Muscle Is More Than Mass

Muscle mass alone says surprisingly little about what a muscle can do. Why the nervous system, tendons and power often make the difference – and what that means for your training.

Two people, same height, same weight, same muscle mass according to a body scan. One casually jumps onto a high box, sprints up the stairs and snatches 80 kilos overhead. The other doesn't. How is that possible?

Because a muscle isn't a block of weight, it's an engine. And how well an engine works doesn't just depend on its size – it depends on how it's controlled, how it's connected to the rest of the machine and how fast it can deliver power.

What Muscles Actually Do

Muscles move us – obviously. But they do much more:

• —They stabilise. Before you raise an arm, your trunk muscles are already tensing so you don't lose your balance.
• —They pre-activate. When running or jumping, your calf muscles tense before your foot hits the ground. That way the system is already stiff and ready on impact.
• —They store energy – and give it back. Together with the tendons, the muscle works like a spring.
• —They brake. When running downhill or landing, muscles work eccentrically and protect your joints.

None of these functions can be read from muscle mass.

The Body as a Spring: the Stretch-Shortening Cycle

Jump from a standstill and you jump lower than with a quick counter-movement. The reason is the stretch-shortening cycle, which Finnish biomechanist Paavo Komi researched for decades.

The principle: if a tensed muscle and its tendon are briefly stretched and then immediately shortened, much more force is produced than when shortening from rest. Part of the energy is stored in the elastic structures – mainly the tendons – and released during push-off. On top of that, reflexes activate the muscle further.

When running, the Achilles tendon takes over a considerable share of the work the calf muscle would otherwise have to do alone. A well-trained tendon and a muscle that is stiff at the right moment are at least as important as a big calf.

Strength Starts in the Head

People who start strength training often get much stronger in the first weeks – without visible muscle growth. Toshio Moritani and Herbert de Vries showed this back in 1979: early strength gains are mostly due to neural adaptations.

The nervous system learns to

• —activate more motor units at the same time,
• —fire them faster,
• —relax opposing muscles better,
• —and coordinate the muscles involved with precise timing.

So strength is also a skill. That's why you get strongest in exactly the exercise you train – and why technique work with moderate loads is so valuable.

How Are Muscle Mass and Strength Connected?

This deserves a nuanced look. That bigger muscles can generally produce more force is undisputed. How much training-induced muscle growth explains strength gains, however, is debated. In 2019, researchers around Jeremy Loenneke provocatively asked whether muscle growth is a meaningful mechanism for strength gains at all – other groups push back and do find a link.

The honest summary: muscle mass is the potential. What you make of it is decided by your nervous system, technique and tendons. Training both makes sense.

With Age, Function Matters Even More

This becomes especially clear as we get older. The large US Health ABC Study (Goodpaster et al., 2006) followed more than 1,800 older adults. Result: strength declined about three times faster than muscle mass. And even those who maintained or gained muscle mass weren't protected from that.

Even more important than maximal strength in older age is power – the ability to produce force quickly. Kathryn Reid and Roger Fielding (2012) describe it as a decisive factor in whether someone can safely climb stairs, get up from a chair or catch a fall. Power is lost faster with age than maximal strength – and it can be trained specifically.

More Muscle Isn't Automatically Better

From a sports perspective there's another point: extra muscle mass is also extra weight. For a runner, every gram on the legs is work; for a climber, every kilo on the upper body. And in sports with weight classes, strength per kilogram of body weight is what counts anyway.

You can see this in Olympic weightlifting. The goal there is to get a barbell overhead explosively in a fraction of a second. Isolation exercises for chest or biceps barely play a role in many lifters' training. Not because those muscles don't matter – but because extra mass in the wrong place doesn't improve performance and may cost mobility or a weight class. The focus is on whatever makes the movement faster and more efficient.

What Does This Mean for Your Training?

You don't have to give up building muscle – quite the opposite. But add to it:

• —1. Compound lifts with a technique focus: Squat, deadlift, pushing and pulling with clean execution and reps in reserve. This trains the control.
• —2. Explosive reps: Even with moderate weight, move every rep up with maximal intent.
• —3. Jumps and throws: Box jumps, medicine ball throws, kettlebell swings – they train the stretch-shortening cycle and power.
• —4. Load your tendons: Tendons adapt slowly. Regular, heavy and controlled loading makes them more resilient.
• —5. Single-leg and unstable exercises: Lunges, split squats, carries – they challenge stabilisation and coordination.

Key Takeaways

• —Muscle mass is potential, not a guarantee of performance.
• —Muscles stabilise, pre-activate, store energy and brake – no body scan shows that.
• —Strength is largely a product of the nervous system and therefore a trainable skill.
• —With age, strength drops faster than mass – and power decides everyday ability.
• —The point isn't to own as much muscle mass as possible, but to make the right muscles capable and use them functionally.

Sources (selection)

• —Komi P. V. (2000): Stretch-shortening cycle: a powerful model to study normal and fatigued muscle. Journal of Biomechanics, 33(10).
• —Moritani T., de Vries H. A. (1979): Neural factors versus hypertrophy in the time course of muscle strength gain. American Journal of Physical Medicine, 58(3).
• —Loenneke J. P. et al. (2019): Is muscle growth a mechanism for increasing strength? Medical Hypotheses, 125.
• —Goodpaster B. H. et al. (2006): The loss of skeletal muscle strength, mass, and quality in older adults: the Health, Aging and Body Composition Study. Journals of Gerontology: Series A, 61(10).
• —Reid K. F., Fielding R. A. (2012): Skeletal muscle power: a critical determinant of physical functioning in older adults. Exercise and Sport Sciences Reviews, 40(1).
• —Suchomel T. J. et al. (2016): The importance of muscular strength in athletic performance. Sports Medicine, 46(10).