Hypertrophy vs Strength in Rehab: Why Light Loads Still Build Muscle

A sore knee that cannot take a heavy squat still has a quadriceps muscle that can be rebuilt. The load needed for muscle growth is far lower than most people assume.

Here is what the evidence separates, what the condition attached to light loading actually means, and how the two goals get sequenced in rehab.

The repetition continuum, and where it holds

The traditional model most gyms and textbooks use assigns each rep range a purpose. Strength sits at 1 to 5 repetitions with loads of 80 to 100 percent of one-repetition maximum. Hypertrophy, meaning muscle growth, sits at 8 to 12 repetitions with loads of 60 to 80 percent. Local endurance sits at 15 or more repetitions with loads below 60 percent.

Schoenfeld, Grgic, Van Every and Plotkin re-examined that model in a 2021 review in Sports. Their conclusion was that one part of the continuum holds up reasonably well and another part does not.

The part that holds is strength. Their reported meta-analysis showed a moderate to large effect favouring high loads for one-repetition maximum strength, with an effect size of 0.58. If the goal is to lift the heaviest possible single load, training with heavy loads is better.

There is a detail inside that finding worth carrying. When strength was tested isometrically, meaning a maximal push against something that does not move, the advantage for heavy training dropped to an effect size of 0.16, which was small and non-significant. Some of what heavy training builds is skill at the specific heavy lift being tested, alongside the underlying strength.

Where load stops mattering

The part of the continuum that did not hold is hypertrophy.

The review reports no significant difference in whole-muscle growth between high loads, above 60 percent of one-repetition maximum, and low loads, below 60 percent. The effect size was 0.03, which is effectively zero. That finding held independent of body region, age, and training experience.

The specific numbers make the point clearer. Loads around 30 percent of one-repetition maximum produced comparable muscle growth to heavy loads. Studies comparing roughly 10-repetition-maximum loads against roughly 30-repetition-maximum loads found similar increases in muscle thickness in the biceps, triceps, and quadriceps.

Thirty percent of maximum is light. For someone whose best leg press is 100 kilograms, that is 30 kilograms. For a knee, a shoulder, or a tendon that flares under heavy load, that difference is the difference between training and not training.

The condition attached to it

The light-load finding comes with one requirement that decides whether it works.

The sets have to be taken to failure, meaning to the point where another full repetition is not possible. The review is explicit about why this matters: studies that reported worse results from low-load training generally used work-matched protocols, where participants performed a set number of repetitions and stopped well short of fatigue. In those designs the light-load group simply did less hard work, and grew less.

When the light-load sets ran to failure, the growth matched. A set at 30 percent of maximum might take 25 or 30 repetitions to reach that point. It burns, and the last several repetitions are unpleasant.

The review names that cost directly. Light-load training produces greater discomfort, greater displeasure, and higher ratings of perceived exertion. That is an adherence problem, and adherence decides outcomes in rehab more than programme design does, which is why we cover it separately in exercise adherence and rehab outcomes.

Heavy loading has its own cost. The review reports that heavy-load training requires more sets to reach comparable muscle growth, which makes it time-inefficient. Both ends of the range have a practical problem, and the review concludes that moderate loads are the most efficient compromise for most people.

What this changes in rehab

The useful part of this evidence for rehab is that pain often sets the ceiling on load, and that ceiling used to be read as a ceiling on progress.

A knee that flares at 80 percent of maximum can still be trained at 30 percent. An irritated tendon that will not tolerate a heavy squat may tolerate a lighter one for higher repetitions. The muscle around that joint can be rebuilt while the joint itself is still settling, which matters because muscle loss during a period of avoidance is one of the things that makes the return harder later.

There are limits to how far that reasoning goes. Maximal strength still favours heavy loads, so a light-load phase is a stage in a plan rather than the whole plan. For an athlete returning to a sport that demands high force, or a person whose goal is to lift heavy again, heavy loading has to come back at some point. Our guide to returning to lifting after back pain covers what that reintroduction looks like.

Tendon problems are a separate case again. Some tendon rehabilitation protocols are built specifically around slow, heavy loading, because the tendon responds to the magnitude of the load rather than only to the effort of the set. Light loads to failure are a good answer for muscle size. They are not automatically the right answer for every tissue.

There is also a way to make light loads harder without adding weight. Blood flow restriction training uses a cuff to limit blood flow during light-load exercise, and it is used in rehab for exactly this situation, where the joint cannot take heavy load but the muscle needs a stimulus.

Older adults and bone

The review reports that in older adults, light-load training appeared at least as effective as heavier loads for muscle growth. That is a helpful finding, because heavy loading is often harder to introduce safely in an older population.

Muscle size is one goal among several in that group. Bone responds to load differently than muscle does, and a programme built only around light loads to failure may not provide the stimulus bone needs. Safe strength training with osteoporosis covers how those goals get balanced when bone density is part of the picture.

The general principle applies here as much as anywhere. The load is chosen for the goal and for what the person in front of you can tolerate, and there is more than one goal in most rehabilitation plans.

What the evidence does not settle

The review names its own limitations, and they are worth stating because this is an area where confident advice outruns the data.

There is a paucity of studies in women, so most of the underlying research was done in men. Research in highly trained athletes is limited, so how well these findings transfer to that group is unclear. Findings on whether different muscle fibre types respond differently to different loads were inconclusive.

Individual response also varies, and averages from a meta-analysis describe a group rather than a person. Two people can follow the same programme and respond differently, which is the argument for measuring progress rather than assuming it. This is the same reason we push back on several common gym beliefs in strength training during rehab: what is myth and what is not.

When to get it looked at

Pain that increases through a set rather than settling, pain that stays high for more than 24 hours after training, or a joint that swells after loading is worth having assessed before you keep adjusting the weight on your own. So is a muscle that visibly wastes despite continued training, or strength on one side that is not catching up to the other.

Some situations need prompt medical attention rather than a programme change: sudden loss of strength after a specific incident, a visible deformity or a gap in a muscle, or joint pain with fever or redness. Those are calls to your physician, and a sudden severe injury during lifting warrants an assessment at Lions Gate Hospital.

If a painful joint has stopped you loading and you want a programme built around what it can tolerate, book a 30-minute assessment. You can see what we treat or reach the clinic here.

This article is general information, not personal medical advice. A regulated practitioner can confirm whether the patterns described apply to you.

Sources

• Loading Recommendations for Muscle Strength, Hypertrophy, and Local Endurance: A Re-Examination of the Repetition Continuum (Sports, 2021)
• College of Health and Care Professionals of BC public registry

This article is for general information only and does not constitute medical advice, diagnosis, or treatment. Individual presentations vary, and assessment findings and treatment plans differ from person to person. If you are experiencing severe symptoms, neurological changes (numbness, weakness, bowel or bladder changes), or a significant trauma, contact your physician or emergency services. Care at Medstar Sport Physio & Health is provided by practitioners registered with their respective British Columbia regulatory colleges.