What’s the optimal type of training to develop strength and hypertrophy? What’s the minimal dose? These are pertinent questions within rehabilitation and the focus of lots of recent research. Recently a research paper suggested comparable strength and hypertrophy adaptations between low- and high-load resistance training. But should we take that on face value? Let’s take a closer look: can you really get stronger with lighter loads? Should I lift heavy or light for size and strength?
Selecting the Load
The evidence is now mounting – that we have a range of resistance training options to build muscle. That is to say to provoke hypertrophy we can train with low loads OR high loads. That’s right. The major factor that determines muscle growth isn’t the load, it’s the volume of work performed by the muscle.
The eminent Prof Stu Philips proposed a nice modification to the Repetitions Maximum continuum to reflect the evidence on this topic. And if you want to read the associated systematic review and meta analysis, here it is.
Data within that paper also showed that whilst lighter loads could bring about strength change, the greatest adaptions were seen with heavier loads.
So, at this point we’re thinking we can use a variety of high-volume prescriptions to grow muscle, but to maximise strength changes we need heavier loads.
Heavy or Light for Strength?
Then, along came this paper. Congratulated on social media, it shows that similar changes in muscle size and also strength were achievable with low and high intensity resistance training. Now this is a great study, and adds to overwhelming evidence for hypertrophy achievable with various rep ranges.
Full text pre-print is available here.
However, I’m not so sure about the conclusion of similar strength changes. Let me explain why.
Compromised Study Design
This was a within-participant design, this means that in this study, participants performed both conditions. One leg was assigned the high-load resistance training (3-5 reps) and the other the low-load resistance training (20-25 reps).
Cumming et al. (2025). https://doi.org/10.1101/2025.04.28.650925
Now within-participant designs can be really useful because they provide higher statistical power by removing individual differences, and thus also require fewer participants. i.e. the control or comparator group isn’t a separate population, it’s the same people, with the same natural biological variability.
Cross-Education Effect
But within this context it confounds the data. Why? Due to something called the cross-education effect. I’ve written and researched this extensively (more info here too) and it refers to the strength gain observed in the opposite homologous untrained muscle caused by strength training the other side.
This means, if you strength train (heavy loads, few repetitions) your right quads, you could expect an adaptation in your left, untrained quads, for free! Amazing, right? The amount of transfer from trained to untrained depends on volume, duration and type of training, however, it’s a very well-know phenomenon and in fact very useful in MSK rehab settings, but that’s another post.
In this context, can you see the problem of the within-participant design? One limb is performing 3-5 rep max training – i.e. strength training, and the other limb is performing endurance-type training. There is likely to be a strong cross-education influence.
In fact we even have data to show that strength gains of low load training were augmented by a high-load strength training program on the other side (Bell et al. 2024).
Should I Lift Heavy Or Light For Size And Strength?
So what’s the consensus, is there one? Taking into account the best available evidence, and acknowledging that other variables such as participant training experience will play a role:
- Muscle hypertrophy (increased size) is achievable with light or heavy loads; the volume of work performed is the key variable
- Muscle strength improvements are possible with lighter loads, however, the extent of change is likely less than that achieved by heavier loads
Summary
So, whilst we do have mounting evidence to show that we can generate similar changes in muscle size from prescriptions involving heavy and light loads, we don’t have the evidence to make this conclusion for strength. If you’re looking for decent changes in muscles strength, you – probably – still need to lift heavy.
As always thanks for reading. If you’re looking to understand this better, particularly within a rehabilitation context then check out my super popular online course. Enrolment is opening SOON!
