The Effect of Shoulder Flexion Angles on the Recruitment of Upper-extremity Muscles during Isometric Contraction
Identifieur interne : 002B00 ( Ncbi/Merge ); précédent : 002A99; suivant : 002B01The Effect of Shoulder Flexion Angles on the Recruitment of Upper-extremity Muscles during Isometric Contraction
Auteurs : Jeheon Moon ; Insik Shin ; Myoungsoo Kang ; Yeonghun Kim ; Kunwoo Lee ; Jaewoo Park ; Kyungnam Kim ; Daehie Hong ; Dohoon Koo ; David O'SullivanSource :
- Journal of Physical Therapy Science [ 0915-5287 ] ; 2013.
Abstract
The purpose of this study was to investigate the differences in muscle activation patterns of the biceps brachii (BB) and flexor carpi radialis (FCR) muscles, while measuring the resultant force (RF) at different shoulder flexion angles. [Subjects] Thirteen healthy males (age 24.85±3.4 years, weight; 77.8±7.9 kg; height, 1.7±0.05 m) were enrolled in this study. [Methods] The resultant force was measured by a force transducer . The elbow angle remained constant and the flexion shoulder angle was changed (30°, 45°, 60°, 75° and 90°). [Results] The results of the surface EMG show the largest muscle activities occurred at a shoulder flexion of 75° for BB and 90° for FCR. The largest resultant force was measured at a shoulder flexion angle of 75°. We conclude, that when performing the biceps curl exercise using an arm curl machine, the shoulder should be flexed at 75° to maximize the focus of the exercise for the BB. [Conclusion] These results are useful from the perspective of design as they highlight the differences in the muscle activation of BB and FCR with postural change. Ultimately this knowledge can be used in the design of rehabilitation training for the shoulder as they show that posture can affect muscle activation.
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DOI: 10.1589/jpts.25.1299
PubMed: 24259780
PubMed Central: 3820192
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<record><TEI><teiHeader><fileDesc><titleStmt><title xml:lang="en">The Effect of Shoulder Flexion Angles on the Recruitment of Upper-extremity
Muscles during Isometric Contraction</title><author><name sortKey="Moon, Jeheon" sort="Moon, Jeheon" uniqKey="Moon J" first="Jeheon" last="Moon">Jeheon Moon</name><affiliation><nlm:aff>NONE</nlm:aff></affiliation></author><author><name sortKey="Shin, Insik" sort="Shin, Insik" uniqKey="Shin I" first="Insik" last="Shin">Insik Shin</name><affiliation><nlm:aff>NONE</nlm:aff></affiliation></author><author><name sortKey="Kang, Myoungsoo" sort="Kang, Myoungsoo" uniqKey="Kang M" first="Myoungsoo" last="Kang">Myoungsoo Kang</name><affiliation><nlm:aff>NONE</nlm:aff></affiliation></author><author><name sortKey="Kim, Yeonghun" sort="Kim, Yeonghun" uniqKey="Kim Y" first="Yeonghun" last="Kim">Yeonghun Kim</name><affiliation><nlm:aff>NONE</nlm:aff></affiliation></author><author><name sortKey="Lee, Kunwoo" sort="Lee, Kunwoo" uniqKey="Lee K" first="Kunwoo" last="Lee">Kunwoo Lee</name><affiliation><nlm:aff>NONE</nlm:aff></affiliation></author><author><name sortKey="Park, Jaewoo" sort="Park, Jaewoo" uniqKey="Park J" first="Jaewoo" last="Park">Jaewoo Park</name><affiliation><nlm:aff>NONE</nlm:aff></affiliation></author><author><name sortKey="Kim, Kyungnam" sort="Kim, Kyungnam" uniqKey="Kim K" first="Kyungnam" last="Kim">Kyungnam Kim</name><affiliation><nlm:aff>NONE</nlm:aff></affiliation></author><author><name sortKey="Hong, Daehie" sort="Hong, Daehie" uniqKey="Hong D" first="Daehie" last="Hong">Daehie Hong</name><affiliation><nlm:aff>NONE</nlm:aff></affiliation></author><author><name sortKey="Koo, Dohoon" sort="Koo, Dohoon" uniqKey="Koo D" first="Dohoon" last="Koo">Dohoon Koo</name><affiliation><nlm:aff>NONE</nlm:aff></affiliation></author><author><name sortKey="O Sullivan, David" sort="O Sullivan, David" uniqKey="O Sullivan D" first="David" last="O'Sullivan">David O'Sullivan</name><affiliation><nlm:aff>NONE</nlm:aff></affiliation></author></titleStmt><publicationStmt><idno type="wicri:source">PMC</idno><idno type="pmid">24259780</idno><idno type="pmc">3820192</idno><idno type="url">http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3820192</idno><idno type="RBID">PMC:3820192</idno><idno type="doi">10.1589/jpts.25.1299</idno><date when="2013">2013</date><idno type="wicri:Area/Pmc/Corpus">002063</idno><idno type="wicri:Area/Pmc/Curation">002063</idno><idno type="wicri:Area/Pmc/Checkpoint">000F82</idno><idno type="wicri:Area/Ncbi/Merge">002B00</idno></publicationStmt><sourceDesc><biblStruct><analytic><title xml:lang="en" level="a" type="main">The Effect of Shoulder Flexion Angles on the Recruitment of Upper-extremity
Muscles during Isometric Contraction</title><author><name sortKey="Moon, Jeheon" sort="Moon, Jeheon" uniqKey="Moon J" first="Jeheon" last="Moon">Jeheon Moon</name><affiliation><nlm:aff>NONE</nlm:aff></affiliation></author><author><name sortKey="Shin, Insik" sort="Shin, Insik" uniqKey="Shin I" first="Insik" last="Shin">Insik Shin</name><affiliation><nlm:aff>NONE</nlm:aff></affiliation></author><author><name sortKey="Kang, Myoungsoo" sort="Kang, Myoungsoo" uniqKey="Kang M" first="Myoungsoo" last="Kang">Myoungsoo Kang</name><affiliation><nlm:aff>NONE</nlm:aff></affiliation></author><author><name sortKey="Kim, Yeonghun" sort="Kim, Yeonghun" uniqKey="Kim Y" first="Yeonghun" last="Kim">Yeonghun Kim</name><affiliation><nlm:aff>NONE</nlm:aff></affiliation></author><author><name sortKey="Lee, Kunwoo" sort="Lee, Kunwoo" uniqKey="Lee K" first="Kunwoo" last="Lee">Kunwoo Lee</name><affiliation><nlm:aff>NONE</nlm:aff></affiliation></author><author><name sortKey="Park, Jaewoo" sort="Park, Jaewoo" uniqKey="Park J" first="Jaewoo" last="Park">Jaewoo Park</name><affiliation><nlm:aff>NONE</nlm:aff></affiliation></author><author><name sortKey="Kim, Kyungnam" sort="Kim, Kyungnam" uniqKey="Kim K" first="Kyungnam" last="Kim">Kyungnam Kim</name><affiliation><nlm:aff>NONE</nlm:aff></affiliation></author><author><name sortKey="Hong, Daehie" sort="Hong, Daehie" uniqKey="Hong D" first="Daehie" last="Hong">Daehie Hong</name><affiliation><nlm:aff>NONE</nlm:aff></affiliation></author><author><name sortKey="Koo, Dohoon" sort="Koo, Dohoon" uniqKey="Koo D" first="Dohoon" last="Koo">Dohoon Koo</name><affiliation><nlm:aff>NONE</nlm:aff></affiliation></author><author><name sortKey="O Sullivan, David" sort="O Sullivan, David" uniqKey="O Sullivan D" first="David" last="O'Sullivan">David O'Sullivan</name><affiliation><nlm:aff>NONE</nlm:aff></affiliation></author></analytic><series><title level="j">Journal of Physical Therapy Science</title><idno type="ISSN">0915-5287</idno><idno type="eISSN">2187-5626</idno><imprint><date when="2013">2013</date></imprint></series></biblStruct></sourceDesc></fileDesc><profileDesc><textClass></textClass></profileDesc></teiHeader><front><div type="abstract" xml:lang="en"><p> The purpose of this study was to investigate the differences in muscle activation
patterns of the biceps brachii (BB) and flexor carpi radialis (FCR) muscles, while
measuring the resultant force (RF) at different shoulder flexion angles. [Subjects]
Thirteen healthy males (age 24.85±3.4 years, weight; 77.8±7.9 kg; height, 1.7±0.05 m) were
enrolled in this study. [Methods] The resultant force was measured by a force transducer .
The elbow angle remained constant and the flexion shoulder angle was changed (30°, 45°,
60°, 75° and 90°). [Results] The results of the surface EMG show the largest muscle
activities occurred at a shoulder flexion of 75° for BB and 90° for FCR. The largest
resultant force was measured at a shoulder flexion angle of 75°. We conclude, that when
performing the biceps curl exercise using an arm curl machine, the shoulder should be
flexed at 75° to maximize the focus of the exercise for the BB. [Conclusion] These results
are useful from the perspective of design as they highlight the differences in the muscle
activation of BB and FCR with postural change. Ultimately this knowledge can be used in
the design of rehabilitation training for the shoulder as they show that posture can
affect muscle activation.</p></div></front><back><div1 type="bibliography"><listBibl><biblStruct><analytic><author><name sortKey="Nelson, Me" uniqKey="Nelson M">ME Nelson</name></author><author><name sortKey="Rejeski, Wj" uniqKey="Rejeski W">WJ Rejeski</name></author><author><name sortKey="Blair, Sn" uniqKey="Blair S">SN Blair</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Harman, E" uniqKey="Harman E">E Harman</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Holcomb, Wr" uniqKey="Holcomb W">WR Holcomb</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Jones, Rm" uniqKey="Jones R">RM Jones</name></author><author><name sortKey="Fry, Ac" uniqKey="Fry A">AC Fry</name></author><author><name sortKey="Weiss, Lw" uniqKey="Weiss L">LW Weiss</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Mohamed, O" uniqKey="Mohamed O">O Mohamed</name></author><author><name sortKey="Perry, J" uniqKey="Perry J">J Perry</name></author><author><name sortKey="Hislopd, H" uniqKey="Hislopd H">H Hislopd</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Bayram, U" uniqKey="Bayram U">U Bayram</name></author><author><name sortKey="Vasfi, K" uniqKey="Vasfi K">K Vasfi</name></author><author><name sortKey="Serkan, B" uniqKey="Serkan B">B Serkan</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Hagberg, M" uniqKey="Hagberg M">M Hagberg</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Oliveira, Lf" uniqKey="Oliveira L">LF Oliveira</name></author><author><name sortKey="Matta, Tt" uniqKey="Matta T">TT Matta</name></author><author><name sortKey="Alves, Ds" uniqKey="Alves D">DS Alves</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Hermens, Hj" uniqKey="Hermens H">HJ Hermens</name></author><author><name sortKey="Freriks, B" uniqKey="Freriks B">B Freriks</name></author><author><name sortKey="Disselhorst Klug, C" uniqKey="Disselhorst Klug C">C Disselhorst-Klug</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Ahamed, Nu" uniqKey="Ahamed N">NU Ahamed</name></author><author><name sortKey="Sundaraj, K" uniqKey="Sundaraj K">K Sundaraj</name></author><author><name sortKey="Ahmad, Rb" uniqKey="Ahmad R">RB Ahmad</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Stegeman, D" uniqKey="Stegeman D">D Stegeman</name></author><author><name sortKey="Hermens, H" uniqKey="Hermens H">H Hermens</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Park, Jw" uniqKey="Park J">JW Park</name></author><author><name sortKey="Kim, Kn" uniqKey="Kim K">KN Kim</name></author><author><name sortKey="Hong, Dh" uniqKey="Hong D">DH Hong</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Ahmadi, S" uniqKey="Ahmadi S">S Ahmadi</name></author><author><name sortKey="Sinclair, Pj" uniqKey="Sinclair P">PJ Sinclair</name></author><author><name sortKey="Foroughi, N" uniqKey="Foroughi N">N Foroughi</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Van Zuylen, Ej" uniqKey="Van Zuylen E">EJ van Zuylen</name></author><author><name sortKey="Gielen, Cc" uniqKey="Gielen C">CC Gielen</name></author><author><name sortKey="Denier, Gj" uniqKey="Denier G">GJ Denier</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Hansen, Ea" uniqKey="Hansen E">EA Hansen</name></author><author><name sortKey="Lee, Hd" uniqKey="Lee H">HD Lee</name></author><author><name sortKey="Barrett, K" uniqKey="Barrett K">K Barrett</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Murray, Wm" uniqKey="Murray W">WM 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<front><journal-meta><journal-id journal-id-type="nlm-ta">J Phys Ther Sci</journal-id><journal-id journal-id-type="iso-abbrev">J Phys Ther Sci</journal-id><journal-id journal-id-type="publisher-id">JPTS</journal-id><journal-title-group><journal-title>Journal of Physical Therapy Science</journal-title></journal-title-group><issn pub-type="ppub">0915-5287</issn><issn pub-type="epub">2187-5626</issn><publisher><publisher-name>The Society of Physical Therapy Science</publisher-name></publisher></journal-meta><article-meta><article-id pub-id-type="pmid">24259780</article-id><article-id pub-id-type="pmc">3820192</article-id><article-id pub-id-type="publisher-id">jpts-2013-134</article-id><article-id pub-id-type="doi">10.1589/jpts.25.1299</article-id><article-categories><subj-group subj-group-type="heading"><subject>Original</subject></subj-group></article-categories><title-group><article-title>The Effect of Shoulder Flexion Angles on the Recruitment of Upper-extremity
Muscles during Isometric Contraction</article-title></title-group><contrib-group><contrib contrib-type="author"><name><surname>Moon</surname><given-names>Jeheon</given-names></name><degrees>PhD, ST</degrees><xref ref-type="aff" rid="aff1"><sup>1</sup></xref></contrib><contrib contrib-type="author"><name><surname>Shin</surname><given-names>Insik</given-names></name><degrees>PhD</degrees><xref ref-type="aff" rid="aff1"><sup>1</sup></xref></contrib><contrib contrib-type="author"><name><surname>Kang</surname><given-names>Myoungsoo</given-names></name><degrees>PhD, ST</degrees><xref ref-type="aff" rid="aff1"><sup>1</sup></xref></contrib><contrib contrib-type="author"><name><surname>Kim</surname><given-names>Yeonghun</given-names></name><degrees>PhD, ST</degrees><xref ref-type="aff" rid="aff2"><sup>2</sup></xref></contrib><contrib contrib-type="author"><name><surname>Lee</surname><given-names>Kunwoo</given-names></name><degrees>PhD</degrees><xref ref-type="aff" rid="aff2"><sup>2</sup></xref></contrib><contrib contrib-type="author"><name><surname>Park</surname><given-names>Jaewoo</given-names></name><degrees>PhD</degrees><xref ref-type="aff" rid="aff3"><sup>3</sup></xref></contrib><contrib contrib-type="author"><name><surname>Kim</surname><given-names>Kyungnam</given-names></name><degrees>PhD, ST</degrees><xref ref-type="aff" rid="aff3"><sup>3</sup></xref></contrib><contrib contrib-type="author"><name><surname>Hong</surname><given-names>Daehie</given-names></name><degrees>PhD</degrees><xref ref-type="aff" rid="aff3"><sup>3</sup></xref></contrib><contrib contrib-type="author"><name><surname>Koo</surname><given-names>Dohoon</given-names></name><degrees>MS</degrees><xref ref-type="aff" rid="aff4"><sup>4</sup></xref></contrib><contrib contrib-type="author"><name><surname>O'sullivan</surname><given-names>David</given-names></name><degrees>PhD</degrees><xref ref-type="aff" rid="aff5"><sup>5</sup></xref><xref rid="cor1" ref-type="corresp"><sup>*</sup></xref></contrib><aff id="aff1"><label>1)</label> Biomechanics Laboratory, Department of Physical Education, Seoul National University, Republic of Korea</aff><aff id="aff2"><label>2)</label> Human-Centered CAD Laboratory, Department of Mechanical and Aerospace Engineering, Seoul National University, Republic of Korea</aff><aff id="aff3"><label>3)</label> Mechatronics and Field Robotics Laboratory, Department of Mechanical Engineering, Korea University, Republic of Korea</aff><aff id="aff4"><label>4)</label> Department of Rehabilitation and Assistive Technology, Korea National Rehabilitation Center Research Institute, Republic of Korea</aff><aff id="aff5"><label>5)</label> Department of Sport Science, Chung-Ang University, Republic of Korea</aff></contrib-group><author-notes><corresp id="cor1"><label>*</label>Correspondence to: O'sullivan David, Department of Sport Science, Chung-Ang University: 72-1
Daedeok-myeon, Anseong-si, Gyeonggi-do 456-756, Republic of Korea. E-mail: <email xlink:href="tkdnutter@yahoo.com">tkdnutter@yahoo.com</email></corresp></author-notes><pub-date pub-type="epub"><day>20</day><month>11</month><year>2013</year></pub-date><pub-date pub-type="ppub"><month>10</month><year>2013</year></pub-date><volume>25</volume><issue>10</issue><fpage>1299</fpage><lpage>1301</lpage><history><date date-type="received"><day>02</day><month>4</month><year>2013</year></date><date date-type="accepted"><day>29</day><month>5</month><year>2013</year></date></history><permissions><copyright-statement>2013©by the Society of Physical Therapy Science</copyright-statement><copyright-year>2013</copyright-year><license license-type="open-access" xlink:href="http://creativecommons.org/licenses/by-nc-nd/3.0/"><license-p>This is an open-access article distributed under the terms of the Creative
Commons Attribution Non-Commercial No Derivatives (by-nc-nd) License. </license-p></license></permissions><abstract><p> The purpose of this study was to investigate the differences in muscle activation
patterns of the biceps brachii (BB) and flexor carpi radialis (FCR) muscles, while
measuring the resultant force (RF) at different shoulder flexion angles. [Subjects]
Thirteen healthy males (age 24.85±3.4 years, weight; 77.8±7.9 kg; height, 1.7±0.05 m) were
enrolled in this study. [Methods] The resultant force was measured by a force transducer .
The elbow angle remained constant and the flexion shoulder angle was changed (30°, 45°,
60°, 75° and 90°). [Results] The results of the surface EMG show the largest muscle
activities occurred at a shoulder flexion of 75° for BB and 90° for FCR. The largest
resultant force was measured at a shoulder flexion angle of 75°. We conclude, that when
performing the biceps curl exercise using an arm curl machine, the shoulder should be
flexed at 75° to maximize the focus of the exercise for the BB. [Conclusion] These results
are useful from the perspective of design as they highlight the differences in the muscle
activation of BB and FCR with postural change. Ultimately this knowledge can be used in
the design of rehabilitation training for the shoulder as they show that posture can
affect muscle activation.</p></abstract><kwd-group><kwd>Biceps brachii</kwd><kwd>Flexor carpi radialis</kwd></kwd-group></article-meta></front><body><sec id="s1"><title>INTRODUCTION</title><p>For the last couple of decades weight training has become increasingly popular due to its
well-documented numerous benefits, such as increasing muscular strength and raising resting
metabolism rates<xref ref-type="bibr" rid="r1">1</xref><sup>)</sup>. According to Nelson, in
a review article about the physical activity and public health of older adults, some of the
benefits of resistance exercise include improvement of physical fitness, management and
prevention of diseases caused by sedentary lifestyles, reduction of chronic health
conditions and unhealthy weight gain<xref ref-type="bibr" rid="r1">1</xref><sup>)</sup>. It
is believed by personal trainers that the use of weight training machines are easier to
learn than free weights. Also, weight training machines help the maintenance of good
posture, and thus, are safer. The advantages and disadvantages of using weight training
machines rather than free weights are well-established<xref ref-type="bibr" rid="r2">2</xref>, <xref ref-type="bibr" rid="r3">3</xref><sup>)</sup>. Jones et al. investigated
and compared the kinetic and kinematic variables of the power clean exercise using free
weights and machine resistance<xref ref-type="bibr" rid="r4">4</xref><sup>)</sup>. Their
results demonstrate that both kinetic and kinematic variables were significantly different.
While the maximum strength was significantly greater for the free weight condition, the
average and peak velocities were significantly higher for the machine condition.</p><p>Isometric exercises have become popular and have been regularly and successfully
implemented by physiotherapists for the rehabilitation of knee flexors and other muscles
after surgery<xref ref-type="bibr" rid="r5">5</xref><sup>)</sup>. Isometric exercises are
especially useful as they can provide a controllable and safe training environment for
patients that have limited range of motion of the joints, and reduce the risk of pain and
re-injury during exercise<xref ref-type="bibr" rid="r6">6</xref><sup>)</sup>. Furthermore,
Hagberg showed there were no significant differences in the endurance times and the
contraction levels sustained during isometric exercises compared to dynamic exercises<xref ref-type="bibr" rid="r7">7</xref><sup>)</sup>.</p><p>It was shown in a study by Oliveria that the trunk and upper arm position can affect the
muscle integrated EMG activation levels<xref ref-type="bibr" rid="r8">8</xref><sup>)</sup>.
In their study surface EMGs were recorded of the classic dumbbell biceps curl, inclined
dumbbell curl and dumbbell preacher curl were presented and compared. The inclined dumbbell
curl and the classic dumbbell biceps curl produced similar surface EMG activation levels,
and both conditions isolated the biceps muscle more than the dumbbell preacher curl. To our
knowledge, there have been no published studies investigating the effect of shoulder flexion
angle on the muscles activated while performing a biceps curl. Thus, the purpose of this
experiment was to investigate the differences in muscle activation patterns of isometric
dumbbell curls among different trunk and upper arm postures.</p></sec><sec sec-type="methods" id="s2"><title>SUBJECTS AND METHODS</title><p>Thirteen healthy male subjects participated in this study (age, 24.8±3.4 years; weight,
77.9±7.9 kg; height, 1.70±0.05 m). All subjects had more than one year of weight training
experience and were right-handed. Prior to the start of the experiment, the subjects were
given a full explanation of the purpose and experimental procedure, and signed Institutional
Review Board consent forms to comply with the ethical standards of the Declaration of
Helsinki (1975, revised 1983).</p><p>All subjects warmed up by light stretching and their skin was prepared<xref ref-type="bibr" rid="r9">9</xref><sup>)</sup>. Two circular Ag-AgCl pre-gelled electrodes, (20 mm in
diameter, 20 mm inter-electrode distance) were positioned on the biceps brachii (BB) (long
head) and flexor carpi radialis (FCR) muscles following the Surface EMG for Non-Invasive
Assessment of Muscles (SENIAM) recommendations<xref ref-type="bibr" rid="r10">10</xref>,
<xref ref-type="bibr" rid="r11">11</xref><sup>)</sup>. MVC testing was performed on a
specially designed table and chair that were constructed for this study. For the BB MVC
test, the trunk was strapped to the chair and the shoulder angle was fixed at an angle of
90°, directing the greatest exertion of force toward the subject's body. In the FCR MVC
test, the forearm was held in position on the table, directing the greatest exertion of
force upwards. During performance of the trials at the various shoulder angles (30°, 45°,
60°, 75° and 90°) a TELEMYO 2400 T G2 system (gain=1000, band-pass=15–500 Hz; Noraxon, USA)
was used to record the data. The force was measured by a 6-axis force/torque sensor (Nano25,
ATI Corp. Canada) installed on a haptic-based resistance-training machine built by the
research team<xref ref-type="bibr" rid="r12">12</xref><sup>)</sup>. Surface EMG and force
were synchronized through a 16-bit A/D converter (DAQ card AI-16XE-50, National Instruments,
TX, USA). Trial positions were executed in a random order according to a table of random
numbers. There was a three-minute break between each MCV trial and a two-minute break
between each main experimental trial<xref ref-type="bibr" rid="r8">8</xref><sup>)</sup>.</p><p>The study was conducted over a period of two days to increase the validity of the data, and
all sEMG data were converted to root mean square (RMS), following the recommendations of
previous research<xref ref-type="bibr" rid="r13">13</xref><sup>)</sup>. To observe the
changes in the neuromuscular activities of BB and FCR at the different shoulder angles, all
sEMG data were obtained using an FIR band-pass filter (80–250 Hz) and FIR high-pass filter
(10 Hz) according to SENIAM recommendations. The sEMG data were rectified and the RMS were
calculated for smoothing, then normalized with the MVC.<disp-formula><inline-graphic xlink:href="jpts-25-1299-e001.jpg"></inline-graphic></disp-formula></p><p>RMS<sub>deg</sub> = root mean square at the shoulder angle of deg, and x is the raw
sEMG.</p><p>T<sub>1</sub> and T<sub>2</sub> = the start and end times, respectively.</p><p>The force was calculated using the formula shown below. All the force points were
normalized to the maximum value for comparison.<disp-formula><inline-graphic xlink:href="jpts-25-1299-e002.jpg"></inline-graphic></disp-formula></p><p>F<sub>force</sub> is the resultant force, F<sub>x</sub> and F<sub>y</sub> are the forces
exerted in the anteroposterior and vertical directions, respectively</p><p>All data were processed using the RMS method, and sEMG, MVC, and RF data were normalized to
the maximum values. One-way ANOVA (Muscle Type × Degree) was used for the statistical
analysis, and Tukey's post-hoc test was conducted on the data of the angles showing
significant differences. The level of significance was chosen as p=0.05. Statistical
analyses were performed using a commercial statistics software program (SPSS v. 19.0,
IBM).</p></sec><sec sec-type="results" id="s3"><title>RESULTS</title><table-wrap id="tbl_001" orientation="portrait" position="float"><label>Table 1.</label><caption><title> Mean (SD) of normalized surface EMG and force at various shoulder flexion
angles</title></caption><table frame="hsides" rules="groups"><thead><tr><td align="center" valign="middle" rowspan="2" colspan="1">Degree</td><td align="center" colspan="2" rowspan="1">Surface EMG</td><td align="center" valign="middle" rowspan="2" colspan="1">Force Sensor(N)</td></tr><tr><td align="center" rowspan="1" colspan="1">BB (%)</td><td align="center" rowspan="1" colspan="1">FCR (%)</td></tr></thead><tbody><tr><td align="center" rowspan="1" colspan="1">30</td><td align="center" rowspan="1" colspan="1">93.8 (16.4)</td><td align="center" rowspan="1" colspan="1">85.0 (14.8)</td><td align="center" rowspan="1" colspan="1">72.5 (9.2)</td></tr><tr><td align="center" rowspan="1" colspan="1">45</td><td align="center" rowspan="1" colspan="1">100.6 (25.9)</td><td align="center" rowspan="1" colspan="1">86.4 (21.4)</td><td align="center" rowspan="1" colspan="1">71.6 (11.9)</td></tr><tr><td align="center" rowspan="1" colspan="1">60</td><td align="center" rowspan="1" colspan="1">109.7 (16.93)</td><td align="center" rowspan="1" colspan="1">90.6 (16.5)</td><td align="center" rowspan="1" colspan="1">84.1 (12.3)</td></tr><tr><td align="center" rowspan="1" colspan="1">75</td><td align="center" rowspan="1" colspan="1">115.6 (22.4)</td><td align="center" rowspan="1" colspan="1">89.0 (17.1)</td><td align="center" rowspan="1" colspan="1">88.8 (11.7)</td></tr><tr><td align="center" rowspan="1" colspan="1">90</td><td align="center" rowspan="1" colspan="1">88.2 (9.1)</td><td align="center" rowspan="1" colspan="1">105.8 (24.9)</td><td align="center" rowspan="1" colspan="1">87.2 (14.3)</td></tr></tbody></table></table-wrap><p>Surface EMG showed the largest force of 115.65% and 88.80%, respectively, at a shoulder
flexion of 75°. The largest FCR activity 90.57%, was at a shoulder angle of 60°. The results
are shown in <xref rid="tbl_001" ref-type="table">Table 1</xref>.</p><p>To investigate the differences among angles, Tukey's post-hoc test was performed. The
results indicate that BB showed significant differences between 60° and 90°, between 75°,
and 30°, and 90° (p<0.05, <italic>p</italic><0.01). The FCR results showed no
significant differences between any angles. The mean and standard deviation values of RF at
the different shoulder angles showed statistically significant results between 75° and 30°,
45° (p<.01) and between 90° and 30°, 45° (p<.05). These results, along with the mean
values of BB, FCR, and RF at the different shoulder angles, were normalized by RMS.</p></sec><sec sec-type="discussion" id="s4"><title>DISCUSSION</title><p>The force-length relationship of a muscle is defined as the maximal force obtained in a
series of discrete contractions performed at different muscle (fiber and sarcomere)
lengths<xref ref-type="bibr" rid="r14">14</xref><sup>)</sup>. In most biomechanical muscle
models, the sub-maximal force-length relationship is a linearly scaled version of the
maximal force-length relationship<xref ref-type="bibr" rid="r14">14</xref><sup>)</sup>. For
skeletal muscles, the size of the force differs as a function of the muscle contraction type
(sub-maximal contraction or maximal contraction) and the degree of flexion<xref ref-type="bibr" rid="r15">15</xref><sup>)</sup>.</p><p>In a computer simulation by Murry et al., the relationship between the length of the moment
arm and force that could be generated at different positions of the elbow and forearm was
investigated and compared<xref ref-type="bibr" rid="r16">16</xref><sup>)</sup>. They
concluded that the moment arm is maximized for the biceps when the arm is in an extended
position with the forearm in supination. They also showed that the peak biceps supination
moment arm decreases as the elbow angle is extended. Some researchers have investigated the
degree of activation of BB and FCR by executing isokinetic and isometric exercises according
to the change in the elbow angle, and it has been reported that these activations are
affected by the type of the muscle contraction and the change in the angle<xref ref-type="bibr" rid="r17">17</xref><sup>)</sup>. In addition, in another study, the forces
exerted in eight directions and the activation of muscles in five postures on the vertical
axis were examined<xref ref-type="bibr" rid="r17">17</xref><sup>)</sup>. These previous
studies were mainly performed to explain the relationship between the force exerted by the
upper arm in the vertical axis and its effect on muscle activation and conclusions were
drawn about the control of the muscles from a neuromechanical perspective, without any
advice on the practical implementation of this knowledge of these relationships.</p><p>In contrast, studies similar to ours have been conducted to investigate the generation of
muscle activation by training when the forearm exerts force in the perpendicular direction,
as in the curl motion. They have reported that the degree of activation of BB differs with
different types of dumbbell curl motion and muscle contraction<xref ref-type="bibr" rid="r18">18</xref><sup>)</sup>. The differences in the degree of activation of the BB can
be understood by the surface EMG-torque relationship, which shows a non-linear increase in
activation levels. The reason for the non-linear increase is that BB is used not only for
flexion but also for supination<xref ref-type="bibr" rid="r14">14</xref>, <xref ref-type="bibr" rid="r19">19</xref><sup>)</sup>.</p><p>The results of our study indicate that a shoulder angle of 75° is the most effective at
stimulating the BB and generating the highest force. For BB, the 75° shoulder angle had a
surface EMG value that was significantly different from the values of 30° and 90°. This
means that this posture may be considered ideal for exercising or the design of a BB curl
exercise machine. Our results, supported by Langenderfer et al., state that the ideal
angle-torque relationship at the elbow joint is between 70° and 80°of shoulder flexion<xref ref-type="bibr" rid="r20">20</xref><sup>)</sup>. Force showed statistically significant
differences at shoulder flexion angles of 30° and 45°.</p></sec></body><back><ack><p>This research was supported by the Basic Science Research Program through the National
Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and
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