Training intensity is one of the most misunderstood aspects of fitness programming. Whether you’re lifting weights for strength, muscle growth, or general health, understanding how hard you should be pushing yourself is crucial for long-term progress. This comprehensive guide will explore why intensity matters, how to measure it effectively using Rate of Perceived Exertion (RPE), and how to implement it accurately in your training.
Why Is Training Intensity Important?
Training progress isn’t simply about short-term results – it’s about long-term consistency. To train for years, you must stay injury-free for years, which requires finding the right balance of intensity that challenges your muscles without overtaxing your system.
The Science Behind Muscle Growth
There are three primary mechanisms that drive muscle growth:
- Mechanical tension: You need to recruit as many muscle fibres as possible during a set. If you aren’t recruiting maximum muscle fibres, they can’t all be stimulated for adaptation and growth.
- Metabolic stress: Metabolite accumulation after exercise in response to low energy causes a flow-on effect and reactions that can result in muscle growth.
- Muscle damage: Micro tears in the muscle from repeated repetitions can stimulate repair and growth.
Understanding Motor Unit Recruitment
Motor unit recruitment refers to the activation of different muscle fibre types (from type 1 to type 2a to type 2x) with increasing force requirements or as fatigue develops during voluntary muscle contraction. We need to give the body a reason to call upon those extra fibres, which is where training intensity becomes crucial.
This recruitment process involves:
- Intra-muscular coordination: The individual muscle’s ability to recruit muscle fibres
- Inter-muscular coordination: The ability of a group of muscles to work together, with working and non-working muscles relaxing and contracting with good timing
What Is ‘Failure’ in Training?
Training failure is the inability to meet and overcome the demands of an exercise, causing an involuntary set endpoint. In other words, you couldn’t keep going even if someone held a gun to your head – your muscles are physically incapable of moving the weight.
There are different types of failure:
- Absolute failure: Even if you change or break your technique, you physically can’t move the weight anymore
- Technical failure: Your form is altered significantly, which technically counts as failure
Do You Need to Train to Failure?
This is where many trainees get confused. Research has consistently shown we can get the same strength and hypertrophy adaptations when training around 2-4 repetitions short of failure as we can from going to complete failure.
Failure can be beneficial and does have its place, but it isn’t necessary for results. In fact, consistently hitting failure can actually make you weaker and impair recovery. The sweet spot appears to be staying around 3-5 reps away from true muscle failure, which recruits enough fibres to maximally stimulate growth without unnecessary fatigue.
Training Intensity: What Is RPE?
RPE stands for ‘Rate of Perceived Exertion’ – putting a number (rate) on how hard (perceived) an activity (exertion) is or was. While typically used during cardiovascular exercise, it’s also extremely valuable after a set of resistance training to gauge intensity.
The RPE scale for resistance training typically runs from 1-10:
- RPE 10: Maximum effort, couldn’t possibly do another rep
- RPE 9: Could maybe do 1 more rep
- RPE 8: Could definitely do 1 more rep, possibly 2
- RPE 7: Could definitely do 2-3 more reps
- RPE 6: Could definitely do 4-5 more reps
- RPE 5 and below: Many reps left in reserve
Why Use RPE To Manage Training Intensity Instead of Percentages?
Critics claim RPE isn’t precise and prefer percentage-based training (% of 1RM). However, percentage-based training has its own issues:
Our strength varies day to day based on numerous factors:
- Whether you’re training alone or with friends
- Your emotional state (happy versus sad)
- Sleep quality
- Nutrition
- And countless other variables
Your body doesn’t know ‘weight’ – it only knows ‘stress’. On a normal day, 100 kg might feel like a 7/10 ‘stress’ on the body, which is the sweet spot for stimulating growth and strength. On a good day, that same 100 kg might feel like only 5/10, allowing you to increase the weight. On a bad day, 100 kg might feel like 9/10, indicating you should reduce the weight.
Even though the absolute weight changes in these scenarios, maintaining the appropriate RPE keeps you in the sweet spot for growth and progress while adjusting to your body’s day-to-day fluctuations.
Using RPE Accurately To Manage Training Intensity
Training and rehabilitation are more reactive than proactive processes. We should aim to progressively refine our approach rather than ‘set and forget’. This aligns with the S.A.I.D. principle (Specific Adaptations to Imposed Demands), which emphasises that training should be specific not just to your goals, but to you as an individual.
Rep Speed as an Indicator of RPE
One of the most reliable ways to gauge your RPE is by monitoring rep speed:
- Training too easy: The speed stays the same for every rep, indicating you haven’t gone anywhere near failure
- Training too hard: The speed drops dramatically at the end, with reps looking significantly different, grindy, and nearly getting stuck
- Training in the sweet spot: Reps gradually slow down towards the end but aren’t ugly or grinding to a halt
You can film your sets and watch them during rest periods to assess rep speed and make adjustments to your intensity accordingly.
Anchoring Sets: Calibrating Your RPE
To improve your ability to estimate RPE accurately, try this ‘anchoring’ approach:
- Choose an exercise (either upper or lower body)
- Begin a set with a challenging weight (maximum 10-12 reps)
- When you think you’ve reached RPE 8, note or call out the repetition
- Continue to absolute failure, pushing as hard as possible
- Compare your estimate to reality:
- If you stopped at exactly 2 more reps after calling RPE 8, your estimate was spot on
- If you completed more than 2 additional reps, you underestimated your capacity and are likely not training hard enough regularly
- If you couldn’t complete 2 more reps, you overestimated your capacity and are likely training too hard regularly
This exercise helps calibrate your internal RPE scale, making future training more effective. Repeat it with different exercises and muscle groups to build a comprehensive understanding of how different movements feel at various intensities.
Practical Applications for Different Training Goals
Training across various rep ranges is ideal, with emphasis dependent on your specific goals:
- For strength and power: Focus more on heavier weights with lower reps. This develops your ability to produce maximum force but can be more demanding on joints and the nervous system.
- For muscle growth with less emphasis on strength: Higher reps will work well, as long as you’re still getting close enough to failure. This approach is generally ‘friendlier’ on the body but can be more tiring and time-consuming.
- For general fitness: A mixture of approaches provides the best overall stimulus and prevents boredom.
Remember that you can build just as much muscle doing sets of 5 reps as you can with 30 reps, provided you’re training close enough to failure. Motor unit recruitment increases steadily as you approach fatigue, plateauing at around 3-5 reps shy of failure regardless of the rep range.
This is why the idea that you need to train in the 8-12 rep range for muscle growth is a myth. It’s not the reps that stimulate muscle growth – it’s the intensity relative to your capacity.
Key Takeaways
- Training intensity is crucial for progress, but going to complete failure isn’t necessary and may be counterproductive
- The sweet spot for most training is around RPE 7-8 (2-3 reps in reserve)
- RPE allows you to adjust your training based on how you’re feeling day to day
- Rep speed is a reliable indicator of how close you are to failure
- You can build strength and muscle in any rep range, provided the intensity is appropriate
- Periodically test your ability to gauge RPE through ‘anchoring sets’
- Training should challenge you but shouldn’t require a spotter to complete your reps on every set
By understanding and properly implementing appropriate training intensity, you’ll be able to make consistent progress while minimising injury risk and avoiding unnecessary fatigue – the true formula for long-term success in any fitness endeavour.
