***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: The purpose of this study is to assess cardiovascular responses to  lower body positive pressure ( LBPP )  and to examine the effects of LBPP unloading on gait mechanics during treadmill ambulation . We hypothesized that LBPP allows comfortable unloading of the body with minimal impact on the cardiovascular system and gait parameters . Fifteen healthy male and female subjects ( 22 - 55 yr ) volunteered for the study . Nine underwent noninvasive cardiovascular studies while st and ing and ambulating upright in LBPP , and six completed a gait analysis protocol . During stance , heart rate decreased significantly from 83 + /- 3 beats/min in ambient pressure to 73 + /- 3 beats/min at 50 mmHg LBPP ( P < 0.05 ) . During ambulation in LBPP at 3 mph ( 1.34 m/s ) , heart rate decreased significantly from 99 + /- 4 beats/min in ambient pressure to 84 + /- 2 beats/min at 50 mmHg LBPP ( P < 0.009 ) . Blood pressure , brain oxygenation , blood flow velocity through the middle cerebral artery , and head skin microvascular blood flow did not change significantly with LBPP . As allowed by LBPP , ambulating at 60 and 20 % body weight decreased ground reaction force ( P < 0.05 ) , whereas knee and ankle sagittal ranges of motion remained unaffected . In conclusion , ambulating in LBPP has no adverse impact on the systemic and head cardiovascular parameters while producing significant unweighting and minimal alterations in gait kinematics . Therefore , ambulating within LBPP is potentially a new and safe rehabilitation tool for patients to reduce loads on lower body musculoskeletal structures while preserving gait mechanics Walking humans conserve mechanical and , presumably , metabolic energy with an inverted pendulum-like exchange of gravitational potential energy and horizontal kinetic energy . Walking in simulated reduced gravity involves a relatively high metabolic cost , suggesting that the inverted-pendulum mechanism is disrupted because of a mismatch of potential and kinetic energy . We tested this hypothesis by measuring the fluctuations and exchange of mechanical energy of the center of mass at different combinations of velocity and simulated reduced gravity .  Subjects walked with smaller fluctuations in horizontal velocity in lower gravity , such that the ratio of horizontal kinetic to gravitational potential energy fluctuations remained constant over a fourfold change in gravity . The amount of exchange , or percent recovery , at 1.00 m/s was not significantly different at 1.00 , 0.75 , and 0.50 G ( average 64.4 % ) , although it decreased to 48 % at 0.25 G. As a result , the amount of work performed on the center of mass does not explain the relatively high metabolic cost of walking in simulated reduced gravity The Hoffmann ( H- ) reflex is an electrical analogue of the monosynaptic stretch reflex , elicited by bypassing the muscle spindle and directly stimulating the afferent nerve . Study ing H-reflex modulation provides insight into how the nervous system central ly modulates stretch reflex responses . A common measure of H-reflex gain is the slope of the relationship between H-reflex amplitude and EMG amplitude . To examine soleus H-reflex gain across a range of EMG levels during human locomotion , we used  simulated reduced gravity  to reduce muscle activity . We hypothesised that H-reflex gain would be independent of gravity level . We recorded  EMG from  eight subjects walking ( 1.25 m s-1 ) and running ( 3.0 m s-1 ) at four gravity levels ( 1.0 , 0.75 , 0.5 and 0.25 G ( Earth gravity ) ) . We normalised the stimulus M-wave and result ing H-reflex to the maximal M-wave amplitude ( Mmax ) elicited throughout the stride to correct for movement of stimulus and recording electrodes relative to nerve and muscle fibres . Peak soleus EMG amplitude decreased by ~30 % for walking and for running over the fourfold change in gravity . As hypothesised , slopes of linear regressions fitted to H-reflex versus EMG data were independent of gravity for walking and running ( ANOVA , P > 0.8 ) . The slopes were also independent of gait ( P > 0.6 ) , contrary to previous studies . Walking had a greater y-intercept ( 19.9 % Mmax ) than running ( -2.5 % Mmax ; P < 0.001 ) . At all levels of EMG , walking H-reflex amplitudes were higher than running H-reflex amplitudes by a constant amount . We conclude that the nervous system adjusts H-reflex threshold but not H-reflex gain between walking and running . These findings provide insight into potential neural mechanisms responsible for spinal modulation of the stretch reflex during human locomotion 
    Output:
    
Studies investigating the immediate effects of partial gravity exposure reveal that cardiopulmonary parameters such as heart rate , oxygen consumption , metabolic rate , and cost of transport are reduced compared to 1 g , whereas stroke volume seems to increase with decreasing gravity levels .
Biomechanical studies reveal that ground reaction forces , mechanical work , stance phase duration , stride frequency , duty factor and preferred walk-to-run transition speed are reduced compared to 1 g. Partial gravity exposure below 0.4 g seems to be insufficient to maintain musculoskeletal and cardiopulmonary properties in the long-term .
To compensate for the anticipated lack of mechanical and metabolic stimuli some form of exercise countermeasure appears to be necessary in order to maintain reasonable astronauts ' health , and thus ensure both sufficient work performance and mission safety