SYNOPSIS. This paper proposes a biomechanical model for locomotor-respiratory coupling (LRC) in galloping mammals in which gait and breathing cycles are phase-locked on a 1:1 basis. It also explores some of the physiological and neuromotor implications of LRC. The mechanical coupling of locomotor and respiratory cycles depends upon the coordinated, reciprocal oscillations of the cranio-cervical and lumbo-pelvic components of the axial system and their attendant actions on the intervening thorax via muscular linkages. Concurrently, accelerational and decelerational forces imparted to the axial system by the limbs help to drive lung ventilation by inducing inertial displacements of a "visceral piston' connected to the diaphragm. Several lines of evidence (including cineradiographic data) suggest that an important function of the crural diaphragm is to control the displacement of the visceral piston. The kinematics of LRC indicate that the interosseous intercostal muscles must simultaneously operate to assure thoracic stability against locomotor stresses as well as to promote breathing. The former may be their more essential role, however. The characteristic design of the rib cage in cursorial mammals (=deep and narrow) appears to maximize the leverage of certain "accessory respiratory muscles" (i.e., sternocleidomastoid, scalenes) while minimizing torsional loading of the thorax during forelimb support. Physiological implications of LRC include the prediction that large mammals will breathe relatively faster and with relatively smaller lung volumes when galloping than small species. An additional prediction, that running mammals could automatically gear lung ventilation to speed by simply linking breathing rate to stride frequency and depth of breath (=tidal volume) to stride length, appears to be supported by experimental data from horses. Finally, the neuromotor basis of LRC probably depends upon the direct interaction of central pattern generators for locomotion and respiration. This interaction might be modulated, however, by afferent input from thoracic mechanoreceptors, particularly the intercostal stretch receptors.
Locomotor-respiratory coupling (LRC), phase-locking between breathing and stepping rhythms, occurs in many vertebrates. When quadrupedal mammals gallop, 1∶1 stride per breath coupling is necessitated by pronounced mechanical interactions between locomotion and ventilation. Humans show more flexibility in breathing patterns during locomotion, using LRC ratios of 2∶1, 2.5∶1, 3∶1, or 4∶1 and sometimes no coupling. Previous studies provide conflicting evidence on the mechanical significance of LRC in running humans. Some studies suggest LRC improves breathing efficiency, but others suggest LRC is mechanically insignificant because 'step-driven flows' (ventilatory flows attributable to step-induced forces) contribute a negligible fraction of tidal volume. Yet, although step-driven flows are brief, they cause large fluctuations in ventilatory flow. Here we test the hypothesis that running humans use LRC to minimize antagonistic effects of step-driven flows on breathing. We measured locomotor-ventilatory dynamics in 14 subjects running at a self-selected speed (2.6±0.1 ms(-1)) and compared breathing dynamics in their naturally 'preferred' and 'avoided' entrainment patterns. Step-driven flows occurred at 1-2X step frequency with peak magnitudes of 0.97±0.45 Ls(-1) (mean ±S.D). Step-driven flows varied depending on ventilatory state (high versus low lung volume), suggesting state-dependent changes in compliance and damping of thoraco-abdominal tissues. Subjects naturally preferred LRC patterns that minimized antagonistic interactions and aligned ventilatory transitions with assistive phases of the step. Ventilatory transitions initiated in 'preferred' phases within the step cycle occurred 2x faster than those in 'avoided' phases. We hypothesize that humans coordinate breathing and locomotion to minimize antagonistic loading of respiratory muscles, reduce work of breathing and minimize rate of fatigue. Future work could address the potential consequences of locomotor-ventilatory interactions for elite endurance athletes and individuals who are overweight or obese, populations in which respiratory muscle fatigue can be limiting.
Mammals must stabilize the head during running to keep angular accelerations of head within the operating range of the vestibulo-ocular (VOR) reflexes. However, several unique aspects of the human body plan and locomotor kinematics make head stabilization more challenging than in other cursors. Most bipedal and quadrupedal cursors have cantilevered heads and necks that act to attenuate forces and counter sagittal head pitching through controlled flexion and extension movements. In contrast, humans have short vertical necks that emerge from near center of head, combined with relatively extended, stiff legs at heel strike (HS), resulting in a strong tendency for the head to pitch forward at the beginning of stance. Using EMG, kinematic, and kinetic measurements of human arm and head movements during running and walking we show that humans stabilize the head following HS using a unique tuned-mass damper system. This mechanism, which links the head with inertial forces in the stance side (ipsilateral) arm, is facilitated by a number of derived aspects of human anatomy and running kinematics. Notably, humans have lost all muscular connections between shoulder girdle and head except for the cleidocranial portion of the trapezius (CCT), which reaches the occiput via a tendon-like nuchal ligament. Additionally, coordinated movements of the arm and thorax position the ipsilateral arm behind the head-neck joint prior to HS, when the ipsilateral CCT fires. Out of phase accelerations of the arm and head then link the counterbalancing mass of the arm and the flexed forearm via a compliant connection to the head, controlling the head’s rate of pitch. Because the nuchal ligament, a key component of the system, leaves a trace on the skull, it is possible to show that this novel mechanism for head stabilization originated within the genus Homo approximately 2 millions years ago.
The Os transiliens, a sesamoid bone in the central raphe of the external adductor muscle, has previously been described in three of the four living species of the tortoise genus Gopherus (G. polyphemus, flavomarginatus, agassizii). The Os transiliens is reported for the first time in Recent G. berlandieri and in the oldest recognized fossil Gopherus, G. laticunea from the Middle Oligocene of Colorado. The sesamoids from 16 G. berlandieri show marked variation in size and shape within and between individuals. There is a low degree of correlation (r = +.405) between sesamoid development and skull size. Evidence from Recent and fossil testudinids indicates that the Os transiliens is restricted to gopher tortoises. The presence of the bony sesamoid appears to be related to mechanical stresses arising from the specialized feeding mechanism of this group. The mechanism, which involves pronounced protraction and retraction of the mandible, permits more efficient utilization of the coarser vegetation types common to xerophytic floras. It is suggested that the specialized feeding mechanism and the associated Os transiliens originated as an adaptive response to North American climatic and vegetational changes during the Late Eocene-Early Oligocene period.
Moore, W. J. The mammalian skull, biological structure and function. Cambridge University Press, New York, 8:1–369 pp., 1981. Price $85.00 Get access Moore W. J.The mammalian skull, biological structure and function. Cambridge University Press, New York, 8: 1–369 pp., 1981. Price $85.00 Dennis M. Bramble Dennis M. Bramble Department of Biology, University of Utah, Salt Lake City, UT 84112 Search for other works by this author on: Oxford Academic Google Scholar Journal of Mammalogy, Volume 63, Issue 3, 26 August 1982, Pages 540–542, https://doi.org/10.2307/1380467 Published: 26 August 1982
