seven able - bodied male subjects ( age 30.7  4.2 yrs ; height 178.9  10.2 cm ; mass 73.9  12.2 kg , mean  sd ) participated in this study
the study was approved by the local ethics committee ( ethics committee of the swiss canton of bern , kek bern ) .
the knee dynamometer is a custom made measurement device , which moves one leg at a constant velocity and measures the isokinetic torque produced during stimulated knee extension .
the lower leg is fixed with a brace to an aluminium load cell ( lcb130 , me - mesysteme gmbh , germany ) , which moves , via a lever arm , a chain drive system connected to a magnetostrictive torque sensor ( s-2220 - 75 , ncte ag germany ) . the torque sensor and the load cell are used to bi - directionally measure the effective torque on the gauge bar in real time . a brushless motor ( ec45 , 250 w , maxon motor ag , switzerland )
is used with a planetary gear head ( gear ratio : 156:1 , gp42 , maxon motor ag , switzerland ) .
the actuator can generate a maximum continuous torque of 90 nm . a position sensor ( vert - x 28 , contelec gmbh , switzerland ) is used for angle measurement with a resolution of 0.648 deg to control the motor torque .
the measurements were performed at a constant angular velocity of ~110 /s , which is equivalent to a cycling cadence of 50 rpm , and the range of motion was set from 45 to 130 knee extension ( where 180 is full extension ) .
the electrical pulses were generated with an eight channel stimulator ( rehastim , hasomed gmbh , germany ) .
co. , ltd , usa ) were placed on the motor points of the m. quadriceps lateralis and medialis and dispersive electrodes were placed 10  15 cm proximal to the corresponding muscle motor point ( fig .
1 ) . the skin was cleaned and the body hair shaved at the position of the electrodes .
muscle motor points were detected for each stimulated muscle prior to measurement with a stimulation pen ( motor point pen , compex , switzerland ) .
subjects were stimulated with rectangular bi - phasic pulses at a constant amplitude of 40 ma .
current was applied using an sdss electrode setup , which consists of four small electrodes with a surface area of 4.5  2.5 cm each and one large electrode ( 9  5 cm ) with the same total area .
each of the four small electrodes used a frequency of 8.75 hz and a phase shift of 90  , which corresponds to an overall stimulation frequency of 35 hz .
two different sdss electrode placements were investigated : distal versus proximal sdss electrodes . in both cases the active part
the mean pulse widths were 73.3  14.2 s for the proximal and 73.3  14.4 s for the distal sdss setup .
each subject participated in two sessions with only one electrode placement tested for each leg within each session .
between the two measurements in each session , subjects had a break of 15 minutes . stimulation setup order and the leg were chosen randomly . before each measurement
subjects were placed on the dynamometer and individual adjustments to body proportions were made . then a two - minute phase was started in which the measured leg was moved by the device without stimulation ( non - stimulation [ ns ] phase ) . then the pulse width was manually increased after every third extension , starting at 0 s . pulse width was increased up to the subjects pain threshold or up to the point they were no longer able to stay relaxed .
80 % of this maximal pulse width ( pwmax ) was then used for the test measurements . after a rest period of 10 minutes ,
the measurement started with an ns - phase of two minutes followed by a stimulation phase ( st - phase ) of 6 minutes .
each session was conducted on a different day with at least one day of rest in between .
electrode positions were marked to ensure identical placement each day only the extension phase of joint motion was evaluated , as the setup was to simulate cycling motion .
the measured torque (  ) , together with the angular speed was used to calculate the instantaneous output power ( pm ) .
the power used to move the leg during the ns - phase was denoted as pns .
the net effective power output of one stimulation cycle is thus pstim = pns  pm . for every knee extension
the following outcomes were calculated : ( a ) mean power output during one extension ( pmean ) , ( b ) peak power output ( ppeak ) and ( c ) the time from onset of the stimulation to 80% of ppeak ( tpeak80 ) . to compare the different stimulation setups ,
pstim was scaled using a reference pulse width of 100 s ( pstim , s ) , e.g. subject a had a pulse width of 80 s , so the scaled mean power output pmean , s of that subject is pmean 
  ( 100/80 ) and the scaled peak power output ppeak , s is ppeak 
  ( 100/80 ) .
all outcomes were calculated for the initial 15 knee extensions and for the final 15 knee extensions .
a fatigue index ( fi ) describes the percentage reduction in pmean from the initial phase ( pinit ) to the final phase ( pfinal ) : fi=1-(pinit - pfinal)/pinit .
the higher the value , the higher the fatigue resistance ; fi=1 means no fatigue .
the data were tested for normality using the shapiro - wilk - test and then a paired t - test for normally distributed data and a wilcoxon test for non - normal data was applied to test differences of means .
seven able - bodied male subjects ( age 30.7  4.2 yrs ; height 178.9  10.2 cm ; mass 73.9  12.2 kg , mean  sd ) participated in this study
the study was approved by the local ethics committee ( ethics committee of the swiss canton of bern , kek bern ) .
the knee dynamometer is a custom made measurement device , which moves one leg at a constant velocity and measures the isokinetic torque produced during stimulated knee extension .
the lower leg is fixed with a brace to an aluminium load cell ( lcb130 , me - mesysteme gmbh , germany ) , which moves , via a lever arm , a chain drive system connected to a magnetostrictive torque sensor ( s-2220 - 75 , ncte ag germany ) .
the torque sensor and the load cell are used to bi - directionally measure the effective torque on the gauge bar in real time . a brushless motor ( ec45 , 250 w , maxon motor ag , switzerland )
is used with a planetary gear head ( gear ratio : 156:1 , gp42 , maxon motor ag , switzerland ) .
a position sensor ( vert - x 28 , contelec gmbh , switzerland ) is used for angle measurement with a resolution of 0.648 deg to control the motor torque .
the measurements were performed at a constant angular velocity of ~110 /s , which is equivalent to a cycling cadence of 50 rpm , and the range of motion was set from 45 to 130 knee extension ( where 180 is full extension ) .
the electrical pulses were generated with an eight channel stimulator ( rehastim , hasomed gmbh , germany ) .
co. , ltd , usa ) were placed on the motor points of the m. quadriceps lateralis and medialis and dispersive electrodes were placed 10  15 cm proximal to the corresponding muscle motor point ( fig .
1 ) . the skin was cleaned and the body hair shaved at the position of the electrodes .
muscle motor points were detected for each stimulated muscle prior to measurement with a stimulation pen ( motor point pen , compex , switzerland ) .
subjects were stimulated with rectangular bi - phasic pulses at a constant amplitude of 40 ma .
current was applied using an sdss electrode setup , which consists of four small electrodes with a surface area of 4.5  2.5 cm each and one large electrode ( 9  5 cm ) with the same total area .
each of the four small electrodes used a frequency of 8.75 hz and a phase shift of 90  , which corresponds to an overall stimulation frequency of 35 hz .
two different sdss electrode placements were investigated : distal versus proximal sdss electrodes . in both cases the active part was placed on the previously detected motor point .
the mean pulse widths were 73.3  14.2 s for the proximal and 73.3  14.4 s for the distal sdss setup .
each subject participated in two sessions with only one electrode placement tested for each leg within each session . between the two measurements in each session , subjects had a break of 15 minutes . stimulation setup order and the leg
subjects were placed on the dynamometer and individual adjustments to body proportions were made . then a two - minute phase was started in which the measured leg was moved by the device without stimulation ( non - stimulation [ ns ] phase ) . then the pulse width was manually increased after every third extension , starting at 0 s . pulse width was increased up to the subjects pain threshold or up to the point they were no longer able to stay relaxed .
80 % of this maximal pulse width ( pwmax ) was then used for the test measurements . after a rest period of 10 minutes ,
the measurement started with an ns - phase of two minutes followed by a stimulation phase ( st - phase ) of 6 minutes .
each session was conducted on a different day with at least one day of rest in between .
only the extension phase of joint motion was evaluated , as the setup was to simulate cycling motion .
the measured torque (  ) , together with the angular speed was used to calculate the instantaneous output power ( pm ) .
the power used to move the leg during the ns - phase was denoted as pns .
the net effective power output of one stimulation cycle is thus pstim = pns  pm . for every knee extension
the following outcomes were calculated : ( a ) mean power output during one extension ( pmean ) , ( b ) peak power output ( ppeak ) and ( c ) the time from onset of the stimulation to 80% of ppeak ( tpeak80 ) . to compare the different stimulation setups ,
pstim was scaled using a reference pulse width of 100 s ( pstim , s ) , e.g. subject a had a pulse width of 80 s , so the scaled mean power output pmean , s of that subject is pmean 
  ( 100/80 ) and the scaled peak power output ppeak , s is ppeak 
  ( 100/80 ) .
all outcomes were calculated for the initial 15 knee extensions and for the final 15 knee extensions . a fatigue index ( fi )
describes the percentage reduction in pmean from the initial phase ( pinit ) to the final phase ( pfinal ) : fi=1-(pinit - pfinal)/pinit .
the higher the value , the higher the fatigue resistance ; fi=1 means no fatigue .
the data were tested for normality using the shapiro - wilk - test and then a paired t - test for normally distributed data and a wilcoxon test for non - normal data was applied to test differences of means .
figure 2 shows the development of pmean , s , ppeak , s and tpeak80 over the 6-minute stimulation phase .
the corresponding outcome measures for the initial and final stimulation phases are summarised in tab .
1 . no significant differences between distal and proximal electrode placement were found for any outcome measures during the initial stimulation phase . in the final stimulation phase , ppeak and ppeak ,
s showed significantly higher values for the distal sdss setup : 25.4  8.1 w vs. 28.2  6.2 w , p=0.0062 and 34.8  9.5 w vs. 38.9  6.7 w , p=0.021 , respectively . in the final phase
3 ) and pmean , s with the distal sdss placement ( 11.8  3.8 w vs. 12.7  3.3 w , p=0.071 and 16.2  4.5 w vs. 17.4  3.4 w , p=0.14 ) , and of longer tpeak80 for distal sdss ( 347.6  29.2 ms vs. 359.4  38.2 ms , p=0.096 ) . the modestly higher mean power output in the final stage with distal sdss , and a lower dispersion of power values , can be conveniently visualised ( fig .
fatigue resistance was not different between the two stimulation setups ( fi 0.61  0.14 vs. 0.64  0.9 , p=0.38 ) .
the aim of this study was to compare the power output , fatigue and activation properties of proximally versus distally placed sdss electrodes in an isokinetic knee extension task simulating knee movement during recumbent cycling .
overall , especially in the final phase of stimulation , the distal sdss setup showed higher power outputs ( the only exception was pmean , s in the initial phase , which was minimally lower for distal sdss ) .
this might be regarded as surprising , since the active electrode was placed exactly on the motor point in the proximal sdss setup .
splitting one large electrode into four small ones of the same overall size , and using a sequential stimulation strategy , was previously shown to give increased power output and better fatigue resistance compared to a standard electrode setup .
the temporal and spatial shift had a significant impact on muscle activation in a dynamic knee extension task .
using a standard electrode setup ( ses ) , it should not make a difference which electrode ( active , dispersive ) is positioned at the motor point : the electrical field is the same and the current direction should not activate the muscle fibres differently or change the number of recruited motor units . on the other hand , the electrical field changes with the size of the electrodes , and the distance between the active and dispersive electrodes has a substantial influence on the torque . to control for these factors in the two setups investigated in the present study , the distance was not changed and the active electrode was placed distally .
that no substantial or significant difference was observed for any power output parameter in the initial phase of the task may indicate that both setups recruited and activated a similar number of motor units . the similar development over time of pmean and ppeak , as well as the different development of tpeak80 ( fig .
2a ) between setups , shows that the muscle fibre recruitment and the power curve of a single extension is not the same over time . although fi was not significantly different , the examination of power development over time ( fig .
2b , c ) shows that distal placement seems to have a slower power decrease and a higher power output in the final stimulation phase .
smaller electrodes increase the current density compared to larger electrodes using the same amplitude and pulse width .
thus , the non - equal size of the electrodes ( active , dispersive ) leads to an asymmetric electrical field , which seems to influence muscle activation .
in contrast to the distal placement , where four small active electrodes are placed around the motor point , in the proximal setup one large active electrode is placed exactly on the motor point . in consequence , the change of the electrical field at this sensitive position is lower in this setup .
less change over time in the pattern of activation has been shown to favourably affect fatigue and power output development ( 13 , 14 ) .
activation time , i.e. the time from stimulation onset to 80% of peak power , plays a crucial role when electrical stimulation is used to produce a functional movement .
usually , more than just one muscle group is involved , so that both the coordination of the force and the activation of the different muscles are of importance . in fes - cycling ,
often only three major muscle groups are involved ( m. quadriceps , m. hamstrings and m. gluteus ) and the coordination of these muscles is one factor for achievement of high power output . during repetitive activation , muscles not only fatigue , but
tpeak80 for the proximal placement starts to flatten out after about 30 extensions , whereas tpeak80 for the distal placement is still increasing at this time and only begins to flatten out after about 65 extensions ( fig .
the mean difference for tpeak80 of 11.8 ms in the final phase corresponds to a phase shift of 6 when cycling 50 rpm , which might have an influence on the overall cycling performance . by positioning the electrodes more precisely in relation to the motor points ( proximal setup ) ,
the activation becomes more efficient and the muscle activation time is less affected by the duration of the task . in conclusion ,
the sdss approach to muscle stimulation seems to provide substantial performance benefits , but the placement of the electrodes is still a crucial factor .
distal placement of the sdss electrodes showed higher power output values in the final stimulation phase but also a slightly increased activation time .
the development of new array electrodes , specifically for sdss , where the initial pulse is applied directly on the motor point and the following pulses are randomly distributed , may combine the positive effects of the proximal and distal electrode placements .
based on the evidence presented here , for practical fes applications , distal placement of the sdss electrodes appears to be preferable .
 spatially distributed sequential stimulation ( sdss ) has demonstrated substantial power output and fatigue benefits compared to single electrode stimulation ( ses ) in the application of functional electrical stimulation ( fes ) . 
 this asymmetric electrode setup brings new possibilities but also new questions since precise placement of the electrodes is one critical factor for good muscle activation . 
 the aim of this study was to compare the power output , fatigue and activation properties of proximally versus distally placed sdss electrodes in an isokinetic knee extension task simulating knee movement during recumbent cycling . m. vastus lateralis and medialis of seven able - bodied subjects were stimulated with rectangular bi - phasic pulses of constant amplitude of 40 ma and at an sdss frequency of 35 hz for 6 min on both legs with both setups ( i.e. n=14 ) . 
 torque was measured during knee - extension movement by a dynamometer at an angular velocity of 110 deg / s . 
 mean power , peak power and activation time were calculated and compared for the initial and final stimulation phases , together with an overall fatigue index . 
 power output values ( pmean , ppeak ) were scaled to a standardised reference input pulse width of 100 s ( pmean , s , ppeak , s ) . 
 the initial evaluation phase showed no significant differences between the two setups for all outcome measures . 
 ppeak and ppeak , s were both significantly higher in the final phase for the distal setup ( 25.4  8.1 w vs. 28.2  6.2 w , p=0.0062 and 34.8  9.5 w vs. 38.9  6.7 w , p=0.021 , respectively ) . with distal sdss 
 , there was modest evidence of higher pmean and pmean , s ( p=0.071 , p=0.14 , respectively ) but of longer activation time ( p=0.096 ) . 
 the rate of fatigue was similar for both setups . for practical fes applications , distal placement of the sdss electrodes 
 is preferable . 