the following detailed description is provided as an enabling teaching of embodiments of the invention . those skilled in the relevant art will recognize that many changes can be made to the embodiments described , while still obtaining the beneficial results . it will also be apparent that some of the desired benefits of the embodiments described can be obtained by selecting some of the features of the embodiments without utilizing other features . accordingly , those who work in the art will recognize that many modifications and adaptations to the embodiments described are possible and may even be desirable in certain circumstances . thus , the following description is provided as illustrative of the principles of the invention and not in limitation thereof , since the scope of the invention is defined by the claims . in an exemplary embodiment , the leg press exercise apparatus serves as a portable , lightweight , and effective exercise system that can be paired with an existing environmentally controlled lbnp box 30 to form a lbnp leg press exercise apparatus as shown in fig1 a . in this illustration , the legs of the user are simulated by links 40 . tubes 42 , 44 represent cooling ducts . four - bar linkage 20 movement is controlled by foot pedals 22 . the lbnp leg press exercise apparatus stresses the user &# 39 ; s lower extremities to counteract the effects of microgravity induced syndrome . fig1 b and 1c illustrate perspective and side elevation views , respectively , of the leg press exercise apparatus including the leg simulation links 40 with the leg press exercise apparatus positioned outside the lbnp box . fig2 a - 2b illustrate perspective views of an embodiment of the leg press exercise apparatus 10 with the four - bar linkage 20 at different positions during a cycle of movement . the leg press exercise apparatus 10 includes a four - bar linkage 20 , with a first pair of spaced - apart bars 19 and a second pair of spaced - apart bars 21 , rectangular frame 28 , foot pedals with strain gages 22 , a coil spring and damper mechanism 16 and 17 for each respective pair of spaced apart bars 19 and 21 , and an adjustable seat or chair 12 . each pair of bars 19 and 21 in the four - bar linkage 20 is pivotally attached at respective ends 23 and 25 at corresponding pivot points 27 on opposite sides 29 and 31 of adjustable sliding frame member 24 . each coil spring and damper mechanism 16 and 17 ( with the latter shown in phantom in fig2 a ) is respectively attached at one end 9 and 11 to bars 19 and 21 by ground pivots 15 and 15 ′, and at each respective second end 5 and 7 to the corresponding side 29 and 31 of adjustable sliding frame member 24 . an upper end 1 and 3 of each respective pair of bars is attached to a support bracket 30 on which a foot pedal 22 is mounted . the sliding frame member 24 is mounted on the rectangular base frame 28 with its position adjustable relative to the rectangular frame 28 by a knob and pin detent mechanism 26 . in some embodiments , a linear actuator can be included in the leg press exercise apparatus 10 as described below . the location of the linear actuator is indicated by reference number 14 in fig2 a ( not illustrated ) and is illustrated schematically in fig1 . fig3 illustrates a two dimensional diagram of a four - bar parallelogram linkage 60 paired with a sliding crank mechanism 66 representing kinematics of the four - bar linkage and coil spring and damper mechanisms . the sliding crank 66 is a spring and damping system that provides a variable resistance . as shown in fig3 , if a force ( f user ) 62 is applied by the user to the foot pedal , the parallelogram linkage 60 will guide the foot pedal along a circular - arc path at a fixed angle relative to the frame 68 . this maintains 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 according to the national osteoporosis foundation . with further reference to fig2 a - 2b , in order to accommodate a wide range of potential users , the location of the pedal system 22 is adjustable relative to the seat 12 location . this is accomplished through the use of a sliding member 24 that allows the user to adjust the position of the device 10 along the rectangular base frame 28 . fig4 - 5 illustrate perspective and side elevation views , respectively , of the leg press exercise apparatus 10 in an exemplary embodiment . as shown in fig4 - 5 , the sliding member 24 is easily adjusted over a range of positions ( e . g ., 14 cm in one embodiment ) by a spring - loaded knob and pin detent system 26 . fig6 - 7 illustrate a perspective and side elevation views , respectively , of the leg press exercise apparatus including a two - position chair in a raised position in an exemplary embodiment . the two - position chair 12 has frame members 11 and 13 . chair frame member 11 folds under the bottom side of frame chair member 13 to position the chair 12 for exercise . the raised position shown in fig5 - 6 enable the user to sit on frame member 11 until the user is ready to exercise . the coil spring and damper system 16 , acting as the prismatic joint in a slider - crank mechanism , provides the resistance for the leg press exercise apparatus 10 . using this force - generating slider - crank system in conjunction with the four - bar linkage 20 creates a nearly optimal resistance curve that approximates the strength curve of the user through the range of motion . this creates the high forces and stresses needed to maintain bone density and optimize the cardiovascular workout . fig8 illustrates the starting position of the leg press apparatus 10 with the spring at a resting position . fig9 illustrates the end position of the leg press exercise apparatus 10 with the spring fully compressed . 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 &# 39 ; s leg is fully extended . this trend in the resistance provides the desired optimized profile in relation to the human strength curve . the leg press exercise apparatus has been optimized to produce desirable force and motion properties using classical techniques in kinematics . fig1 illustrates a kinematic diagram for an exemplary embodiment of the leg press mechanism . the loop closure equation ( eq . 1 ) and velocity loop equation ( eq . 2 ), described below , 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 &# 39 ; s motion profile ( θ - dot ). assumed user motion profiles and their effect on dynamic resistance are discussed below . 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 eq . 3 . the inertial term in eq . 3 , i *{ umlaut over ( θ )}{ dot over ( θ )} is based on a position - dependent equivalent inertia approach known in the art . the motion of the user is expected to be slow , therefore , dynamic effects , including the force of the damper , are expected to be small . the damper is incorporated to prevent rapid movement in the event that the user &# 39 ; s foot slips off the pedal . the damper also helps to discourage high - speed exercise motion . l 0 ĵ − l 1 − l 2 e jθ − se jγ = 0 ( 1 ) { circumflex over ( j )} l 2 { dot over ( θ )} e jθ −{ dot over ( s )} e jγ −{ circumflex over ( j )} s { dot over ( γ )} e jγ = 0 ( 2 ) l *{ umlaut over ( θ )}{ umlaut over ( θ )}+ f user { dot over ( θ )} l 3 + f spring { dot over ( s )} = 0 ( 3 ) 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 eq . 3 by calculating an equivalent inertia of the system , i *, that varies with position . eq . 4 shows how such an equivalent inertia is determined . 1 2 ⁢ i * = ∑ i = 1 n ⁢ 1 2 ⁢ m i ⁡ ( x . i 2 + y . i 2 ) + 1 2 ⁢ i i ⁢ θ . i 2 ( 4 ) eq . 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 i 3 , it has the largest peak velocities and accelerations . the foot pedal is also the most massive element in the leg press exercise apparatus . one goal in designing the leg press exercise apparatus is to minimize inertial forces . this enables shaping 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 . in another embodiment , to further accommodate users of different strengths , an additional adjustment can be included in the leg press exercise apparatus . this additional feature personalizes the device by changing the initial preload in the spring along with the displacement curve . the geometry of the slider - crank mechanism can be changed by lowering the ground pivot on the right side of the mechanism as shown at 14 in fig1 b , 1c , and 4 - 7 and schematically in fig1 . lowering this pivot ( i . e ., changing the value of l 0 ) generally causes a vertical shift in the resistance curve . during testing , the largest single - leg forces during resistance exercise were 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 apparatus , equals 187 degrees and the minimal leg force was at 0 . 68 bw ( 136 lbs ) when γ equals 177 degrees . the leg press exercise apparatus was able to elicit loads comparable to exercise on earth since the forces were greater than 1 bw . when paired with lbnp , the maximum resistance load could be as low as 196 lbf when the lbnp is set for the recommended 50 mm hg to achieve a maximum of 2 bw . in a further embodiment , a linear actuator can be incorporated into the leg press exercise device to control the position of the ground pivot . the adjustment will occur automatically based on the user &# 39 ; s heart rate ( hr ). the user will be required to keep a steady target heart rate that will be determined , using eq . 5 , before testing and monitored throughout the workout . the spring can be changed manually with the use of quick release pins . ground reaction forces ( grf ) are created by static and dynamic loading . the forces experienced in 1g are due to the user &# 39 ; s weight ( static ) and the dynamic loading is due to movement . to simulate forces equivalent to those experienced in 1g , the ground reaction force must be equal to or greater than 1 bw . as indicated in eq . 6 , the grf are directly related to the pressure differential force and the total user force applied to move the foot pedal . it should be noted that the vacuum feature of the lbnp box was not used during preliminary testing . eq . 6 states that the pressure differential force ( the product of the body cross - sectional area multiplied by the pressure differential across the lbnp box , which will be assumed to equal 50 mm hg ) plus the total user force ( the inertial forces caused by geometry and the force required to overcome the resistance of the coil spring and damper system ) equals the ground reaction force . the total user &# 39 ; s force includes the inertial forces caused by the leg press exercise apparatus and the force required to overcome the resistance of the coil spring and damper system . the posterior side of the lower extremities are accustomed to ⅔ bw between six and eight hours a day . the chair simulates this daily activity of sitting by translating a fixed linear force to the active areas . the force applied is simulated from the negative pressure in the lbnp box . as indicated in eq . 7 , if the user is sitting or remains static , the total user force term in eq . 6 equals zero . the grf then becomes : the chair is adjustable in both angle and linear distance by the use of quick release pins and a sliding member . the chair can be folded easily and has a resting position horizontal to the center bar . the chair can be cushioned by foam and covered with leather allowing the user to both exercise and sit comfortably . the leg press exercise apparatus can be manufactured to be removable , without disassembly , from the lbnp inner structure . the leg press exercise apparatus can be attached to a trolley system , making the apparatus maneuverable and easily accessible which allows the user to adjust his 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 . fig1 illustrates a perspective view of an upright device for supporting the leg press exercise apparatus in a vertical position . the physiological and biomechanical responses of each user can be recorded in both 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 the upright position can be compared to similar data in the supine position . if the lbnp is effective , user forces , heart rate , and expended energy should be comparable between the two configurations . it is well known that the human body is a highly nonlinear mechanical device from the standpoint of generating forces over a given cycle of motion . the leg press provides a good example of this . a plot of the maximum force a user can produce at each point in the outward cycle of a leg press would show , not surprisingly , that the user is 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 . mechanical work and physiology stress in the muscles will be nearly optimized when the resistance provided by a leg press exercise machine most nearly matches this strength curve . stated simply , 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 as does the resistance curve , the general shape of the curves is approximately maintained . the leg press exercise apparatus approximately matches the resistance provided by the apparatus with the human strength curve in a leg press exercise . an adjustment could be provided to raise and lower the magnitude of resistance while keeping the general shape of the curve . the slider - crank mechanism used in the leg press exercise apparatus creates an excellent approximation to the human strength curve when considering only the resistance of the spring . by limiting dynamic forces , the overall apparatus exhibits an excellent resistance curve under typical operating conditions . the theoretical resistance provided by the leg press exercise apparatus can be calculated under a set of assumed conditions . the analysis uses the actual link masses and inertias from the apparatus . the most important assumption necessary to perform a complete analysis is the user &# 39 ; s motion profile . since the foot petals reciprocate , 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 . results from testing indicate that a typical user moves at about one cycle of motion per second . two different motion profiles were used to calculate the inertia and user force . the first profile used 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 had no period of constant velocity motion separating the periods of positive and negative constant acceleration . the results of these two analyses are shown in fig1 and fig1 , respectively . in both figures , the user force curve shows the user force on the foot pedal due to the resistance of the spring , the inertia curve shows the user force on the pedal due to dynamic effects , and the superposition curve is the net user force on the pedal through a 0 . 5 second stroke . in one test of the multi - user device , an electrogoniometer was applied to the subject &# 39 ; s left knee , centered directly over the rotational joint . fig1 illustrates the results from the electrogoniometer test when the use is in the supine position . the electrogoniometer limits were calibrated for 0 ° when the user &# 39 ; s knee was straight , the top limit equaled 200 volts , and for 90 °, when the user &# 39 ; s knee was bent , the lower limit equaled zero volts . the vertical line in fig1 indicates a maximum voltage of 141 . 5 volts at roughly 90 degrees . this curve indicates that the user is generally accelerating or decelerating the foot pedal , with little or no constant velocity in the middle . 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 fig1 show the variation in user foot pedal force as the spring preload increases through a change in the adjustable dimension l 0 . the top curve used a dimension l 0 = 0 . 38 . the middle curve used a dimension l 0 = 1 . 38 . the bottom curve used a dimension l 0 = 2 . 0 . exercising in space is the most effective known method of counteracting the deleterious effects of living for prolonged periods in low gravity conditions . however , even with rigorous exercise , astronauts typically lose 0 . 4 - 1 . 0 % of their bone density per month in space . although astronauts gradually recover their muscle tissue and most of their bone mass when they return to earth , it is important that they are strong enough to perform emergency procedures during landing . coupling appropriate resistance exercise with a lower body negative pressure ( lbnp ) box will improve on the current state of the art in preventing deconditioning and bone loss . the leg press exercise apparatus was designed to fit within an existing lbnp box , which placed significant constraints on its dimensions and operation . this caused the length requirement for the subject &# 39 ; s lower extremities , waist to sole of foot , to range from 70 cm to 82 cm . a linear actuator in another embodiment would change the level of resistance based directly off the subject &# 39 ; s heart rate . after testing the embodiment illustrated in fig4 - 5 , it was found that the angle of the foot pedal is preferably adjusted so that the user &# 39 ; s foot maintains an angle closer to 90 ° throughout the entire cycle rather than just toward the beginning and the end of the stroke . if too much of the force from the subject &# 39 ; s foot is directed along the link , user forces will be somewhat higher than desired for the first half of the pedal stroke . overall , the combination of the multi - platform and the lbnp box provides the advantage of minimizing deconditioning in a safe , compact , lightweight and efficient way for space travelers to exercise . the leg press exercise apparatus can also incorporate automated adjustments and feedback control to maintain the user &# 39 ; s heart rate while providing near optimal resistance curves . the corresponding structures , materials , acts , and equivalents of all means plus function elements in any claims below are intended to include any structure , material , or acts for performing the function in combination with other claim elements as specifically claimed . those skilled in the art will appreciate that many modifications to the exemplary embodiments are possible without departing from the scope of the present invention . in addition , it is possible to use some of the features of the embodiments disclosed without the corresponding use of the other features . accordingly , the foregoing description of the exemplary embodiments is provided for the purpose of illustrating the principles of the invention , and not in limitation thereof , since the scope of the invention is defined solely by the appended claims .
a compact , portable , lightweight , easily transportable leg press exercise apparatus to simulate both exercise and the daily activity of sitting in a microgravity environment . the exercise portion of the apparatus creates stress on the lower extremities by supplying a variable resistance to a reciprocating foot pedal by way of a coil spring and damper system acting through a four - bar linkage . the chair is adjustable in angle to fit each user and to simulate a force that is two - thirds of body weight . by combining resistance exercise and lower body negative pressure with the lbnp leg press exercise apparatus , the users experience one or more times body weight in stress on their musculoskeletal , cardiovascular and nervous systems . by achieving one times bw or greater during exercise and two - thirds bw during sitting , the gap between the precondition and post condition syndrome will become smaller .