Mechanical deficit persists during long-term muscle hypertrophy.

Area of Science:

• Muscle physiology
• Skeletal muscle adaptation
• Exercise science

Background:

• Muscle hypertrophy is often associated with increased force production.
• However, the relationship between muscle size and specific force (force normalized to cross-sectional area) can be complex.
• Previous studies suggest that hypertrophy may not always lead to proportional increases in specific force.

Purpose of the Study:

• To investigate if the deficit in specific force of hypertrophied muscles recovers over time.
• To compare the adaptation of specific force in soleus and plantaris muscles following synergistic ablation.
• To determine if increased muscle cross-sectional area fully contributes to isometric force production after ablation.

Main Methods:

• Induce muscle hypertrophy in female Wistar rats via synergistic muscle ablation (gastrocnemius and plantaris, or gastrocnemius and soleus).
• Measure isometric force (Po) and muscle cross-sectional area (CSA) in vitro over 240 days post-ablation.
• Compare hypertrophied muscles with control muscles from age-matched rats.

Main Results:

• Soleus CSA increased significantly, but Po only showed small increases at specific time points.
• Plantaris CSA increased significantly, with Po increasing initially but returning to control levels by day 240.
• Specific force (Po/CSA) was consistently depressed (11-28%) in both hypertrophied muscles compared to controls.
• Increased CSA did not fully translate to proportional increases in isometric force production.

Conclusions:

• The deficit in specific force of hypertrophied muscles does not fully recover with time.
• The adaptation of specific force differs between soleus and plantaris muscles.
• Increased muscle size following synergistic ablation does not guarantee a proportional increase in specific force.