Visible changes in muscle size during the first weeks of gym training do not necessarily represent lasting muscle growth. Exercise can cause temporary swelling, and studies suggest that inflammation linked to muscle damage can account for some early increases in measured size.
A 2016 study of 10 young people who had not trained before found that their quadriceps area was 2.7% larger after three weeks and 10.4% larger after 10 weeks. The researchers noted that the early increase included edema, so the measurements could not be treated as pure muscle hypertrophy without accounting for inflammation.
A separate study published in 2011 followed 25 sedentary men and found that structural muscle growth began to become established around weeks three or four. Participants’ muscle size was up 9.6% after eight weeks, although the study also linked much of that apparent growth to repair following exercise-induced muscle damage.
A recently published meta-analysis, which is still under review, examined 2,237 observations from 480 studies. It included studies that measured muscle growth using MRI or ultrasound, rather than more subjective bioimpedance scales. Its results indicate that average muscle growth continues over time but slows: 8.9% at 20 weeks, 9.4% at 24 weeks, 10.5% at 36 weeks and 12.6% after a year and a half.
Those averages do not predict what will happen to any one person. In a 2005 study, 585 people completed 12 weeks of training, with muscle size changes ranging from a 2% decrease to a 59% increase. The findings underscore that people can respond differently to the same training, and that there is no precise timetable for when an individual will see results.
Sources: Xataka.