The biggest influences on strength and\/or size increase are muscle fibre composition \u2013 the ratio of different fibre types within a given muscle \u2013 genetics, and usage in day-to-day life, Harcoff says. Though muscles\u2019 relative starting sizes, hormones, training history, and your current workout routine all play a role, too.<\/p>\n
Below, experts delve deeper into exactly what influences muscle growth. Plus, they offer advice on what you can do if one muscle group seems to be lagging behind the rest.<\/p>\n
Meet the experts: Alena Luciani, MS, CSCS, is a strength and conditioning specialist and the founder of Training2xl<\/a>. Melody Schoenfeld, CSCS, is a certified strength and conditioning specialist and the 2019 NSCA Personal Trainer of the Year, and founder of Flawless Fitness<\/a> in Pasadena, California. Jake Harcoff, CSCS, is a certified strength and conditioning specialist and the head coach and owner of AIM Athletic<\/a>. Mark Kovacs<\/a>, PhD, FACSM, CSCS, is a certified strength and conditioning specialist and human performance scientist. Milica McDowell, MS, DPT, is a doctor of physical therapy and the author of the upcoming book WALK<\/a>.<\/p>\n The factors that influence muscle growth<\/p>\n From training to sleep and genetics, muscle growth depends on far more than just lifting heavier.<\/p>\n Muscle fibres<\/p>\n Much like your fave mock-neck sweater is made up of different types of threads, your muscle tissue is made up of different types of fibres. And that makeup can influence how they respond to training.<\/p>\n The two main types are slow-twitch (known as type I) and fast-twitch (type II). Fast-twitch muscle fibres are built to produce force quickly and on demand, which gives them great potential for muscle hypertrophy (the term for increasing muscle size), says Harcoff. As such, ‘muscles with a higher percentage of fast-twitch fibres, like the chest, shoulders, and arms, tend to grow more quickly,’ he says.<\/p>\n Slow-twitch muscle fibres, on the flip side, are better suited for endurance, says Harcoff. Because they\u2019re designed for efficiency rather than size, they generally don\u2019t grow as dramatically, says Harcoff. With that, muscles with a higher proportion of slow-twitch fibres \u2013 such as the calves, lower back, and muscles along the spine \u2013 often take longer to show visible growth, he says.<\/p>\n Muscle size<\/p>\n ‘Some muscle groups are simply bigger at baseline than others,’ says Milica McDowell, MS, DPT, author of the upcoming book WALK<\/a>. And bigger muscles have more total fibers available to grow, which can lead to faster noticeable changes in size.<\/p>\n The glutes and quads, for example, are among the largest muscles in the body. So, it can appear that those areas are making progress faster compared to smaller muscle groups, such as the biceps or triceps.<\/p>\n Body fat<\/p>\n To be clear: having body fat around muscles doesn\u2019t stunt a muscle\u2019s ability to change. ‘Adjacent fat has nothing to do with muscle growth rate, speed, or capacity,’ says McDowell. What overall fat mass can do, however, is cloud how defined those muscles look.<\/p>\n ‘If you are trying to build muscle in an area where you carry more fat, it will be more difficult to see the results,’ she says. (This is why it\u2019s so difficult to see abdominal definition in most individuals unless they have a very low body fat percentage. Still, you\u2019re building muscle, even if you\u2019re not seeing muscle definition, she notes.<\/p>\n Training reminder: You can\u2019t spot-reduce body fat, says Luciani. If fat distribution is obscuring your progress, focus on the fundamentals\u2014nutrition, sleep, hydration, and stress management\u2014holistically rather than overtraining one area.<\/p>\n Genetics<\/p>\n The goods in your DNA impact your muscle gains \u2013 and in more ways than one. For one thing, ‘the number of fast- and slow-twitch fibers you have is genetically based,’ says Melody Schoenfeld, CSCS, 2019 NSCA Personal Trainer of the Year and founder of Flawless Fitness<\/a> in Pasadena, California. In fact, scientists<\/a> have identified dozens of genes that significantly change the amount of various muscle fibres you have, and where.<\/p>\n Genetics also influence your metabolism, hormone levels, and where your body tends to store fat \u2013 all of which can shape how (and how quickly) muscle definition becomes visible.<\/p>\n Important to remember, though: influenced doesn\u2019t mean predetermined. With the right training stimulus, nutrition, and recovery, you can build strength and muscle across your entire body \u2013 even if some areas take a little longer to catch up.<\/p>\n Hormones<\/p>\n ‘Your hormone levels don\u2019t impact where you build muscle,’ says McDowell, ‘but they do impact how, and how much, muscle you can build.’<\/p>\n Building muscle requires support from hormones like testosterone and growth hormone, and even estrogen. So, if your hormone levels are imbalanced \u2013 meaning, your endocrine system is pumping out too many hormones, and suboptimal levels of others \u2013 muscle growth can slow down, she says.<\/p>\n Research has shown that hormonal conditions like diabetes<\/a>, hypothyroidism<\/a>, and menopause-related hormonal shifts<\/a> can all reduce muscle-building capacity.<\/p>\n Life stage<\/p>\n Similarly, your current life stage can influence how quickly you build muscle \u2013 and how visible that growth is.<\/p>\n ‘Estrogen has protective effects on muscle mass and influences fat distribution,’ says human performance scientist Mark Kovacs<\/a>, PhD, FACSM, CSCS. ‘During menopause, declining estrogen levels can lead to reductions in muscle mass, particularly in the lower body, and an increase in central fat storage,’ he says. Taken together, these shifts impact how quickly muscle is gained and how visible it is.<\/p>\n Schoefeld adds that menopause-related changes can also impact the habits and physiological processes that support muscle growth. ‘The disruptions in sleep [common during menopause] can affect both workout performance and recovery, making it harder to train effectively,’ she says \u2013 thus impacting the rate of muscle growth throughout the body.<\/p>\n Muscle memory<\/p>\n Turns out, some of what feels like \u201cstubborn\u201d muscle growth may have less to do with biology and more to do with your training history.<\/p>\n Cultural messaging has long discouraged women from prioritising upper-body strength, which can quietly shape how your body responds to training later on. ‘That misconception that lifting weights with the upper body will make women look bulky historically led many to focus more on lower-body training \u2013 or avoid upper-body work altogether,’ says Harcoff.<\/p>\n As a result, the lower body often has a longer training history, while the upper body is essentially playing catch-up. ‘That difference in training age and exposure can make it feel like the lower body responds much faster,’ he says, ‘when in reality, the upper body just hasn\u2019t had the same opportunity to adapt over time.’<\/p>\n Muscle memory also plays a role. ‘The more consistently you\u2019ve trained a muscle in the past, the more efficiently it can rebuild and grow,’ explains Harcoff. The catch? ‘Many women simply haven\u2019t had the same long-term exposure to upper-body strength training that builds that foundation,’ he says. Which means those muscles may take longer to respond \u2013 not because they can\u2019t grow, but because they\u2019re newer to the process.<\/p>\n How your training habits can create “lagging” muscles<\/p>\n Don\u2019t hate the messenger: ‘If you notice a muscle group is responding more slowly than others, it may be human error,’ says McDowell.<\/p>\n In fact, as long as the list of biological influences may be, the following training habits are often the biggest factor in slow muscle growth.<\/p>\n 1. Your workout splits are out of whack<\/p>\n Commonly, the issue is your workout splits or schedule. ‘Many people spend more time training the muscle groups they enjoy training, even without noticing it,’ says Luciani. Even small differences, like cutting your arm workouts short or always adding in a lower-body exercise when you have extra time, can add up.<\/p>\n 2. You\u2019re not progressively overloading all of your muscles<\/p>\n Across all muscle groups, the most efficient way to put on muscle is to progressively overload them \u2013 which means you gradually increase load, volume, or intensity over time, says Kovacs. Many people unintentionally apply this approach to some muscles but not others, he says.<\/p>\n For instance, ‘if your glutes are progressing but your arms are not, it may be that you\u2019re applying the principle to your lower-body exercises for efficiency, while not graduating your upper-body in the same way,’ he says.<\/p>\n Sometimes, this comes down to confidence. People may feel more comfortable pushing heavier loads with their lower body \u2013 or know how to safely fail lower-body movements \u2013 while playing it safe with upper-body movements. Other times, it\u2019s an equipment issue: the weights available, especially dumbbells, may not increase in small enough increments to allow for gradual progression.<\/p>\n 3. Your form flubs are adding up over time<\/p>\n ‘Improper form when doing certain moves or exercise selection could all be at play, too,’ says McDowell. If you\u2019re cranking out back squats with hopes of growing your peach, consistently shifting your weight into your toes, your quads will take on more of the load than your glutes, explains Luciani.<\/p>\n Over time, that can lead to faster growth in the anterior chain (front of the body) than the posterior chain (back of the body), she says.<\/p>\n 4. You\u2019re cutting the range of motion<\/p>\n Failing to take a movement through its full range of motion can also hold you back, adds Luciani. ‘If, for example, you avoid breaking parallel every time you squat, you\u2019re missing out on part of the muscle\u2019s full stretch and contraction,’ she says. Similarly, if you end overhead presses with arms only partially extended, you\u2019re robbing your shoulders of the muscle growth of those extra few inches.<\/p>\n That reduced stimulus limits how much the muscle is challenged, she says, which ultimately slows hypertrophy.<\/p>\n What to do if a muscle group is falling behind<\/p>\n Whether one muscle group is falling behind or you want to maximise your time at the gym more generally, doing these things will ensure maximum effectiveness and efficiency:<\/p>\n 1. Refine your technique<\/p>\n Because your form and range of motion affect how much bang you get from each rep, Kovacs suggests meeting with a fitness professional for a movement check. ‘This will help ensure you\u2019re utilising proper form and hitting the full range of motion necessary to maximise muscle activation and growth,’ he says.<\/p>\n 2. Check your intensity<\/p>\n ‘Are you actually challenging the muscle enough to stimulate growth?’ asks Luciani. Muscles need a strong enough stimulus to adapt \u2013 and that only happens when you\u2019re consistently pushing yourself.<\/p>\n If you haven\u2019t increased your weights in months, are coasting through sets, or stopping well short of technical failure, that muscle may not be getting the signal it needs to grow, she says.<\/p>\n Most programs written by a professional will follow the progressive overload principle, which keeps you working hard by design, says Kovacs. But if you\u2019re doing your own thing or following a workout routine without prescribed weights, Luciani suggests aiming for a rate of perceived exertion (RPE) of 7 to 9. This allows you to achieve optimal muscle breakdown and, therefore, growth following recovery without significant risk of injury or burnout.<\/p>\n 3. Use targeted accessory work<\/p>\n If your compound movements (like squats, deadlifts, and presses) aren\u2019t doing the trick, accessory work might. ‘Isolation exercises can help bring up weaker areas,’ says Kovacs. These movements allow you to zero in on specific muscles that may not be getting enough stimulus during bigger, full-body lifts, he says.<\/p>\n For the upper body, this might include bicep curls and tricep extensions, while for the lower body, it might mean split squats or leg extensions. Here, ‘utilising moderate-to-heavy loads in the 6 12 rep range will be highly effective,’ he says.<\/p>\n 4. Hone in your lifestyle habits<\/p>\n Sure, what you do in the gym is important, but for muscle growth, you can\u2019t overlook the importance of a good diet and a sound sleep schedule.<\/p>\n ‘Eating enough calories and protein to support muscle growth is also important,’ says Schoenfeld. ‘Building muscle is a lot more challenging if you\u2019re eating in a significant deficit, or if your protein intake is inadequate.’ That\u2019s roughly 0.7 to 1.0 grams<\/a> of protein per pound of goal body weight, which is optimal for gains for most people.<\/p>\n
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