The role of sensory feedback from carpal sinus hairs in locomotor kinematics of rats (Rattus norvegicus, Rodentia) during walking on narrow substrates.

Carpal sinus hairs on the forearms are assumed to have evolved within the stem lineage of Theria. The presence and similar position of these specialized tactile hairs in scansorial and terrestrial species as well as earlier studies on rats indicate a biological role in sensing substrate irregularities in high structured environments to ensure the dynamic stability of the body during locomotion. While these sensors were considered as one functional unit until so far, the present study deals with the biological role of the single tactile hairs of the trident, assuming a role in sensing substrate diameters and adapting limb coordination and body posture to different arboreal inclinations. To investigate the influence of each hair, we studied the locomotion of rats on poles of two different diameters whereby we selectively removed individual carpal sinus hairs. The rats walked at speeds ranging from 0.12 m/s to 0.58 m/s. Normal-light high-speed cameras and x-ray fluoroscopy visualized the hairs and body dynamics during locomotion. The time lag between first contact of the hairs to the branch until contact of the forepaw was 56-108 ms. Within this time window the pronation/supination of the paw and anterior body posture are adjusted to the substrate diameter. We presume that the most proximal sinus hair (located between the medial and lateral one) senses the paw-substrate distance through the increasing bend from its first branch-contact until the contact of the paw. The medial and the lateral hairs touch the pole sides and thereby, may collect information about the properties of the small-diameter substrate. The removal of single hairs from the group results in minor changes of kinematic parameters, but locomotor stability is seriously impaired when more than one hair is cut. The kinematic responses span from a more crouched body posture and higher forearm pronation to paw slipping, muscle tremor or complete refusal to walk on the narrow substrate.

Three-dimensional polygonal muscle modelling and line of action estimation in living and extinct taxa.

Biomechanical models and simulations of musculoskeletal function rely on accurate muscle parameters, such as muscle masses and lines of action, to estimate force production potential and moment arms. These parameters are often obtained through destructive techniques (i.e., dissection) in living taxa, frequently hindering the measurement of other relevant parameters from a single individual, thus making it necessary to combine multiple specimens and/or sources. Estimating these parameters in extinct taxa is even more challenging as soft tissues are rarely preserved in fossil taxa and the skeletal remains contain relatively little information about the size or exact path of a muscle. Here we describe a new protocol that facilitates the estimation of missing muscle parameters (i.e., muscle volume and path) for extant and extinct taxa. We created three-dimensional volumetric reconstructions for the hindlimb muscles of the extant Nile crocodile and extinct stem-archosaur Euparkeria, and the shoulder muscles of an extant gorilla to demonstrate the broad applicability of this methodology across living and extinct animal clades. Additionally, our method can be combined with surface geometry data digitally captured during dissection, thus facilitating downstream analyses. We evaluated the estimated muscle masses against physical measurements to test their accuracy in estimating missing parameters. Our estimated muscle masses generally compare favourably with segmented iodine-stained muscles and almost all fall within or close to the range of observed muscle masses, thus indicating that our estimates are reliable and the resulting lines of action calculated sufficiently accurately. This method has potential for diverse applications in evolutionary morphology and biomechanics.

Exploring the functional morphology of the Gorilla shoulder through musculoskeletal modelling.

Musculoskeletal computer models allow us to quantitatively relate morphological features to biomechanical performance. In non-human apes, certain morphological features have long been linked to greater arm abduction potential and increased arm-raising performance, compared to humans. Here, we present the first musculoskeletal model of a western lowland gorilla shoulder to test some of these long-standing proposals. Estimates of moment arms and moments of the glenohumeral abductors (deltoid, supraspinatus and infraspinatus muscles) over arm abduction were conducted for the gorilla model and a previously published human shoulder model. Contrary to previous assumptions, we found that overall glenohumeral abduction potential is similar between Gorilla and Homo. However, gorillas differ by maintaining high abduction moment capacity with the arm raised above horizontal. This difference is linked to a disparity in soft tissue properties, indicating that scapular morphological features like a cranially oriented scapular spine and glenoid do not enhance the abductor function of the gorilla glenohumeral muscles. A functional enhancement due to differences in skeletal morphology was only demonstrated in the gorilla supraspinatus muscle. Contrary to earlier ideas linking a more obliquely oriented scapular spine to greater supraspinatus leverage, our results suggest that increased lateral projection of the greater tubercle of the humerus accounts for the greater biomechanical performance in Gorilla. This study enhances our understanding of the evolution of gorilla locomotion, as well as providing greater insight into the general interaction between anatomy, function and locomotor biomechanics.