Clients train hard, show up for sessions, try to stay consistent… and sometimes still feel flat, tired or stuck. Every PT has seen this. Programmes that once worked start losing their punch. Strength stalls. Energy drops. Mood changes. You try a few tweaks, maybe adjust some volume, but it just doesn’t quite click. This is exactly the sort of problem that the recent review, Mechanisms of the Biological Response Cascade to Exercise-Induced Stress, helps explain. It gives us coaches a structured way to understand how training stress works inside the body and why clients can slide from progress into frustration when stress tips into the wrong zone. This type of information gives us a biological framework for decisions we make every day on the gym floor.
The more you dig into the paper, the easier it becomes to see where training starts to feel heavy for people, even when volume and intensity look sensible. That’s one reason I often encourage new coaches on any personal trainer course to study exercise stress in a bit more depth. It gives you language for explaining things clients can feel but can’t articulate, like the difference between tired muscles and a nervous system that’s had enough for the week.
Plenty of fitness professionals are also refreshing their knowledge through a variety of different fitness courses which cover stress physiology and recovery science, and this paper gives a fresh lens through which to read that material. It connects the dots between the mechanical side of training, the metabolic side and the hormonal or immune responses that clients experience days or even weeks later.
Stress, Signals and the Bidirectional Threshold
The authors of this study describe exercise as a layered stressor. A single session triggers mechanical tension, metabolic strain, hormonal shifts and immune activation. All of this is normal. The interesting part is how the body interprets the stress and the point where stress flips from useful to unhelpful. The review calls this the bidirectional threshold. That’s the moment where training stress stops stimulating growth and starts creating fatigue, inflammation and poor recovery.
The study breaks the response to exercise into three levels. The primary level begins with mechanical stress inside muscle fibres. Integrins, focal adhesion kinase (FAK), Piezo1 channels and other mechanosensors pick up load instantly. These sensors set off biochemical cascades that support hypertrophy, vascular growth or mitochondrial improvements. The catch is that the same sensors, when pushed for too long or too often, shift into signals associated with inflammation or fibrosis. The study notes how Piezo1, for example, helps with normal tissue remodelling under moderate load but promotes calcium overload and inflammatory signalling when the activation becomes excessive or prolonged.
The secondary level runs through metabolic pathways. Adenosine Monophosphate–Activated Protein Kinase (AMPK), Mechanistic Target of Rapamycin Complex 1 (mTORC1) and Nicotinamide Adenine Dinucleotide (oxidised form) (NAD+) pathways react to energy stress. AMPK steps in when energy is low, helping cells switch fuel and stay alive during hard sessions. mTORC1 comes alive during recovery and supports muscle growth. When training load is balanced, the two take turns. When the balance tilts too far in one direction, progress slows or stops. Chronic AMPK dominance creates delayed recovery, constant fatigue and a sense of never being quite ready for the next session. The study also highlights how mitochondrial and Endoplasmic Reticulum (ER) stress accumulate when recovery is insufficient, creating a deeper layer of fatigue that doesn’t go away with a bit of stretching.
The systemic responses include hormones, myokines, Brain-Derived Neurotrophic Factor (BDNF) and immune shifts. These don’t show up instantly but develop across days. Moderate training promotes a healthy endocrine response, boosts immune function and improves mood. High or unresolved stress pushes Interleukin-6 (IL-6) higher for longer, elevates cortisol and suppresses immunity. The review repeats this point several times: the training stress that helps a body adapt is the same type of stress that can make recovery feel impossible when the threshold is crossed.
How Much Stress Is Too Much?
Most clients don’t realise how easily total weekly stress creeps up. A training plan might look perfectly reasonable on paper, but stress adds up from load, life, sleep, nutrition and everything happening outside the gym. The threshold described in the study shifts all the time. A well-rested client at the start of the month sits at a different threshold from a stressed client in the middle of a long work week. Many coaches already sense this intuitively, but the study helps explain what’s happening underneath.
The mechanical sensors inside muscle react instantly to load, and mechanosensitive ion channels (Piezo1) in particular seem sensitive to chronic activation. When sessions stack up with very little space for recovery, these channels run hotter for longer. The ER struggles to process proteins efficiently, mitochondria stay in a stressed state and the nervous system tightens its grip. Clients don’t always describe this directly. They’ll talk about feeling drained, irritated or flat. They’ll still train, but the spark drops.
Programming with thresholds in mind means paying attention to these subtle cues. A client repeatedly reporting poor sleep, long-lasting soreness or unexpected irritability is showing signs that the underlying stress systems are staying switched on. Early adjustments help. Small changes to weekly structure, slightly lower volume across a block or placing a high-stress session after a genuine rest day all help clients stay on the right side of the threshold.
Why Clients Stop Progressing
Plateaus are common. The study explains why in a way that gives coaches a clearer strategy for handling them. Maladaptation doesn’t arrive overnight. It accumulates through repeated stress signals that never quite resolve. In the early stages, the body still adapts. Gains continue. Then the recovery curve shortens, fatigue lingers and progress slows.
The review describes several biological events underneath these plateaus. Mitochondria begin to falter when the permeability transition pore stays open for longer than it should, which drains energy production. ER stress shifts from adaptive to pro-apoptotic signalling when recovery stays incomplete. Inflammatory markers like IL-6 stay elevated for longer. Cortisol stays slightly raised. BDNF responses blunt, altering cognitive resilience and motivation. These events don’t feel dramatic to the client. They show up as subtle reductions in strength, slower bar speed, frequent low moods, reduced enthusiasm for training and extended DOMS.
The fix is rarely to push harder. Clients often think they need a tougher programme, especially in fat loss phases, but the biology points in the opposite direction. Coaches who spot early signs of maladaptation can change the overall stimulus before progress drifts further away. Sometimes it’s as simple as bringing weekly volume down by 20–30%, adjusting intensity zones or replacing a heavy compound day with a mixed session that drives less mechanical stress. The more consistent the recovery, the easier it becomes for the stress pathways to swing back into the adaptive zone.
Other research supports this. Studies on short-term intensified training in endurance athletes show how quickly mitochondrial function dips when recovery is inadequate. High-intensity eccentric work also shows links to increased ER stress and poor recovery. The signs aren’t always obvious at first, but they tend to follow patterns the review has laid out clearly.
The Missing Deload
Deloads often create tension. Clients worry they will lose progress or feel like they’re “doing less”. The study gives a helpful explanation for why deloads are a normal and healthy part of training. The ER handles protein processing and folding, which are essential for muscle repair. Mitochondria handle energy production. Both need space to reset. When they don’t get this, they enter a stressed state that lingers and accumulates.
ER stress that stays switched on begins to activate C/EBP Homologous Protein (CHOP) and c-Jun N-terminal Kinase (JNK) pathways, which sit on the maladaptive end of the stress spectrum. Mitochondria that sit in a prolonged stressed state struggle to maintain efficient ATP output. These processes are invisible. Clients won’t feel them directly. They feel the consequences. Low energy. Heavy legs. Unexpected fatigue halfway through a session. A sense of hitting a wall too early in the week.
A deload gives these systems time to unwind. Reducing overall volume, adjusting intensity or shifting to technique-focused work lets the body process the backlog of stress. Once ER and mitochondrial stress ease off, normal recovery pathways start working again. Many clients report feeling fresher, stronger and more motivated after a planned deload. It also prevents those long slow plateaus that creep through a training block.
Good deload programming doesn’t have to follow a single rule. Some coaches drop volume sharply and keep intensity moderate. Others reduce intensity and maintain technique work. Some extend the deload to lifestyle factors such as sleep hygiene or low-stress conditioning sessions. Every method works as long as the underlying purpose is respected, and that’s the stress pathways need a break before they start lifting again.
Smarter Stress Management for Clients
Training works best when stress and recovery rotate smoothly. The study gives us as PTs a clear view of the internal processes behind this rotation. Clients load their muscles. Mechanical and metabolic stress systems activate. The body adapts when the stress resolves. The problems start when the stress doesn’t resolve.
Using this framework helps us write cleaner, more responsive programmes. A block of high-stress work followed by a lighter week. Conditioning that fits comfortably around lifting rather than completely overwhelming it. A clearer eye for signs that a client’s stress threshold is drifting. A less rigid approach to planning when a client’s life outside the gym becomes demanding.
The biggest win is confidence. When you know how these systems work, you can explain to clients why you’ve designed a particular block the way you have. They trust the process more easily when they know recovery is part of the plan and not a sign of backing off. Plateaus become easier to understand and easier to fix. Progress becomes steadier across whole training blocks, not just isolated weeks.
Training stress doesn’t need to feel mysterious. The body is responsive, but it needs space to recover. When we write programmes with the stress cascade in mind, clients feel better, adapt faster and enjoy training for longer. That is always the goal.
Reference
- Xu, J., Zhang, J. & Sang, K. (2025). Mechanisms of the biological response cascade to exercise-induced stress: a comprehensive review. Frontiers in Sports and Active Living, 7:1691779. Click here to review the full research article.
- Robson-Ansley, P. et al. (2010). The effect of exercise on plasma soluble IL-6 receptor concentration: a dichotomous response. Exercise Immunology Review, 16, 56–76. Click here to review the full research article.
- Pereira, B. C. et al. (2016). Excessive eccentric exercise-induced overtraining model leads to endoplasmic reticulum stress in mice skeletal muscles. Life Sciences, 145, 144–151. Click here to review the full research article.
- Cardinale, D. A. et al. (2021). Short-term intensified training temporarily impairs mitochondrial respiratory capacity in elite endurance athletes. Journal of Applied Physiology, 131, 388–400. Click here to review the full research article.
Why Our Diplomas Match What the Research Is Telling Us
The TRAINFITNESS Gym Instructor & Personal Trainer Practitioner, Specialist & Master Diplomas™ set you up to coach with confidence from day one. Our courses give you the practical skills and real-world coaching strategies that line up with what the latest research is showing about how bodies actually respond to training stress. In the recent review on exercise-induced stress, researchers noted that IL-6 can rise by roughly fivefold within just a few hours of endurance exercise, and that BDNF, a key player in mood and cognitive resilience, can climb by tens of percent after moderate activity. These shifts are exactly why smart programme design matters, and why our Diplomas teach you how to work with the body, not against it. You’ll learn how to build sessions that create adaptation instead of fatigue, spot early signs of recovery issues, and coach clients in a way that keeps them progressing. If you want a qualification that’s built around real science and real coaching, this is where you start.
Gym Instructor & Personal Trainer Course – Distance Study, In-Person & Live-Virtual
