Many lifters notice that their muscles keep growing while their lifts stall, or that someone visibly smaller can outlift someone more muscular. The question of muscle size vs strength comes down to one central point: muscle size is one determinant of strength, but it is not the only one.
Larger muscles generally have a greater capacity to produce force, and across groups of people, muscle size and strength are positively related. However, the force a person can demonstrate in a specific lift also depends on the nervous system, muscle architecture, skill, leverage, fatigue, and how strength is measured. When these factors change at different rates from muscle size, hypertrophy and strength gains can drift apart.
Is muscle size related to strength at all?
Yes. A larger muscle cross-sectional area generally contains more contractile protein arranged in parallel, which gives the muscle greater potential to produce force. Across individuals, differences in muscle size account for a meaningful share of differences in strength.
The relationship is far from one-to-one, however. In training studies, the amount of muscle a person gains and the amount of strength they gain are often only weakly correlated over the same period.
Researchers continue to debate how much hypertrophy directly causes strength gains. Some have argued that exercise-induced changes in muscle size contribute little to exercise-induced changes in strength (Loenneke et al., 2019), while others maintain that myofibrillar hypertrophy is a contributory cause, particularly over longer time frames (Taber et al., 2019).
The most defensible conclusion is that muscle growth can contribute to strength, but it is not always the dominant driver of strength changes, especially over short periods.
Why do strength and muscle size change at different rates?
Neural adaptations
Strength depends on how effectively the nervous system activates muscle. This includes how many motor units are recruited, how rapidly they fire, and how well the muscles involved in a movement are coordinated.
In novice lifters, strength typically increases before measurable hypertrophy occurs, a pattern attributed largely to neural adaptation (Moritani & deVries, 1979). This is why beginners often gain strength quickly in the first weeks of training even when their muscles look similar.
The opposite pattern can also occur. Muscle can continue to grow during periods when the neural and technical qualities needed to express maximal strength are not being developed to the same extent.
Coordination between muscles
Lifting a heavy load in a compound movement requires the primary movers, assisting muscles, and stabilizers to work together while opposing muscles avoid unnecessary resistance.
Improvements in this coordination can raise the load a person can lift without any change in muscle size. Conversely, gaining muscle does not automatically improve the coordination required for a particular movement.
Strength is partly a skill
Strength is specific to the movement, range of motion, speed, and load used in training. A one-repetition maximum (1RM) in a particular lift is partly a learned skill, not a pure measure of muscle size.
A systematic review and meta-analysis comparing low-load and high-load resistance training found similar hypertrophy between the two approaches, but greater 1RM strength gains with heavier loads (Schoenfeld et al., 2017). In other words, two people can gain a similar amount of muscle while showing different improvements in maximal strength, depending on how they trained.
Strength gains also transfer imperfectly between exercises. Getting stronger in one movement does not guarantee the same improvement in a different movement that uses similar muscles.
Training approach matters
Muscle growth can be produced by many different approaches, including relatively infrequent sessions, as discussed in our article on building muscle once a week. Maximal strength in a specific lift, however, also depends on practicing that movement with loads heavy enough to develop the related skill.
How strength is measured matters
Strength can be tested in different ways, including 1RM testing, repetitions to failure at a given load, isometric tests, and machine-based tests. Changes in muscle size may show up more clearly in some of these measures than in others.
For example, a lifter may perform more repetitions with a moderate load after a period of hypertrophy-focused training while their 1RM changes relatively little.
Not all measured hypertrophy is the same
Early swelling can resemble growth
In the early weeks of a new training program, measured increases in muscle size can partly reflect swelling (edema) associated with muscle damage rather than new contractile tissue (Damas et al., 2016).
Strength would not be expected to increase in proportion to that kind of size change, because fluid accumulation does not add force-producing protein.
The composition of muscle growth is debated
Muscle growth is usually assumed to reflect proportional increases in contractile proteins. Some researchers have suggested that, under certain conditions, fiber enlargement may involve disproportionate expansion of non-contractile components, sometimes described as sarcoplasmic hypertrophy.
The evidence on this question is mixed and depends heavily on measurement methods (Haun et al., 2019). It should therefore be treated as a possible contributor to the gap between size and strength rather than an established explanation.
Where growth occurs matters
Hypertrophy is not evenly distributed throughout the body or even within a single muscle. Growth in muscles or regions that are not limiting a particular lift may add visible size without meaningfully improving performance in that lift.
Muscle architecture, connective tissue, and leverage
Muscle architecture
Many muscles are pennate, meaning their fibers attach to the tendon at an angle. As these muscles grow, the pennation angle can increase. This allows more contractile tissue to be packed into the muscle, but a smaller portion of each fiber’s force is transmitted along the line of the tendon (Aagaard et al., 2001).
The net effect of training is generally still an increase in force capacity, but architectural changes help explain why force does not necessarily rise in exact proportion to size.
Individual differences in force per unit of muscle
The amount of force produced per unit of muscle size, sometimes called specific tension, can vary between individuals and between muscle fiber types. Connective tissue and tendon properties can also influence how efficiently force is transmitted to the skeleton.
Leverage and anatomy
Limb lengths, tendon insertion points, and joint structure affect the mechanical advantage a person has in a given lift. These features are largely fixed, so two equally muscular people can differ substantially in how much they can lift.
Can fatigue hide strength gains?
Yes, it can. High-volume training phases often aimed at hypertrophy can leave a lifter carrying accumulated fatigue. Strength tested under these conditions may underestimate the person’s underlying capacity.
In some cases, strength that has developed during a period of hard training becomes more apparent after training stress is reduced. This is one reason why a temporary plateau in maximal lifts does not necessarily mean that progress has stopped.
Does this also apply to muscle gained with steroids?
The same principles apply regardless of how muscle is gained. Controlled research has shown that supraphysiological testosterone can increase both muscle size and strength, with greater gains when combined with resistance training (Bhasin et al., 1996).
However, strength in a specific lift still depends on neural adaptation, skill, leverage, and training specificity. Our article on building muscle with anabolic steroids explains the mechanisms behind steroid-associated muscle growth, which is separate from the question of how that muscle is used in a particular movement.
Appearance therefore does not reliably indicate strength, whether muscle is gained naturally or not.
What this means for lifters
Hypertrophy and maximal strength are related training outcomes, but they are not identical. Approaches that emphasize higher training volume can be effective for muscle growth, while heavier loads and repeated practice of a specific lift tend to favor improvements in maximal strength in that lift.
Over the long term, additional muscle may increase a person’s potential for strength. Whether that potential is expressed in a specific lift depends on whether the neural, technical, and specific strength qualities for that movement are also trained.
The bottom line
The relationship between muscle size vs strength is real but imperfect. Larger muscles can provide greater force potential, and muscle size explains part of the differences in strength between people. However, strength also depends on neural adaptations, coordination, skill, muscle architecture, leverage, fatigue, and how strength is measured.
This is why hypertrophy does not always translate into proportional strength gains, particularly over short periods. Growing muscle while a lift stalls is not necessarily a sign that training has failed, but it does suggest that strength and size are separate adaptations that respond to training in different ways.
References
Aagaard P, Andersen JL, Dyhre-Poulsen P, et al. A mechanism for increased contractile strength of human pennate muscle in response to strength training: changes in muscle architecture. Journal of Physiology. 2001;534(Pt 2):613–623.
Bhasin S, Storer TW, Berman N, et al. The effects of supraphysiologic doses of testosterone on muscle size and strength in normal men. New England Journal of Medicine. 1996;335(1):1–7.
Damas F, Phillips SM, Libardi CA, et al. Resistance training-induced changes in integrated myofibrillar protein synthesis are related to hypertrophy only after attenuation of muscle damage. Journal of Physiology. 2016;594(18):5209–5222.
Haun CT, Vann CG, Roberts BM, et al. A critical evaluation of the biological construct skeletal muscle hypertrophy: size matters but so does the measurement. Frontiers in Physiology. 2019;10:247.
Loenneke JP, Buckner SL, Dankel SJ, Abe T. Exercise-induced changes in muscle size do not contribute to exercise-induced changes in muscle strength. Sports Medicine. 2019;49(7):987–991.
Moritani T, deVries HA. Neural factors versus hypertrophy in the time course of muscle strength gain. American Journal of Physical Medicine. 1979;58(3):115–130.
Schoenfeld BJ, Grgic J, Ogborn D, Krieger JW. Strength and hypertrophy adaptations between low- vs. high-load resistance training: a systematic review and meta-analysis. Journal of Strength and Conditioning Research. 2017;31(12):3508–3523.
Taber CB, Vigotsky A, Nuckols G, Haun CT. Exercise-induced myofibrillar hypertrophy is a contributory cause of gains in muscle strength. Sports Medicine. 2019;49(7):993–997.
