musculoskeletal diseases due to degenerative changes in bones and joints have tremendously 
 increased with the rapid aging of the populations in developed countries1,2,3 . the estimated number of individuals with 
 knee oa among musculoskeletal diseases is approximately 24 million in japan4 . individuals with knee oa describe a variety 
 of symptoms including knee pain , stiffness , and limited range of motion5 , 6 . it is well known that stair descent predisposes the elderly to falling and is 
 one of the most difficult activities during the early stage of knee oa patients . accordingly , there is an urgent need to clarify the cause of the difficulty in stair descent 
 in knee oa patients . . however , to 
 date , no previous studies have conducted kinematic and kinetic analysis of stair descent in 
 knee oa patients to clarify the biomechanical factors related to their difficulty in 
 descending stairs . therefore , the purposes of this study were to investigate the lower 
 extremity joint kinematics and kinetics of patients with knee oa during stair descent and 
 clarify the biomechanical factors related to their difficulty in descending stairs . the participants were four elderly individuals diagnosed with early stage unilateral knee 
 oa ( mean age , 767 yr ; mean height , 150.25.4  cm ; mean weight , 55.48.4  kg ; a male and three 
 females ) and eight healthy elderly individuals ( mean age , 696 yr ; mean height , 
 154.72.5  cm ; mean weight , 56.54.9  kg ; five males and three females ) . all procedures were 
 approved by the international university of health and welfare research ethics committee , 
 and participants provided written informed consent prior to enrollment . the severity of oa 
 was assessed by using the kellgren - lawrence grading system12 . this study was performed on three patients and a patient who were 
 diagnosed as grade iiand iii , respectively . subjects participated in the present study on a 
 voluntary basis . a 3-d motion analysis system that included 12 infrared cameras ( vicon 612 ; 
 vicon motion systems , oxford , uk ) and 6 force plates ( amti , watertown , ma , usa ) were used to 
 record kinematics and kinetics data at sample frequencies of 120 and 1080  hz , respectively . a total of 34 reflective markers with a diameter of 14  mm were attached to each subjects 
 body with reference to the report of kito et al13 . two staircases consisting of five steps were placed separately on 
 either side of the force plates ( one to the right and one to the left ) . this experimental 
 setup made it possible to measure the floor reaction force individually for the right and 
 left sides . the tread depth and riser height for both staircases were 300  mm and 160  mm , 
 respectively . all components of floor reaction forces were reset to zero to eliminate the 
 effect of the weights of the staircases before initiating the experimental trials14 . all participants were instructed to 
 descend while barefoot at a self - selected speed without any assistive devices . we measured 
 kinematic and kinetic parameters of the dominant limbs of the elderly individuals and those 
 of intact limbs of the knee oa subjects . lower extremity joint angles , ranges of motion , moments , powers , and the ratios of 
 contribution of the powers were calculated for one gait cycle . only the negative values of 
 each joint power were used to determine the impact absorption in the early stance phase . we chose the mean peak value 
 of these parameters in the three trials as the representative value for analysis . all data were analyzed with the spss 17.0 
 statistical software ( spss japan inc . , the mann - whitney - u test was used to 
 compare the representative values described above between the two subject groups . no significant changes in ankle joint kinematic parameters were observed between the two 
 subject groups . the knee joint angle of knee oa subjects was smaller than that of the 
 healthy elderly subjects during 1223% of the gait cycle . the hip joint angle of the knee oa 
 subjects was smaller than that of the healthy elderly subjects during 920% of the gait 
 cycle ( table 1table 1 . significant differences in kinematic and kinetic parametersanglemomentpowerankle joint4649 , 6467 and 7175%3039%knee joint1223%727%1016%hip joint920%3840%79% ) . range of motion of the knee and hip joints of the knee oa subjects were 
 smaller than those of the healthy elderly subjects ( table 2table 2 . range of motion of the lower extremities in the early stance phasehealthy elderlyknee oaankle joint ( deg)34.5  3.635.2  2.6knee joint ( deg)18.3  3.911.3  2.0*hip joint ( deg)4.8  0.81.8  0.9 * * p < 0.05 ) . significant differences in ankle joint moment were observed between subject 
 groups in 4649% , 6467% , and 7175% of the gait cycle . in addition , significant differences 
 in knee and hip joint moments were observed during 727% and 3840% of the gait cycle , 
 respectively ( table 1 ) . significant differences 
 in ankle , knee , and hip joint powers were observed between subject groups in 3039% , 1016% , 
 and 79% of the gait cycle , respectively ( table 
 1 ) . the ratio of contribution of the knee joint power of the knee oa subjects was 
 significantly smaller than that of the healthy elderly subjects , but this was not the case 
 for the ankle and hip joint powers ( table 
 3table 3 . ratios of contribution to power in the lower extremities in the early stance 
 phasehealthy elderlyknee oaankle joint ( % ) 45.3  8.151.0  15.2knee joint ( % ) 8.3  5.51.5  1.8*hip joint ( % ) 4.4  3.42.2  1.7 * p < 0.05 ) . double knee action , which is mostly observed in level walking , has roles in impact 
 absorption and weight bearing that move the center of gravity ( cog ) downward during stair 
 descent15 , 16 . kito et al.13 
 reported that the action was almost completely gone during level walking in knee oa 
 patients . in the present study , knee joint range of motion of knee oa patients was smaller 
 than that of healthy elderly subjects while descending stairs , as reported in the case of 
 level walking . in addition , negative power of the knee joint , indicating eccentric 
 contraction of knee extensor muscles , was not exerted during the early stance phase in stair 
 descent . for this reason , significant differences in ankle joint power were observed between 
 the subject groups in 1016% of the gait cycle . also , knee oa subjects might not be able to 
 sufficiently absorb external impact force using the knee joint because the ratio of 
 contribution of the knee joint power in them was smaller than that in healthy elderly 
 subjects during the early stance phase . other joints of ipsilateral limbs , that is , the 
 ankle and the hip , could potentially compensate for the function of the intact knee joint 
 and absorb the external impact force in this phase . but muscles around the ankle joint would 
 play a most prominent role in this phase . eccentric contraction of ankle plantar flexor 
 occur as a braking force of downward movement of cog . the ratios of contribution of the 
 ankle joint power of the knee oa and healthy subjects were 51.015.2 and 45.38.1 in the 
 early stance phase , respectively . these results might be a clue indicating ankle joint 
 compensation for the intact knee joint in the early stance phase while descending stairs . the hip 
 joint flexed at the same time that the knee joint absorbed external impact force . however , 
 the hip joint range of motion of the knee oa subjects was significantly smaller than that of 
 the healthy elderly subjects . the ratios of contribution of the hip joint power of the knee 
 oa and healthy subjects were 2.21.7 and 4.43.4 , respectively . accordingly , negative hip 
 joint power of the knee oa subjects was significantly smaller than that of the healthy 
 subjects , and knee oa subjects might also have difficulty in using the hip joint to absorb 
 external impact force in the early stance phase . these findings suggest that the main cause 
 of difficulty of knee oa subjects could be observed in the early stance phase during stair 
 descent . the present study reported the differences in lower extremity joint kinematics and kinetics 
 during stair descent between healthy elderly and knee oa subjects . first , our study analyzed the kinetics and kinematics on the 
 sagittal plane but not on the frontal and transverse planes . thus , further study should be performed with a larger number of knee oa 
 patients and should clarify the biomechanical factors related to their difficulty in 
 descending stairs on three - dimensional planes while descending stairs .
[ purpose ] the purposes of this study were to investigate the lower extremity joint 
 kinematics and kinetics of patients with the knee osteoarthritis ( knee oa ) during stair 
 descent and clarify the biomechanical factors related to their difficulty in stair 
 descent . [ subjects and methods ] eight healthy elderly persons and four knee oa patients 
 participated in this study . a 3-d motion analysis system and force plates were employed to 
 measure lower extremity joint angles , ranges of motion , joint moments , joint powers , and 
 ratios of contribution for the joint powers while descending stairs . 
 [ results ] knee joint 
 flexion angle , extension moment , and negative power during the early stance phase in the 
 knee oa group were smaller than those in the healthy subjects group . 
 however , no 
 significant changes in these parameters in the ankle joint were observed between the two 
 subject groups . 
 [ conclusion ] knee oa patients could not use the knee joint to absorb 
 impact during the early stance phase of stair descent . 
 hence , they might compensate for 
 the roles played by the intact knee joint by mainly using ipsilateral ankle kinematics and 
 kinetics .