The Effects of Mechanical Scale on Neural Control and the Regulation of Joint Stability.

Cristiano Alessandro,Matthew C. Tresch,Gil Serrancolí

doi:10.3390/ijms22042018

Abstract

Recent work has demonstrated how the size of an animal can affect neural control strategies, showing that passive viscoelastic limb properties have a significant role in determining limb movements in small animals but are less important in large animals. We extend that work to consider effects of mechanical scaling on the maintenance of joint integrity; i.e., the prevention of aberrant contact forces within joints that might lead to joint dislocation or cartilage degradation. We first performed a literature review to evaluate how properties of ligaments responsible for joint integrity scale with animal size. Although we found that the cross-sectional area of the anterior cruciate ligament generally scaled with animal size, as expected, the effects of scale on the ligament’s mechanical properties were less clear, suggesting potential adaptations in passive contributions to the maintenance of joint integrity across species. We then analyzed how the neural control of joint stability is altered by body scale. We show how neural control strategies change across mechanical scales, how this scaling is affected by passive muscle properties and the cost function used to specify muscle activations, and the consequences of scaling on internal joint contact forces. This work provides insights into how scale affects the regulation of joint integrity by both passive and active processes and provides directions for studies examining how this regulation might be accomplished by neural systems.

Highlights

Central pattern generators (CPGs) for rhythmic locomotor outputs have been demonstrated in vertebrates, from zebrafish to humans [1,2,3,4]
We examined several aspects of how the mechanical scale of an organism affects the regulation of joint stability, evaluating the scale-dependent properties of passive structures in joints and how neural control strategies for joint stabilization are altered across scales
We found that the scaling of anatomical and mechanical properties of the anterior cruciate ligament (ACL) did not generally have a clear relationship with body mass, suggesting the possibility that changes in ACL properties may be specific to each animal depending on their specific behavioral demands, in order to better ensure joint integrity

Summary

Introduction

Central pattern generators (CPGs) for rhythmic locomotor outputs have been demonstrated in vertebrates, from zebrafish to humans [1,2,3,4]. Might be expected to apply in this situation as well: the forces that destabilize a joint due to mass/inertia should scale with the cube of the length scale of an animal, whereas the passive forces that resist these destabilizing forces should scale with the square of the length scale This idea suggests that, in the absence of other adaptations, passive joint elements should be less able to stabilize joints in larger animals and so neural control will become more critical. We demonstrate the importance of the cost function used to resolve muscle redundancy in determining the scaling of muscle activations with animal size and show how these changes in muscle activation affect the internal joint forces that might compromise joint integrity These analyses and literature reviews provide new perspectives on how joint stabilization is affected by changes in animal size, highlighting the importance of animal scale in the role of both passive limb structures and neural control strategies

Scale-Dependent Contributions of Passive Structures to Joint Stabilization

Effects of Scaling on the Neural Control of Joint Stabilization

Conclusions

The Effects of Scaling on Passive Elements Involved in Joint Integrity

The Effects of Scaling on the Neural Control of Joint Stabilization