Intensity of activation and timing of deactivation modulate elastic energy storage and release in a pennate muscle and account for gait-specific initiation of limb protraction in the horse.

The research paper explores how the horse’s biceps brachii muscle, the muscle responsible for moving the horse’s forelimb forward, uses a “catapult mechanism” to store and release elastic energy, aiding in movement efficiency at varying speeds. It proposes that the biceps modulate the output of energy under different locomotion circumstances and work in tandem with the muscle-tendon unit (MTU) to optimize the muscle’s effectiveness.

Understanding the Horse’s Biceps Brachii

• The biceps of a horse are responsible for initiating limb protraction, which is the move of the limb forward through a catapult mechanism.
• The muscle fibers in the equine biceps are short and have a limited range of length changes, making them unfit for driving rapid limb protraction.
• The research suggests these muscle fibers help in adapting the elastic energy output of the MTU to meet diverse locomotion demands.

Research Hypothesis

• The paper hypothesizes that during high-speed locomotion, more elastic strain energy is stored and released in the biceps MTU to accommodate the increased energy needed for limb protraction.
• The paper posits that changes in muscle length and activation conditions can assist in achieving this.

Methodology and Findings

• The researchers employed inverse dynamics analysis (IDA) to study the work carried out by the biceps during trot and canter, two different gaits.
• Using excised biceps muscles, they measured how much work the muscle could accomplish in active and passive stretch-shorten cycles.
• They noted more work being done by the biceps MTU and an increased length change in the unit during canter as compared to trot.
• Furthermore, the ex vivo muscles could perform more work by being active and increasing muscle length change.

Conclusion

• The researchers developed a muscle model that showed changes in activation parameters during stretch-shorten cycles influenced the storage and return of elastic energy from the biceps.
• Increased energy storage and return were observed after the activation of the muscle and by increasing muscle length change.
• The study concluded that the contractile component in combination with the equine biceps MTU acts as an adjustable spring to modulate energy required for fast forelimb protraction at different speeds. They suggest that the MTU plays a significant role in adapting to speed-specific locomotion demands.