Strength training intensity: time matters

Regular readers will be aware that the subject of maximizing strength training workouts by optimizing the workload – for example, the weight/resistance used, numbers of reps and sets, how close you should train to failure and the frequency of workouts – has been much discussed in previous SPB articles. The underlying principle however remains the same: to generate enough training load and intensity to create a training adaptation, but to do it in a way that maximizes the strength gains achieved while minimizing fatigue and time invested.

The role of overload

Different athletes have different goals when resistance training. Some seek to build general strength and resilience, while others want specific strength gains to help with a sport or activity, or to prevent/recover from injury. But whatever the goal, you can’t escape the basic laws of exercise physiology; specifically, if you want to improve muscle function, you need to generate an ‘overload’ stimulus in that muscle⁽¹⁾, and then allow the process of rest and recovery to take place. The term ‘overload’ in this context doesn’t mean subjecting to the muscle to harmful and damaging amounts of physical abuse, but it does mean challenging the muscle by asking it to do significantly more work than its current level of conditioning allows for, which then creates a training adaptation⁽²⁾.

Overload and training intensity

Ask most athletes how to make their strength workouts harder and more intense in order to generate more a bit more overload and you’ll probably get an answer that involves performing more reps per set, more sets, more weight in each set, or some kind of combination of these factors. But while this kind of answer is commonplace and true to a degree, it doesn’t really get to the nub of what intensity is all about.

Intensity is not a measure of how much resistance is used, or even the number of sets/reps that are performed. Intensity is all about packing a lot of high-quality muscular work into a small volume of time. This doesn’t mean running around the gym like a headless chicken, throwing weights around at high speed! What it does mean is that for each muscle group, strength exercises should be performed with enough high quality repetitions and resistance performing in order to properly challenge the target muscle fibres – and that athletes perform sets and move onto different exercises with minimal rest.

This approach to generating intensity in a strength workout (ie condensing a given workload into a short time span with minimal rest in between sets was first pioneered by the eccentric (no pun intended!) but legendary Arthur Jones, the father of the Nautilus training technique – see this article⁽³⁾. Jones had long understood the importance of generating intensity in muscle conditioning and his Nautilus machines were designed to generate intensity. This was executed by performing just one set of strictly executed reps for each major muscle group in the body, performed to ‘failure’, moving from one machine to another without resting or pausing, with a complete body workout taking just 12-15 minutes!

The science of strength training intensity

Let’s unpack the concept of strength training intensity a little further. The traditional view of ‘strength training intensity’ defines it as the load/resistance lifted across a particular exercise or training session divided by the number of reps in which it is achieved⁽⁴⁾. Of course, athletes who are naturally heavy and powerfully built will lift more for the same effort, so an alternative way of defining intensity is to relate total load lifted ÷ reps to the bodyweight of the athlete – ie effort per kilo of bodyweight⁽⁵⁾. This then becomes (total load lifted ÷ reps)/weight in kilos. So for example, a 65kg distance runner might have an ‘total load lifted ÷ reps’ score that is only two thirds that of a 90kg athlete, but the amount lifted per kilo of bodyweight will actually be very similar in both cases.

Time matters in intensity

The average load per rep per kilo of bodyweight sounds like a very useful way to measure strength workout intensities, and indeed it is - if you are only considering the intensity of the work itself – ie the external workload. However, what this intensity measure does not do is measure the internal loading on the body of the athlete – ie what the athlete experiences and feels in terms of perceived exertion. That’s because it takes no account of the inter-set rest periods during resistance exercise, which affects the ‘internal intensity’ of a session⁽⁶⁾. Adding in rest periods ‘dilutes’ the intensity of the session, with longer rest periods creating more dilution.

As a rather extreme analogy, imagine a well-trained runner running his/her faster ever marathon, and the fatigue that would be experienced afterwards - not to mention the recovery time to resume normal training. Now compare that with running at that same marathon pace for just 3.75 miles per day for seven days (ie with over 23.5 hours rest in between each run). At the end of that week of accumulating the marathon mileage, he or she would almost certainly feel zero fatigue, but will have also generated virtually no marathon training effect!

A better way of measuring real intensity (external and internal) therefore is to take into account inter-set rest durations and therefore total exercise time – ie work done per kilo of bodyweight per unit of time. This is calculated by dividing the total work or loading by the training session duration⁽⁷⁾ or summed duration of inter-set rest periods⁽⁸⁾. When calculated directly from measured work, this measure equates to the mean power output across an entire exercise session⁽⁹⁾.

Possible downsides

Performing strength training sessions at a higher work rate – ie resting less in between sets, thereby performing the same number of sets at the same weights and reps in a shorter period of time - elevates levels of blood lactate and growth hormone, and stimulates positive longer term muscular adaptations such as greater muscle cross-sectional area, and higher levels of strength, and endurance⁽¹⁰⁾. There is a big possible downside however; if this increased workout intensity is not monitored appropriately, over an extended period of time, the greater metabolic demand can increase an athlete’s risk of staleness, burnout or overtraining syndrome⁽¹¹⁾.

Given the balance between the rewards that high-intensity training brings and the potential risks, what is needed is an understanding of exactly how manipulating the work rate (by reducing rest times and condensing a given volume of work into a shorter time period) affects measures of internal loading – ie perceived exertion and fatigue, heart rates, blood lactate etc. Surprisingly however, up until now there’s not been any research into this topic, nor has there been any comparison between the more traditional ways that measure intensity purely by external means (ie work done/reps but not taking total time and rest periods into account).