A. C. Vailas

Mechanical, morphological and biochemical adaptations of bone and muscle to hindlimb suspension and exercise

The influences of weightbearing forces on the structural remodeling, matrix biochemistry, and mechanical characteristics of the rat tibia and femur and surrounding musculature were examined by means of a hindlimb suspension protocol and highly intensive treadmill running. Female, young adult, Sprague-Dawley rats were designated as either normal control, sedentary suspended, or exercise suspended rats. For 4 weeks, sedentary suspended rats were deprived of hindlimb-to-ground contact forces, while the exercise suspended rats experienced hindlimb ground reaction forces only during daily intensive treadmill training sessions. The suspension produced generalized atrophy of hindlimb skeletal muscles, with greater atrophy occurring in predominantly slow-twitch extensors and adductors, as compared with the mixed fiber-type extensors and flexors. Region-specific cortical thinning and endosteal resorption in tibial and femoral diaphyses occurred in conjunction with decrements in bone mechanical properties. Tibial and femoral regional remodeling was related to both the absence of cyclic bending strains due to normal weightbearing forces and the decrease in forces applied to bone by antigravity muscles. To a moderate extent, the superimposed strenuous running counteracted muscular atrophy during the suspension, particularly in the predominantly slow-twitch extensor and adductor muscles. The exercise did not, however, mitigate changes in bone mechanical properties and cross-sectional morphologies, and in some cases exacerbated the changes. Suspension with or without exercise did not alter the normal concentrations of collagen, phosphorus, and calcium in either tibia or femur.

Effects of short-term immobilization versus continuous passive motion on the biomechanical and biochemical properties of the rabbit tendon

Little work has been done on the effect that continuous passive motion (CPM) may have on maintaining the mechanical and biochemical characteristics of tendons deprived of normal load-bearing stimuli. This study compared changes in mechanical properties and collagen concentrations of the tibialis anterior tendon between two groups of rabbits: control animals that received no treatment and experimental animals that received CPM to one ankle and immobilization to the other. Treatment duration was three weeks. Mechanical testing yielded values for elastic modulus, stiffness, stress, strain, and strain energy density at linear and maximum loads. Hydroxyproline concentrations were measured to determine tendon collagen content. Findings indicated that cyclic tensile loading of tendons by CPM lessened the harmful effects of immobilization. Linear load and stress for CPM and control tendons were similar, while the same measures for immobilized tendons were 16% and 25% less than control tendons. Ultimate strength of the control tendons, however, exceeded CPM tendons by 16% and immobilized tendons by 20%. Hydroxyproline levels were comparable for the CPM, control, and immobilized tissues. For a short duration (three weeks), CPM reduced the detrimental effects of immobilization in the tendon, primarily in the linear-load region.

Effects of spaceflight on rat humerus geometry, biomechanics, and biochemistry.

The effects of a 12.5‐day spaceflight (Cosmos 1887 biosatellite) on the geometric, biomechanical, and biochemical characteristics of humeri of male specific pathogen‐free rats were examined. Humeri of age‐matched basal control, synchronous control, and vivarium control rats were contrasted with the flight bones to examine the influence of growth and space environment on bone development. Lack of humerus longitudinal growth occurred during the 12.5 days in spaceflight. In addition, the normal mid‐diaphysial periosteal appositional growth was affected; compared with their controls, the spaceflight humeri had less cortical cross‐sectional area, smaller periosteal circumferences, smaller anterior‐posterior periosteal diameters, and smaller second moments of area with respect to the bending and nonbending axes. The flexural rigidity of the flight humeri was comparable to that of the younger basal control rats and significantly less than that of the synchronous and vivarium controls; the elastic moduli of all four groups, nonetheless, were not significantly different. Generally, the matrix biochemistry of the mid‐diaphysial cross sections showed no differences among groups. Thus, the spaceflight differences in humeral mechanical strength and flexural rigidity were probably a result of the differences in humeral geometry rather than material properties.—Vailas, A. C.; Zernicke, R. F.; Grindeland, R. E., Kaplansky, A.; Durnova, G. N., Li, K.‐C.; and Martinez, D. A. Effects of spaceflight on rat humerus geometry, biomechanics, and biochemistry. FASEB J. 4: 47‐54; 1990.

Biomechanical response of bone to weightlessness

“Our job is not only to make sure astronauts can function adequately in space, but also that they can function on their return to earth.”

Regional biochemical and morphological characteristics of rat knee meniscus

The biochemical and morphological characteristics of the anterior and posterior regions of the rat knee meniscus were studied. The anterior meniscal horn was thicker and contained a lower concentration of DNA, hydroxyproline, and uronic acid as compared to the posterior region. The calcium concentration in the anterior region, however, was significantly greater than the calcium concentration in the posterior horn. Presence of a significant concentration of calcium in the normal rat knee meniscus is unique to rats and uncommon in other mammalian species.