The Science Behind Muscle Hypertrophy

Anabolic Signals and Strength Training in Sports Medicine

Research recap of M Behringer, Heinrich C, Franz A. Anabolic signals and muscle hypertrophy – Significance for strength training in sports medicine. Sports Orthopaedics and Traumatology. Published online February 8, 2025. doi:10.1016/j.orthtr.2025.01.002

Muscle hypertrophy—the increase in muscle mass and cross-sectional area—is a fundamental process for both athletic performance and rehabilitation in sports medicine. A new study by Michael Behringer, Christine Heinrich, and Alexander Franz explores the complex biological signals driving muscle growth and how these can be optimized in strength training, injury prevention, and rehabilitation.

Understanding Anabolic Signals and Muscle Growth

Muscle hypertrophy is driven by anabolic signals—biological mechanisms that stimulate muscle growth in response to exercise. The study categorizes these signals into three primary types: mechanical tension, metabolic stress, and muscle damage.

• Mechanical tension, created when muscles contract against resistance, has long been considered the primary driver of hypertrophy. However, recent research suggests that high-intensity resistance training is not the only way to stimulate muscle growth.
• Metabolic stress, often induced by high-repetition, low-rest workouts, triggers anabolic responses through the accumulation of metabolic byproducts such as lactate, hydrogen ions, and inorganic phosphate. This process promotes cell swelling and hormonal responses that contribute to muscle hypertrophy.
• Muscle damage, traditionally thought to be necessary for hypertrophy, may actually be a byproduct rather than a requirement for muscle growth. While damage-induced inflammatory responses can stimulate muscle repair and growth, excessive muscle damage is not an efficient strategy for hypertrophy and can hinder recovery.

Key Molecular Pathways of Muscle Growth

The study outlines several molecular mechanisms underlying hypertrophy, with mTORC1 (mechanistic target of rapamycin complex 1) playing a central role. This pathway regulates protein synthesis and muscle cell growth in response to resistance training. Other key elements include:

• Satellite cell activation, essential for muscle repair and regeneration.
• Hormonal responses, particularly testosterone, growth hormone, and insulin-like growth factor-1 (IGF-1), which enhance protein synthesis.
• Reactive oxygen species (ROS) and cytokines, which act as secondary messengers to stimulate growth-related pathways.

Training Strategies for Hypertrophy

The review highlights the importance of training variables in modulating anabolic signals, including exercise selection, contraction type, training volume, intensity, and frequency.

• Low, moderate, and high loads (30–100% 1RM) can all induce similar muscle growth, as long as sets are performed near failure. This contradicts the traditional belief that only heavy lifting stimulates hypertrophy.
• Blood Flow Restriction (BFR) training, which involves restricting blood flow to working muscles, is an effective way to increase metabolic stress, even with lower training intensities.
• Stretch-induced hypertrophy is gaining attention, with evidence suggesting that prolonged stretching alone can promote muscle growth, making it a potential tool for rehabilitation and populations unable to perform traditional resistance training.

The Role of Nutrition in Maximizing Hypertrophy

Proper nutrition is crucial for optimizing the hypertrophic response. The study underscores:

• Protein intake of 1.6–2.2 g/kg of body weight as essential for muscle repair and growth.
• Even distribution of protein intake throughout the day to maximize muscle protein synthesis (MPS).
• Pre-sleep protein consumption as a strategy to enhance overnight muscle recovery.
• Carbohydrate intake to support energy demands and improve recovery.

Applications in Sports Medicine: Prehabilitation and Rehabilitation

The findings have direct implications for injury prevention, prehabilitation (preparing the body before surgery), and rehabilitation.

• Prehabilitation strategies should focus on increasing muscle mass and strength before surgery or injury to accelerate recovery. BFR training and metabolic stress-based approaches may be particularly useful.
• Rehabilitation programs should aim to minimize muscle atrophy and restore function without causing excessive muscle damage. Lower-intensity training methods, metabolic stress-based approaches, and stretching can be key tools in this process.

Conclusion

This study provides a comprehensive framework for leveraging anabolic signals in strength training and rehabilitation. By understanding the diverse mechanisms behind muscle hypertrophy, sports medicine professionals can design more effective training programs that balance mechanical tension, metabolic stress, and recovery strategies.

These insights challenge traditional beliefs about hypertrophy and expand the range of effective training methods, particularly for clinical applications where heavy resistance training may not be feasible. With proper training, nutrition, and recovery, individuals can optimize muscle growth, prevent injuries, and enhance rehabilitation outcomes.