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Study examines link between giraffe limb length and walking efficiency

Jul 08, 2022 02:58 STI

Washington [US], July 8 (ANI): Published in the journal “Proceedings of the National Academy of Sciences” (PNAS) on July 7, a study examines the trade-offs between limb length and walking efficiency in giraffes. In many mammals, long, straight limbs confer a mechanical advantage that improves speed and muscle efficiency.
However, giraffes’ extremely long limbs, which aid in eating, may not promote athleticism. Christopher Basu and John Hutchinson produced musculoskeletal models of the forelimbs of the giraffe and two related species: Okapia johnstoni, a living species, and Sivatherium giganteum, an extinct species.
The authors measured muscle efficiency in walking giraffes using motion and force capture data, and estimated efficiency in the other two species. Giraffe muscle efficiency was four times lower than expected based on extrapolated data from a variety of small animals, indicating that giraffe muscles must generate relatively large forces. Muscle efficiency varied between the three species tested; giraffe and Sivatherium have similar body mass, but giraffe has lower muscle efficiency.
The giraffe and the okapi have similar muscle efficiency, even though the giraffe has 4-6 times the mass of the okapi. The results suggest that from a certain height, the increase in muscle efficiency associated with the increase in body mass reaches a plateau. According to the authors, large animals are likely to have relatively lower muscle efficiency, unless a parameter known as the arm of muscle moment, which is the distance between the muscle and the pivot point of articulation, is proportional to the length of their limb.

Giraffes are the tallest living animals, using their size to access food inaccessible to their competitors. The impact of their specialized anatomy on their athletic ability is unclear. We created musculoskeletal models of the forelimbs of a giraffe and two close relatives and used motion and force capture data to measure their effectiveness when walking in a straight line. A horse, for example, uses only 1 unit of muscle strength to oppose 1 unit of ground strength. The Giraffe’s limbs are at a relative disadvantage – their muscles must develop 3 units of force to oppose 1 unit of force on the ground. This explains why giraffes walk and run at modest speeds.
Giraffes (Giraffa camelopardalis) have a specialized locomotor morphology, namely elongated and slender distal limbs. While this contributes to their overall height and improves eating behavior, we propose that the combination of long limb segments and modest muscular lever arms results in a low effective mechanical advantage (EMA, the ratio of inner lever arms and exteriors), when relative to other mammalian sliders. To test this, we used a combination of experimentally measured kinematics and ground reaction forces (GRF), musculoskeletal modeling, and inverse dynamics to calculate the EMA of the giraffe’s forelimbs during walking. Giraffes walk with an EMA of 0.34 (+-0.05 SD), with no obvious association with speed in their walking gait. Giraffe EMA was approximately four times lower than expectations extrapolated from other mammals, ranging from 0.03 to 297 kg, providing further evidence that EMA plateaus or even declines in taller mammals. of the horse.
We further tested the idea that limb segment length is a factor that determines EMA, by modeling GRF and muscle moment arms in the extinct giraffid Sivatherium giganteum and the other extant giraffid, Okapia johnstoni. Giraffa and Okapia shared a similar EMA, despite a four- to six-fold difference in body mass (Okapia EMA = 0.38). In contrast, Sivatherium, sharing a similar body mass to Giraffa, had a larger EMA (0.59), which we believe reflects behavioral differences, such as somewhat increased capacity for athletic performance. Our modeling approach suggests that limb length is a determinant of GRF moment arm magnitude and that unless muscle moment arms are isometrically adapted to limb length, large mammals are prone to a low EMA.
We have evidenced that giraffes use a lower than expected effective mechanical advantage because their musculoskeletal morphology (such as the olecranon process of the ulna) is insufficient to maintain the observed trend of EMA in animals until ‘at 300 kg. Our results from an analysis of modeled GRF moment arms and muscle moment arms suggest that the EMA of giraffes is similar to okapi, a giraffid with lower body mass and more plesiomorphic locomotor traits.
Low EMA was not ubiquitous among giraffids, as S. giganteum was predicted to have larger, but still low EMA compared to small mammals, even horses. The differential EMA between Sivatherium and Giraffa may reflect behavioral or athletic differences between these two similarly sized giraffids, which more elaborate methods, such as simulations, could test. While giraffe foraging ability is determined by extreme height, it appears that extreme cursoriality comes with a functional trade-off with locomotor performance. (ANI)