ABSTRACT Mammals must stabilize the head during running to keep angular accelerations of head wit... more ABSTRACT 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. Anthropology Human Evolutionary Biology
This paper proposes a simple, unorthodox model for use in the study of vertebrate jaw mechanics. ... more This paper proposes a simple, unorthodox model for use in the study of vertebrate jaw mechanics. Central to the new “bifulcral model” is the assumption that the bite point may be regarded as a distinct and independent “occlusal fulcrum” equal in status to the jaw articulation or “joint fulcrum” of more traditional biomechanical models. The bifulcral model allows all mechanical forces acting on the feeding system to be evaluated in terms of their purely rotational and purely translational components defined relative to the occlusal and joint fulcra. The major benefit of this analytical approach is that it permits a new and substantially different perspective on the functional consequences of morpho-geometric organization in feeding systems. The bifulcral model clearly establishes the dynamic relationships among muscle alignment, bite point and the resultant patterns of mechanical stress at the craniomandibular joint (CMJ). It also reveals potentially important modes of competitive interaction between otherwise seemingly synergistic jaw muscles. Further, the bifulcral model encourages preliminary investigations into the possible contribution of intramuscular dynamics to the overall operational plasticity of the feeding machinery.Specific application of the new model to structural and functional problems concerning the origin of the mammalian feeding complex lead to the following tentative conclusions. (1) Contrary to current opinion, the CMJ of most cynodont therapsids probably did not experience positive vertical loads (= compressive) when the cheek teeth were utilized for mastication. Instead, net CMJ loads were either neutral or somewhat negative (= tensile). (2) The development of a pronounced coronoid process in cynodonts was more directly related to promoting the differentiation of the masseter complex than to either improving the mechanical advantage or prehensile capacity of the temporalis muscle. (3) Differential motor activity within the complex temporalis musculature of cynodonts could have resulted in “derived lines of action” markedly different from the reconstructed lines of action employed in previous analyses of feeding mechanics in these reptiles. Such derived lines of action may have been optimal configurations for the integrated activity of the temporalis and masseter musculature. (4) Selection in cynodonts favored the evolution of a superficial masseter rather than the elaboration of the preexisting and geometrically similar pterygoideus musculature. This occurred because the masseter held the greater potential for improving bite force, motor control and facilitating the reduction of the postdentary bones of the mandible while still preserving the basic spatial economy of the cranial region. (5) The rearward growth of the condylar process of the dentary in cynodonts promoted the reduction of the postdentary unit primarily by shifting the point of load application between the dentary and postdentary units, thereby reducing bending stresses in the postdentary unit. (6) The enlarged squamosal sulcus of cynodonts was occupied mainly by a hypertrophied depressor mandibulae muscle; an auditory tube was also present in the sulcus deep to the muscle. (7) The depressor mandibulae played a major role in CMJ stabilization in therapsids; this was possibly its only function in later cynodonts. The peculiar downturned retroarticular process of the mandible in therapsids appears to have been related to this same function.
The Os transiliens, a sesamoid bone in the central raphe of the external adductor muscle, has pre... more 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.
... Dissections of fresh or pre-served specimens were made on the following taxa: Staurotypus sal... more ... Dissections of fresh or pre-served specimens were made on the following taxa: Staurotypus salvini, S. triporcatus; Claudius angustatus; Sternotherus odoratus, S. minor; Kino-sternon bauri, K. flavescens, K. herrerai, K. hirtipes, K. integrum, K. leucostomum, K. scorpioides, K ...
American Journal of Physical Anthropology, Jan 25, 2021
ObjectivesThe main objective was to test the hypothesis of a neuromechanical link in humans betwe... more ObjectivesThe main objective was to test the hypothesis of a neuromechanical link in humans between the head and forearm during running mediated by the biceps brachii and superior trapezius muscles. We hypothesized that this linkage helps stabilize the head and combats rapid forward pitching during running which may interfere with gaze stability.Materials and methodsThirteen human participants walked and ran on a treadmill while motion capture recorded body segment kinematics and electromyographic sensors recorded muscle activation. To test perturbations to the linkage system we compared participants running normally as well as with added mass to the face and the hand.ResultsThe results confirm the presence of a neuromechanical linkage between the head and forearm mediated by the biceps and superior trapezius during running but not during walking. In running, the biceps and superior trapezius activations were temporally linked during the stride cycle, and adding mass to either the head or hand increased activation in both muscles, consistent with our hypothesis. During walking the forces acting on the body segments and muscle activation levels were much smaller than during running, indicating no need for a linkage to keep the head and gaze stable.DiscussionThe results suggest that the evolution of long distance running in early Homo may have favored selection for reduced rotational inertia of both the head and forearm through synergistic muscle activation, contributing to the transition from australopith head and forelimb morphology to the more human‐like form of Homo erectus. Selective pressures from the evolution of bipedal walking were likely much smaller, but may explain in part the intermediate form of the australopith scapula between that of extant apes and humans.
ABSTRACT Mammals must stabilize the head during running to keep angular accelerations of head wit... more ABSTRACT 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. Anthropology Human Evolutionary Biology
This paper proposes a simple, unorthodox model for use in the study of vertebrate jaw mechanics. ... more This paper proposes a simple, unorthodox model for use in the study of vertebrate jaw mechanics. Central to the new “bifulcral model” is the assumption that the bite point may be regarded as a distinct and independent “occlusal fulcrum” equal in status to the jaw articulation or “joint fulcrum” of more traditional biomechanical models. The bifulcral model allows all mechanical forces acting on the feeding system to be evaluated in terms of their purely rotational and purely translational components defined relative to the occlusal and joint fulcra. The major benefit of this analytical approach is that it permits a new and substantially different perspective on the functional consequences of morpho-geometric organization in feeding systems. The bifulcral model clearly establishes the dynamic relationships among muscle alignment, bite point and the resultant patterns of mechanical stress at the craniomandibular joint (CMJ). It also reveals potentially important modes of competitive interaction between otherwise seemingly synergistic jaw muscles. Further, the bifulcral model encourages preliminary investigations into the possible contribution of intramuscular dynamics to the overall operational plasticity of the feeding machinery.Specific application of the new model to structural and functional problems concerning the origin of the mammalian feeding complex lead to the following tentative conclusions. (1) Contrary to current opinion, the CMJ of most cynodont therapsids probably did not experience positive vertical loads (= compressive) when the cheek teeth were utilized for mastication. Instead, net CMJ loads were either neutral or somewhat negative (= tensile). (2) The development of a pronounced coronoid process in cynodonts was more directly related to promoting the differentiation of the masseter complex than to either improving the mechanical advantage or prehensile capacity of the temporalis muscle. (3) Differential motor activity within the complex temporalis musculature of cynodonts could have resulted in “derived lines of action” markedly different from the reconstructed lines of action employed in previous analyses of feeding mechanics in these reptiles. Such derived lines of action may have been optimal configurations for the integrated activity of the temporalis and masseter musculature. (4) Selection in cynodonts favored the evolution of a superficial masseter rather than the elaboration of the preexisting and geometrically similar pterygoideus musculature. This occurred because the masseter held the greater potential for improving bite force, motor control and facilitating the reduction of the postdentary bones of the mandible while still preserving the basic spatial economy of the cranial region. (5) The rearward growth of the condylar process of the dentary in cynodonts promoted the reduction of the postdentary unit primarily by shifting the point of load application between the dentary and postdentary units, thereby reducing bending stresses in the postdentary unit. (6) The enlarged squamosal sulcus of cynodonts was occupied mainly by a hypertrophied depressor mandibulae muscle; an auditory tube was also present in the sulcus deep to the muscle. (7) The depressor mandibulae played a major role in CMJ stabilization in therapsids; this was possibly its only function in later cynodonts. The peculiar downturned retroarticular process of the mandible in therapsids appears to have been related to this same function.
The Os transiliens, a sesamoid bone in the central raphe of the external adductor muscle, has pre... more 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.
... Dissections of fresh or pre-served specimens were made on the following taxa: Staurotypus sal... more ... Dissections of fresh or pre-served specimens were made on the following taxa: Staurotypus salvini, S. triporcatus; Claudius angustatus; Sternotherus odoratus, S. minor; Kino-sternon bauri, K. flavescens, K. herrerai, K. hirtipes, K. integrum, K. leucostomum, K. scorpioides, K ...
American Journal of Physical Anthropology, Jan 25, 2021
ObjectivesThe main objective was to test the hypothesis of a neuromechanical link in humans betwe... more ObjectivesThe main objective was to test the hypothesis of a neuromechanical link in humans between the head and forearm during running mediated by the biceps brachii and superior trapezius muscles. We hypothesized that this linkage helps stabilize the head and combats rapid forward pitching during running which may interfere with gaze stability.Materials and methodsThirteen human participants walked and ran on a treadmill while motion capture recorded body segment kinematics and electromyographic sensors recorded muscle activation. To test perturbations to the linkage system we compared participants running normally as well as with added mass to the face and the hand.ResultsThe results confirm the presence of a neuromechanical linkage between the head and forearm mediated by the biceps and superior trapezius during running but not during walking. In running, the biceps and superior trapezius activations were temporally linked during the stride cycle, and adding mass to either the head or hand increased activation in both muscles, consistent with our hypothesis. During walking the forces acting on the body segments and muscle activation levels were much smaller than during running, indicating no need for a linkage to keep the head and gaze stable.DiscussionThe results suggest that the evolution of long distance running in early Homo may have favored selection for reduced rotational inertia of both the head and forearm through synergistic muscle activation, contributing to the transition from australopith head and forelimb morphology to the more human‐like form of Homo erectus. Selective pressures from the evolution of bipedal walking were likely much smaller, but may explain in part the intermediate form of the australopith scapula between that of extant apes and humans.
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