Electromyographic Activity and Applied Load During High Intensity Elastic Resistance and Nautilus Machine Exercises

                                               Journal  of  Human  Kinetics  volume  30/2011,  5-­‐‑13                                                

Author Percival Weaver

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                                               Journal  of  Human  Kinetics  volume  30/2011,  5-­‐‑13                                                      Section  I  –  Kinesiology    


Electromyographic  Activity  and  Applied  Load  During  High   Intensity  Elastic  Resistance  and  Nautilus  Machine  Exercises   by   Saied  J  Aboodarda1,  Mohamad  A.H.  Shariff2,  Ahmad  Munir  Che  Muhamed3,   Fatimah  Ibrahim4,  Ashril  Yusof1   This  study  was  designed  to  quantify  and  compare  Electromyographic  activity  (EMG)  and  applied  load  in   quadriceps  muscle  within  performing  high  intensity  knee  extension  exercises  by  Elastic  Resistance  (ER)  and  Nautilus   Machine  (NM).  Sixteen  male  and  female  subjects  (22.4  ±  4.7  yrs)  completed  8  RM  seated  knee  extension  by  NM,  elastic   tubing  with  original  length  (E0)  and  elastic  tubing  with  30%  decrement  of  original  length  (E30).  The  mean  value  of   EMG   and   external   force   were   calculated   and   synchronized   across   various   segments   of   motion   for   the   three   modes   of   training.  The  results  demonstrated  that  in  the  early  concentric  and  late  eccentric  segments  of  contraction,  NM  elicited   significantly   higher   muscle   activation   than   both   E30   and   E0   (p   <   0.05).   However,   in   the   mid-­‐‑concentric   and   mid-­‐‑ eccentric  as  well  as  late  concentric  and  early  eccentric  segments  no  significant  differences  were  observed  between  NM   and  E30.  These  findings  supported  the  approach  that  developing  external  recoil  of  force  in  ER  device  by  reducing  30%   of  initial  length  of  elastic  material  can  offer  similar  neuromuscular  activation  compared  with  NM.  On  this  basis,  E30   can  be  suggested  as  an  affordable  and  non-­‐‑gym  based  exercise  device  which  has  the  capacity  to  provide  an  appropriate   high  resistance  stimulus  to  meet  the  training  requirement  of  athletes.   Key  words:  electromyogram,  elastic  tubing,  variable  resistance  training,  multiple  repetitions  maximum     Introduction     whole   ROM   (Elliott   et   al.,   1989;   Manning   et   al.,   Resistance   exercises   have   become   an   1990).  However,  the  resistance  training  apparatus   inevitable   part   of   training   schedule   that   is   which  can  meet  the  above  criterion,  the  isokinetic   considerably   recommended   to   develop   dynamometer,   is   very   costly   and   often   used   in   musculoskeletal  health  and  fitness  (ACSM,  2002).   rehabilitational  setting.     The   three   modalities   of   apparatuses   that   are   Based   on   these   disadvantages,   the   typically   used   to   perform   resistance   training   are   attention   of   athletes   and   coaches   has   been   free   weights,   pulley   machines   and   isokinetic   directed   toward   what   is   now   known   with   the   dynamometers.  Despite  extensive  application,  the   term   Variable   Resistance   Training   (VRT).   use  of  free  weights  and  pulley  machines  has  been   Manning   et   al.   (1990)   define   VRTs   as   training   widely   controversial;   because,   these   training   devices   which   attempt   to   accommodate   the   devices   provide   constant   external   force   rather   muscle  changing  level  of  force  output  throughout   than   constant   muscular   tension   throughout   the   the   ROM   by   changing   the   provided   external   range  of  motion  [(ROM),  (Fleck  &  Kraemer,  2004;   resistance.   Among   modalities   of   VRT,   CAM-­‐‑ Wallace   et   al.,   2006)].   In   theory,   the   provided   Nautilus   Machine   (NM)   and,   recently,   Elastic   external   resistance   must   be   accommodative   to   Resistance  exercises  (ER)  have  increasingly  gained     result   in   maximal   muscle   stimulation   across   the     1

 -­‐‑  Sports  Center,  University  of  Malaya,  Malaysia    -­‐‑  Faculty  of  Medicine,  University  of  Malaya,  Malaysia   3  -­‐‑  Advanced  Medical  and  Dental  Institute,  University  Sains  Malaysia,  Malaysia   4  -­‐‑  Dept  of  Biomedical  Engineering,  Faculty  of  Engineering,  University  of  Malaya,  Malaysia   2

Authors  submitted  their  contribution  of  the  article  to  the  editorial  board.   Accepted  for  printing  in  Journal  of  Human  Kinetics  vol.  30/2011  on  December  2011.  

6       popularity  among  athletes  and  recreational  lifters   (Anderson  et  al.,  2008;  Page  et  al.,  1993;  Treiber  et   al.,   1998).   However,   to   date   no   research   has   addressed   the   pattern   and   rate   of   applied   load   and   muscle   activation   during   intensive   ER   and   NM  exercises.     Elastic  resistance  is  well  established  as  an   affordable   and   effective   mode   of   training   in   rehabilitation   and   fitness   settings   (Page   and   Ellenbecker,   2003;   Hintermeister   et   al.,   1998;   Schulthies   et   al.,   1998).   However,   there   is   controversial  evidence  regarding  utilization  of  ER   for  high  intensity  training  protocols  (Treiber  et  al.,   1998).   Reportedly,   ER   cannot   lead   muscle   to   its   maximal   activation   level   due   to   inadequate   external   force   (Page   et   al.,   1993;   Matheson   et   al.,   2001).   Elastic   tubing   is   being   produced   in   several   colour-­‐‑codes   and   each   colour   denotes   a   specific   resistance   (Patterson   et   al.,   2001;   Simoneau   et   al.,   2001).   Hughes   (1999)   reported   the   magnitude   of   resistance  in  elastic  device  (Hygienic  Corporation,   Akron,   Ohio)   from   3.3N   to   70.1N   for   yellow   and   silver   colour   when   elastic   materials   were   at   18   and   159%   of   deformation   from   resting   length,   respectively.  Based  on  this  viewpoint,  utilizing  ER   has   been   confined   to   the   initial   and   intermediate   stages  of  rehabilitation  protocols  (Hintermeister  et   al.,  1998;  Hopkins  et  al.,  1999).     Accordingly,   in   this   investigation   two   strategies  are  employed  to  increase  the  magnitude   of  elastic  force.  Firstly,  additional  elastic  bands  are   utilized  in  parallel  to  the  current  unit  (Page  et  al.,   1993).   Secondly,   the   initial   length   of   the   elastic   material  are  reduced  to  enhance  the  magnitude  of   force  at  the  beginning  of  concentric  phase  (Treiber   et   al.,   1998;   Hodges,   2006).   We   hypothesize   that   applying   these   strategies   can   improve   the   tensile   force   in   ER   and   result   in   achieving   comparable   muscle  activation  as  NM  during  performing  high   intensity   knee   extension   exercise.   The   results   of   this   investigation   may   facilitate   better   understanding   for   prescribing   high   intensity   training   protocols   in   contribution   of   these   two   modes   of   exercise.   The   purpose   of   this   investigation,   therefore,   was   to   quantify   and   compare   the   magnitude   of   applied   load   and   muscle   activation   (EMG)   during   8-­‐‑RM   seated   knee  extension  in  contribution  of  ER  and  NM.  

Methods   Subjects   Seven   female   (mean   ±   SD;   22.4   ±   4.7   age,    

Journal  of  Human  Kinetics  volume  30/2011,  

Electromyographic  Activity  and  Applied  Load     60.05  ±  6.17  kg,  158  ±  3  cm)  and  9  male  (24.0  ±  3.6   age,  78.14  ±  7.2  kg,  174  ±  7cm)  healthy  volunteers   were  recruited  for  this  study.  None  of  the  subjects   had   experience   of   participating   in   any   resistance   training   program   in   the   past   12   months.   This   study   was   approved   by   the   ethics   committee   of   Sports   Centre,   University   of   Malaya   and   all   participants  signed  the  informed  consent  forms.     Experimental  design   The   measurement   scheme   of   the   present   investigation   was   started   with   an   orientation   session   in   which   the   benefits   and   risks   of   participating   in   the   study   were   clarified   for   the   participants.   After   seven   days,   subjects   attended   the   main   testing   session.   To   avoid   inaccurate   location  of  electrodes  from  day  to  day  testing,  all   data   were   collected   within   one   experimental   session.   The   order   of   the   measurement   was   randomized   across   3   exercise   modalities.   The   three   types   of   resistance   exercise   in   this   research   consisted  of  NM,  elastic  tubing  with  initial  length   (E0)   and   elastic   tubing   with   30%   decrement   of   initial   length   (E30).   The   subjects   undertook   8-­‐‑RM   knee  extension  trials  by  3  modes  of  training  with  5   min   recovery   periods   between   training   modalities.   Ten   subjects   were   randomly   selected   to   duplicate   similar   procedure   after   5   days   to   compute   the   test-­‐‑retest   reliability   on   two   occasions.     Instrumentation   EMG   muscle   activation   was   measured   with   a   sample   rate   of   1000   Hz   using   a   16-­‐‑bit   acquisition   mode   with   an   eight-­‐‑channel   TeleMyo™   2400T   G2   EMG   system   (Noraxon,   Scottsdale,  Arizona,  USA).  Pre-­‐‑gelled  silver/silver   chloride   adhesive   surface   electrodes   (Meditrace,   Canada)   were   used   to   detect   electromyographic   signals.   EMG   signals   were   passed   through   a   build-­‐‑in   preamplifier   leads   (Input   impedance   of   500  MΩ  common  mode  rejection  ratio  of  130  dB).   Receiver  unit  collected  the  telemetry  signals  from   the  receiver  amplified  and  filtered  (15  Hz  to  1000   Hz)   the   signals.   Range   of   motion   of   dominant   knee   was   monitored   using   a   2-­‐‑D   electrogoniometer   (Noraxon,   Scottsdale,   Arizona,   USA).   In   order   to   avoid   any   biomechanical   interference   in   position   of   the   subjects,   the   nautilus   knee   extension   chair   (Nautilus,   Vancouver,  WA)  was  used  for  ER  exercise  testing   as   well.   The   lever   arm   in   nautilus   machine   and     http://www.johk.pl  

by  Aboodarda    S.J  et  al.       the   ankle   cuff   in   elastic   device   were   equipped   with   a   force   transducer   (Noraxon,   Scottsdale,   Arizona,   USA)   to   measure   magnitude   of   applied   force.  Data  were  collected  and  synchronized  using   the   data   acquisition   package   Myoresearch-­‐‑XP,   Master   Edition   (Noraxon,   Scottsdale,   Arizona,   USA).   Measurement  procedure   A   week   prior   to   the   experiment   all   subjects  attended  an  orientation  session  in  which,   for  familiarization,  they  were  required  to  practice   Maximal   Voluntary   Isometric   Contractions   (MVIC)   and   8-­‐‑RM   trials   with   three   modes   of   exercise.   The   external   load   was   either   added   or   removed   to   achieve   the   actual   8-­‐‑RM   for   each   mode   of   training   (Treiber   et   al.,   1998).   This   goal   for   elastic   material   was   fulfilled   by   examining   different   combination   of   elastic   colour   codes   (but   similar   length)   to   meet   the   actual   number   of   repetitions   (8-­‐‑RM).   The   initial   length   of   elastic   material   (Hygienic   Corporation,   Akron,   OH)   was   determined   for   every   subject   by   measuring   the   distance   from   the   origin   of   the   elastic   device   (elastic   tubing   was   anchored   to   the   base   of   the   NM  chair)  to  the  axis  (ankle  cuff).  In  this  way,  the   combination   of   colour   code   as   well   as   the   length   of   elastic   device   was   personalized   for   each   subject.   In   addition,   subjects   were   asked   not   to   participate   in   any   training   48   hours   before   the   main  testing  session.   The   main   testing   session   for   each   participant   started   at   8   a.m.   The   warm   up   was   performed  comprising  static  stretching  and  5  min   biking   on   an   ergometer   with   a   self-­‐‑selected   pace.   Following   the   warm   up,   the   subjects   were   allowed   to   rest   for   5   min,   during   which   time   the   electrodes  were  located  parallel  to  the  direction  of   the   Vastus   Lateralis   (VL)   muscle   fibres   above   the   midpoint   of   the   muscle   belly   as   assessed   by   palpation.   The   ground   electrode   was   placed   on   the   patella   bone.   Before   placement   of   electrodes,   the   subject`s   skin   was   shaved   and   cleaned   with   alcohol  to  reduce  skin  impedance.   Each   subject   was   then   seated   on   the   knee   extension   NM   according   to   the   procedure   reported   by   Manning   et   al.   (1990).   A   five-­‐‑second   baseline   signal   was   collected   from   the   muscles   to   ensure   no   artefacts   existed.   All   the   subjects   completed   3   trials   of   unilateral   MVIC   with   the   dominant  leg.  For  this  aim,  the  NM  chair  was  set   to  obtain  a  120°  knee  extension.  Each  trial  lasted  5     ©  Editorial  Committee  of  Journal  of  Human  Kinetics  

7     seconds   and   two   minutes   rest   intervals   were   assigned   between   trials   to   prevent   fatigue   (Matheson   et   al.,   2001).   The   MVIC   was   determined   as   an   average   of   amplitude   over   one   second   window   of   the   highest   rectified   EMG   signals   (automatically   selected   by   Myoresearch-­‐‑ XP).   It   was   used   as   a   reference   value   for   normalizing   muscle   activation   data   during   dynamic  exercises  (%MVIC).     The  subjects  then  completed  8-­‐‑RM  knee   extension  trials  by  3  modes  of  training  with  5  min   recovery   periods   between   training   modalities.   Repetitions   were   completed   within   80   to   180º   of   knee   extension   with   the   cadence   of   1.5   s   concentric  and  1.5  s  eccentric,  set  by  a  metronome.   One   second   pause   between   every   repetition   was   assigned   to   avoid   potential   stretch-­‐‑shortening   cycle   interference   in   the   concentric-­‐‑eccentric   merging   phase.   To   control   the   position   of   shank   during   dynamic   contractions,   two   laser   beams   connected  to  an  alarm  system  limited  the  range  of   motion   at   each   extremity.   Therefore,   an   alarm   sounded   if   subject`s   foot   would   touch   laser   spectrums.   An   attempt   at   8-­‐‑RM   was   deemed   successful   if   all   repetitions   were   performed   in   accordance   with   the   pace   of   the   metronome   without   any   compromise   in   ROM.   Therefore,   the   cadence   of   performing   exercises   and   the   range   of   motion   were   limiting   factors   on   the   amount   of   external   resistance.   Ten   subjects   were   randomly   selected   to   duplicate   a   similar   procedure   after   5   days   to   compute   reliability   of   testing   on   two   occasions.  The  test-­‐‑retest  reliability  for  magnitude   of  external  force  at  each  phase  of  contraction  was   0.92,   0.89   and   0.95   for   E30,   E0   and   NM,   respectively.       Although   data   were   collected   from   all   repetitions   during   8-­‐‑RM,   the   first   (initial),   the   5th   (middle)   and   the   8th   (last)   repetitions   were   selected   for   further   analysis.   Appointed   repetitions   were   partitioned   into   concentric   and   eccentric   components   based   on   the   end   points   determined  by  the  electrogoniometer  traces.  Then,   the  value  of  every  component  was  divided  into  3   equal   phases.   Accordingly,   the   division   of   movement   into   six   phases   (3   concentric   and   3   eccentric)  across  the  entire  ROM  (80  -­‐‑  180°  of  knee   extension)  comprised  of  80  –  113°,  113  –  146°  and   146  –  180°  for  the  1st  ,  2nd  and  3rd  concentric  phases   respectively,  and  in  a  reverse  order  for  the  4th,  5th   and   6th   phases   of   eccentric   contraction.   The   Root    

8         Mean  Square  (RMS)  of  rectified  EMG  signals  and   the   average   of   external   force   (N)   were   calculated   for   each   phase.   The   average   of   6   phases   for   the   EMG   and   the   force   were   then   used   to   represent   the   value   of   each   repetition.   Then,   the   values   of   1st,  5th  and  8th  repetitions  were  used  for  calculating   the   total   value   for   each   exercise   modality   (6   phases  ×  3  repetitions).       Statistical  Analysis   Differences   in   EMG   and   external   force   values  were  examined  within  various  phases  (1  to   6),   repetitions   (1,   5   and   8)   and   modalities   of   exercise   (NM,   E0   and   E30)   using   a   6   ×   3   ×   3   Repeated   Measure   Analysis   of   Variance   (ANOVA).   If   significant   results   were   obtained   from  ANOVA,  a  series  of  pair  sample  t-­‐‑tests  were   used   to   compare   analogous   phases   and   repetitions   among   modalities   of   exercise.   Significance  was  defined  as  p  <  .05.  

Results     Applied  Force   The   data   addressing   the   magnitude   of   applied   force   for   the   main   effects   (phases,   repetitions   and   exercise   types)   are   listed   in   Table   1.   The   analysis   of   variance   demonstrated   statistically  significant  values  for  the  interaction  of   the   main   effect   phases   ×   repetitions   ×   training   modes   (p   <   .01).   Subsequently,   a   series   of   pair   sample   t-­‐‑tests   among   Total   Average   force   (the   value  of  each  exercise  mode  comprised  of  3  rep  ×   6   phases)   indicated   that   there   was   a   considerable   difference   between   NM   and   both   ER   modalities   (Figure   1).   In   addition,   E30   exhibited   a   significantly   higher   overall   value   compared   with   E0  (p  <  .05).   The   results   concerning   the   pattern   of   applied   force   during   each   mode   of   exercises   are   depicted  in  Figure  2.  The  force-­‐‑angle  relationship   was   an   inverted   “U”   for   E30   and   E0   throughout   the  whole  ROM.  The  data  indicate  that  during  the   concentric   contraction   both   types   of   ER   devices   provided  significantly  greater  external  force  in  the   2nd   and   3rd   phases   compared   with   the   1st   phase;   though,   no   significant   difference   was   observed   between   the   2nd   and   3rd   phases.   During   eccentric   contraction   however   a   significant   decline   in   magnitude  of  external  force  was  observed  during   which   the   6th   phase   <   the   5th   phase   <   4th   phase.   However,  for  NM  the  only  systematic  change  was     Journal  of  Human  Kinetics  volume  30/2011,  

Electromyographic  Activity  and  Applied  Load     an   insignificant   decline   in   magnitude   of   force   toward  the  3rd  and  4th  phases  (p  <  .05).     EMG.   The   results   addressing   the   means   and   standard   deviations   for   the   main   effects   (phases,   repetitions   and   exercise   types)   are   listed   in   a   hierarchical   structure   in   Table   2.   Analysis   of   variance   demonstrated   a   significant   value   for   the   interaction   effects   of   phases   ×   repetitions   ×   training   mode   (p   =   .025;   p   <   .05).   Subsequently,   a   series  of  pair  sample  t-­‐‑tests  among  Total  Average   EMG  (the  value  of  each  exercise  mode  comprised   of   3   rep   ×   6   phases)   indicated   that   there   was   no   significant   difference   between   E30   and   NM   (Figure   3);   though,   either   of   these   exercise   types   exhibited   a   significantly   higher   value   compared   with  E0  (all  p  <  .05).     The   EMG   values   for   various   phases   of   contraction   in   the   three   exercise   types   are   presented   graphically   in   Figure   4.   These   results   indicated   that   in   the   1st   and   6th   phases   NM   generated   significantly   higher   muscle   activation   than   both   elastic   modalities   and   E30   attained   a   significantly  higher  value  than  E0  (p  <  .05).  In  the   2nd   through   5th   phases   only   E30   showed   a   significantly   higher   muscle   activity   than   E0,   but   no  other  significant  differences  were  observed  (p  <   .05).    

Discussion     The  purpose  of  this  descriptive  study  was   to  quantify  and  compare  the  EMG  activity  and  the   applied   load   during   high   intensity   seated   leg   extension   exercises   by   NM   and   ER   devices.   To   enhance  the  provided  external  force  by  ER  device,   two   strategies   were   employed:   firstly,   additional   elastic   bands   were   utilized   in   parallel   to   the   current  unit  to  develop  overall  tensile  force  (Page   et   al.,   1993);   secondly,   the   initial   length   of   the   elastic   material   was   reduced   to   enhance   the   magnitude   of   force   at   the   beginning   of   the   concentric   phase   (Treiber   et   al.,   1998;   Hodges,   2006).   The   data   presented   in   Figure   3   suggested   that   applying   a   combination   of   these   two   strategies   could   result   in   producing   equal   overall   muscle   activation   (EMG)   by   E30   compared   with   NM.   Furthermore,   reducing   the   initial   length   of   the   elastic   device   (the   only   source   of   difference   between   E30   and   E0)   could   significantly   improve   muscle   activation   (28.8%)   and   applied   force   (25.8%)  for  E30  compared  with  E0.     The   effectiveness   of   these   strategies     http://www.johk.pl  

by  Aboodarda    S.J  et  al.       becomes   even   more   evident   when   the   pattern   of   muscle  activity  is  compared  across  the  three  types   of   training.   As   depicted   in   Figure   4,   significantly   higher  EMG  was  achieved  by  E30  compared  with   E0   in   the   1st,   2nd,   4th,   5th   and   6th   segments.   In   addition,   E30   generated   muscle   activation   equal   to   that   of   NM   in   the   2nd   to   5th   segments.   Such   results   suggest   E30   as   a   modified   form   of   elastic   device   which   can   partially   overcome   a   chronic   drawback   of   ER   exercise   in   eliciting   adequate   muscle   activation   throughout   the   ROM,   particularly   at   the   beginning   of   the   concentric   phase   (Lim   and   Chow,   1998;   Matheson   et   al.,   2001).     Despite   the   above   findings,   significantly   less   EMG   activity   at   the   1st   and   6th   phases   and   considerably   less   applied   force   of   E30   compared   with   NM   across   the   entire   ROM   indicate   that   caution   should   be   taken   before   accepting   E30   as   an   inclusive   mode   of   training   for   high   intensity   resistance   exercise   protocols.   In   fact,   despite   reducing   by   30%   the   initial   length,   E30   could   not   provide   adequate   external   resistance   to   meet   the   force   generating   capability   of   quadriceps   muscle   at   these   two   particular   segments.   This   finding   is   in   accordance   with   the   results   reported   by   Hodges   (2006).   He   demonstrated   that   even   shortened   elastic   tubing   provides   less   average   resistance  and  consequently  lower  neuromuscular   adaptation  than  that  of  traditional  free  weights  at   the   beginning   of   concentric   and   end   of   eccentric   segments   of   motion.   These   results   point   to   the   need   for   more   studies   to   elucidate   if   a   reduction   of   initial   elastic   device   length   (e.g.   40%   or   50%)   would  result  in  more  muscle  activity  and  provide   elastic  force  in  these  segments  of  motion.   A  unique  aspect  of  the  data  in  the  present   study   is   the   observation   of   equal   Total   Average   EMG   activity   between   E30   and   NM,   despite   a   considerably   smaller   external   load   (111.4%)   that   was   employed   during   E30   compared   with   NM   (Figure   1).   This   result   propounds   the   question   “how  could  a  smaller  external  force  in  E30  elicit  a   similar   rate   of   muscle   activation   compared   with   NM?”  This  discrepancy  was  also  evident  across  a   whole  range  of  motion  (Figures  2  and  4)  where  a   higher   applied   force   within   NM   exercise   was   not   reflected   in   EMG   values.   Interestingly,   the   data   indicated   that   although   extensively   greater   external   load   was   employed   by   NM   during   the   whole   ROM,   insignificantly   higher   EMG   activity     ©  Editorial  Committee  of  Journal  of  Human  Kinetics  

9     was   detected   for   E30   compared   with   NM   in   the   2nd,   3rd   and   5th   phases.   In   the   4th   phase   E30   demonstrated  significantly  higher  muscle  activity   than  NM.  On  the  other  hand,  E0  demonstrated  an   equal   EMG   activity   to   NM   in   the   2nd,   3rd   and   4th   phases  of  contraction,  although  137.1%,  46.7%  and   60%   greater   load   was   employed   by   NM   than   E0   during  these  phases,  respectively.     The   reason   behind   this   relatively   higher   EMG   for   E30   is   unclear.   However,   since   distal   extremity  of  lower  leg  (ankle)  had  a  higher  degree   of   freedom   during   knee   extension   by   ER   device   (compared   with   restricted-­‐‑unidirectional   NM   lever   arm)   more   control   over   the   movement   was   required   to   keep   lower   leg   motion   aligned   in   sagittal  plane  (McCaw  &  Friday,  1994).  Therefore,   muscle   activity   during   E30   could   have   been   partially  devoted  to  control  lower  leg  movements   throughout   the   assigned   range   of   motion.   In   advocate  of  this  idea  Richards  and  Dawson  (2009)   indicated  that  performing  exercises  in  a  multiaxial   direction   could   potentially   change   the   rate   of   muscle   activation   via   altering   motor   unit   recruitment.   Overall,   the   data   supported   the   idea   of   Bosco   et   al.   (2000)   regarding   the   concept   of   exercise  intensity.  They  stated  that  contrary  to  the   classical   thought   which   had   defined   exercise   intensity  as  the  magnitude  of  the  load  employed,   it  must  have  been  defined  as  the  rate  of  the  work   performed.   In   the   1st   and   6th   phases,   E30   and   E0   generated   significantly   less   EMG   activity   compared   with   NM   (Figure   4).   This   result   could   be  attributed  to  the  necessity  of  less  muscle  effort   to   overcome   the   inertia   of   much   lower   external   load   in   ER   exercises   during   the   early   concentric   and   late   eccentric   phases   of   contraction.   Nonetheless,   the   findings   of   the   present   study   highlight   the   effect   of   reducing   the   initial   length   of   elastic   material   in   achieving   significantly   higher   muscle   activation   and   applied   lead   by   elastic   resistance   device   (Figures   2   and   4).   The   data   demonstrated   dramatically   higher   EMG   values   for   E30   compared   with   E0   in   all   phases   of   contraction,  except  in  the  3rd  phase  in  which  equal   EMG   observed   between   the   two   modes   of   training.   Based   on   similar   finding,   Hodges   (2006)   concluded  that  after  reducing  the  initial  length  of   elastic   material,   a   shifting   occurs   in   the   distribution  of  muscle  tension  from  late  concentric   to  early  concentric  and  from  early  eccentric  to  late    

10         eccentric   range   of   motion.   Accordingly,   E30   exhibited  significantly  higher  EMG  than  E0  in  the   1st   (48%)   and   the   6th   (84.31%)   phases.   These   data   disclose   the   importance   of   reducing   the   initial   length   as   an   essential   strategy   to   develop   muscle   activation  by  ER  devices.    

Conclusion   Many   athletes   rather   use   various   modalities  of  resistance  exercise  (e.g.  free  weights,   pulley  machines,  isokinetic  dynamometers,  elastic   resistance,  etc)  within  their  conditioning  program   with   the   prevailing   view   that   each   type   of   strength   training   offers   a   unique   mechanical   and    

Electromyographic  Activity  and  Applied  Load   physiological   muscle   stimulation   (Welsch   et   al.,   2005).  On  this  basis,  undertaking  several  types  of   resistance   exercise   might   facilitate   better   development   of   the   muscle   performance.   Based   on  equal  average  EMG  between  E30  and  NM,  the   findings   of   the   present   study   suggest   that   E30   could   be   an   alternative   to   the   use   of   NM   in   high   exercise   intensity   (8-­‐‑RM).   However,   since   NM   displayed  higher  EMG  compared  with  E30  in  the   early  concentric  and  late  eccentric  phases  and  E30   demonstrated  higher  muscle  activation  in  the  late   concentric   and   early   eccentric   phases   of   contraction,  a  training  protocol  comprised  of  both   modes  of  exercise  seems  to  be  ideal.  

Acknowledgement     For  this  investigation  a  research  grant  was  provided  by  University  of  Malaya,  Malaysia  (PS008/2008C).  

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