s 339 having a higher strength in the longitudinal than in the transverse direction. Mechanical roles of both networks are proposed and age etfects on fiber structure are discussed. BEHAVIOR OF CARTILAGE LAYERS SUBJECTED TO PERIODIC DYNAMIC REGIMES S. TEPIC, T. J. MACIROWSKI, R. W. MANN (Massachusetts Institute of Technology. 77 Mass. Ave., Rm. 3-147, Cambridge, MA 02139, U.S.A.) W. H. HARRIS (Massachusetts General Hospital, Orthopaedic Research Laboratory. Boston. MA, U.S.A.) The response of cartilage layers of the human hip joint subjected to periodic dynamic load and motion is simulated using a model based on the actual layer geometries measured in uirro by a noncontacting ultrasonic technique. Ulltrasound is concurrently used to measure the local uniaxial strainequivalent. high-frequency elastic modulus of the cartilage, (E ,). The local zero-frequency modulus, (E,), is estimated from ,!? I based on the ratios of the two from the literature. For most physiologically relevant dynamic regimes (such as walking) fundamental harmonic frequency is above the region of complex viscoelastic response of cartilage; thus the viscoelastic behavior is characterized at but two points of the frequency domain-zro and “infinity”. Since the articulating surfaces contact problem prohibits a one-step solution, an iterative computational procedure was developed to lrolve the equilibrium equations. Another iteration loop around the equilibrium-search procedure solves the problem of relaxation under zero-frequency content of strains. (Supported by the Whitaker Professorship of Biomedical Engineering, M.I.T. ; The Kroc Foundation ; and NIH Grant AM161 16.) THE EFFECT OF WATER ON THE MECHANICAL PROPERTIES OF TENDON COLLAGEN D. F. BETSCH, A. HILTNER and E. BAER (Case Western Reserve University, Cleveland, Ohio 44106, U.S.A.) Tendon employs a hierarchically arranged multicomposite of collagen, polysaccharides, and water to achieve a specific mechanical function. The mechanical contribution of water to rat tail tendon is presented. The uniaxial stress-strain curve for native mature tendon shows a non-linear “toe” region, modulus region, and two yield plateaux. Thils behavior is essentially maintained if free water remains in the system (above 209; water). A low strain “shoulder” appears when there is no free water in the tissue. The maximum strength and toughness occur at 11% water, where the fracture stress and strain are 50 kg/mm’ and 15% for the native tendon. Coilagen fib& trace a planar zig-zag waveform through the tendon. As a result, deformation involves sliding offibrous units past one another as the crimp straightens. The mechanical results are discussed qualitatively in terms of “crosslinking” interactions among various fibrous moieties in the hierarchy. (This work is generously supported by NIH Grant No. 00361-20). A BIOMECHANICAL ANALYSIS OF THE INTERRELATIONSHIPS OF PROPUlSlVE FORCE, VELOCITY, AND STROKE RATE IN THE FRONT CRAWL STROKE *T. M. ADAMS II, *R. A. YEA~ER and tR. B. MARTIN (*Human Performance Laboratory, torthopedic Research Laboratory, West Virginia University, Morgantown, WV 26505, U.S.A.) This study evaluated the interrelationship between propulsive force, stroke rate, and velocity in male and female varsity swimpers in the front crawl stroke. Fully tethered force (FTF) and free swimming velocity (FSV) were measured as functions of stroke rate and partially tethered swimming (PTS) was measured as a function of release velocity. As would be expected both FTF and FSV increased with stroke rate. The partially tethered experiments confirmed a negative linear relationship between tether force and release velocity. Individual graphs provide a practical method of determining ideal stroke rates and events for which a subject would be most adapted. Finally, it was observed that women had a greater capacity than men to choose a stroke rate that wauld produce the greatest tether force of free swimming velocity.