Meeting Abstract
Terrestrial animals navigate complex and unpredictable terrain with agility and safety during everyday cyclic tasks. Birds achieve this by a proximo-distal gradient in their joint motor control strategy (Daley et al., 2007). However, the mechanical behavior and neuromuscular control at the individual muscle-tendon unit (MTU) level during perturbations remains unclear. To address this, we used a Hill-type MTU with an antagonist catch in a hopping paradigm (Roberston et al 2014). The MTU was cyclically stimulated by a combination of feedforward and positive force feedback control signals. A substrate height perturbation was applied and simulated until steady state behavior. This was done for varying levels of feedback and feedforward signals. Our results suggest that feedforward dominated strategies have adequate settling times, the lowest muscle strains, the highest amount of energy dissipation, and the lowest energetic demands. This is achieved by a shorten-stretch cycle where the muscle is pre-activated and shortens against the tendon due to delay in ground contact while falling in a hole. This allows the tendon to rapidly stretch and initially absorb the energy of the fall whilst enabling the muscle to re-absorb it over a longer period with the added benefit of force-enhancement. This is achieved without requiring complex neural control and is a fundamental property of the perturbation response of feedforward dominated MTUs during cyclic tasks which has also been experimentally shown in animals and humans. In conclusion, we propose that sensory feedback may not be the dominant strategy for maintaining stability in the face of uneven terrain. However, it may play an important role in triggering antagonist co-activation to generate a muscle-based latch mechanism which enables muscle pre-shortening to maintain stability and prevent muscle damage.
