How do you learn to produce force?

01 / The force pathway

From intention to force.

Pause motion

Select a stage. The moving line follows the instruction; it does not measure nerve speed or force.

Structure / relationship Illustrative process

01 · Brain 02 · Spinal cord 03 · Motor neuron 04 · Motor unit 05 · Muscle 06 · Force

Brain

Movement begins with a goal and a coordinated plan across brain networks. The drawing is a simplified route, not a map of every area involved.

Spinal cord

Descending commands interact with spinal circuits and sensory feedback. The spinal cord is more than a passive cable.

Motor neuron

A motor neuron carries an electrical signal to the muscle fibres it controls. At the neuromuscular junction, chemical signalling activates the fibre.

Motor unit

One motor neuron and all its muscle fibres form a motor unit. Recruitment and discharge rate help adjust the force produced.

Muscle

Calcium enables the contractile proteins actin and myosin to interact. ATP supports the repeating cross-bridge cycle.

Force

The force expressed in a task also depends on joint position, leverage, coordination and skill. Muscle size is one contributor.

Begin with the ability.

Strength is often discussed through exercises, weights and repetitions. A better starting point is the ability itself: producing force against resistance.

That ability can matter when you lift a suitcase, carry a child, climb stairs, control a landing or maintain independence later in life. The relevant expression of strength changes with the task.

ESSENTIAL TERMS

Strength and hypertrophy.

Hypertrophy means an increase in muscle size. A larger muscle can offer greater potential for force, but size does not determine strength on its own.

The nervous system decides which motor units are recruited, how strongly they are activated and how the muscles work together. Technique and familiarity with a movement also matter. That is why strength can improve before a visible change in muscle size, and why performance does not transfer perfectly from one exercise to every other task.

The nervous system learns too.

Muscle fibres act when they receive signals from motor neurons. A motor neuron and the fibres it controls form a motor unit. Strength training can improve how motor units are recruited and coordinated. These changes are often called neural adaptations.

In a six-week randomised study of 26 men who had not recently trained their legs, 80% and 30% 1RM leg-extension training produced similar increases in muscle thickness. The heavier-load group gained more maximal strength and showed greater changes in voluntary activation. The study does not create a universal rule, but it shows why muscle size and strength should not be treated as the same outcome.

Maximum strength and power.

A one-repetition maximum, or 1RM, is the heaviest load that can be completed once in a defined exercise. It is one way to measure maximal strength. It does not describe every useful expression of force.

Power describes how quickly work is done; in movement, it reflects the interaction between force and velocity. Rising from a chair and reacting quickly after a trip can both depend on force, but the second task leaves much less time to produce it.

How training progresses.

Progressive overload means adjusting the training demand as capacity changes. More weight is one option. More repetitions, an additional set, a larger range of motion, greater movement control or a more demanding variation can also change the stimulus.

Progress should serve the goal. Adding difficulty without a reason may increase fatigue without improving the capacity you care about.

Load.

Heavier loads are generally more specific to maximal strength. Hypertrophy can occur across a broader range of loads when the training provides sufficient effort and volume. This helps explain why two programmes can both be useful while emphasising different outcomes.

Volume.

Volume describes how much work is performed. It may be counted through sets, repetitions or load lifted. More volume can support muscle growth up to a point, but it also creates more fatigue and takes more time. The useful dose is one that can be recovered from and repeated.

Repetitions in reserve.

RIR estimates how many technically acceptable repetitions you could have completed when a set stops. Finishing with two repetitions in reserve means you believe two more were possible. It is a practical way to discuss effort, though the estimate becomes more accurate with experience.

Every set does not need to end in failure. Research does not show that momentary failure is consistently required for strength or hypertrophy. The set still needs to be sufficiently challenging for the intended adaptation.

ILLUSTRATION / WHAT “2 RIR” MEANS

Stop after repetition 10 when about 12 were possible.

1 2 3 4 5 6 7 8 9 10 11 12

Solid: completed repetitions   ·   Outlined: estimated reserve. RIR is a practical judgement, not an exact laboratory measurement.

WHAT THE 2026 ACSM REVIEW FOUND

137 systematic reviews.
More than 30,000 participants.

The review concluded that progressive resistance training improves strength, muscle size, power, endurance and several measures of physical function in healthy adults.

The number describes the evidence base reviewed; it does not prescribe one programme for everyone.

Why strength matters across life.

Muscle strength can decline faster than muscle mass with age. In an observational study of 1,880 older adults, leg strength fell roughly three times as quickly as leg lean mass over three years. Even participants who maintained or gained lean mass were not necessarily protected from strength loss.

This is one reason to think about neuromuscular function: the practical relationship between nervous system and muscle. Preserving tissue matters, but so does preserving the ability to use it.

Strength training cannot stop ageing or remove every limitation. It can, however, train capacities that are relevant to carrying, rising, climbing, balancing and producing force.

FROM GOAL TO EMPHASIS

A programme has to fit a person.

The best plan on paper is of little use if it cannot be performed consistently. Exercise selection, load, frequency and progression should reflect the person’s goal, experience, access, preferences and current health.

A beginner may benefit from a small number of repeatable movements. An experienced lifter pursuing maximal strength may need much more specific loading. The logic is the same: decide what should improve, choose a demand that can create that adaptation, then review the response.

From the archive / Starting points and responses

Different starting points do not determine the same training response.

“Beginner gains” combine learning the exercise, improved coordination, changes in neural drive and, with repeated exposure, growth of muscle tissue. The balance changes over time. An early increase in the weight lifted is not a direct measurement of new muscle, and a slower later phase does not mean a programme has failed.

This connects the Jenkins study above with Muscle. Archive: beginner gains.

Sex differences: specify the comparison.

Average differences in muscle mass, body dimensions, haemoglobin and hormonal development help explain sex differences in absolute sporting performance. A comparison of elite performances, however, is a different question from how much a particular person can improve with training. Sex and gender also describe different concepts.

Absolute capacity

How much force or power can the person produce now?

Relative change

How much did performance improve compared with the person's own starting point?

The actual task

Which muscles, contraction speed, duration and relative intensity were tested?

A 2020 meta-analysis of matched resistance-training protocols found similar relative hypertrophy and lower-body strength responses in young to middle-aged men and women, with a larger relative upper-body strength response in women. This does not imply identical absolute strength or erase variation within either group. It does undermine the idea that one group has little to gain from resistance training.

Women are often less fatigable in sustained or repeated isometric tasks at comparable relative intensity. The size and even presence of the difference depend on the muscle and task. “Women recover faster from everything” is not a supported programming rule. Set the load from the person's capacity, then adjust from observed performance and recovery.

Hunter et al. (2023), ACSM consensus — biological and contextual determinants; Senefeld & Hunter (2024), Endocrinology — hormonal perspective; Roberts et al. (2020), training-response meta-analysis; Hunter (2014), task-specific fatigability review. Archive: sex differences.

Does a sled automatically make training safer or more functional?

A sled is a way to apply resistance. Its value depends on load, surface, direction, speed and the task being trained. Resisted sprinting, heavy pushing and gentle backward dragging are different interventions; a study of one cannot validate all the others.

A 2024 review of 15 studies found no clear acceleration advantage over volume-matched unresisted sprint training. A 2025 review of 14 studies and 344 team-sport athletes reported benefits over its comparisons for 5- and 10-metre performance, but the overall risk of bias was high. Differences in comparisons and methods help explain the different conclusions.

Neither review establishes that sled work prevents injury, corrects every strength imbalance or treats foot and ankle disorders. “Low impact” does not mean low tissue load. Choose an exercise for a defined goal and tolerated demand; rehabilitation requires an appropriate assessment.

Aldrich et al. (2024); Salazar-Orellana et al. (2025). Archive: sled training. Apply task specificity with the Strength Planner or discuss a starting point through Private Health.

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Sources & references.

1. Currier BS et al. Resistance training prescription for muscle function, hypertrophy and physical performance. Medicine & Science in Sports & Exercise. 2026. doi:10.1249/MSS.0000000000003897
2. Jenkins NDM et al. Greater neural adaptations following high- vs low-load resistance training. Frontiers in Physiology. 2017. doi:10.3389/fphys.2017.00331
3. Goodpaster BH et al. Loss of skeletal muscle strength, mass and quality in older adults. J Gerontology A. 2006. doi:10.1093/gerona/61.10.1059
4. Plotkin D et al. Progressive overload without progressing load. PeerJ. 2022. doi:10.7717/peerj.14142
5. Refalo MC et al. Resistance training proximity-to-failure and hypertrophy. Sports Medicine. 2023. doi:10.1007/s40279-022-01784-y
6. Jinha A, Herzog W. Muscle power: A simple concept causing much confusion. Journal of Sport and Health Science. 2025;14:101005. PubMed.

Educational information, not individual treatment advice. Zacharias Razvi is a final-semester physiotherapy student and is not yet an authorised physiotherapist.