Swimming’s Hidden Problem: How Coaches Accidentally Traded Physiology for Logistics

Introduction

For many years, a common swim training method has existed to solve a logistical problem: crowded pools. This method is the bundled-rest interval, where swimmers start each repetition on a fixed time interval (a bundle of active time plus rest). This was an effective solution for managing a large number of swimmers simultaneously, but it created a conflict between convenient pool management and the principles of physiological science.

Today, this conflict has new consequences, especially in modern coaching that uses data and artificial intelligence (AI). The practice of bundling rest creates a fundamental problem with data quality. Because the actual time a swimmer rests between swims is not recorded, an athlete's training history becomes inaccurate and misleading. This means the sport collects large amounts of data, but cannot use that data to generate reliable conclusions.

This is more than a technical problem; it also negatively affects athlete development by causing unnecessary fatigue and burnout. It is time to question this standard training method and adopt a more intentional and scientific approach to the most important variable for improvement: rest.

A Swimmer's Story of Burnout

I grew up in the "No pain, no gain" culture of swimming, where exhaustion was treated as the primary measure of success. To be clear: significant improvement requires intense effort, and an athlete must be willing to do the hard work required to reach their potential. However, there is a very large difference between the necessary pain of pushing your limits and the avoidable suffering caused by a poorly designed training session. This avoidable suffering—which results from poor design, not a lack of determination—is the source of many problems in our sport.

I honestly do not remember a time when I was not tired. I would fall asleep in class, doze off while doing homework, and ask for five more minutes of sleep on the way to morning practice. This constant exhaustion was a direct result of my training in the pool. When I was a slower swimmer in my lane, every repetition was a desperate effort to catch up, which meant I sacrificed my rest time to stay with the group. When I eventually became the fastest swimmer in the lane, the type of pressure changed; I had more rest time, but I felt compelled to swim faster than the planned intensity to maintain my lead. I firmly believed that to win a race, a swimmer must always be the practice leader.

I survived that training system, and I still love the sport, but many of my promising teammates did not. Their careers were ended by constant fatigue, preventable injuries, and the physical consequences of over-training.

Years later, my education in Sports Science connected my personal experience with a new professional understanding. As I transitioned from an athlete to a coach leading a team with diverse abilities, I started to see this long-established training method from a new perspective. I began to question if our methods were truly designed to produce the best physiological results or if they were simply a compromise that everyone had accepted. We measure swimming volume and intensity with high precision, down to the metre and the fraction of a second, but we treat rest as an inconvenient part of the schedule.

This overlooked variable is the central point of the story—a story that is not unique to me, but one that resulted from a compromise made across the entire sport.

When Logistics Override Physiology

The bundled rest interval was not created by sports scientists; it was a practical solution to a problem. As training groups grew larger and more diverse while pool space remained limited, coaches needed a timing rule to keep many swimmers moving in an organised way. The solution was the repeat interval, for example: “10 × 100 @ 1:40—everyone leaves on the beep.” This solved a difficult management problem for the coach, but it created a physiological problem. It combined the work and recovery periods into a single unit, which made the rest period the part that could be sacrificed.

This convenience has a significant, often unseen, negative consequence: it creates a major gap in the training data. By treating rest as a random and unrecorded variable, the resulting training data becomes fundamentally unreliable. This is a critical flaw in modern, data-driven coaching.

This idea is not new, but it is not widely understood or applied. Daniel L. Carl, Ph.D., wrote an article on SwimSwam that explained this exact problem in detail: swimming coaches often use repeat intervals as a solution for logistics, even when this method compromises the physiological goals of the training.

The comments section under that article is also very revealing. The responses are mixed: some coaches are unaware of the problem, and others acknowledge it, but very few offer practical solutions. This accurately reflects the current situation in the swimming community: the problem is real and known to some, but it remains largely unsolved in practice.

This year, coach Brett Hawke provided a rare, real-world confirmation of this issue. While preparing sprint champion James Magnussen for the “Enhanced Games,” they added heavy gym workouts to high-intensity pool sessions without increasing the recovery time. As a result, Magnussen’s progress stopped. Hawke's public honesty about this was remarkable. It started a discussion that many people in the sport avoid, because they incorrectly believe that over-training is not a real phenomenon (Abnormal Podcast, 2025).

So why is a method based on convenience so common in high-performance swimming? The usual justification is that it is "fair" for a lane with swimmers of different abilities. Ironically, this diversity of ability is the strongest argument against bundling rest. When faster and slower athletes share a fixed send-off time, one might rest for fifty seconds while another rests for only twenty. This difference in rest has no physiological basis.

Research is very clear: even small changes in rest time alter the body’s response to exercise. Deliberately shortening rest periods increases the body's use of its aerobic metabolism and hinders the recovery of phosphocreatine, which is the body's fuel for explosive power (Laursen & Buchheit, 2019). For example, adding just ten seconds of rest can significantly restore peak power because it allows these anaerobic pathways to recover more completely (Laursen & Buchheit, 2019). When the swim time and distance are fixed, it is the rest period that changes. This causes athletes to switch unpredictably between energy systems, which undermines the goal of the training set.

The negative effects are widespread. The direct consequences are that an athlete's power output decreases, periods of no improvement last longer, and rates of injury or illness increase. The indirect consequences are even more systemic. Swimmers are still tired in their lives outside of swimming, which affects their school, jobs, and family life. Coaches are left with inaccurate monitoring data that leads to poor decisions about future training. Most critically for the sport's future, this practice creates a fundamental problem with data quality. As recent analyses have explored, entire training histories become unreliable because the most important variable—the actual recovery time—is never recorded accurately. The result is a sport that possesses large amounts of data but cannot extract meaningful knowledge from it (Wise Racer, 2025).

The Science of Rest: Understanding the Third Variable in Training

When coaches design a workout, they typically focus on distance and pace. However, neither of these variables will produce the desired result unless the body has enough time to recover from and adapt to the training stress. Recovery is not one single process. Instead, it is a complex combination of different energetic, structural, and regulatory processes, and each of these operates on its own unique timeline. If a training plan does not respect these different timelines, the intended goal of a session and the actual adaptation the body makes will become very different.

Sports science provides many methods for prescribing exercise intensity, but the prescription of rest remains a neglected area of study. This oversight becomes more critical during high-intensity training because efforts above the lactate threshold heavily use the anaerobic energy systems, which deplete their fuel rapidly. Therefore, the faster an athlete swims, the more important precise recovery becomes.

The amount of recovery is a primary factor that determines which energy system the body uses and how the body adapts to training. By not controlling the rest period, coaches unintentionally lose control over several key factors. These include which energy system is dominant, the availability of fuel (substrates), the accumulation of fatigue, and VO2​ dynamics. This means the athlete may not be training in the intended physiological zone.

To understand why this happens, we must look at more than just a single energy system. The body does not rely on one source of energy, like a car with one engine and one fuel tank. Instead, the body has a collection of interconnected systems that provide energy for movement together on a continuum. Each of these systems is stressed by exercise and then repaired on its own unique schedule. The table below summarises information from current scientific literature about these recovery timelines.

System/Substrate	Type of Major Stressor	Recovery Duration	Key Notes	References
Phosphocreatine (ATP-CP System)	Anaerobic	~3–5 minutes (65% in 90s, ~95% in 6 min)	Biphasic resynthesis (fast then slow) critical for interval training design; aerobic fitness accelerates recovery.	(McMahon & Jenkins, 2002; Bogdanis et al., 1996; Dawson et al., 1997)
Muscle & Liver Glycogen	Aerobic & Anaerobic	24–48 hours (24-36h for full restoration with proper nutrition; longer after very high volume)	Biphasic resynthesis (rapid insulin-independent, slower insulin-dependent); "magic hour" crucial for rapid replenishment.	(Burke et al., 2017; Ivy, 1998; Jentjens & Jeukendrup, 2003; Burke et al., 2004; Aragon & Schoenfeld, 2013; Betts et al., 2010)
Skeletal Muscle	Anaerobic (intense/eccentric)	24–72 hours (age-dependent: teens 24-48h, middle-aged 48-72h, older 4-7 days)	Recovery varies by exercise intensity/load; age-related decline necessitates adapted strategies (sarcopenia, hormonal changes, brain-muscle connection).	(Kim et al., 2005; Peake et al., 2017; Damas et al., 2018)
Connective Tissue (Tendons & Ligaments)	Anaerobic (high intensity, explosive loads)	Acute soreness 48–72 h; structural remodelling weeks-months (e.g., tendon collagen turnover); long-term >6 months for significant adaptation.	Slowest recovery; susceptible to chronic injury; very limited collagen turnover in mature tendons (focus on adaptation, not rapid repair).	(Bohm et al., 2015; Cook & Purdam, 2009; Shaw et al., 2017; Purdam et al., 2004; Malliaras et al., 2015)
Autonomic Nervous System (ANS)	Aerobic & Anaerobic	24–48 hours (up to 24h low intensity, 24-48h threshold, ≥48h high intensity aerobic/HIIT)	ANS balance is key indicator of training stress and fatigue; low HRV correlates with health risks; HRV reflects overall lifestyle stress.	(Buchheit & Gindre 2006; Buchheit & Laursen 2014; Bellenger et al., 2016; Borresen & Lambert, 2009; Stanley et al., 2013)
Central Nervous System (CNS)	High-intensity anaerobic / prolonged exhaustive endurance	Minutes to days (20 mins to several days; often 24-72h after intense work)	Distinct from muscular fatigue; can persist longer, leading to "flat" feeling; impacts motor skill coordination significantly.	(Gandevia, 2001; Thomas et al., 2015; Meeusen et al., 2006; Kellmann et al., 2018; Kreher & Schwartz, 2012; Vaile et al., 2008; Issurin, 2010)
Hormonal System	Aerobic & Anaerobic	24–48 hours (acute responses 48-72h post-RE)	Acute endocrine responses normalize in 24-48h; prolonged imbalance signals over-reaching; T/C ratio is powerful biomarker for anabolic-catabolic balance and recovery status.	(Kraemer & Rogol, 2008; Urhausen & Kindermann, 2002; Cadegiani & Kater, 2017; Ho et al., 1988)
Immune System	Aerobic (prolonged)	Up to 24 hours ("open window" of susceptibility)	High-volume aerobic training more likely to suppress immune function temporarily; "open window" necessitates proactive, multi-pronged recovery.	(Pedersen & Ullum, 1994; Gleeson, 2007; Walsh et al., 2011; Gleeson, 2016; Nieman, 1997; Walsh, 2019)
Vascular and Endothelial Function	Aerobic & Anaerobic (intensity-dependent)	~24 hours (moderate); longer (intense); deeper changes months	Regular exercise benefits endothelial function, but excessive intensity can impair it ("exercise paradox"); moderate intensity optimal long-term.	(Green et al., 2017; Laughlin et al., 2008; Tinken et al., 2009; Corretti et al., 2002)

The most important conclusion from the data in the table is the significant variation in recovery periods. For example, the phosphocreatine that fuels a single sprint can be replenished in minutes, but the structural repair of connective tissue can take 48 to 72 hours or longer, and the central nervous system, which is critical for speed, can take up to 72 hours after intense efforts. A swimmer might feel "recovered" after one day of rest, but their central nervous system could still be significantly fatigued from an intense session.

This complex reality, which involves many different recovery timelines, is precisely why the bundled-interval model is ineffective. That model operates on a single timeline for logistics, while the athlete’s body must manage many different physiological timelines simultaneously. To manage this complexity, effective training is often structured using a zone-based framework. This framework clarifies the specific physiological purpose of each training set. This principle is the basis for different systems, such as a 5-zone framework for general swimming for fitness and a more detailed 9-zone framework for competitive swimming athletes. Both frameworks are designed to match the training stimulus with the necessary recovery time.

The Three Scales of Recovery

To be effective, training must be planned according to the body's biological timelines. Recovery from training stress occurs on three distinct but overlapping scales:

1. Interval Rest (Recovery Between Repetitions): This is the pause between individual swims within a single set. For high-intensity sprint work, passive rest (standing or floating) is the most effective way to replenish phosphocreatine (PCr). For efforts over a longer duration, a low-intensity active recovery helps remove metabolic byproducts from the muscles. If this rest period is too short, PCr cannot regenerate sufficiently, power output decreases sharply, and the set no longer trains the intended energy system (Laursen & Buchheit, 2019).
2. Set Rest (Recovery Between Sets): This is the rest period that separates different blocks of work within a single training session. After intense work that uses the glycolytic system, light activity helps to clear lactate more quickly, which helps the athlete maintain a high level of performance in later sets. For sets focused only on maximum speed, however, passive rest is better for maintaining the focus on peak power. Skipping this rest period causes the second half of the practice to become slow, low-quality aerobic swimming. This defeats the original purpose of the session.
3. Session-to-Session Recovery (Recovery Between Workouts): This includes everything that happens after athletes leave the pool, such as nutrition, sleep, and low-intensity movement. The muscle micro-trauma, depleted glycogen stores, and neural fatigue from one workout can last for several days; markers of muscle damage can reach their peak 48 hours after a workout. If the next workout is planned without considering these biological timelines, athletes will train before their bodies have fully recovered. Protection against this is achieved through careful weekly planning, for example, by not scheduling two maximum-effort days consecutively and by placing easy sessions after the most intense ones.

Because these different systems recover at different rates—and because age, genetics, sleep, and nutrition influence each timeline—using a single, fixed send-off time for everyone produces an unpredictable result. For example, two swimmers completing a 100-metre swim in 60 seconds and 75 seconds will arrive at the next start with very different levels of energetic and neural readiness, even though the pace clock indicates they are on the same schedule.

While training volume and intensity provide the stimulus for adaptation, recovery time determines the quality of the performance and the training outcome. If you ignore these recovery timelines, the result is random fatigue instead of targeted physiological adaptation.

A Better Approach: From Standard Practice to Intentional Design

We must acknowledge the real-world challenges that coaches face every day. With crowded pools and limited time, the bundled-rest interval is, and will remain, a helpful tool for managing the logistics of a complex session. It ensures swimmers continue to move and that the planned activities for the workout are completed.

The goal is not to eliminate this method, but to redefine its purpose. It should be used as a specific tool for a specific training goal—such as an aerobic set that uses the pace clock to create pressure—rather than being used as the standard method for all training.

When pool space is not a limiting factor, when resources are available, and when technology can help manage complexity, prioritizing logistics over physiology will hinder an athlete's development. For goals like developing maximum power, improving technique, or targeting specific anaerobic pathways, the physiological need for precise, individualised rest must be more important than convenience. This is how modern coaching must evolve. Technology should be developed to help coaches balance the demands of physiology and logistics, without adding excessive stress or complexity to their work.

Personalising rest is still a new and developing area in coaching, but we do not need to have perfect data to begin taking action. The following recommendations are based on scientific principles and can make rest a true competitive advantage.

Top 5 Recommendations for Coaches

1. Prescribe Rest as a Separate Variable: Instead of writing "10x100 on 1:50," prescribe "10x100 @ Zone 3 + 30s rest." This method isolates the training stimulus to ensure you are training the intended energy system. It also ensures that the data you collect is accurate, reliable, and ready for future coaching tools.

2. Match Rest to the Goal of the Set: Use long, passive rest (2-5 minutes) for maximum-quality speed. Use a shorter rest (1-3 minutes) to develop anaerobic capacity. Use very short rest (less than 60 seconds) for aerobic and threshold training.

3. Coach the Athlete, Not Just the Plan: Be a responsive coach. Adjust rest based on what you observe (like technique breaking down), what you measure (like heart rate or HRV), and what the athlete communicates to you. Each athlete is different and may require a different approach.

4. Teach the Importance of Rest: Explain that rest is a key part of training that leads to adaptation, not just downtime. Use simple analogies, like a "recharging battery," to help athletes understand and support this approach. An informed team will be able to manage their own rest periods correctly.

5. Plan Recovery on All Scales: During practice, focus on the details of the rest interval. For the week, consider the big picture and plan a schedule with proper recovery days. Always promote the essential elements of recovery: sleep, nutrition, and hydration.

Top 5 Recommendations for Athletes

1. Become an Expert on Your Own Body: Pay attention to your body's signals, such as poor technique when you are tired. Record important data, like your swim times and sleep quality. Over time, you will see patterns that reveal your personal method for achieving peak performance.

2. Understand the Purpose, Then Execute the Method: Understand the goal of each set (Is it for speed? Or for endurance?). Then, follow the prescribed rest period, because it is designed specifically for that goal. Executing the plan correctly is more effective than training hard without a specific purpose.

3. Master Recovery Outside of the Pool: Real improvement is achieved in the time between training sessions. Master your recovery by consistently focusing on the three most important elements: Sleep, Fuel, and Hydration.

4. Rest with Purpose: Do not simply wait for the next repetition. Use every rest interval to actively prepare your body and mind for the next swim. You can do this with calm breathing and by focusing on your next technical goal.

5. Your Feedback is Essential Information: Tell your coach the things they cannot see. Instead of saying, "I'm tired," provide specific information like, "My HRV is lower than normal, and my swim times get much slower when I only have 15 seconds of rest." Specific feedback helps your coach make smarter training decisions.

Note: This article was originally written in English. It has been translated into other languages using automated AI tools to share this information with a wider audience. We have tried to ensure the translations are accurate, and we encourage community members to help us improve them. If there are any differences or errors in a translated version, the original English text should be considered the correct version.

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