From Sunlight to Strength

Where does the energy to lift this glass come from? We could start with your food, but let's go a little further back: all the way to the Sun.

From light to your food

Light arrives in packets of energy called photons. When one reaches a leaf, chlorophyll absorbs some of that light and sets a series of transformations in motion. Using that energy, the plant makes sugars, such as glucose, from water and carbon dioxide. That process is photosynthesis.

There is an important distinction here: light supplies energy; water and the gas supply matter. The photon does not turn into a sugar molecule.

Vaclav Smil describes it with a striking image: the energy of roughly one in every 500 photons reaching Earth's ice-free surface ends up stored in new plant matter. That is less than 0.2% of the incoming solar energy, a global average, not the efficiency of each leaf. Yet that capture supports an enormous web of life.

The plant uses those sugars to live and grow, and joins glucose units into cellulose. The cow arrives and eats it: microbes in its digestive system help break the cellulose down. The cow uses the nutrients to stay alive, move, build tissues and produce milk, and some of the energy ends up as heat. The cow has expenses, too.

That is how we get energy, by eating plants or foods from animals that ate plants. One unchanged glucose molecule doesn't travel all the way into your arm: materials are transformed and energy is transferred.

ATP: the intermediary

Carbohydrates and fats are two important energy sources for our muscles. To put that energy to use, cells rely on an intermediary: ATP, adenosine triphosphate. In cellular work, ATP can become ADP and phosphate, and making it again takes a fresh input of energy from nutrients and stored reserves. The molecules are recycled, but each round has a cost. Some of the energy supports work and some ends up as heat.

From intention to movement

You decide to lift the glass and your nervous system organizes the action. Motor neurons activate groups of muscle fibers; each neuron and its fibers form a motor unit, like a team receiving the same signal.

Inside each fiber, two proteins produce force. Imagine a rail and little hands that can grip it: actin is the rail and myosin is the set of hands. They attach, pull, release and get ready again, in huge numbers of interactions at once.

The nerve signal releases calcium from an internal store. Calcium opens a kind of barrier that covered the grip sites. And ATP keeps the cycle going: it allows myosin to let go and get ready to pull again. It isn't a spark that produces one tug, but part of the whole cycle.

Getting energy on time

To lift something heavy, having plenty of stored energy isn't enough: it must become available at the rate the effort demands. Muscle stores only a little ATP and regenerates it as it goes, through three cooperating pathways:

• Phosphocreatine: an immediate but limited reserve.
• Glycolysis: produces ATP quickly and doesn't directly require oxygen.
• The oxidative system: depends on oxygen, uses fuels from carbohydrates and fats, and takes place largely in the mitochondria.

These are not three separate shifts. They work together, and each one's contribution changes with the task.

Holding the glass still in the air also costs energy, even though it isn't moving. When you lower it slowly, muscles produce force while lengthening. So muscle contraction means activation and force production, not always shortening. Even relaxing takes work: the pumps that return calcium to its store use ATP.

Strength is also a skill

An empty glass and a full one don't require the same force. To adjust, the nervous system can bring more motor units into the task, which is called recruitment, and change the frequency of its signals. Some muscles move, others stabilize and others brake, while vision, touch and proprioception report back. It isn't a command you send and forget: it's a continuous adjustment.

If part of the result depends on how you organize all this, you can improve before muscle growth becomes obvious. This connects with an idea from Pavel Tsatsouline's Power to the People!: strength is also a skill you practice. Pavel calls one approach grease the groove: practice frequently, in small doses, stopping short of exhaustion so you can repeat with quality. The name is a metaphor; nobody is lubricating nerves.

This doesn't mean a secret reserve of strength is waiting to be unlocked by wanting it badly enough. It means the nervous system also adapts to training.

That's why I care so much about teaching you, at the gym, to know how you're producing force: set your position, start applying tension, maintain the path and control the finish. I want every repetition to be a chance to practice that.

Does slower mean more force?

Not necessarily. To make a weight gain upward speed, you need more force than gravity exerts on it, so a sharp initial pull can require a large force peak. After that, the weight already has velocity and can keep rising for a while as you reduce force: that's what we take advantage of when we use momentum.

If you raise it continuously, without that initial launch, you have to keep supplying force through the whole path. The difference lies in how you distribute force through the repetition: a peak at the start followed by less, or a more sustained effort.

One clarification: lifting the same weight to the same height, starting and ending at rest, does the same mechanical work against gravity. Taking longer changes the time and the average power, not that work. Your body's internal energy cost is a different calculation.

Slowness can be a tool for learning to produce, sustain and regulate force. And not every fast movement is careless: you can train an explosive action with intention and technique. The useful question is what ability you want to practice, and what a good repetition should look like.

Think of lifting a load and playing the piano. Both need muscular control, but learning one doesn't automatically teach you the other. At the piano, squeezing everything harder can get in the way. What matters is organizing useful force for the task.

When the muscle itself changes

Training can also change the muscle itself. Tension against resistance can trigger adaptation signals and, with an appropriate stimulus over time, fibers can become larger. That's hypertrophy, and our rails and little hands are among the things that can increase.

Building that machinery requires parts. Just as the plant needed energy and matter, food gives us both. The proteins we eat are digested into amino acids, nine of which must come from food. Eating protein doesn't insert ready-made muscle into another muscle, and having parts available doesn't by itself guarantee growth.

Activity supplies the stimulus, food supplies materials and energy, and recovery helps sustain adaptation. The body is continually making and breaking down proteins; to accumulate muscle protein, more has to be built than is lost over a period. That's why hypertrophy isn't the same as feeling pumped after a workout. And resting doesn't mean everything switches off: the body keeps renewing tissues and replenishing resources.

Look at the glass again

At first it seemed like one simple thing: I want to move it, so I move it. Now we can see what was happening in between. Next time you lift it, it may look like that's all you did, but you'll have coordinated signals, transformed energy and put to work a structure that can learn and change with you.

Sources

• Vaclav Smil, Energy: A Beginner's Guide.
• G. Gregory Haff and N. Travis Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., NSCA.
• Robert R. Wolfe, A Guide to Amino Acid and Protein Nutrition.
• Pavel Tsatsouline, Power to the People!