Start a new strength programme and the first few weeks feel like magic. The weights go up every session, but in the mirror nothing has changed. That isn’t your imagination. Early strength gains come mostly from your nervous system learning to use the muscle you already have.
This article explains what changes, how long it takes for muscle to actually grow, and what it means for training and rehab.
- Early gains are mostly neural. Muscle growth takes over later.
- Motor units fire faster after just 4 weeks of strength training.
- Strength is specific. You get best at the movements you train.
- Training one side strengthens the other by about 12% on average.
- Real muscle growth may take around 18 sessions to show clearly.
The first few weeks are mostly your nervous system
In a classic but small 1979 study, researchers tracked strength and muscle activity while 15 people trained for 8 weeks. They estimated that neural factors accounted for most of the early strength gain, with muscle growth becoming the dominant factor after the first 3 to 5 weeks [1].
That basic picture has held up. A 2021 review concluded that the early increases in force with resistance training are thought to be driven mainly by neural adaptations, although exactly where in the nervous system those changes happen is still not fully understood [2].
What actually changes
A muscle is made up of motor units: a nerve cell in the spinal cord and the muscle fibres it controls. To produce more force, your nervous system can switch on more motor units, or make each one fire faster.
In a 2019 study, 14 people trained their shin muscles with isometric contractions for 4 weeks while a matched group did not train. The trained group’s motor units fired faster, by 3.3 pulses per second on average, and switched on earlier. The authors concluded that the strength gain came from greater output from the spinal cord to the muscle [3]. Put simply, the muscle got a stronger signal.
Strength is specific to what you train
Because so much early strength is learned, it is tied to the movement you practise. A meta-analysis of 12 studies found that the effect of training on strength in the trained movement was more than twice as large as in movements that were not directly trained [4].
One study shows how specific this can be. After 4 weeks of squat training, people improved their one-rep max squat by 35% and an isometric squat test by 49%. Their strength on a knee extension machine, which works the same thigh muscles in a different movement, rose by just 1%. Muscle thickness didn’t change at all [5].
If you want to get better at a lift, a sport or a daily task, you need to practise that movement, not just train the muscles involved.
When muscle actually grows
Muscle size changes can be measured surprisingly early. In one study, thigh muscle size had increased by 3.5% to 5.2% after 20 days of intense training [6]. But some of that early increase is swelling caused by the training itself [7]. A later review proposed that true muscle growth shows up only after around 18 training sessions [8], which would put the switch from mostly neural to mostly muscular gains later than the 1979 study suggested.
So if you are six weeks in and your strength has jumped but your clothes fit the same, you are right on schedule.
Training one side strengthens the other
One of the stranger effects of neural adaptation is cross-education. Train one arm or leg, and the untrained one gets stronger too. A meta-analysis of 31 studies with 785 people found an average strength gain of 11.9% in the untrained limb, though the studies carried a high risk of bias [9].
This has a practical use in rehab. In a study of 30 people, 20 had one wrist and hand put in a cast for 3 weeks. In those who did nothing, the casted arm lost strength. In those who trained the free arm, the casted arm kept its strength [10].
After ACL reconstruction, the results are mixed. One trial of 48 people found that training the uninjured leg improved strength recovery in the operated leg, but not hop tests or knee scores [11]. Another trial of 43 people found no benefit [12]. It is a reasonable extra while the injured side is protected, but it doesn’t replace direct rehab of the injured leg.
Does the mind-muscle connection matter?
In one 8-week trial, 30 untrained men either focused on squeezing the muscle (an internal focus) or on moving the weight (an external focus). The internal focus group grew their biceps more, 12.4% against 6.9%, but thigh growth and strength gains were similar in both groups [13]. It is a single small study, so treat it as a minor detail.
What about activation drills before training?
Many gyms push activation drills or a heavy set before the main work to switch the muscles on. A heavy warm-up set can produce a short-lived performance boost. A review of the research found it is unclear whether this adds much once you have done a thorough, active warm-up [14]. A good general warm-up that builds up to your working weights covers it.
What this means for your training
- Expect fast progress early, and don’t mistake it for muscle growth.
- Practise the exact movements you want to get better at.
- Stick with a programme well past the first month if you want muscle, not just strength.
- After an injury, keep training the healthy side while the injured side recovers.
After an injury, the nervous system often holds back the muscles around the injured area, which is a separate problem with its own fixes. We cover it in why muscles switch off after injury.
At Aspire Physio Bangkok, rehab moves from the treatment room to the gym floor in the same building. See sports injury rehab and post-surgery rehab, or book a free first consultation through our contact page.
Common questions
How quickly do you get stronger when you start lifting?
Quickly. Most people gain noticeable strength in the first few weeks, and research suggests that early gain comes mainly from the nervous system learning to use the muscle better. Muscle growth takes over as the main driver later, though studies disagree on exactly when.
When will my muscles start to grow?
Changes in muscle size can be measured from about 3 weeks, but some of that early increase is swelling from the training itself. One review proposed that true muscle growth shows up after around 18 sessions.
Does training one arm or leg make the other one stronger?
Yes, a little. Across 31 studies, training one limb increased strength in the untrained limb by about 12% on average. It can help limit strength loss when one limb is in a cast or sling, though trials after ACL surgery have given mixed results.
Do I need activation exercises before lifting?
A proper warm-up is enough for most people. A review of the short-term performance boost from a heavy warm-up set found it is unclear whether it adds much once you have already done a thorough active warm-up.
References
- Moritani T, deVries HA. Neural factors versus hypertrophy in the time course of muscle strength gain. Am J Phys Med. 1979;58(3):115-130. PMID 453338. pubmed.ncbi.nlm.nih.gov/453338/
- Škarabot J, Brownstein CG, Casolo A, Del Vecchio A, Ansdell P. The knowns and unknowns of neural adaptations to resistance training. Eur J Appl Physiol. 2021;121(3):675-685. doi.org/10.1007/s00421-020-04567-3
- Del Vecchio A, Casolo A, Negro F, et al. The increase in muscle force after 4 weeks of strength training is mediated by adaptations in motor unit recruitment and rate coding. J Physiol. 2019;597(7):1873-1887. doi.org/10.1113/JP277250
- Spitz RW, Kataoka R, Dankel SJ, et al. Quantifying the generality of strength adaptation: a meta-analysis. Sports Med. 2023;53(3):637-648. doi.org/10.1007/s40279-022-01790-0
- Ansdell P, Brownstein CG, Škarabot J, et al. Task-specific strength increases after lower-limb compound resistance training occurred in the absence of corticospinal changes in vastus lateralis. Exp Physiol. 2020;105(7):1132-1150. doi.org/10.1113/EP088629
- Seynnes OR, de Boer M, Narici MV. Early skeletal muscle hypertrophy and architectural changes in response to high-intensity resistance training. J Appl Physiol. 2007;102(1):368-373. doi.org/10.1152/japplphysiol.00789.2006
- Damas F, Phillips SM, Lixandrão ME, et al. Early resistance training-induced increases in muscle cross-sectional area are concomitant with edema-induced muscle swelling. Eur J Appl Physiol. 2016;116(1):49-56. doi.org/10.1007/s00421-015-3243-4
- Damas F, Libardi CA, Ugrinowitsch C. The development of skeletal muscle hypertrophy through resistance training: the role of muscle damage and muscle protein synthesis. Eur J Appl Physiol. 2018;118(3):485-500. doi.org/10.1007/s00421-017-3792-9
- Manca A, Dragone D, Dvir Z, Deriu F. Cross-education of muscular strength following unilateral resistance training: a meta-analysis. Eur J Appl Physiol. 2017;117(11):2335-2354. doi.org/10.1007/s00421-017-3720-z
- Farthing JP, Krentz JR, Magnus CRA. Strength training the free limb attenuates strength loss during unilateral immobilization. J Appl Physiol. 2009;106(3):830-836. doi.org/10.1152/japplphysiol.91331.2008
- Harput G, Ulusoy B, Yildiz TI, et al. Cross-education improves quadriceps strength recovery after ACL reconstruction: a randomized controlled trial. Knee Surg Sports Traumatol Arthrosc. 2019;27(1):68-75. doi.org/10.1007/s00167-018-5040-1
- Zult T, Gokeler A, van Raay JJAM, et al. Cross-education does not improve early and late-phase rehabilitation outcomes after ACL reconstruction: a randomized controlled clinical trial. Knee Surg Sports Traumatol Arthrosc. 2019;27(2):478-490. doi.org/10.1007/s00167-018-5116-y
- Schoenfeld BJ, Vigotsky A, Contreras B, et al. Differential effects of attentional focus strategies during long-term resistance training. Eur J Sport Sci. 2018;18(5):705-712. doi.org/10.1080/17461391.2018.1447020
- Blazevich AJ, Babault N. Post-activation potentiation versus post-activation performance enhancement in humans: historical perspective, underlying mechanisms, and current issues. Front Physiol. 2019;10:1359. doi.org/10.3389/fphys.2019.01359
