How does strength training affect muscles and tendons?

Strength training induces several important structural and functional changes in your muscles and connective tissues:

• Increased Musculo-Tendinous Stiffness: Your tendons (like the Achilles) and associated muscles become stiffer. This allows them to store and return elastic energy more efficiently with each foot strike, much like a spring. This reduces the amount of active muscular work needed for propulsion and directly improves running economy.

• Enhanced Stretch-Shortening Cycle (SSC) Function: The SSC is the rapid sequence of muscle lengthening (eccentric) followed by shortening (concentric) that occurs during running. Strength training, especially plyometrics, improves SSC efficiency, allowing you to better absorb impact and transition to a powerful push-off.

• Functional Muscle Fiber Hypertrophy (Growth): Strength training can lead to an increase in the size of both Type I (slow-twitch) and Type II (fast-twitch) muscle fibers. Importantly for runners, this hypertrophy typically occurs without a significant increase in overall body mass, meaning you get stronger without a weight penalty. This is likely due to the concurrent endurance training.

• Potential Fiber Type Shift: Some evidence suggests strength training might promote a shift from the very fast-twitch Type IIx fibers towards the more fatigue-resistant Type IIa fibers, which can further enhance endurance.

• Improved Bone Health: Resistance training stimulates your bones to maintain or increase their mineral density, making them stronger and more resilient to the repetitive impacts of running, which can help reduce the risk of stress fractures.