Effects of leg immobilization and recovery resistance training on skeletal muscle-molecular markers in previously resistance-trained versus untrained adults

We sought to examine how resistance training (RT) status in young healthy individuals, either well resistance trained (T, <i>n</i> = 10) or untrained (UT, <i>n</i> = 11), affected molecular markers with leg immobilization followed by recovery RT. All participants underwent 2 wk of left leg immobilization via a locking leg brace. Afterward, all participants underwent 8 wk (3 days/wk) of knee extensor-focused progressive RT. Vastus lateralis (VL) ultrasound-derived thickness and muscle cross-sectional area were measured at baseline (PRE), immediately after disuse (MID), and after RT (POST) with VL muscle biopsies also being collected at these time points. Both groups presented lower ultrasound-derived VL size metrics at MID versus PRE (<i>P</i> ≤ 0.001), and values increased in both groups from MID to POST (<i>P</i> < 0.05); however, VL size increased from PRE to POST in UT only (<i>P</i> < 0.001). Mean and type II myofiber cross-sectional area values were greater at PRE and POST versus MID (<i>P</i> < 0.05), with T being greater than UT throughout (<i>P</i> ≤ 0.012). In both groups, satellite cell number was not affected by leg immobilization but increased in response to RT (<i>P</i> ≤ 0.014), with T being greater than UT throughout (<i>P</i> = 0.004). Total RNA (ribosome content) decreased (<i>P</i> = 0.010) from PRE to MID while total RNA and certain endoplasmic reticulum stress proteins increased from MID to POST regardless of training status. Immobilization-induced muscle atrophy and recovery RT hypertrophy outcomes are similar between UT and T participants, and the lack of molecular signature differences between groups supports these findings. However, results are limited to younger adults undergoing noncomplicated disuse.<b>NEW & NOTEWORTHY</b> Formerly trained and untrained individuals demonstrate similar atrophic responses to disuse while untrained individuals exhibited a greater hypertrophic response to subsequent resistance training. The molecular responses accompanying these changes were largely similar between groups and included increases in satellite cell content with resistance training and increases in ribosome biogenesis, which was largely driven by the formerly trained group.