Force development during sustained locomotion: a determinant of gait, speed and metabolic power.

The research article discusses the role of force development during sustained locomotion in determining an animal’s gait, speed, and metabolic energy consumption. It presents the idea that muscle-tendon units respond like springs during locomotion and this affects the force developed by the muscle and its metabolic needs.

Muscle-Tendon Units Function as Springs

• The paper starts by presenting the first idea of muscle-tendon units acting like springs during locomotion. This is due to their alternate stretch-shorten pattern of activity. This spring-like function influences the amount of force that a cross-sectional area of a muscle can develop, plus the metabolic requirements of the muscles for creating such force.
• The animals adjust their speeds and stride frequencies to optimize the performance of these natural springs, enhancing their movement efficiency.

Muscle Stress Dictates Speed and Gait

• The second part of the paper explains that muscle stress, which is defined as the force per cross-sectional area, determines when animals change their gait, how fast they run, and their peak accelerations and decelerations.
• The research proposes that terrestrial birds and mammals develop similar muscle stresses under equivalent conditions (e.g. preferred speed within a gait). It further proposes that animals switch their gait to reduce peak stresses as they increase their speed, helping to regulate their energy use and protect their muscle structures.

Link between Time Course of Force Development and Metabolic Cost

• Lastly, the article provides evidence to support the third idea that it is the timeline of force development during locomotion, not the mechanical work the muscles perform, that determines the metabolic cost of locomotion. This finding highlights the importance of efficient force development to keep the metabolic cost low during sustained locomotion.