Resistance Exercise Machine within Lower Body Negative Pressure for Counteracting Effects of Microgravity

Gravity has had an integral effect on the development of life on Earth over millions of years and has shaped the anatomy and physiology of human beings. Exposure to microgravity has been shown to affect the body, causing it to undergo a reduction in heart size (Dorfman et al., 2007) and blood volume (Watenpaugh, 2001), central nervous head and spine impaired balance control (Mulavara et al., 2010), changes in nervous system sensitivity, specifically the vestibular system (Benson, 2002), decreases in bone and muscle mass (Morgan et al., 2012), and reduction of the immune function (Martinelli et al., 2009). Astronauts in space during short or long-term missions have demonstrated these physiological changes, known as space deconditioning, which may lead to undesirable health consequences and to operational difficulties, especially in emergency situations during the return to Earth (Hughson et al., 2012; Baqai et al., 2009).

In the future, it is expected that greater numbers of astronauts will work and live in low-gravity environments, and the need to understand the in-flight and post-flight consequences of microgravity exposure will become more significant. The physiological adaptations to weightlessness seem to be better tolerated by the astronauts than the changes suffered after they return to Earth. In space, the mechanical unloading affects the musculoskeletal system (Smith et al., 1998), causing significant atrophy, especially affecting the anti-gravitational group of muscles. It has been demonstrated that the vastus meadilias, vastus lateralis, gastrocnemius medialis, and tibialis anterior presented an important degree of atrophy after five weeks of a ground-based microgravity simulation (Berg et al., 2007). During actual exposure to microgravity, the calf muscle can decrease 13% after five months on board the ISS (Shackelford et al., 2004). The decrease in bone mass is also of great concern to space physiologists and physicians, as the normal processes of bone formation and resorption are disturbed, favoring a loss of bone tissue (Caillot-Augusseau et al., 1998). This process begins after the introduction into microgravity and can range between 1% and 2% of bone mass loss per month, affecting astronaut’s health during the return to Earth (Lang et al., 2004). One of the first responses to spaceflight is the shift of blood and body fluids towards the upper body, with subsequent adaptations occurring over a few days to lower overall blood volume through activation of several mechanisms (Charles and Lathers, 1991; Thornton et al., 1977). It is upon return to Earth that the cardiovascular deconditioning raises concerns by producing significant orthostatic intolerance and decreasing aerobic performance (Convertino and Cooke, 2005).

Many different types of countermeasures have been developed (ranging from specific diets to heavy exercise protocols) that must be performed periodically by the astronauts during a space mission. However, it is believed that a very efficient way to counterbalance space deconditioning is by improving the exercise countermeasures protocols, devices, and systems aiming to provide the terrestrial gravitational stress to the osteomuscular and cardiovascular systems. Among the countermeasures currently under testing, resistance exercise training regimen in space seems to be the most complete because it prevents increased bone resorption (Shackelford et al., 2004; Trappe et al., 2009), muscle loss, and cardiovascular deconditioning. However, the effectiveness of exercise protocols (aerobic and resistance exercise) and equipment (treadmill, cycloergometer, and Resistive Exercise Device (RED)) for astronauts in space is unresolved and still under discussion. Studies indicate that all exercise in space to date has lacked sufficient mechanical and physiological loads to maintain preflight musculoskeletal mass, strength, and aerobic capacity (Davis and Cavanaugh, 1993; Gazenko et al., 1976; Vorobyev et al., 1976; Watenpaugh and Hargens, 1996). Recently, researchers have been pairing exercise with LBNP. The LBNP Box is a sealed chamber into which the human subject is partially inserted. A seal near the waist allows a vacuum to be applied to the chamber, thus creating a lower relative pressure on the subject’s lower body. This lower pressure helps pull bodily fluids toward the feet (Russomano et al., 2005).

Combining the resistive force from exercise and a uniformed pressure distribution to the lower extremities has shown to be an efficient solution for counteracting microgravity-induced deconditioning during terrestrial testing. A study of the addition of a treadmill to an LBNP Box has demonstrated that it is able to simulate physiological and biomechanical features of upright exercise (Boda et al., 2000). However, its mechanical design lacks mobility and is both large and heavy, making it unsuitable for spaceflight.

The research presented in this paper offers a new method that combines both, endurance and resistance exercise protocols. Preliminary data from our early test phases using gas analysis and electromyography showed that the physical effort required by the subject is considered enough to maintain cardiovascular and skeletal muscle fitness. The purpose was to design a lightweight, compactable exercise machine combined with a collapsible chair that could be easily integrated into a smaller, existing LBNP Box. The exercise device is to offer a constant load path to maintain compressive loads on the musculoskeletal system and aid to the human body as much as possible. The human body is a highly nonlinear mechanical device from the standpoint of generating forces over a given cycle of motion. The human strength curve for the leg-press exercise shows that the maximum force a user can produce at each point in the outward cycle gradually increases. Not surprisingly, we are able to generate far more force at the extreme position (when the knee joint is at full extension) than when the knee is sharply bent (Schulz et al., 2012). Mechanical work and effort in the muscles will be nearly optimized in relation to the muscle mechanics when the resistance provided by a machine most nearly matches this trend (Zwart et al., 2007). The resistance curve should match the human strength curve for optimal efficiency in strengthening muscle and stressing bone. Although the strength curve varies from user to user, the general shape of the curve is approximately maintained.

Our goal in designing this system was to match the resistance provided by the machine with the human strength curve in a leg press exercise. This requires an adjustable level of resistance that will lead to a vertical shift in magnitude of resistance while keeping the general shape of the curve to accommodate each user. In this paper the alternative system is referred to as a multi-platform. The multi-platform is to be a compact system that offers a constant load path throughout the cycle, and is to conform to the most natural movement of the human body as possible. The design was driven by both the dimensions of an existing LBNP Box and by the average size astronaut. Averaging the size and weight of astronauts (NASA, 2000) allowed for an initial range of resistance the multi-platform would impose on the user to simulate forces equal to one or more of their body weight (BW).

A 3-D SolidWorks model of the multi-platform is shown in Figure 1. The integrated system, the multi-platform, and the existing LBNP Box, is shown in Figure 2 where the brown-colored links simulate the human legs and feet. The green and red tubes represent cooling ducts that provide an environmentally controlled atmosphere. The system will stress the lower extremities of the human body by providing both a resistance force due to the exercise machine, and a pressure force caused by the LBNP Box. It is believed that these forces might help to counteract some of the undesirable effects of microgravity.

Figure 1.

Figure 2.

Parallelogram Linkage in Conjunction with Coil Spring and Damper System

Kinematics

Classic techniques in kinematics were used to design and optimize the geometry and resistance which would produce desirable motion and force properties (Tidwell et al., 1996). As shown in Figure 3, if a force is applied by the user to the foot pedal, the parallelogram linkage will guide the foot pedal along a circular-arc path at a fixed angle relative to the frame of the machine. This is important for maintaining a generally perpendicular relationship between the lower leg and the foot. Applying forces in this manner to the musculoskeletal system is believed to be one of the most efficient ways to counteract osteoporosis (Gianoudis et al., 2012).

Figure 3.

Referring to Figure 4, loop closure, Equation (1), and velocity loop, Equation (2), will yield the position, s, and velocity, s-dot, of the slider crank mechanism given the input position, θ, and velocity, θ-dot. Static resistance is dependent only on the value of θ, which determines the compression of the spring and the geometry of the device. Dynamic resistance depends on the user’s motion profile (θ-dot).Equation (1):l0ȷ^−l1−l2ejθ−sejγ=0l_{0} \hat{\jmath}-l_{1}-l_{2} e^{j \theta}-s e^{j \gamma}=0Equation (2):ȷ^l2θ˙ejθ−S˙ejγ−ȷ^sγ˙ejγ=0\hat{\jmath} l_{2} \dot{\theta} e^{j \theta}-\dot{S} e^{j \gamma}-\hat{\jmath} s \dot{\gamma} e^{j \gamma}=0

Figure 4.

Two different motion profiles were used to calculate the inertia and damping force. The first profile had constant angular acceleration of the foot pedal link to start and end the motion cycle, and a period of constant velocity in between. The second motion profile was similar, but with no constant velocity motion period separating the periods of positive and negative constant acceleration. An electrogoniometer was used as a method in confirming which assumed motion profile was most accurate. The meter was applied to the subject’s left knee, centered directly over the rotational joint.

Once the position and velocity loop equations have been solved, virtual work can be used to find the resistive force, F_user, as a function of position, θ, from Equation (3).Equation (3):I∗θ¨θ˙+Fuser θ˙l3+Fspring s˙=0I^{*} \ddot{\theta} \dot{\theta}+F_{\text {user }} \dot{\theta} l_{3}+F_{\text {spring }} \dot{s}=0

The inertial term in Equation (3), is based on a position-dependent equivalent inertia approach, as described in Suh and Radcliffe (1978). Note that the motion of the user is expected to be slow, so dynamic effects, including the force of the damper, are expected to be small. According to Equation (3), if the velocity and acceleration increases, the resistance counter effect also increases. Thus, the damper is incorporated to help prevent the spring from creating a rapid return since it could be a source of risk for the user. Additionally, the damping effect limits the occurrence of high-speed movement during exercise.

Resistance

A coil spring and damper system, acting as the prismatic joint in a slider-crank mechanism, provides resistance. Using this force-generating slider-crank system in conjunction with the 4-bar linkage creates a nearly optimal resistance curve that approximates the strength curve of the user through the range of motion. The range of motion is defined by θ; from 45 degrees, the start position, to 125 degrees, the end position. This system creates the high forces and stresses needed to maintain bone density and optimize the cardiovascular workout. The slider-crank mechanism compresses the linear spring, creating an increasing resistance throughout the movement and causing the largest load to be applied when the user’s leg is fully extended. This then provides the desired optimized profile in relation to the human strength curve.

Resistance Due to Inertial Forces

The user must overcome the static spring forces, the damping forces, and the inertia forces generated by acceleration of the links of the exercise device. Inertia forces are incorporated in Equation (3) by calculating an equivalent inertia of the system, I^*, that varies with position. Equation (4) from Suh and Radcliffe (1978) shows how such an equivalent inertia is found.Equation (4):12I∗=∑i=1n12mi(x˙i2+y˙i2)+12Iiθ˙i2\frac{1}{2} I^{*}=\sum_{i=1}^{n} \frac{1}{2} m_{i}\left(\dot{x}_{i}^{2}+\dot{y}_{i}^{2}\right)+\frac{1}{2} I_{i} \dot{\theta}_{i}^{2}

Equation 4 takes into account the mass (m) and inertia (I) of every moving link in the mechanism. While all links contribute to the total user force, the mass of the foot pedal is of special concern. Because the foot pedal is at the extreme end of link 3, it has the largest peak velocities and accelerations. It is also the most massive element in the prototype system. One goal in designing the device is to minimize inertial forces. This allows us to shape the static resistance curve (through kinematics) to be as similar to the human strength curve as possible. Dynamic forces will change the shape of this curve as a function of how rapidly the user moves the foot pedal. Further analysis will show that the dynamic forces can be kept small.

Biomechanics

Ground reaction forces (GRF) are created by static and dynamic loading. The forces experienced in 1G are due to the user’s weight (static), and the dynamic loading due to movement. To simulate forces equivalent to those experienced in 1G, the GRF must be equal to or greater than 1 BW. As shown in Equation (5), the GRF are directly related to the pressure differential force and the total user force applied by the user to move the foot pedal. Note that the vacuum feature of the LBNP Box will not be used during preliminary testing.Equation (5):GRF=( Pressure Differential Force )+( Total User Force )\mathrm{GRF}=(\text { Pressure Differential Force })+(\text { Total User Force })

Equation 5 states that the GRF during exercise in LBNP while supine and in microgravity, equals the pressure differential force plus the total user’s force. The pressure differential force equals the product of the body cross-sectional area (A_xy) and the pressure differential (∆P) across the LBNP Box, which will be assumed to equal 50 mmHg. The total user’s force includes the inertial forces caused by the geometry of the exercise portion of the multi-platform and the force required to overcome the resistance of the coil spring and damper system.

The Integration of the Multi-Platform Device and the Existing LBNP Box

The multi-platform device is manufactured to be removable, without disassembly, from the LBNP Box inner structure. It attaches to the trolley system, shown in Figure 5, making it maneuverable and easily accessible, which allows the user to adjust it to their personal settings outside of the LBNP Box. The parallel arms and seat collapse horizontally to the center bar, allowing the removal process to be quick, easy, and safe.

Figure 5.

The Integration of the Multi-Platform Device and the Upright Device

The physiological and biomechanical responses of each subject will be recorded in the supine and upright position in order to collect comparative data. In the upright position, there will be no added negative pressure or suction force, only the effects of gravity. Data collected in upright position will be compared to similar data taken in the supine position. If the LBNP is effective, the user’s responses should be most similar between the two configurations. The design and integration of the multi-platform device in the upright position are shown in Figure 6.

Figure 6.

In the next design iteration, a linear actuator will be incorporated to control the position of the above-mentioned ground pivot. The adjustment will occur automatically based on the user’s heart rate (HR). The user will be required to keep a steady target HR that will be determined using Equation (8) and monitored throughout the workout.Equation (8):HRtarget = (( HRmax− HRrest ) * %intensity ) + HRrest{\rm{HR}}_{{\rm{target}}} {\rm{ = (( HR}}_{{\rm{max}}} - {\rm{ HR}}_{{\rm{rest}}} {\rm{ ) * \% }}_{{\rm{intensity}}} {\rm{ ) + HR}}_{{\rm{rest}}}

Static Resistance

The multi-platform was to approximate the resistance provided by the machine with the human strength curve in a leg press exercise. As shown in Figure 7, the slider-crank mechanism used in the multi-platform creates a feasible approximation to the human strength curve when considering only the resistance of the spring. By limiting dynamic forces, the results show that the overall machine exhibits an increasing resistance curve under typical operating conditions.

Figure 7.

Total Theoretical Resistance

The theoretical resistance provided by the multi-platform device has been calculated under a set of assumed conditions. This analysis uses the actual link masses and inertias from the prototype with the exception of the foot pedal’s mass. In the final design, these values should be reduced. This should result in improved resistance profiles. The most important assumption necessary to perform a complete analysis is the user’s motion profile. Since the foot pedals reciprocate, we know that their angular velocity will be zero at the beginning and end of each stroke. Velocity should ramp up to a peak somewhere between these endpoints. But, there is no way to precisely predict how the user will accelerate and decelerate. We do know from testing that a typical user moves at about one cycle of motion per second. The results from the two assumed motion profiles are shown in Fig. 8 and Fig. 9. In both figures, the red curve shows the user force on the foot pedal due to the resistance of the spring, the green curve shows the user force on the pedal due to dynamic effects, and the blue curve is the net user force on the pedal through a 0.5 second stroke.

Figure 8.

Figure 9.

The output data from the electrogoniometer indicated that the user is generally accelerating or decelerating the foot pedal, with little or no constant velocity in the middle. As a result, the second velocity profile will be assumed for all subsequent analysis.

The analysis also considered the effect of varying the spring preload and the effect of the LBNP Box pressure difference on the foot pedal forces exerted by the user. The graphs in Fig. 10 show the variation in user foot pedal force as the spring preload increases through a change in the adjustable dimension l_o.

Figure 10.

The compact, easily transportable, multi-platform device is designed to simulate both exercise and the daily activity of sitting. The exercise portion of the device creates stress on the lower extremities by supplying a variable resistance to a reciprocating foot pedal. This resistance is created from a coil spring and damper system acting through a 4-bar linkage. The resisting force increases as a function of leg extension to maximize work done by the user in each cycle of motion. The sitting portion of the multi-platform device creates a resistance applied to the posterior side of the lower extremities by the use of a chair. The chair is adjustable in angle to fit each subject and to simulate a force 2/3 BW, mimicking the posterior forces equivalent to the human activity of sitting between 6-8 hours a day.

The multi-platform is paired with an existing LBNP Box to add an evenly distributed pressure-induced stress to the lower extremities. However, the LBNP Box constrains the length of the subject’s lower extremities, waist to sole of foot, to range from 70 cm to 82 cm. By combining resistance exercise and lower body negative pressure, the subject will experience one or more times BW in stress on their musculoskeletal, cardiovascular and nervous systems. By achieving 1 BW or greater (artificial gravity) during exercise and 2/3 BW during sitting, the gap between the precondition and post condition syndrome will become smaller. The largest single-leg forces during resistance exercise are 1.16 BW (232 lbs) during supine position when γ, the angle between the horizontal and the ground pivot on the right side of the mechanism, equals 187 degrees and minimal at 0.68 BW (136 lbs) when γ equals 177 degrees. We conclude that the exercise portion of the multi-platform was able to elicit loads comparable to exercise on Earth since the forces were greater than 1 BW and predict that when paired with LBNP the maximum resistance load can be as low as 196 lbf when the LBNP is set for the recommended 50 mmHg to achieve, at maximum, 2 BW.

Future versions of the machine should have lighter links and hence improved overall resistance curves. The multi-platform is fabricated from steel, which causes the inertia forces in the above calculations to be larger than desired. The angle of the foot pedal needs to be adjusted so that the user’s foot maintains an angle closer to 90° throughout the entire cycle rather than just toward the beginning and the end of the stroke. Currently, too much of the force from the subject’s foot is directed along the link, resulting in user forces that are somewhat higher than desired for the first half of the pedal stroke. Another future improvement includes a linear actuator to change the level of resistance based directly off the subject’s heart rate.

Overall, the combination of the multi-platform and the LBNP Box show great promise for minimizing deconditioning and for providing a safe, compact, lightweight and efficient way for space travelers to exercise.