, again, you have to slow down a bit more.
- If exercise continues beyond this time frame, the oxidative system will start to provide most of the energy transfer for ATP regeneration. And again, because the oxidative systems are slower than the anaerobic systems, the pace must slow again. In fact, if the pace is slow enough, the exercise can last for quite a long time.
There are two main types of exercise: anaerobic exercise and aerobic exercise. Anaerobic exercise is defined as higher-intensity, shorter-duration (less than 2 minutes) activity, whereas aerobic exercise occurs when the exercise is longer than 2 minutes in which the oxidative system must kick in to provide the remaining energy for ATP regeneration. As our initial energy stores can only supply energy for about three seconds, our ATP must be regenerated in large amounts, and quickly, to support this type of exercise.
Short-burst activities such as the following:
- The golf swing
- Field events (shot put, discuss)
- The tennis swing
- The 100-meter sprint
- The baseball swing
Oxidative energy transfer takes place in the mitochondria of our cells and utilizes a combination of muscle glycogen, intramuscular fatty acids, free fatty acids, and amino acids. As the oxidative processes utilize breakdown products from both glycolysis (glucose through to pyruvate) and beta oxidation (fatty acids through to acetyl-coA), energy transfer occurs at a slower rate. However, what this system lacks in speed, it makes up for in ATP regeneration. As a result, oxidative metabolism can support activities including the following:
- 800-meter run
- 2000-meter rowing
- 1500-meter skating
- Cross-country skiing
- Long-distance swimming
Indeed, any activity done at a high intensity for longer than two minutes derives a large percentage of its energy transfer from the oxidative system. There is a “switchover point” at which an activity moves from anaerobic to aerobic, as seen in this interesting comparison.
- 200-meter run: 29% aerobic; 71% anaerobic
- 400-meter run: 43% aerobic; 57% anaerobic
- 800-meter run: 66% aerobic: 34% anaerobic
- 1500-meter run: 84% aerobic; 16% anaerobic
The primary muscle fiber types that contribute to aerobic exercise are the oxidative type I and type IIA fibers. As aerobic exercise is heavily oxygen dependent, training adaptations occur in order to support oxygen transport and delivery in these fibers. Specifically, aerobic exercise can increase the number and size of the blood vessels. This occurs through increased capillarization. Specifically, with aerobic training, there is a greater number of capillaries per unit of muscle. This allows for enhanced delivery of oxygen (fuel) to muscle cells, enhanced removal of CO2 and waste products, and the transfer of heat away from the muscle. In addition to enhanced oxygen delivery, there is an increase in the size and number of mitochondria along with greater myoglobin content within cells. While the greater capillarization leads to more oxygen transport, the greater myoglobin leads to increased muscle oxygen uptake, and the larger and more numerous mitochondria allow for greater oxygen use. Of course, in addition to these adaptations, the enzymes involved with aerobic energy transfer will adapt as well.
Let’s talk a bit about oxygen and the adaptations that occur with regular exercise.
After a full exercise session, or even after a single interval within an entire exercise session, the oxygen deficit that’s accumulated must be paid back. This means that after you’ve stopped exercising and the amount of mechanical work you’re doing is no different than you’d be doing at rest, you still continue to consume more oxygen. This period of increased oxygen consumption and energy demand has been called the period of oxygen debt or EPOC (excess post-exercise oxygen consumption). In essence, after exercise, the amount of oxygen consumed can be elevated for minutes to hours. This is due to the fact that the body must:
- a) metabolize additional nutrients,
- b) replenish the energy stores that have been used up, and
- c) reload the depleted oxygen stores in the muscle and blood.
In addition to these recovery-type activities, the following also contribute to the EPOC:
- Elevated post-exercise body temperature
- Increased activity of the heart and respiratory muscles
- Elevated levels of metabolism-boosting hormones
- Increased conversion of energy transfer products such as lactate into other substrates
- Increased protein synthesis
- Recovery of muscles stressed and damaged with the activity
It is important to note, however, that the energy systems do not work independently from one another. During various types of exercise, from aerobic to anaerobic, all three energy systems are activated. However, the extent to which they are activated, and the amount of ATP they regenerate relative to the total ATP regeneration required, determines the description of the activity. For example, during short-burst activity, the ATP–PCr system is most important. When immediate and explosive movement is desired, the brain initiates the contraction with a signal that’s passed along the nerves to the muscles. The muscles then contract, using ATP and depleting these immediate energy stores within a second or two. In order for the muscles to continue to contract, the resulting ADP and P must be regenerated to ATP.
Adaptations to Exercise.
In response to regular exercise training, whether anaerobic or aerobic, certain changes occur in the muscle. These changes improve the body’s ability to respond to similar exercise challenges in the future. Each of these processes is regulated by protein synthetic mechanisms initiated within our genetic material (our DNA). Cellular communication through hormones is intimately involved in this process. The hormone insulin, in the presence of adequate nutrient availability, encourages the stimulation of protein synthesis and a positive nitrogen balance. Insulin availability is greatest during well-fed conditions and during periods of energy surplus. Protein and amino acid intake is key here as protein-containing meals stimulate a positive protein status. In addition, hormones like testosterone and growth hormone have a stimulatory effect on muscle adaptation.
On the other hand, the counter-regulatory hormones such as glucagon, catecholamines, and glucocorticoids have a contradictory effect, promoting protein breakdown and a negative nitrogen balance. These hormones are released in large numbers during periods of fasting or energy deficit
Protein synthesis and exercise adaptation are also affected by:
- The amount of mRNA in our cells
- Ribosomal number
- Ribosomal activity
- Amino acid availability
- The hormonal environment
- Our native genetic code
Interestingly, even the process of recovering and adapting to our exercise training demands is metabolically costly. As proteins are degraded and amino acids re-synthesized into proteins, this process of protein turnover builds more functionally adapted enzymes, contractile units, etc. And this process accounts for between 10% and 25% of resting energy expenditure. Therefore, as you can see, not only does exercise increase total daily energy expenditure during the activity, it also increases post-exercise expenditure through two mechanisms. Energy expenditure is increased due to both the oxygen debt being paid back and to the increased protein turnover and synthesis just described.
Adaptations of anerobic exercise such as weight training and sprint training:
- muscle fibers both increase in size and in myofibrillar number
- mitochondrial size and number
- increases in myoglobin number
- increases in intracellular storage capacity and availability (such as stored glycogen)
- increases in intracellular glycogen storage can also contribute to muscle hypertrophy
- In addition to changes in muscle cross-sectional area, anaerobic exercise can enhance the activity of ATP–PCr system enzymes (creatine kinase, myokinase) and the glycolytic system enzymes (glycogen phosphorylase, phosphofructokinase).
These changes help to increase the rate of energy transfer within the muscle, allowing for more rapid responses to energy demands in the future.
Adaptations to aerobic exercise such as jogging, steady state cardio or swimming:
Please note, this type of lower-intensity, longer-duration activity primarily influences muscle quality (as opposed to muscle size).
- Enhancement of oxidative or mitochondrial enzyme activity
- Increase in intramuscular glycogen and triglyceride content
- Increase in blood volume due to increase uptake and delivery of aerobic activity- due to increase in red blood cell content and the oxygen-carry capacity of the body.
- Capillary density of trained muscles increases- meaning there will be a great number of capillaries per muscle fiber
- With this lengthened border between blood vessels and muscle fibers, oxygen delivery, carbon dioxide removal, waste removal, fuel delivery to muscle, and the transfer of heat are all amplified.
- Beyond this, the myoglobin content of skeletal muscles will increase, improving oxygen delivery across muscle cells
- Finally, the number and size of mitochondria are increased with aerobic activity of high enough intensity. This promotes greater oxygen utilization through the process of pyruvate, fatty acid, and ketone utilization through the Krebs cycle and electron transport chain
Additional benefits of both aerobic and anaerobic exercise training include:
- The attenuation of sympathetic nervous system activity.In essence, “stress” to the body with exercise is minimized over time, and therefore greater workloads are required to promote the same amount of adaptation.
- Greater insulin sensitivity.With exercise training, the body responds to carbohydrate intake with less insulin release, allowing insulin to act in carbohydrate update and protein synthesis without preventing fat loss/stimulating fat gain.
- Improved fatty acid uptake and transport.Another positive response to exercise training is that fats can be more easily mobilized from adipose tissue, transported, taken up, and broken down.
- Less lactate produced per intensity.At every intensity, less lactate will be produced. This is due to greater aerobic production of ATP at every intensity, lower catecholamine response, reduced carbohydrate metabolism, and changes in the isoenzymes of lactate dehydrogenase to forms that favor the conversion of lactate to pyruvate.
- More lactate removed per unit of intensity.At every intensity, more lactate will be removed. Increased rates of lactate removal are due to increased blood flow to the liver and enhanced uptake of lactate by cardiac and skeletal muscles.
- Better lactate tolerance.With training at the highest intensities, the body can better deal with high acid conditions and high levels of lactate. This means higher intensities can be achieved and sustained for short periods of time.
sympathetic nervous system: One division of the autonomic nervous system that is always active and provides sympathetic tone. Its activity increases during times of bodily stress.
So, what qualifies as “intense exercise”? Resistance training (strength training), interval training(through activities such as running, climbing, cycling, and rowing), circuit training, rope jumping, running hills, squat thrusts, plyometrics, explosive medicine ball work, explosive kettlebell exercises, and strongman activities are all high-intensity activities.
Basically, high-intensity activity includes any physically demanding task that:
- a) incorporates many muscle groups, and
- b) is done near your maximum heart rate.
The high-intensity activities listed above require a maximum of muscle activity, which leads to high amounts of cellular stress and the need for muscle adaptation. It’s this muscle stress and adaptation that brings about the maximum number of benefits, including increased protein turnover, muscle preservation and building, a high energy cost, and even cardiovascular benefits.
However, as important as exercise is to this process, nutrition is equally critical.
Firstly, nutritional status can impact energy transfer. Therefore, a good nutrition program will help facilitate top performance of each of the energy systems: ATP–PCr, glycolysis, and oxidative phosphorylation. Both macronutrients and micronutrients are important here.
A sub-optimal nutritional intake can reduce enzyme efficiency (due to deficiencies of co-enzymes and co-factors) and lead to substrate deficiencies. And this means poor exercise performance and fewer calories burned both at rest and during exercise. So much for the metabolic and muscle preserving and building benefits of exercise. In addition, with an inadequate intake of dietary protein and fat, amino acid availability and the ratio of anabolic to catabolic hormones can be compromised. This can lead to an inability to build and preserve muscle mass, even in the face of a solid exercise program.
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As many of you know, I am a fitness instructor with over 15 years of teaching experience in group fitness and personal training. I am launching an app that includes a series of exercise videos targeted for increasing fat free body mass and reducing body fat. Recently I have started to incorporate high intensity interval training (HIIT) into my classes. An interesting observation I have made is that the interval style training seems to contribute to greater feelings of competence and adherence. My participants feel successful, accomplished and always return for more! This type of response I have seen in all my classes, but particularly in the interval style classes. However, I have noticed there is a sweet spot within ratio of intensity and duration. If the class is too hard or too long, the dropout rate is greater. For example I have experimented with HIIT classes that are 75 minutes and I notice about 25% will leave the class within the first 45 minutes. I have found that 20-30 minutes and a rate of perceived exertion not to exceed 8 or 9 during the intervals drives the most positive response from my participants of all age groups. For this review, I conducted some research to see if there is any data to demonstrate that interval training improves competence and perceived adherence to support or refute my own personal experience. These are very important variables that will support practitioners in making their future exercise prescriptions.
We are all told we must exercise because it has important health benefits. Unfortunately, few adults meet the weekly guidelines of 150+ minutes of moderate –intensity or 75+ minutes of vigorous-intensity aerobic physical activity, and 2+ days of resistance exercises.(Heinrich, Patel, O’Neal, & Heinrich, 2014) For weight loss, the minimum requirements double. The good news is that high intensity interval training (HIIT) provides fitness and health improvements in less time per week than current guidelines. (Heinrich et al., 2014) There are two main methods of exercise that I have worked with in my career as a trainer: continuous training (CT) and high-intensity interval training (HIIT). The main difference between these two modalities of training is that CT is characterized by a lower-intensity (sub-maximal) effort for a longer duration, whereas HITT is characterized by repeated bouts of short duration at higher intensities above lactate threshold. “Compared to CT, HIIT induces similar-to-greater improvements in fitness and cardiovascular function but in a shorter amount of time.”(Heisz, Tejada, Paolucci, & Muir, 2016) There are many types of HIIT protocols. The one that I use often is called Tabata. In Tabata, we work hard for 20s and rest for 10 seconds, and continue that pattern for a total of 8 rounds. It comes out to 4 minutes per set. I often superset two different exercises, sometimes one strength and one cardio, so they are only doing 4 sets instead of 8. The possibilities with Tabata are endless and we have a lot of fun with it! The video I am posting is an example of a strength only Tabata. There are other protocols used by trainers as well, but I have found that the higher duration intervals are not perceived as enjoyable as the lower duration. “Protocols with 120s high-intensity intervals are rated as being less enjoyable than protocols with 30s or 60s high-intensity intervals.”(Heisz et al., 2016) According to Heiz, this reduced enjoyment of the more strenuous protocols may be related to the individual’s ability to complete the exercise, or their competence. I have found the Tabata to elicit the most favorable feeling of competence with my participants, especially when the intensities were higher. “Alternatively, the accumulated fatigue or physical stress from chronically performing a strenuous exercise may actually increase negative feelings and reduce enjoyment for the exercise over time.”(Heisz et al., 2016)
Another benefit to the HIIT I have found is the shorter time commitment being favorable among my participants. This also contributes to the differences in enjoyment for HIIT compared to CT even though the HIIT is more physically strenuous. My participants have often told me, “I know that the exercise session is shorter so I am able to push harder mentally knowing that is will end soon!” The great thing I have noticed that even with the shorter duration, the physiological benefits I have observed have been comparable or even better than the longer duration CT. According to Heiz et al (2016):
Even with the shorter time commitment, HIT induced similar physiological adaptations as indicated by pre-to-post change on maximal aerobic fitness test for both VO2 peak and PPO. Together, these results support the growing evidence that HIT is similarly effective but more time-efficient at improving aerobic fitness compared to more traditional moderate forms of continuous exercise.
According to Heinrich et al, although the intensity requirement for HIIT may be intimidating, the reduced time requirement may be appealing to many adults, showing potential for higher rates of adherence.(Heinrich et al., 2014) Interestingly, there is also mention that combining aerobic and resistance training may result in greater weight and fat loss and fitness improvement than each modality alone. This is exactly the way I teach my classes and it has been received very favorably thus far among my participants!
Perceived pleasure, according to Oliveira et al, has been reported to be an important contributor to exercise adherence. (Oliveira, Slama, Deslandes, Furtado, & Santos, 2013) There are mixed results according to the literature I read in regards the HIIT and perceived pleasure. In a few studies, I read that HIIT is correlated to a great enjoyment of exercise, but other studies show if the intensity is too high or difficult to complete it can lead to a higher dropout rate and a lower perceived competence and adherence. In one study it was observed that changes in exercise enjoyment were predicted by increases in workload, suggesting that strength adaptions may be important for promoting exercise enjoyment. (Heisz et al., 2016) I have encountered this as well with my beginners. I have to start them slowly and gradually build up the intensity over time. If the intensity is too hard at first, they drop out. “For sedentary individuals, a key barrier to starting an exercise program is the preconceived notion that exercising is not enjoyable and failing to find enjoyment from exercise can make it more difficult to adhere to an exercise program over time.”(Heisz et al., 2016) According to a 6 week study conducted by Heisz et al., “across the six weeks of training, increases in workload predicted increases in enjoyment.”(Heisz et al., 2016) Typically it takes my participants about 4 weeks of consistent training with gradual builds in intensity for the pleasure to stick. If my participants continue with me past this point, the enjoyment of the exercise seems to outweigh the discomfort, and I typically see an increase in adherence. This seems consistent with the data from the 6 week study conducted by Heisz et al.
Not everyone shares my enthusiasm for HIIT, however. One study discusses that sprint interval training as being inappropriate for a largely sedentary population. “An inactive population is unlikely to engage in sprint interval training (SIT) due to poor aﬀective responses, low self-eﬃcacy and motivation, and increased challenges to self-regulation.”(Robertson-Wilson, Eys, & Hazell, 2017). According to Robertson-Wilson et. al, one of the key concerns with HITT/ SIT is negative affective responses with faced with arduous physical activity of this nature. It appears that differences in the methodological procedures may explain the divergent results. The studies that utilized longer work sessions with inadequate recovery were the ones that demonstrated negative affective responses during the HITT sessions.
Under these conditions, over 50% of the participants were unable to finish the HIIT session. Although it was not the focus of this study, this is an important fact because self-efficacy may be negatively influenced in cases of participant dropout. It is possible that HIIT sessions with longer recovery periods would provide better affective responses than the HIIT sessions used in the present study.(Oliveira et al., 2013)
The rule of thumb I use when designing my classes is when the work portion is longer (greater than 45s), then the intensity is lower and the rest is longer. But when the work portion is shorter (as in Tabata), the intensity is higher and the rest is shorter. When I stick to these basic rules, I noticed the most adherence and satisfaction among my participants. According to Oliveria et. al, the affective response seems to have been influenced by the magnitude of the stimulus intensity and by the predominant metabolic pathway engaged by the exercise. “It is possible that other HIIT configurations with greater recovery periods could result in positive affective responses, and this hypothesis should be tested in future studies.”(Oliveira et al., 2013) I have personally tested this informally in my classes and have found this to be true.
As a final note, another area I have found quite intriguing are the psychological components involved exercise. Oliveira et al. have conducted numerous studies in regards to intrinsic and extrinsic motivation, and they discuss the opponent process theory and its involvement in the enjoyment of exercise. According to Oliveira et al.(2013):
The opponent process theory, postulates that after every affective perception (pleasant or unpleasant), an opponent process occurs. Thus, according to this theory, a feeling of pleasure can occur after an aversive stimulus or stress, which can activate the reward system and can then lead to a repetition of that stimulus. The increased production of neuromodulatory substances such as anandamide, dopamine, serotonin and endorphins may be associated with decreased anxiety and increased pleasure after intervals of intense stimulation. However, this hypothesis was not objectively investigated, and it is not well known whether individuals choose to continue to engage in physical activities based on perceptions experienced during or after exercise. Future studies should investigate this issue.
In conclusion, I think any exercise is better than no exercise. However in order for exercise to be effective, it must be consistent. In order for participants to be motivated to continue exercising, the workout should be enjoyable, achievable, and effective. I have found that HIIT training meets all these requirements. The classes I teach are short (no more than 30 minutes), achievable ( the intensity I select is appropriate for my participants) and effective (my participants report increases in strength, flexibility, fat free body mass and decreases in body fat). These variables thus make the HIIT classes I teach enjoyable. Because my participants enjoy my classes, they feel competent and come back for more! I have been known to have a pretty strong following, and I think these simple guidelines I use contribute greatly to my success as a fitness instructor.
Heinrich, K. M., Patel, P. M., O’Neal, J. L., & Heinrich, B. S. (2014). High-intensity compared to moderate-intensity training for exercise initiation, enjoyment, adherence, and intentions: an intervention study. BMC Public Health, 14, 789. doi:10.1186/1471-2458-14-789
Heisz, J. J., Tejada, M. G., Paolucci, E. M., & Muir, C. (2016). Enjoyment for High-Intensity Interval Exercise Increases during the First Six Weeks of Training: Implications for Promoting Exercise Adherence in Sedentary Adults. PLoS ONE, 11(12), e0168534. doi:10.1371/journal.pone.0168534
Oliveira, B. R., Slama, F. A., Deslandes, A. C., Furtado, E. S., & Santos, T. M. (2013). Continuous and high-intensity interval training: which promotes higher pleasure? PLoS ONE, 8(11), e79965. doi:10.1371/journal.pone.0079965
Robertson-Wilson, J., Eys, M., & Hazell, T. J. (2017). Commentary: Why sprint interval training is inappropriate for a largely sedentary population. Front Psychol, 8, 1603. doi:10.3389/fpsyg.2017.01603