Human mechanics

The Anterior Cruciate Ligament (ACL) resists excessive anterior translation and
internal rotation of the tibia during athletic activities and stabilizes the knee. In the US,
annually, over 200,000 cases of ACL disruption are reported. The impact on the quality of life of the subject and its cost to healthcare is tremendous. The objectives of this study were to determine any significant associations between the size of the tibial eminence and ACL injury and to develop a finite element model for structural analysis. The results suggest that the size of the tibial eminence plays a role in loading the ACL and is therefore a risk factor. In addition to the epidemiological analysis, a finite element model of the knee was developed that with added modifications can be used for complex knee loading situations. The results in this thesis may be used to develop strategies for ACL injury prevention and rehabilitation.

In this thesis the transition region between two modes of behavior is explored using a novel technique, delayed feedback, and a variety of dynamical systems measures. In a previous study, Engstrom, Kelso, and Holroyd (to appear) established the existence of a transition between anticipatory and reactive behavior in a sensorimotor coordination task as a control parameter (frequency) was varied. Here, in order to explore the hypothesis that the behavioral dynamics during this transition are intermittent in character, subjects were asked to synchronize with a metronome that was actually a delayed copy of their own response pattern. The use of delayed feedback was expected to destabilize the behavioral dynamics enough to allow the observation of hypothesized intermittent phenomena. Use of delayed feedback was shown to destabilize synchronization, resulting in the emergence of a new behavioral pattern in the transition region that exhibited complex "bursting" dynamics. Analysis revealed that this bursting behavior displays many of the characteristics common to intermittency, which supports the idea that the anticipation-reaction transition is the result of a neurobehavioral dynamical system losing stability. Living in the vicinity of instabilities may be an important mechanism for biological organisms to maintain both flexibility and stability.

A delayed response paradigm was used to investigate the cerebral electroencephalographic (EEG) signal preceding bimanual finger flexions of continuously increasing and decreasing movement rates. The Bereitschaftspotential displayed larger amplitudes at faster required response rates as did two spectral frequency modes, which also showed magnitude reversals depending on the initiating finger. Furthermore, at these specific frequency modes, the averaged relative phase between electrode locations C3 and C4, as well as the variance in this measure was found to correspond closely to the variance in inter-response times derived from the subjects' movements. The results suggest the existence of possible signatures on the neurophysiological level which may yield information regarding the efficacy and parametric properties of the impending movement.

The aim of this research was to study the coordinative dynamics of multijoint arm movements as a function of forearm spatial orientation. Six subjects rhythmically coordinated flexion and extension of the right elbow and wrist under the following conditions: (1) forearm supine: wrist flexion/elbow flexion and vice versa; and (2) forearm prone: wrist flexion/elbow extension and vice versa. Starting in either pattern, subjects rotated the forearm in eight 20 steps, producing 15 cycles of motion at a frequency of 1.25 Hz. Switching from pattern (1) to pattern (2) and vice versa was observed at a critical spatial orientation. The critical point depended on the direction of forearm rotation, thus revealing the hysteretic nature of the switching. En route to the transition, regardless of direction of change, critical fluctuations and critical slowing down were observed in the relative phasing between the joints. Such results provide definitive evidence that relative phase is a viable order parameter, spatial orientation a relevant control parameter and loss of stability the chief mechanism leading to observed changes in coordination.

Humans are often faced with tasks that require stabilizing inherently unstable situations. We performed four experiments to explore the nature of functional stabilization. In Experiment 1 participants balanced a pole until a time criterion was reached. The geometry, mass, and characteristic "fall time" of the pole were manipulated. Distributions of timing between pole and hand velocities showed strong action-perception coupling. When actions demonstrated a potential for failure, the period of hand oscillation correlated significantly with the "time to balance" (t bal=theta/theta.), where q is pole angle re: the vertical balance point, but not other quantities such as theta and theta. alone. This suggested that participants were attending to available t bal information during critical situations. In a model analysis and simulation, we demonstrated how discrete t bal information may be used to adjust the parameters of a controller to perform this task. In Experiment 2 participants balanced a virtual inverted pendulum under manipulations designed: (1) to decouple the mechanics of the system from its visual image; (2) to alter the mapping of perception and action; and (3) to perturb successful balancing. A replication of the correlation analysis of Experiment 1 revealed that across all conditions, significant relationships existed between visually specified t -variables and hand oscillation during critical motions of the pole. These results suggested that participants use the same t bal information to successfully stabilize both virtual and physical unstable systems, despite quite dramatic visual and mechanical transformations. In Experiments 3 and 4 we investigated how parts of the body, or individuals in a social dyad cooperate to perform a functional stabilization task. Participants balanced a pole either intermanually (using 2 separate hands) or interpersonally (2 persons each using their preferred right hand) until a time criterion was reached. Although the magnitudes of the forces exerted by each hand were different, an analysis of the timing of the forces revealed that intermanual (interpersonal) participants developed a consistent antiphase (inphase) coordination pattern. These different coordination patterns allowed for the recruitment of previously unavailable efferent and afferent connections to produce the net forces that served to stabilize the pole via theta. (see Experiment 1).

The dynamics of recruitment and suppression processes are studied in the coupled pendulum paradigm developed by Kugler and Turvey (1987). Experimentally, the main concern is whether pendulum motion in this task is purely planar. Theoretically, the main concern is whether one-dimensional phase equations developed originally by Haken, Kelso and Bunz (1985) and the symmetry breaking extension by Kelso, Delcolle and Schoner (1990), can capture the richness of the dynamics of this experimental model system. In experiment 1, subjects swung single hand-held pendulums in time with an auditory metronome whose frequency increased. Bifurcations from planar to spherical pendulum motion occurred at critical cycling frequencies. Typically, these frequencies were above the pendulum's eigenfrequency. Spectral measures showed that spherical pendulum motion was generated through the recruitment of wrist abduction and adduction. The spectral measures revealed that elbow flexion and extension was recruited as movement rate increased, presumably to stabilize pendulum motion. When recruited, both components frequency- and phase-entrained with the primary pendulum mover, wrist ulnar flexion-extension. In experiment 2, subjects swung coupled pendulums in either an in-phase or anti-phase coordinative mode as movement rate increased. Transitions between coordinative modes were not observed. Pattern stability, as defined by the variability of the phase relation between the pendulums, was not affected to any large degree by increasing movement rate. Bifurcations from planar to spherical motion emerged at critical cycling frequencies. Spectral measures demonstrated that this motion was generated by abduction and adduction of the wrist. Elbow flexion-extension motion was also recruited. The newly active components frequency- and phase-entrained with wrist ulnar flexion-extension. When the same neuromuscular components were recruited simultaneously, e.g., elbow motion in both arms, the components exhibited frequency- and phase-entrainment with the task defined pattern. The results demonstrate that recruitment processes stabilize the coordinative modes, thereby reducing the need to switch patterns. Both experiments revealed a much richer dynamics than ever observed in the coupled pendulum paradigm and question the application of one-dimensional phase equation models to the coupled pendulum paradigm.