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 decision-support tools. Because the actual time a swimmer rests between swims is often not recorded, an athlete's training history can become harder to interpret. The sport may collect large amounts of data while still missing one of the variables needed to understand what stimulus the athlete actually received.

This is more than a technical problem. Poorly matched work and recovery can turn a targeted training set into unnecessary fatigue. It is time to question this standard training method and adopt a more intentional and scientific approach to one of the most important variables for improvement: rest.

Bundled-rest interval: A timing method where the swim and the rest are combined into one fixed send-off, such as "10 x 100 on 1:40." The faster swimmer gets more rest, and the slower swimmer gets less. This happens even when the written set looks identical.

Key idea: Rest is part of the training dose. If rest is hidden inside the send-off, coaches lose one of the variables that helps define the intended stimulus.

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 promising teammates did not stay in it. Some were worn down by constant fatigue, injury problems, and the physical consequences of training that did not match the recovery they actually received.

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 organized way. The solution was the repeat interval, for example: "10 x 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 record. By treating rest as a variable that is left over after swim time, the resulting data becomes harder to interpret. This matters for modern, data-informed coaching because a set record should preserve not only distance and pace, but also the structure and intent of the work.

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 (Carl, 2017).

The comments section under that article is also useful context. The responses are mixed: some coaches are unaware of the problem, and others acknowledge it, but very few offer practical solutions. It is a good snapshot of the current coaching challenge: the issue is known to some, but it remains difficult to solve in day-to-day pool practice.

In 2025, coach Brett Hawke provided a public, real-world example of this discussion while describing James Magnussen's preparation for the Enhanced Games. In that public conversation, the issue was not simply whether the athlete trained hard. It was whether the total load, recovery time, and nervous-system demands were being managed well enough for the intended performance goal (Abnormal Podcast, 2025). The details of that case should not be generalized to every swimmer, but the lesson is relevant: high-intensity work cannot be separated from the recovery architecture that makes it possible.

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. That difference is a logistical artifact, not a physiological prescription.

Research shows that rest duration changes the body's response to exercise. Deliberately shortening recovery can limit phosphocreatine restoration, change the energy contribution of the next high-intensity effort, and alter the stimulus of the set (Laursen & Buchheit, 2019; McMahon & Jenkins, 2002; Bogdanis et al., 1996; Dawson et al., 1997). When the swim time and distance are fixed, it is the rest period that changes. This can cause athletes to shift between energy-system demands in ways that undermine the goal of the training set.

The negative effects can spread beyond a single set. Power output can fall, the intended training stimulus can drift, fatigue can accumulate in ways the coach did not intend, and coaches are left with records that do not show the recovery dose each athlete actually received. That makes it harder to interpret future performance, fatigue, and training response. As previous Wise Racer analysis has argued, training histories become less useful when one of the most important variables, actual recovery time, is missing from the record (Buchheit, 2014; Kellmann et al., 2018; Wise Racer, 2025).

Why it matters: A coach may think the group completed the same set, but each swimmer may have received a different work-to-rest dose. That difference changes the physiology and the meaning of the session record.

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 can drift apart.

Sports science provides many methods for prescribing exercise intensity, but in day-to-day swimming practice, rest is often less visible than distance and pace. This oversight becomes more critical during high-intensity training because repeated fast efforts depend heavily on rapidly changing energy-system contributions. HIIT programming literature treats work duration, recovery duration, recovery intensity, repetition number, and density as variables that can change the session stimulus, which is exactly why rest should be visible in swimming-session design (Laursen & Buchheit, 2019). Therefore, the faster and more power-dependent the set becomes, the more important intentional recovery becomes.

The amount of recovery is one of the primary factors that influences relative energy-system contribution and how the body responds to training. By not controlling the rest period, coaches unintentionally lose control over several key factors. These include which energy-system demands are emphasized, the availability of fuel or substrates, the accumulation of fatigue, neuromuscular readiness, and how confidently later monitoring data can be interpreted.

To understand why this happens, we must look at more than one 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 restored, repaired, or regulated on its own timeline.

The table below is a coaching synthesis from the cited literature, not a rigid prescription. Recovery windows vary with athlete age, training history, session design, nutrition, sleep, health, and measurement method. The point is not that every swimmer should follow these timelines mechanically. The point is that "recovery" is not one variable with one clock.

System / Substrate	Main Stress Context	Approximate Recovery Window	Key Coaching Notes	Selected Support
Phosphocreatine (ATP-PC system)	Anaerobic / maximal or repeated sprint work	Roughly minutes; often discussed around ~3-5 minutes for substantial restoration, with slower completion after repeated or longer maximal work	PCr resynthesis is biphasic and oxygen-dependent; incomplete restoration can reduce peak power and change the stimulus of repeated efforts.	McMahon & Jenkins (2002); Bogdanis et al. (1996); Dawson et al. (1997)
Muscle and liver glycogen	Aerobic and anaerobic work, especially high-volume or high-intensity sessions	Often 24-48 hours depending on depletion, carbohydrate availability, and recovery strategy	Glycogen restoration is nutrition- and time-dependent; incomplete replenishment can affect subsequent training quality.	Burke et al. (2017); Ivy (1998); Jentjens & Jeukendrup (2003); Burke et al. (2004); Aragon & Schoenfeld (2013); Betts & Williams (2010)
Skeletal muscle	Intense, high-load, eccentric, or unfamiliar work	Often 24-72 hours, with large variation by athlete and stimulus	Muscle damage, protein turnover, soreness, and force recovery do not always move together; visible readiness can hide lingering tissue stress.	Kim et al. (2005); Peake et al. (2017); Damas et al. (2018)
Connective tissue: tendons and ligaments	High-intensity, explosive, or repeated mechanical loading	Acute symptoms may settle over days; structural adaptation/remodelling takes weeks to months	Tendons adapt more slowly than many metabolic systems; chronic loading errors can accumulate when high-intensity work is repeated without adequate planning.	Bohm et al. (2015); Cook & Purdam (2009); Shaw et al. (2017); Purdam et al. (2004); Malliaras et al. (2013)
Autonomic nervous system (ANS)	Aerobic and anaerobic load; whole-athlete stress	Often 24-48+ hours depending on load, fitness, and life stress	HRV and heart-rate recovery can help interpretation, but they are not interchangeable and must be read with context, training load, and athlete feedback.	Buchheit & Gindre (2006); Buchheit (2014); Bellenger et al. (2016); Borresen & Lambert (2009); Stanley et al. (2013)
Central nervous system (CNS)	High-intensity anaerobic work, sprinting, skill demands, prolonged exhaustive work	Minutes to days; practically relevant after hard or exhaustive work depending on task and athlete state	Neural fatigue is distinct from local muscular fatigue and can affect speed, coordination, reaction, and "feel" in the water.	Gandevia (2001); Thomas et al. (2015); Meeusen et al. (2006); Kellmann et al. (2018); Kreher & Schwartz (2012); Vaile et al. (2008); Issurin (2010)
Hormonal and endocrine regulation	Aerobic and anaerobic load; cumulative stress	Acute responses may shift over hours to days; chronic imbalance requires broader review	Endocrine markers can reflect training stress and overreaching risk, but interpretation is complex and should not be reduced to one number.	Kraemer & Rogol (2005); Urhausen & Kindermann (2002); Cadegiani & Kater (2016); Ho et al. (1988)
Immune system	Prolonged, heavy, or repeated exertion	Often discussed around a transient post-exercise window, commonly up to ~24 hours, depending on load and athlete state	Heavy training can temporarily alter immune function; sleep, nutrition, recovery, and load management matter.	Pedersen & Ullum (1994); Gleeson (2007); Walsh et al. (2011); Gleeson (2016); Nieman (1997); Walsh (2019)
Vascular and endothelial function	Aerobic and anaerobic work, intensity-dependent	Acute responses can occur over hours to a day; structural adaptations occur over longer periods	Exercise generally supports vascular function, but intensity, volume, and recovery influence the acute response.	Green et al. (2017); Laughlin et al. (2008); Tinken et al. (2009); Corretti et al. (2002)

The most important conclusion from this table is the significant variation in recovery periods. The phosphocreatine that fuels a single sprint can be substantially restored in minutes, while glycogen restoration, connective-tissue adaptation, autonomic balance, and neural readiness can operate on longer or more variable timelines. A swimmer might feel "recovered" after one day of rest, but that does not mean every relevant system has returned to the same state.

This complex reality is precisely why the bundled-interval model is limited. It operates on a single timeline for logistics, while the athlete's body responds to the actual work and recovery received. 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 help connect the intended stimulus with the rest, density, and recovery context needed to interpret it.

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, rest duration strongly affects phosphocreatine (PCr) restoration. If this rest period is too short, PCr cannot regenerate sufficiently, power output can fall, and the set no longer trains the intended energy system (Bogdanis et al., 1996; Dawson et al., 1997; McMahon & Jenkins, 2002). For longer efforts, active recovery may be useful in some contexts, while pure speed work often needs enough passive or very low-intensity rest to preserve peak quality (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. It determines whether the next block starts with enough recovery to preserve the planned intensity, technique, and physiological purpose. Skipping or compressing this rest can turn the second half of practice into a different session than the one the coach intended.
3. Session-to-Session Recovery (Recovery Between Workouts): This includes everything that happens after athletes leave the pool, such as sleep, nutrition, hydration, life stress, and the next day's training load. If the next workout is planned without considering accumulated fatigue and recovery feedback, training can drift from useful overload toward non-functional overreaching or underrecovery (Meeusen et al., 2006; Kellmann et al., 2018; Kreher & Schwartz, 2012).

Because these different systems recover at different rates, and because age, genetics, sleep, nutrition, life stress, and training history influence each timeline, using a single fixed send-off time for everyone can produce a less predictable 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 shapes the quality of the performance and the likely training outcome. If you ignore these recovery timelines, the result can become less controlled 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 weakens the quality of the training record and can undermine the intended stimulus. For goals like developing maximum power, improving technique, or targeting specific anaerobic pathways, the physiological need for adequate rest should carry more weight than convenience. Technology should help coaches balance physiology and logistics without adding excessive stress or complexity to their work.

Personalising rest is still a developing area in coaching, but we do not need 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 makes the training stimulus easier to interpret because work and recovery are no longer hidden inside the same number.

2. Match Rest to the Goal of the Set: For maximum-quality speed, use enough rest to preserve power, technique, and intent; several minutes may be needed after repeated maximal work. Shorter rests can be appropriate when the goal is aerobic pressure, threshold work, or controlled fatigue.

3. Coach the Athlete, Not Just the Plan: Be a responsive coach. Adjust rest based on what you observe, what you measure, and what the athlete communicates. Heart-rate and HRV data can help, but they should be interpreted with training context and athlete feedback.

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. Across the week, monitor whether athletes are coping with the total load. Encourage the basics that make recovery possible: sleep, nutrition, hydration, and honest feedback.

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 depends on what happens 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 times fall apart when I only have 15 seconds of rest," or "I slept poorly and my effort feels higher than normal." Specific feedback helps your coach make smarter training decisions.

Note: This article was originally written in English and translated into other languages using automated AI tools so we can share this information with more people. We do our best to keep translations accurate and easy to understand, and we welcome help from the community to improve them. If anything in a translated version is unclear, incorrect, or differs from the English version, the original English text should be considered the official version.

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Wise Racer. (2025, February 20 — updated May 29, 2025). Are Swimming's Fitness and Competitive Industries Data Fit for AI? Part 2. Wise Racer Blog.