Zone 2 Training for Spinal Cord Injuries?Unlocking the Benefits of Low-Intensity Cardio for Health and Performance

In this article, we explore whether Zone 2 training can aid individuals with spinal cord injuries (SCI). Zone 2 cardiovascular training, characterised by moderate-intensity exercise where the body primarily uses fat as fuel and maintains a steady heart rate, can benefit persons with spinal cord injuries (SCI). However, its suitability depends on the level and completeness of the injury, as well as the individual's physical condition and training goals.

You may be thinking, what is Zone 2 training and why should it benefit us? We will cover all of this. We begin by examining the fundamental purpose of Zone 2 training, its benefits, and implementation methods. Next, we will consider its applicability to those with SCI and discuss the potential limitations.

I've pursued activities like powerlifting, karate, and wrestling throughout the years. In my 70s, I still enjoy strength training with barbells and kettlebells, though I've never been keen on "pure" cardiovascular-focused workouts. However, exercise is undeniably medicinal, and we all require a mix of cardio and strength-focused training. Research shows that this is best for longevity. That being the case, I can no longer deceive myself into believing that elevating my heart rate through weightlifting alone suffices, so I'm increasing my Zone 2 training. If it's good enough for me, I wondered if it would also benefit many of our clients with spinal cord injuries who wish to remain healthy for the long term.

What's Zone 2 training?

Zone 2 training is a heart rate-focused training method that has recently surged in popularity due to its diverse health benefits, accessibility, and effectiveness in enhancing general fitness and performance. Zone 2 involves exercising at a demanding but sustainable level of effort for relatively long periods. This will positively influence metabolism, longevity, and cardiovascular health when done correctly. As we will see, this training involves exercising at a level of effort at which the body predominantly uses fat as fuel whilst maintaining a steady, elevated heart rate. This type of training stimulates mitochondrial growth and function, as well as lactate clearance. We will have a more formal definition later in this article.

In the Beginning - Heart Rate Training Zones

The concept of heart rate training zones, including Zone 2, stems from the work of exercise physiologists and sports scientists who sought to classify different levels of exercise intensity based on the body's physiological responses, such as heart rate and metabolism. However, it's difficult to attribute the specific term "Zone 2" to one individual or researcher.

The concept of heart rate zones gained popularity in the 1980s and 1990s, thanks to endurance sports coaches and physiologists who introduced a low-intensity aerobic training method centred around heart rate. As mentioned above, this approach emphasises maintaining a heart rate level that primarily utilises fat as fuel. Today, wearable technology has made monitoring heart rate zones effortless and accessible, as we will see.

To understand whether Zone 2 training is practical and effective, we must first understand how it impacts our metabolism.

Metabolism and Life

Most people think of calorie consumption when thinking of their metabolism, but that doesn't do the topic justice because our metabolism is fundamental to life. Our food is subject to metabolic processes that are responsible for literally creating us and keeping us alive.

Ultimately, we are all made up of stored energy in the form of fats, carbohydrates, and protein, plus information in the form of nucleic acid (the building blocks of DNA) and quite a lot of water. Our metabolism operates with both anabolic and catabolic processes. The catabolic processes take the fats, carbohydrates, and protein we ingest and breaks them into smaller components such as fatty acids, amino acids, glucose, and nucleotides. The anabolic processes then use these components to build our bodies. Both anabolic and catabolic processes are essential to life.

A straightforward way to understand health is to recognise that our metabolism manages the body's internal environment, maintaining stability in a process known as homeostasis. Homeostasis is a highly complex process, as our bodies must preserve this balance even when external conditions fluctuate. It accurately regulates body temperature, pH levels, blood pressure, hydration, blood sugar, and more, ensuring they remain within normal ranges irrespective of the demands we place on our bodies in daily life. Living, exercising, maintaining our tissues, and adapting to an environment filled with stressors all require energy.

The simplest stressors are environmental changes, such as temperature. Whatever

the temperature in our environment, hot or cold, our bodies must precisely regulate our internal temperatures, or we die. Of course, nutritional and mental stressors, lack of sleep, etc, might seem more subtle, but they can profoundly affect our health. We all know that even thinking about something stressful can increase our heart rate or blood pressure.

The key takeaway is that health fundamentally depends on our metabolic resilience, which, in turn, hinges on our ability to regulate and maintain our internal environment whilst adapting to events in a constantly changing external environment. Mitochondria, the tiny intracellular organelles produce much of our energy and are key to metabolic resilience. Healthy mitochondria can burn (use as fuel) both glucose and fat. Persons with poor metabolic function can use only glucose but not fat.

We should recognise that a healthy body must adapt to exercise. When we lift weights in particular ways, we become stronger. Engaging in cardio exercise for long enough enhances our cardiovascular system, making us more efficient. This adaptation process epitomises survival of the fittest, ensuring our bodies can adjust to demands, which is metabolically taxing as it requires energy. Energy is needed for working out, rebuilding tissue, and adapting to whatever life brings.

This metabolic balancing act is dynamic. As we age, men and women lose cardio and respiratory fitness and tend to lose muscle mass. We recover more slowly from injury and become less metabolically resilient. Ultimately, if we aim to slow the ageing process, our goal should be to cultivate a healthy metabolism. If you want to live a long life with the highest quality possible, focusing on building and maintaining a robust metabolism is key.

We began this section by emphasising that metabolism encompasses far more than just the number of calories you burn daily. While calories are indeed an indicator of metabolism, they merely represent the gross output. They reveal nothing about how that energy is utilised, your metabolic resilience, or your ageing process. Focusing solely on metabolic function via calorie output overlooks the broader picture, which is a significant oversight. Concentrating only on burning calories is not the correct approach to enhancing your metabolism's resilience and health.

Energy systems

Our bodies consume energy to do work of any kind or even exist. It's crucial to grasp the fundamental pathways to comprehend how the body generates and uses energy. We've delved into this topic in previous articles, so here's a concise overview. The body utilises two primary pathways:

• the aerobic energy pathway, which relies on oxygen to produce energy, and

• the anaerobic energy pathway, which functions without oxygen.

Both pathways produce the same energy molecule, Adenosine Triphosphate (ATP). As the primary energy carrier for all living organisms, ATP plays a vital role in biology. Cells rely on ATP to execute various functions, including muscle contraction and nerve impulse transmission. It is the essential catalyst for overall cellular health, extending beyond muscle activity.

The aerobic system fulfils most of our energy requirements, operating continuously from birth until death, and is highly adaptable. It utilises oxygen to generate ATP from carbohydrates, fats, and proteins for bodily functions.

In contrast, the anaerobic system is more constrained and not intended for prolonged use.

Zone 2 training is focused on developing the aerobic system at the point at which it is utilising fat as the main energy source.

As exercise intensity increases for the average person, the body uses less fat and more carbohydrates. This shift occurs because generating energy from carbohydrates is quicker than from fat due to various biochemical reasons.

FAT Oxidation versus work done

A research study by Millán and Brooks (reference below) compared the fat oxidation rate of professional athletes (PA) in grams per minute relative to their power output (in Watts) with that of moderately active (MA) people and those with metabolic syndrome (MtS).

The results demonstrated a pronounced difference between these groups, showcasing the impact of fitness and metabolic health on energy utilisation. Professional athletes exhibited a higher fat oxidation rate at given power outputs than moderately active individuals. Those with metabolic syndrome were least able to use fat as fuel.

This study also looked at blood lactate. As more lactate accumulates in the blood, it's another indicator that the body is utilising more carbohydrates than fat. With professional athletes, we don't see lactate accumulation at the same rate as in those with lower levels of fitness. For example, professional cyclists can generate huge amounts of power in cycling events whist still primarily burning fat as fuel for their efforts.

This finding underscores the fact that improved cardiovascular fitness correlates with enhanced metabolic flexibility, allowing the body to switch between fuel sources efficiently.

The ability to oxidise fat effectively at varying intensities indicates metabolic health.

These differences may be attributed to the conditioning and metabolic adaptations that occur with rigorous training, suggesting that regular and targeted exercise can significantly bolster metabolic pathways and overall energy efficiency. This highlights the vital importance of a structured fitness regimen for improved metabolic function, particularly in those with compromised metabolic health. Hence, prioritising aerobic and anaerobic conditioning can be pivotal in optimising energy use, fostering metabolic resilience, and advancing long-term health outcomes.

Muscle Fibres

When we use energy to do physical work, we involve muscle fibres. As you know, there are different muscle fibre types. Traditionally, these were referred to as slow twitch and fast twitch, and each type is better suited to utilising different forms of energy and doing different forms of work.

Slow twitch muscle fibres, now known as Type I fibres, are highly efficient at utilising oxygen to produce ATP. These fibres are ideally suited for endurance activities because they can sustain prolonged muscle contractions. Their high mitochondrial density and capillary networks enable them to supply oxygen to mitochondria, optimising aerobic metabolism. As a result, activities such as long-distance running or cycling primarily recruit slow twitch fibres.

In contrast, fast twitch muscle fibres, or Type II fibres, are designed for rapid, powerful bursts of movement. They rely more heavily on anaerobic pathways to produce ATP and are less dependent on oxygen. Hence, they are better suited for short-duration, high-intensity activities like sprinting or weightlifting. Type II fibres can be further subdivided, with Type IIA fibres possessing some aerobic capacity, while Type IIX fibres are geared almost exclusively towards anaerobic metabolism.

The proportion of slow-twitch to fast-twitch fibres in a person is largely determined by genetics. Still, through targeted training regimens, individuals can somewhat enhance the efficiency and capacity of both fibre types to improve overall muscle performance. Understanding and integrating different training approaches to address the unique characteristics of muscle fibre types can significantly amplify an individual's athletic performance and metabolic resilience.

Heart Rate Training Zones

There are several ways of examining heart rate training zones with different pros and cons.

One approach is metabolic testing, which can accurately identify how much oxygen is used and carbon dioxide is being generated at different work and power output levels. Metabolic testing can determine the aerobic threshold at which carbohydrate consumption starts to dominate, the anaerobic threshold and VO2 max. These values are quite individual to the person being tested at the time of the test. Although these metabolic markers can infer appropriate zones for individualised heart rate training, they are not practical for routine use by most people. Apart from the cost, driving your body to the maximum is challenging, and the values obtained only represent your state on a particular day. Re-testing a day later, you will likely find different values.

Hear rate measurement is widely available but may not always be reliable with spinal cord injury

A second method is to look directly at the heart rate and relate this to the intensity of effort. It makes sense that increasing exercise intensity leads to a higher heart rate, so there is certainly a relationship between the two. We are all probably familiar with the idea that your heart rate maxima is based on 220 minus your age. So, someone aged 60 would supposedly have a maximum heart rate of 160. This is, however, an oversimplification, as so many factors influence the relationship between intensity and heart rate.

Many hardware and software products are available to monitor heart rate during exercise. However, if you compare these products, you will likely note that they describe heart rate zones and their supposed physiological effects differently.

Different products may take your supposed heart rate maximum and divide that range into 3, 4, or 5 linear zones, from the lowest level of intensity to the highest. These linear intensity models are approximations; this does not match how our physiology works. The relationship between intensity and heart rate is not linear and changes daily based on our stress levels.

A product I use for heart rate measurement is Morpheus, which employs a non-linear model to connect intensity with heart rate. It seeks to estimate aerobic and anaerobic thresholds, acknowledging that these are dynamic and affected by factors like fatigue, sleep quality and recovery stage. Morpheus aims to establish low, moderate and high-intensity training zones. The aim of these zones is as follows:-

• low intensity (blue) - aims to train the slow twitch muscle fibres, build higher cardiac output and the vascular network.

• Moderate intensity (green) - develops slow twitch and mixed muscle fibres. It increases enzymes for aerobic metabolism and lactate oxidation.

• High intensity (red) - targets all muscle fibres, builds VO2 max.

The transition from blue to green is approximately at the aerobic threshold, and the threshold from green to red is approximately at the anaerobic threshold.

What sets Morpheus apart is its dynamic heart rate zones. For instance, your green zone might range from 149 to 166 beats per minute one day and shift to 141 to 157 the next. Such fluctuations could indicate low recovery levels, possibly due to inadequate sleep or incomplete recovery from previous exercise. To estimate these zones, using Morpheus, it is necessary to use the product to take heart rate variability measurements each day.

A Definition of Zone 2

The creator of Morpheus, Joel Jamieson, described Zone 2 as:-

The highest level of intensity at which power output is mainly generated by slow-twitch muscle fibres using fat as the primary fuel source. Zone 2 is an intensity level that can be sustained for long durations without significant fatigue.

Another way to describe this level is the effort we can maintain for a long time without accumulating lactate.

Of course, this level is very individual. I heard Tadej Pogacar, the professional cyclist, describe on Dr Peter Attia's podcast that he likes to stay in Zone 2 for 5 hours, and his heart rate will be 140 to 145 when he’s fatigued, and when fresh, it’s 150-155. He will be generating some 320 to 340 Watts for all that time. For us mortals, 5 hours at that level of effort is impossible. Elite athletes work out at intensities that are considered low for them but would be high for others, thanks to their exceptionally high and efficient mitochondrial density.

Whilst Zone 2 training is beneficial to elite athletes we could argue that it is even more important for non-athletes.

This is because it builds a base of endurance for everything else we choose to do in life. It also is suggested to play a role in preventing chronic disease by improving the health and efficiency of your mitochondria.

Benefits of Zone 2 Training for SCI Patients:

For individuals with SCI, engaging in Zone 2 training offers several specific benefits:

1. Improved Cardiovascular Health: SCI typically leads to reduced physical activity levels, increasing the risk of cardiovascular diseases. Zone 2 training helps strengthen the heart and circulatory system, lowering blood pressure and improving cardiovascular function.

2. Enhanced Metabolic Efficiency: Due to decreased muscle activity, people with SCI may experience metabolic disorders, including insulin resistance and obesity. Zone 2 training boosts metabolic resilience and improves insulin sensitivity, aiding in better glucose regulation and weight management.

3. Reduced Risk of Overexertion: High-intensity workouts can be challenging and sometimes unsafe for individuals with SCI, especially those with autonomic dysfunctions. Zone 2 training provides a moderate intensity that reduces the risk of overexertion and associated complications like autonomic dysreflexia. Individuals with lesions above T6 may experience autonomic dysreflexia during exercise, where blood pressure rises dangerously high. It is crucial to monitor blood pressure and heart rate carefully in these patients to avoid triggering AD during cardiovascular training.

4. Muscle Preservation and Endurance: Even with limited mobility, consistent Zone 2 training can help preserve muscle mass and improve endurance in functional muscles. This is crucial for maintaining independence in daily activities and preventing muscle atrophy.

5. Autonomic Nervous System Regulation: Moderate-intensity exercise can help stabilise the autonomic nervous system, which is often impaired in SCI patients. This stabilisation can improve heart rate variability and better control of bodily functions regulated by the autonomic system.

6. Mental Health Benefits: Regular aerobic exercise reduces stress, anxiety, and depression. For individuals coping with the challenges of SCI, the mood-enhancing effects of Zone 2 training can contribute significantly to overall well-being.

Research Support:

• Studies have demonstrated that moderate-intensity aerobic exercise improves cardiovascular fitness and metabolic health in individuals with SCI.

• Research indicates that consistent Zone 2 training enhances mitochondrial function, leading to more efficient energy production at the cellular level, which is beneficial for muscle health and endurance.

Precautions and Recommendations:

• Medical Supervision: Individuals with SCI should consult healthcare professionals before starting any exercise program. A tailored exercise plan should consider the level and completeness of the injury.

• Monitoring Exercise Intensity: Due to possible alterations in heart rate responses, alternative methods, such as perceived exertion scales, or the "talk test" may be used to gauge exercise intensity effectively. Individuals with SCI, especially those with higher-level injuries, may have impaired thermoregulation, making them more prone to overheating or struggling with temperature regulation during exercise. Ensuring a cool, well-ventilated environment and monitoring for signs of heat stress is crucial.

FES Cycling is one way of getting moderate intensity exercise even with paralysed muscles

• Adaptive Equipment: Depending on the level of the injury, Zone 2 cardiovascular training may be limited to certain muscle groups. For those with paraplegia, for example, upper body exercises such as hand cycling, arm ergometry, or using specialized equipment may be required. Quadriplegic patients may have even more limitations, necessitating assistive devices or functional electrical stimulation (FES) to activate muscles. Utilizing specialized equipment such as arm ergometers or functional electrical stimulation (FES) bikes can make Zone 2 training more accessible and effective.

• Gradual Progression: Start with shorter durations and gradually increase the length and frequency of training sessions to build endurance safely. For example, 20 minutes, 2 times per week initially then progressing to 30 minutes, 3 times per week for optimal benefits

Conclusion:

Zone 2 cardiovascular training can be highly beneficial for individuals with SCI when appropriately modified and carefully monitored. Tailoring the exercise regimen to account for the individual’s injury level, autonomic function, and cardiovascular response is key to maximising benefits and avoiding complications. Regular assessments are essential to ensure the training remains safe and effective.

We have seen that

- Use of adaptive equipment like arm ergometers, FES cycles, or recumbent hand bikes will be helpful

- Perceived exertion levels can be monitored using RPE (Rate of Perceived Exertion) or the Talk Test rather than relying on heart rate alone.

- We should pay close attention to hydration and temperature regulation during exercise.

- Incorporating functional electrical stimulation (FES) to engage muscles that are otherwise paralysed or weakened can be helpful. FES cycling has been shown to improve aerobic fitness, including peak power and oxygen uptake, in individuals with SCI. This method is particularly effective for enhancing lower-body muscle health and increasing aerobic capacity[8]

- Shorter, more frequent training sessions, might be needed depending on tolerance and fatigue levels.

These studies highlight the importance of tailored exercise testing and training interventions to accurately assess and improve cardiovascular fitness in individuals with spinal cord injuries.

References

San-Millán I, Brooks GA. Assessment of Metabolic Flexibility by Means of Measuring Blood Lactate, Fat, and Carbohydrate Oxidation Responses to Exercise in Professional Endurance Athletes and Less-Fit Individuals. Sports Med. 2018 Feb;48(2):467-479. doi: 10.1007/s40279-017-0751-x. PMID: 28623613.

Vavřička J, Brož P, Follprecht D, Novák J, Kroužecký A. Modern Perspective of Lactate Metabolism. Physiol Res. 2024 Aug 31;73(4):499-514. doi: 10.33549/physiolres.935331. PMID: 39264074; PMCID: PMC11414593.

"Zone 2 Training: Build Your Aerobic Capacity" by Iñigo San Millán, PhD. https://www.trainingpeaks.com/blog/zone-2-training-for-endurance-athletes/

#85 – Iñigo San Millán, Ph.D.: Zone 2 Training and Metabolic Health. https://peterattiamd.com/inigosanmillan/

Morpheus product. https://trainwithmorpheus.com/

Pelletier C. Exercise prescription for persons with spinal cord injury: a review of physiological considerations and evidence-based guidelines. Appl Physiol Nutr Metab. 2023 Dec 1;48(12):882-895. doi: 10.1139/apnm-2023-0227. Epub 2023 Oct 10. PMID: 37816259.

Eitivipart AC, de Oliveira CQ, Arora M, Middleton J, Davis GM. Overview of Systematic Reviews of Aerobic Fitness and Muscle Strength Training after Spinal Cord Injury. J Neurotrauma. 2019 Nov 1;36(21):2943-2963. doi: 10.1089/neu.2018.6310. Epub 2019 May 23. PMID: 30982398.

van der Scheer JW, Goosey-Tolfrey VL, Valentino SE, Davis GM, Ho CH. Functional electrical stimulation cycling exercise after spinal cord injury: a systematic review of health and fitness-related outcomes. J Neuroeng Rehabil. 2021 Jun 12;18(1):99. doi: 10.1186/s12984-021-00882-8. PMID: 34118958; PMCID: PMC8196442. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8196442/