Early skeletal muscle hypertrophy and architectural changes in response to high-intensity resistance training

TLDR

Whole‑muscle hypertrophy onset following overload is poorly documented. The study aimed to assess early muscle size and architectural changes during a 35‑day high‑intensity resistance training program. Seven healthy young adults performed three weekly bilateral leg extensions on a flywheel ergometer, with quadriceps cross‑sectional area, architecture, MVC, and EMG measured at baseline and after 10, 20, and 35 days. After 35 days, MVC rose 38.9 %, EMG 34.8 %, quadriceps CSA increased 6.5–7.4 %, fascicle length 9.9 %, and pennation angle 7.7 %; architectural remodeling appeared after 3 weeks, preceding CSA gains, indicating early hypertrophy and flywheel training’s effectiveness.

Abstract

The onset of whole muscle hypertrophy in response to overloading is poorly documented. The purpose of this study was to assess the early changes in muscle size and architecture during a 35-day high-intensity resistance training (RT) program. Seven young healthy volunteers performed bilateral leg extension three times per week on a gravity-independent flywheel ergometer. Cross-sectional area (CSA) in the central (C) and distal (D) regions of the quadriceps femoris (QF), muscle architecture, maximal voluntary contraction (MVC), and electromyographic (EMG) activity were measured before and after 10, 20, and 35 days of RT. By the end of the training period, MVC and EMG activity increased by 38.9 +/- 5.7 and 34.8% +/- 4.7%, respectively. Significant increase in QF CSA (3.5 and 5.2% in the C and D regions, respectively) was observed after 20 days of training, along with a 2.4 +/- 0.7% increase in fascicle length from the 10th day of training. By the end of the 35-day training period, the total increase in QF CSA for regions C and D was 6.5 +/- 1.1 and 7.4 +/- 0.8%, respectively, and fascicle length and pennation angle increased by 9.9 +/- 1.2 and 7.7 +/- 1.3%, respectively. The results show for the first time that changes in muscle size are detectable after only 3 wk of RT and that remodeling of muscle architecture precedes gains in muscle CSA. Muscle hypertrophy seems to contribute to strength gains earlier than previously reported; flywheel training seems particularly effective for inducing these early structural adaptations.