gait initiation ( gi ) is the transition from an upright stance to a steady - state gait .
it is 
 a movement that can be completed in the absence of plantar surface feedback1 and is presumably preprogrammed2 . in quiet stance , the inhibition of the 
 soleus ( sol ) and the activation of the tibialis anterior ( ta ) result in a dorsiflexion 
 torque , forward propulsive force , and a backward movement of the center of pressure 
 ( cop)3,4,5 . the cop and center of mass ( com ) become 
 decoupled , and a fall forward is initiated .
in addition , the stepping - limb hip abductors 
 create movement of the cop toward that limb6 . thus , muscle activity at the ankle and hip propels the com forward 
 and toward the intended stance limb to allow stepping - limb toe - off ( to ) and the first 
 step7,8,9 .
of interest has been the 
 modulation of ground reaction forces ( grfs ) and muscle activity before stepping - limb to with 
 changes in intended velocity . as would be expected ,
the posterior displacement of the cop 
 and the propulsive force have been shown to increase , and the time to this peak force 
 decrease , with an increase in the intended velocity of gi4 , 10,11,12,13 .
the duration of both stepping and stance ta activities increase 
 with the velocity of gi4 , 5 , 14 .
significant 
 correlations between kinetic events , time to stepping - limb to , amplitude and duration of ta , 
 and intended velocity of gi have been reported5
. similar patterns of modulation seem to occur when stepping to a new 
 height15 or over an obstacle7 , 16,17,18,19,20,21 .
previous studies have described the velocity - dependent characteristics of gi . however , no 
 single study has reported electromyogram ( emg ) and grf data of both the stance and stepping 
 limbs and their relation to the intended velocity of gi .
one reason for this has been the 
 use of a single force plate12 , 13 , 22,23,24,25,26,27 and the other is that emg activity was not measured1 , 28,29,30,31,32,33,34,35,36,37 in many studies .
information on both the stepping and stance limbs is valuable in a rehabilitation setting 
 where accurate patient education and documentation of change is required .
this information 
 has uses when an asymmetrical lower - limb function is caused by injury or disease , and the 
 patient not only has to generate the required forces , but also strategize as to which limb 
 should be the stepping limb .
for example , time to stepping - limb to has been shown to be 
 slower in stroke patients when the hemiplegic limb is the stance limb28 , yet others have reported the initiation velocity to be 
 greater if the hemiplegic limb is the stance limb30 , 
 35 .
tokuno and eng35 reported decreased anteroposterior ( a - p ) forces from the 
 hemiplegic limb , regardless of whether it was leading or trailing , and questioned whether 
 the hemiplegic limb should be the stepping or trailing stance limb .
those with a lower - limb 
 amputation tend to initiate gait with the prosthetic limb .
the force generated by this limb 
 is low , and a rescaling of the force of the sound limb is necessary for an adequate first 
 step length36 .
information on the interaction of the stance and stepping limbs during gi could help guide 
 rehabilitation decisions .
moreover , how this interaction changes with the velocity of gi is 
 also important , given that an increase in gait velocity is a positive indicator of 
 recovery38,39,40,41 . if the velocity and strategy employed to initiate gait is 
 important in rehabilitation , the collection of more comprehensive data from healthy subjects 
 is required .
the purpose of this study was , therefore , to investigate the effects of 
 accuracy constraints ( targets ) placed on the stepping - limb heel - strike ( hs ) on muscle 
 activities and grf during gi .
we hypothesized that the velocity of gi and stepping over an 
 obstacle would decrease when accuracy constraints were placed on the stepping - limb hs , and 
 our results show this decrease was the result of the modulation of grfs and muscle 
 activities of both the stance and stepping limbs before stepping - limb to .
the study sample consisted of twenty healthy participants ( ten males and ten females ; mean 
 age , 29.2  2.9  years ; range , 2434  years ) with no known neurological or orthopedic 
 deficits .
the study was approved by the university institutional review board , and all 
 participants signed an informed consent form . the study participants characteristics are 
 summarized in table 1table 1.subject characteristicsgendernumberage ( years)height ( cm)weight ( kg)male1030.2 ( 2.78)174.8 ( 5)71.2 ( 3.79)female1028 ( 2.69)164.6 ( 5.24)53.4 ( 3.09)total2029.2 ( 2.9)169.9 ( 7.3)62.8 ( 9.72 ) .
surface electrodes were applied to the center of the muscle bellies of the ta and sol of 
 the stance and stepping limbs . each recording electrode consisted of two silver - silver 
 chloride 1-cm - diameter electrodes , embedded in an epoxy - mounted preamplifier system ( 35 ) , 
 and their centers were spaced 2  cm apart .
the emg signals were high - pass filtered ( 20  hz4 khz ) using a zero - lag 
 filter ( therapeutics unlimited , iowa city , ia , usa ) to remove movement artifacts and 
 full - wave rectified online .
the final amplification was 10  k. the collection and processing 
 of emg data followed seniams recommendations42 . two force platforms ( advanced mechanical technology , inc . ,
newton , ma , usa ) , embedded in a 
 level walkway ( 5  m length , 1.22  m width ) , measured the grfs of the stance and stepping 
 limbs .
the grfs ( kinetic data ) were collected at 1,000  hz and later filtered with a 4th 
 order , butterworth , zero lag filter , with a cutoff frequency of 50  hz .
foot switches ( b 
 & l engineering , los angeles , ca , usa ) were placed in the shoes to measure the hs of the 
 stepping limb .
the emg and foot switch signals were synchronized with the kinetic data to 
 simultaneously start both the emg and foot switch data capture .
both the emg and the force 
 platform data were recorded simultaneously on a personal computer at a sampling rate of 
 1,000  hz for 4  s ( biopac systems , goleta , ca , usa ) .
the test conditions included the use of 
 an obstacle ( 10  cm height , 10  cm depth , and 140  cm width ) made of wood .
for each trial , participants stood in a predetermined position with each foot on a force 
 platform .
participants were asked to begin walking or stepping over the obstacle , at a fast 
 speed , with the limb they naturally preferred to initiate gait with , after receiving the 
 verbal cue  go ,  and to continue to walk a minimum of three steps . before the experimental 
 trials , the average position of the stepping hs was determined for each subject using a 
 video analysis . for half of the experimental trials ,
a 3-cm - diameter target was placed on 
 the ground to dictate the position and accuracy of the stepping - limb hs .
all participants 
 performed two practice trials to familiarize themselves with the experimental procedure . 

participants completed 10 trials under each of the following four conditions : gi , gi to the 
 target , stepping over the obstacle , and stepping over the obstacle to the target .
all participants were required to wear flat - soled shoes normally used for everyday 
 walking or sports activities .
two - way analysis of variance ( initiation condition  accuracy ) for repeated measures was 
 performed to determine the main and interaction effects of a - p grfs , muscle response , and 
 temporal events .
post hoc analysis using turkey s hsd was used to determine the 
 between - group mean differences if the analysis of variance found a significant main effect 
 or interaction of initiation condition  accuracy
the independent variables were initiation condition ( gi and stepping over an obstacle ) and 
 accuracy ( target , no target ) .
the dependent measures included emg amplitude , slopes and peak 
 of the propulsive grf , and timing events of gi .
the amplitude of stance- and stepping - limb 
 ta emg was determined between the onset of movement and time to peak a - p grf of the stepping 
 limb ( taemg1 ) , and between time to stepping - limb peak a - p grf and stepping - limb to ( taemg2 ) . 
 the amplitude and duration of stepping - limb sol activity were also calculated . the onset and 
 offset of sol activity
time to stepping - limb to , 
 stepping - limb hs , stance - limb heel - off , and stance limb to were also determined .
timing data 
 were referenced from the onset of movement , which was defined as the first detectable onset 
 of force platform activity .
an investigator blinded to the experimental trials performed all 
 determinations of muscle activity timing and change in grf .
the temporal events are shown in table 
 2table 2.means ( sd ) of temporal events ( ms)dependent variablesgait initiationsteppingno targettargetno targettargetstepping limb peak fx340 ( 49)423 ( 42)329 ( 47)368 ( 73)stepping limb toe - off511 ( 76)613 ( 81)475 ( 69)548 ( 91)stepping limb heel - strike890 ( 74)1008 ( 86)968 ( 71)1071 ( 108)stance limb toe - off1024 ( 76)1219 ( 156)1093 ( 85)1257 ( 153)values represent mean  standard deviation ( sd ) . * significant main effect of target 
 ( p<0.01 or p<0.05 )
. significant main effect of initiation condition 
 ( p<0.01 or p<0.05 ) . significant interaction ( p < 0.05 ) . 

significant difference between conditions ( p<0.01 or 
 p<0.05 ) .. although there was an interaction ( f(1 , 8) = 6.47 , p < 
 0.05 ) of the time to stepping - limb peak a - p grf ( the target condition for stepping over an 
 obstacle was greater than that for gi ) , the mean times to peak a - p grf in the target 
 conditions were significantly greater than in the no - target conditions .
the time to 
 stepping - limb to showed significant main effects of both the initiation and target 
 conditions ( f(1 , 8) = 10.87 , p
< 0.05 and f(1 , 8) = 
 19.29 , p < 0.01 , respectively ) .
time to stepping - limb to was longer in stepping to the 
 target but shorter in the obstacle condition .
the times to stepping - limb hs and stance - limb 
 to showed main effects of the initiation and target conditions ( f(1,7 ) = 
 7.83 to 18.24 , p < 0.01 to p < 0.05 ) . the times increased in both the target and 
 stepping conditions .
values represent mean  standard deviation ( sd ) . * significant main effect of target 
 ( p<0.01 or p<0.05 ) .
significant main effect of initiation condition 
 ( p<0.01 or p<0.05 ) . significant interaction ( p < 0.05 ) . 

the amplitude of the ta emg was determined from both the onset of force platform activity 
 to stepping - limb peak a - p grf ( taemg1 ) , and from stepping - limb peak a - p grf to to of the 
 stepping limb ( taemg2 ) .
there was a significant main effect of target on the stepping - limb 
 taemg1 ( f(1 , 8) = 7.75 , p < 0.05 ) which showed decreased amplitudes in 
 the target conditions ( table 3table 3.means ( se ) of emg dependent variables ( ms.v)dependent variablesgait initiationsteppingno targettargetno targettargetstance limb taemg1340 ( 49)423 ( 42)329 ( 47)368 ( 73)stance limb taemg2511 ( 76)613 ( 81)475 ( 69)548 ( 91)stepping limb taemg1890 ( 74)1008 ( 86)968 ( 71)1071 ( 108)stepping limb taemg21024 ( 76)1219 ( 156)1093 ( 85 ) 1257 ( 153)values represent mean  standard deviation ( sd ) .
however , there was no significant effect on the taemg2 of the stepping 
 limb .
taemg : tibialis anterior 
 electromyogram . * significant main effect of target ( p<0.01 or p<0.05 ) . 
 significant interaction ( p<0.05 ) .
stance taemg1 showed a significant interaction ( f(1 , 8) = 12.12 , p < 
 0.05 ) ( table 3 ) .
stance taemg1 was greater in gi 
 than in the stepping over the obstacle with no target , but showed no difference between the 
 target conditions .
stance taemg2 was significantly greater in the no - target conditions than 
 in the target conditions ( f(1,8 ) = 13.31 , p < 0.01 ) ( table 3 ) .
the mean data show that except for taemg2 of the 
 stepping limb , the ta amplitude was clearly greater in the no - target conditions .
the 
 stepping - limb sol emg amplitude was similar in all the conditions ; however , its duration was 
 24  ms greater in the target conditions ( f(1 , 8) = 8.05 , p < 0.05 ) .
there was a significant main effect of target on both the stepping - limb peak a - p grf 
 ( f(1 , 8) = 30.88 , p < 0.001 ) and the slope to stepping - limb peak a - p 
 grf ( f(l , 8) = 18.2 , p < 0.01 ) .
values were greater in the no - target 
 conditions ( table 4table 4.means ( sd ) of peak ( % bw ) and slope ( % bw / s ) force plate dependent 
 variablesdependent variablegait initiationsteppingno targettarget no targettargetstepping limb peak fx39.6 ( 6)26 ( 7.8)40.8 ( 9.6)25.7 ( 7.2)stepping limb slope fx125.2 ( 42)62 ( 22.5)120.6 ( 26.4)75.6 ( 32.3)stance limb peak fx81.8 ( 13)52.6 ( 12)66.3 ( 15)47.5 ( 13)stance limb slope fx318.2
( 63)208.2 ( 53)313.1 ( 87)192.5 ( 54)values represent mean  standard deviation ( sd ) . fx : anteroposterior ground reaction 
 force ; % bw : percent body weight ; % bw / s : percent body weight per second .
there was no significant difference between the gi and stepping over the 
 obstacle conditions for either dependent variable .
although there was a significant 
 interaction of the stance - limb peak a - p grf ( f(1 , 8) = 13.64 , p < 0.01 ) , 
 there were also main effects of the stepping ( f(1 , 8) = 30.07 , p < 0.01 ) 
 and target ( f(1 , 8) = 24.80 , p < 0.01 ) conditions .
stance - limb peak a - p 
 grf was greater in the gi and no - target conditions ( table 4 ) .
the slope to peak a - p grf showed a main effect of target only and the 
 slope was greater in the no - target conditions ( f(1 , 8) = 22.27 , p < 

fx : anteroposterior ground reaction 
 force ; % bw : percent body weight ; % bw / s : percent body weight per second . *
significant interaction 
 ( p<0.05 ) . significant difference between conditions ( p<0.01 or 
 p<0.05 ) .
a unique relation exists between force profiles and muscle activity in different movement 
 strategies .
an increase in the degree of accuracy of a task causes a decrease in the 
 velocity of movement43,44,45 .
therefore , 
 there must be a change in strategy with an accuracy constraint , but only if the accuracy 
 demands are sufficiently sensitive to dictate movement velocity46 .
furthermore , on the basis of previous studies of gi and 
 stepping7 , 8 , it could be concluded that by manipulating the limb trajectory , for 
 example , stepping over an obstacle as opposed to gi , forces would remain invariant . by 
 investigating
the component of gi thought to dictate the velocity of movement , better data 
 on how forces are modulated during voluntary movement from a quiet stance could be 
 provided .
the current study investigated both gi and stepping over an obstacle and the interaction of 
 the stance limb with the stepping limb , when target constraints were placed on the 
 stepping - limb hs .
the results show that the initiation velocity increase in no - target 
 conditions was due to the modulation of the stance- and stepping - limb grfs and the 
 corresponding increase of the ta activity in both limbs before stepping - limb to .
this was 
 achieved by significantly increasing the stepping- and stance - limb taemg1 in the no - target 
 conditions .
it seems , therefore , that taemg1 and the slope to stepping - limb peak a - p grf 
 contributed to the intended velocity of initiation ( gi or stepping over an obstacle ) . in the present study ,
stepping - limb to marks the first significant event , and in the no - target condition it 
 occurred approximately 90  ms earlier than in the target conditions .
this decrease in time 
 was , in part , due to a 15% ( 61  ms ) decrease in time to stepping - limb peak a - p grf of the 
 no - target conditions .
this decrease in time to peak a - p grf was due to a 78% increase in 
 slope to peak a - p grf that resulted in a 55% increase in peak a - p grf of the stepping limb . 

given the relative changes in time and slope , it seems that subjects modulated the rate of 
 increase of force and kept the time to peak force relatively constant to achieve the 
 increase in the peak acceleration force of the stepping limb . as the ta controls the 
 backward movement of the cop3 , 4 , 10 , 47
, there was a significant increase in stepping - limb taemg1 
 in the no - target conditions .
the relation between the backward movement of the cop10 , 48 
 or ta activity and gait velocity has previously been recognized . of interest ,
the time from 
 stepping - limb to to stepping - limb hs remained invariant for both gi and stepping over the 
 obstacle , a finding supported by the earlier data of brenire and colleagues3 , 10 , 47 . in the present study , the mean difference 
 between the accuracy conditions was only 23  ms .
furthermore , there was no main effect of the 
 amplitude of stepping - limb taemg2 or sol , and it has been previously recognized that sol 
 creates heel rise in preparation for the first step but does not contribute toward gait 
 velocity48 .
the contribution of the 
 stepping limb to the velocity of initiation ( gi or stepping ) seems , therefore , to be 
 determined by taemg1 and the slope to stepping - limb peak a - p grf48 . for the stance limb , there was a significant target effect on both taemg1 and taemg2 . in 
 both stepping over the obstacle and gi ,
the amplitude of taemgi was far greater than that of 
 taemg2 , and only minimal acceleration force was generated until the slope to stance - limb a - p 
 grf appeared .
the onset of this slope coincided with the peak a - p grf of the stepping limb , 
 our selected time division between taemg1 and taemg2 . during this phase of gi and stepping 
 over an obstacle ,
this transition 
 from unloading to loading occurs slightly before peak a - p grf of the stepping limb . given 
 the greater amplitude of taemg1 of the stance limb but smaller peak force
, we consider that 
 the smaller a - p grf is probably related to the unloading of the stance limb , and slope to 
 stance - limb peak a - p grf to the loading of that limb before stepping - limb to .
the slope to 
 stance - limb peak a - p grf was the same for gi and stepping over the obstacle , but peak a - p 
 grf was less when stepping over the obstacle .
peak a - p grf coincides with stepping - limb to , 
 and the earlier to for stepping ( and therefore smaller peak a - p grf ) is thought to be 
 related to the trajectory of the stepping limb7 . because of the greater trajectory , the time to stepping - limb hs 
 from to was longer for stepping over the obstacle .
the slope to stance - limb peak a - p grf increased in the no - target condition .
there was an 
 increase of approximately 50% in the no - target condition , with only a 14% difference in time 
 to peak a - p grf .
the increase in the slope to peak a - p grf of the stepping limb was 78% . for 
 the stance limb
, these data differ from a previous report8 that used both a large and a small target .
however , it should be 
 noted that the target size used in the present study was 50% smaller than the small target 
 used in the previous study8 . in an 
 isometric plantar flexion task , the slopes to peak forces
were also found to be the same 
 once the target size had reached a critical level .
the fact 
 that an explicit instruction to increase the velocity of gi ( as opposed to accuracy 
 constraints ) resulted in an increase in the slope to stance - limb peak a - p grf supports this 
 interpretation7 .
because the experiment was 
 only conducted on a relatively small sample of healthy young adults ( gi in a 
 non - pathological setting ) , our findings can not be generalized to the population with a 
 pathological gi .
moreover , much of the pathological gi condition is asymmetrical , yet data 
 is only presented for right foot gait initiation .
furthermore , data was only collected at a 
 fast speed , and we used experimental manipulations such as a target and obstacle - crossing 
 instead of asking the participants to simply initiate their gait at slow , preferred , and 
 fast speeds . in conclusion , this study investigated changes in emg and grf responses during gi at 
 different intended velocities of initiation .
the velocity of gi and stepping over an 
 obstacle increased when no accuracy constraints were placed on the stepping - limb hs .
this 
 increase was due to the modulation of both the stance- and stepping - limb grfs before 
 stepping - limb to .
the time to these peak forces remained relatively constant ( 100-ms less in 
 the no - target conditions ) .
that is , the slope to the stepping- and stance - limb peak a - p grf 
 was modulated , whereas the time to peak a - p grf remained relatively constant .
this notion is 
 supported by the strong correlations between the slope to stepping - limb a - p grf and time to 
 stepping hs ( r = 0.84 ) , and the more modest correlation ( r 
 = 0.64 ) of the slope to stance - limb peak a - p grf .
furthermore , there were no differences in 
 the slopes to either stepping- or stance - limb peak a - p grf between gi and stepping over the 
 obstacle .
this concurs with upper - extremity studies showing that the distance moved does not 
 affect the rate of increase of force . on the basis of these data , gi and/or stepping over an 
 obstacle
this would be preferable in 
 upper - extremity experiments , for which the subject sits and the extremity is stabilized . 

voluntary movement from an upright stance may be useful for assessing changes in performance 
 after rehabilitation .
hemiparetic patients show an asymmetric gait pattern and prefer initiating gait with the 
 affected leg to utilize the higher stability of the supporting limb .
however , hemiparetic 
 patients are encouraged to initiate gait with the intact limb to increase the weight - bearing 
 ability of the affected leg .
similarly , unilateral amputees in rehabilitation training are 
 also encouraged to lead with the intact limb when stepping up , and with their prosthetic 
 limb when stepping down49 .
however , a 
 recent study30 questions whether 
 initiating the gait of hemiplegic patients with the intact limb is related to a higher risk 
 of fall . the movement pattern of the cop and com is altered in patients with hemiparetic 
 stroke when initiating gait with the unaffected leg
. this difference may be attributable to 
 the higher incidence of falls when initiating gait with the unaffected leg due to the 
 altered movement pattern of the cop and com .
the present study provides insight into the 
 working mechanisms of the stepping and stance limbs and shows the clear need to further 
 investigate whether the intact or affected limb should be used to initiate gait during 
 rehabilitation and prosthetic training .
 [ purpose ] this study investigated the effects of accuracy constraints ( targets ) placed on 
 the stepping - limb heel - strike ( hs ) on the electromyogram ( emg ) and ground reaction forces 
 ( grfs ) during gait initiation . [ subjects and methods ] twenty healthy subjects ( 29.2  
 2.9  years ) were asked to begin walking or stepping over a 10-cm - high obstacle at a fast 
 speed . 
 a 3-cm - diameter target was placed on the ground to dictate the position and 
 accuracy of the stepping - limb hs . 
 [ results ] the results showed that the initiation 
 velocity increase in the no - target conditions was due to modulation of the stance- and 
 stepping - limb grfs and a corresponding increase in the tibialis anterior ( ta ) activities 
 of both limbs before stepping - limb toe - off . 
 this was achieved by significantly increasing 
 the stepping- and stance - limb taemg1 ( determined between the onset of movement and time to 
 peak anteroposterior ( a - p ) grf of the stepping- and stance- limb ) for the no - target 
 conditions . 
 it seems , therefore , that taemg1 and the slope to stepping - limb peak a - p grf 
 contributed to the intended velocity of initiation . 
 [ conclusion ] these data indicate that 
 gait initiation and/or stepping over an obstacle may prove to be tasks by which motor 
 control can be measured . 
 the present study provides insight into the working mechanisms of 
 the stepping and stance limbs and shows a clear need to further investigate whether the 
 intact or affected limb should be used to initiate gait during rehabilitation and 
 prosthetic training . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION