Musculoskeletal Modeling and Countermeasures to Space Flight
Poster Abstract

A comprehensive approach is necessary to obtain practical and effective measures to counter the bone loss and muscle atrophy associated with reduced gravity, non-terrestrial environments (space, Moon, Mars). The main objectives of this work are to develop exercise countermeasures (primarily resistive exercise) designed to enhance human musculoskeletal strength and function in the most efficient manner possible. To accomplish this objective, a thorough understanding of exercise biomechanics and musculoskeletal adaptation is required. We are using a combined analytical, numerical and experimental modeling approach to I) study the forces acting on the musculoskeletal system during exercise, II) predict the adaptive response of the musculoskeletal system, and III) develop a resistive exercise program/system that can be utilized in a microgravity environment.

An important first step toward understanding musculoskeletal adaptation necessitates a precise knowledge of muscle strength and loads acting on the skeleton. An analytical model to predict forces and moments associated with high-intensity exercise has been developed to quantify trunk muscle strength and to predict forces and moments acting on the human spine during isometric trunk flexion, extension, lateral flexion and rotation movements [1]. The next step is to predict how the musculoskeletal system responds to the imposed forces. A simple density/load adaptive theory has been proposed to determine the exercise activity level required to maintain a specific skeletal density/strength in a given gravity environment [2]. This work indicates, for example, that skeletal strength is maintained above the fracture risk level for normal activity performed in a lunar gravity environment (1/6 g). Simulated weightlessness and exercise research performed by this investigator also suggest that the cortical skeleton adapts its geometry and material properties in order to preserve an optimal strength/tissue stress ratio of about 3-5 [3,4]. Moreover, in comparison to cortical bone, exercise/disuse produces a more rapid and approximately 10-fold greater change in trabecular bone properties [5], which suggests that efforts should focus on adaptation of trabecular bone tissue. Consequently, we have begun to investigate large scale, anatomic, finite element modeling studies of trabecular bone tissue stress and stress adaptation. This work shows that trabecular tissue stresses are many fold greater than the actual apparent stresses applied to skeletal structures, and suggests that an important mechanism of skeletal adaptation may be local tissue microdamage [6-8]. In order to better understand skeletal tissue adaptation to stress, an iterative, finite element method (FEM) scheme has been investigated, which involves analysis of stresses and strains within a 2D or 3D finite element domain, followed by successive iterations with the goal to minimize strain energy density of the structure [9]. Structures have been constructed using this "self-optimization" numerical approach that have remarkable similarities to trabecular bone structures found in the axial skeleton of humans. Current analytical and numerical efforts focus on expanding the analytical model to predict dynamic forces and moments acting on the axial skeleton and integrating a microdamage-based adaptive regime within the framework of the self-optimization scheme. This work will provide the baseline information and insight necessary to develop efficient exercise countermeasures for space flight.

Previous research has identified constant resistance or isoinertial exercise as an essential element for efficient maintenance of muscle and bone mass in non-terrestrial environments [10]. In this work, a strategy for biomedical support of humans during long periods of space flight and/or non-terrestrial environments is presented that is consistent with current NASA mandates for efficient utilization of crew time (HEDS54). Ultimately, however, a new resistive exercise device is being sought by NASA for the ISS, which will allow weight training type exercise in zero gravity. At the University of Vermont, we have begun to examine the effect of load intensity, duration and postural effects of resistive, standing cycle exercise on musculoskeletal adaptation response. This research, dubbed "Resist-Stance", targets a broad spectrum of exercise, cross-training and rehabilitation applications. The Resist-Stance methodology utilizes a standing cycle ergometer and emphasizes relatively low RPM¹s, while targeting greater resistance as work effort increases. In contrast to conventional harnessed treadmill and horizontal cycling space exercise countermeasures methods, Resist-Stance emphasizes simultaneous activation of the entire musculoskeletal system, provides a closed kinetic chain between the hands and feet, and promotes muscle activation using an unconstrained, standing posture, cyclic exercise. With deliberate use this maximizes the utilization of the body's combined segmental degrees of freedom while applying variable to maximal resistance, and is thereby capable of simulating the gravitational, centripetal and centrifugal forces acting on all intersegmental joints and musculature. The method targets increasing upper extremity and trunk musculature recruitment to assist lower extremity work effort as resistance is increased, and integrates the upper and lower body to actively stabilize the mid body. The net effect is to increase forces and the degrees of freedom of the individual joints involved. Simultaneous activation of the entire musculoskeletal system should produce an efficient exercise prescription for long duration space missions. Our goal is to develop a standardized sub-set of closed chain Resist-Stance exercises that optimize skeletal forces and muscle activation patterns commensurate with maintenance of optimal levels of bone and muscle mass during long-duration space missions.

References: [1], D Guzik, T Keller, M Szpalski, et al. (1996) A Biomechanical Model of the Lumbar Spine During Upright Isometric Flexion, Extension, and Lateral Bending: Spine 21:427-433; [2] T Keller and A Strauss (1993) Predicting Skeletal Adaptation in Altered Gravity Environments: Journal of the British Interplanetary Society 46:87-96;[3], A Abram, T Keller, and D Spengler (1988) The Effects of Simulated Weightlessness on Bone Biomechanical and Biochemical Properties in the Maturing Rat: J. Biomechanics 21:755-767, 1988; [4] T Keller and D Spengler (1989) Regulation of Bone Stress and Strain in the Immature and Mature Rat Femur: J. Biomechanics 22:1115-1128; [5] T Hansson and T Keller (1996) Osteoporosis of the Spine (In: The Lumbar Spine, 2nd Edition, Wiesel S., Weinstein J., Herkowitz H., et al. Eds.), WB Saunders Company, New York, Vol. 2, pp 969-988; [6] R Saxena, T Keller and J Sullivan (1999) A Three-Dimensional Finite Element Scheme to Investigate the Apparent Mechanical Properties of Trabecular Bone. Computer Methods in Biomechanics and Biomedical Engineering, 2:285-294; [7] R Saxena and T Keller (1999) A Volumetric Finite Element Scheme to Investigate the Mechanical Properties of Normal and Osteoporotic Trabecular Bone. (In: IUTAM Symposium on Synthesis in Bio Solid Mechanics, P Pedersen & M Bensøe, Eds), Kluwer Academic Publishers, Dordrecht, pp 373-386; [8] R Saxena and T Keller (1999) Computer Modeling for Evaluating Trabecular Bone Mechanics: (In: Mechanical Testing of Bone and the Bone-Implant Interface, Y An & R Draughn, Eds.), CRC Press, Boca Raton, FL, pp. 407-436; [9] Finite Element Modeling of Bone Tissue Stress-Adaptation: T.S. Keller, SPIE International Conference on Complex Adaptive Structures, Hutchinson, Island, June 4-6, 2001; [10] T Keller, A Strauss, M Szpalski (1992) Prevention of bone loss and muscle atrophy during manned space flight: Microgravity Quarterly 2:89-102.