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Innervation zone location of the biceps brachii, a comparison between genders and correlation with anthropometric measurements
Journal of Electromyography and Kinesiology 20 (2010) 76–80
Contents lists available at ScienceDirect
Journal of Electromyography and Kinesiology journal homepage: www.elsevier.com/locate/jelekin
Innervation zone location of the biceps brachii, a comparison between genders and correlation with anthropometric measurements Jason M. DeFreitas, Pablo B. Costa, Eric D. Ryan, Trent J. Herda, Joel T. Cramer, Travis W. Beck * Biophysics Laboratory, Department of Health and Exercise Science, University of Oklahoma, 1401 Asp Avenue, Norman, OK 73019, USA
a r t i c l e
i n f o
Article history: Received 4 June 2008 Received in revised form 12 September 2008 Accepted 15 September 2008
Keywords: EMG Electrode array Isometric
a b s t r a c t Avoiding the innervation zone (IZ) is important when collecting surface electromyographic data. The purposes of this study were threefold: (1) to examine the precision of two different techniques for expressing IZ location for the biceps brachii, (2) to compare these locations between men and women, and (3) to determine if IZ movement with changes in elbow joint angle is related to different anthropometric measures. Twenty-four subjects (mean ± SD ages = 21.8 ± 3.5 yr) performed isometric contractions of the right forearm flexors at each of three separate elbow joint angles (90°, 120°, and 150° between the arm and forearm). During each contraction, the location of the IZ for the biceps brachii was visually identified using a linear electrode array. These IZ locations were expressed in both absolute (i.e. as a distance (mm) from the acromion process) and relative (i.e. as a percentage of humerus length) terms. The results suggested that the estimations of IZ location were more precise when expressed in relative versus absolute terms, and were generally different for men and women. The shift in IZ location with changes in elbow joint angle was not, however, related to height, weight, or humerus length. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction One of the most important practical issues in surface electromyography (EMG) is determining proper positioning of the electrodes on the surface of the skin. Of particular importance when addressing the issue of electrode location is the innervation zone (IZ) of the muscle. The IZ is a collection of neuromuscular junctions that occupy a relatively small, localized region of a muscle (Masuda et al., 1985). Action potentials originate within this region and travel in opposite directions away from the IZ toward the tendonous regions. Previous studies have shown that when bipolar electrode arrangements were placed over or near the IZ, the absolute EMG amplitude values were lower (Farina et al., 2001; Piitulainen et al., 2008; Rainoldi et al., 2000), the frequency values were higher (Farina et al., 2001; Li and Sakamoto, 1996; Piitulainen et al., 2008; Roy et al., 1986), and conduction velocity estimates were unstable (Nielsen et al., 2008; Roy et al., 1986) when compared to electrode arrangements that were away from the IZ. Thus, it has been suggested (Farina et al., 2001; Nielsen et al., 2008; Piitulainen et al., 2008; Rainoldi et al., 2004; Rainoldi et al., 2000) that the IZ should be avoided when recording surface EMG signals. A multichannel, linear electrode array has been recommended to non-invasively determine the location of the IZ (Merletti et al., 2003). The EMG channel from the array that demonstrates minimal amplitude and phase reversal is then used to estimate the location * Corresponding author. Tel.: +1 405 325 1378; fax: +1 405 325 0594. E-mail address: [email protected] (T.W. Beck). 1050-6411/$34.00 Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jelekin.2008.09.009
of the IZ (Merletti et al., 2003). However, the linear electrode array is a relatively new methodology. Thus, there are many surface EMG laboratories that do not have the necessary resources to record multiple surface EMG signals from a muscle. In these circumstances, it is important to be able to estimate the IZ location such that it can be avoided when positioning the electrodes. Previous studies have also shown that the IZ moves with changes in joint angle. For instance Rainoldi et al. (2000), examined the effects of knee joint angle changes on the IZ locations for the vastus lateralis and vastus medialis muscles. The authors found that when the leg was extended over a 90° joint angle change (75°165°, 180° = full extension of the leg), there were 1 cm proximal shifts in the IZ locations for both muscles. Martin and MacIsaac (2006) also examined IZ location movement with changes in joint angle. These authors (Martin and MacIsaac, 2006) reported a shift of up to 30 mm in IZ location for the biceps brachii with changes in elbow joint angle from 50° to 130° (180° = full extension of the arm). The results from these studies (Martin and MacIsaac, 2006; Rainoldi et al., 2000) suggested that perhaps the location of the IZ should be estimated separately for each joint angle being tested in the experimental protocol. Additionally, it is unclear if the location of the IZ, or the distance that it moves with changes in joint angle, differs on a subject-bysubject basis. Specifically, it is possible that differences in height and/or limb length could cause the absolute location of the IZ (i.e. the distance of the IZ from an anatomical landmark) to differ between subjects. For example, a subject with a relatively long humerus length, and subsequently a long biceps brachii muscle,
J.M. DeFreitas et al. / Journal of Electromyography and Kinesiology 20 (2010) 76–80
should have an IZ that is located further away from the acromion process than a subject with shorter limbs and muscle lengths. Therefore, it is possible that estimations of IZ locations could be made more precise (i.e. less inter-subject variability) by expressing them as a percentage of limb or muscle length. Rainoldi et al. (2004) provided estimates of IZ locations for 13 different lower limb muscles. However, no studies have given a similar recommendation for IZ location for the biceps brachii. Thus, the purposes of this study were three fold: (1) to examine the precision of two different techniques for expressing IZ location for the biceps brachii, (2) to compare these locations between men and women, and (3) determine if IZ movement with changes in elbow joint angle is related to different anthropometric measures. 2. Methods 2.1. Subjects Seventeen healthy men (mean ± SD age = 21.7 ± 4.1 yr; height 1.80 ± 0.09 m; mass 87.3 ± 16.6 kg) and seven healthy women (age = 21.9 ± 1.5 yr; height 1.61 ± 0.08 m; mass 57.0 ± 4.0 kg) volunteered to participate in this investigation. Each participant completed a pre-exercise health and exercise status questionnaire, which indicated no current or recent (within the past six months) neuromuscular or musculoskeletal problems specific to the shoulder, elbow, or wrist joints. The study was approved by the University Institutional Review Board for Human Subjects, and all participants signed an informed consent form prior to testing. 2.2. Isometric testing The isometric testing was performed on a calibrated Biodex System 3 isokinetic dynamometer (Biodex Medical Systems, Inc., Shirley, NY). The participants were seated with restraining straps over the shoulders and waist, and the elbow was rested on a limb support pad so that the input axis of the dynamometer bisected the longitudinal axis of the shaft of the humerus in accordance with the manufacturer’s instructions (Biodex, 1998). Each participant performed submaximal isometric contractions of the right forearm flexors at randomly ordered joint angles of 90°, 120°, and 150° between the arm and forearm. These submaximal contractions were 6 s in duration and used to provide EMG signals for finding the location of the IZ. The participants were instructed to provide an effort corresponding to approximately 50% of their perceived maximal voluntary contraction (MVC) during each contraction, and the
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location of the IZ was usually identified within three to five trials. Force was not directly measured; however, the dynamometer did guarantee the isometric nature of the effort by not permitting movement of the dynamometer lever arm. Two minutes of rest were allowed between all submaximal contractions, and after testing was completed at the first joint angle, the participant’s forearm was moved to the next randomly ordered joint angle, and the testing was repeated. 2.3. EMG measurements During each contraction, fifteen separate bipolar surface EMG signals were detected from the biceps brachii using a linear 16 electrode array and surface EMG16 data acquisition system (EMG16, LISiN-Prima Biomedical & Sport, Treviso, Italy). The skin over the belly of the biceps brachii was prepared prior to testing by shaving, careful abrading, and cleansing with rubbing alcohol. Conducting gel was then applied to the skin, three reference electrode straps were wrapped around the subject’s wrist to reduce electromagnetic noise as much as possible (EMG16, 2006), and a 15-channel electrode array (1 1 mm prongs, 2.5 mm interelectrode distance, Ottino Bioelettronica, Torino, Italy) was placed over the belly and most prominent part of the muscle to find the location of the IZ. Specifically, the participant was asked to contract the forearm flexors at approximately 50% of his/her perceived MVC, and the location of the IZ was visually identified by the investigator as the EMG channel that demonstrated minimum amplitude and phase reversal (EMG16, 2006; Shiraishi et al., 1995; Tokunaga et al., 1998; Yamada et al., 1987). The IZ location was identified to the nearest 1.25 mm (half of the interelectrode distance). The 15-channel electrode array was then removed, and the location of the IZ at each joint angle was marked on the skin with a permanent marker. In a few cases in which two innervation zones were present, the one that most clearly showed minimum amplitude and phase reversal was marked. In the case of a wide IZ that occupied multiple channels, the middle of the IZ was used. An example of the linear electrode array and its resulting signals are shown in Fig. 1. 2.4. Anthropometric measurements Humerus length (mm), height (m), mass (kg), and the three IZ locations were measured upon completion of the isometric testing. All measurements were taken with the subject in the standing position with their arm and forearm relaxed and hanging at their side. Humerus length was estimated as the distance from the
Fig. 1. (A) A representation of a linear electrode array, (B) an example of a narrow-band innervation zone (IZ), and (C) an example of a wide-band IZ. The two signals came from two separate subjects and show the between subject variability in IZ distribution. The wide-band could be due to either a wide IZ or due to multiple motor units that occupy different IZs in close proximity of each other being recruited at the same time.
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acromion process to the lateral epicondyle of the humerus and recorded to the nearest mm. The distance (mm) of the marked IZ location from the acromion process for each joint angle (90°, 120°, and 150° between the arm and forearm) were then recorded as the absolute IZ locations. In addition, relative IZ locations were calculated by expressing the absolute locations as a percentage of humerus length. All measurements were taken using a cloth tape with a Gulick handle.
imal direction. However, the distances that the IZ shifted with decreases in elbow joint angle were not significantly correlated with height, mass, or humerus length (Table 3). Table 3 also shows that there were significant, positive linear relationships between the absolute IZ locations at all three elbow joint angles and height, mass, and humerus length.
2.5. Statistical analysis
4.1. IZ location
Twelve separate independent-samples t-tests were used to compare men versus women for height, mass, humerus length, absolute and relative IZ locations for each joint angle, and the distances that the IZ shifted proximally with decreases in elbow joint angle from 150° to 120°, 120° to 90°, and 150° to 90°. Additionally, zero-order correlations were used to determine if the distances that the IZ shifted with changes in elbow joint angle, as well as the absolute IZ locations, were related to height, mass, and humerus length. An alpha level of p 6 0.05 was considered to be statistically significant. Coefficients of variation (COVs) were used to examine the precision of the absolute and relative IZ locations and were calculated using the following equation:
These results are important from a practical standpoint because they indicated that when expressed in absolute terms, the location of the IZ for the biceps brachii was different in men compared to women. As stated previously Rainoldi et al. (2004), recently conducted a similar study in which the IZ locations for 13 muscles of the lower limb were examined in men. Like the present investigation Rainoldi et al. (2004), provided reference lines for each muscle and identified where the IZ was located in relation to those lines. Although several studies (Masuda et al., 1985; Masuda and Sadoyama, 1991; Saitou et al., 2000) have examined the distribution patterns of the IZs for the biceps brachii, these investigations did not provide an exact distance from an anatomical landmark that could be used for estimating its location. For example Masuda and Sadoyama (1991) reported that the IZ for the biceps brachii was generally positioned at the belly of the muscle, but its exact location varied considerably between individuals. In addition, the authors (Masuda and Sadoyama, 1991) indicated that the width of the IZ usually ranged from 30–60 mm, but a reference point that could be used to estimate its exact location was not provided. The present study is useful in this regard because it describes these reference points so that the location of the IZ for the biceps brachii may be estimated without the use of a linear electrode array. It is also important to note that for the men in the present study, the COVs for the absolute IZ locations at each elbow joint angle were higher than those for the relative IZ locations. Conversely, the women showed similar COVs between the absolute and relative IZ locations. Although the exact cause for this difference is unclear, it is possible that discrepancies in height, mass, and/or humerus length between men and women were the primary factor
COV ¼
SD 100 Mean
ð1Þ
3. Results Means ± standard deviations (SD) for anthropometric data and absolute and relative IZ locations for the men and women are shown in Tables 1 and 2. There were significant mean differences between genders for height, mass, relative IZ location at a 150° elbow joint angle, and absolute IZ locations for all three elbow joint angles. There were no significant mean differences between genders for humerus length or the relative IZ locations at the 90° and 120° elbow joint angles. With a 60° change in elbow joint angle (i.e. 150° to 90° between the arm and forearm), the mean shifts in IZ locations were 14.4 mm (4.3% of humerus length) for the men and 10.3 mm (3.4% of humerus length) for the women in the prox-
4. Discussion
Table 1 Height (m), mass (kg) and absolute innervation zone (IZ) location for all 24 participants. Absolute IZ location is the distance (mm) from the IZ to the acromion process for each elbow joint angle. Height (m)
Mass (kg)
Absolute IZ location (mm) 90°
*
*
120° *
150° *
254.2 ± 21.2* 8.33
Men n = 17
Mean ± SD COV
1.80 ± 0.09 –
87.3 ± 16.6 –
239.8 ± 20.5 8.54
247.4 ± 20.9 8.44
Women n = 7
Mean ± SD COV
1.61 ± 0.08* –
57.0 ± 4.0* –
207.4 ± 20.7* 9.99
214.7 ± 16.8* 7.82
217.7 ± 17.1* 7.86
<0.001
<0.001
0.002
0.001
0.001
p-values for gender comparisons *
Indicates a statistically significant difference between genders (p < 0.05).
Table 2 Humerus length (mm) and relative innervation zone (IZ) location for all 24 participants. The IZ location is expressed as a percentage of humerus length for each elbow joint angle. Humerus length (mm)
Relative IZ location (%) 90°
120°
150°
Men n = 17
Mean ± SD COV
335.9 ± 30.2 –
71.6 ± 4.8 6.77
73.9 ± 5.1 6.87
75.9 ± 5.6* 7.39
Women n = 7
Mean ± SD COV
310.7 ± 17.2 –
66.8 ± 6.3 9.36
69.2 ± 5.3 7.68
70.2 ± 5.8* 8.31
0.052
0.057
0.055
0.035
p-values for gender comparisons *
Indicates a statistically significant difference between genders (p < 0.05).
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Table 3 Zero-order correlations between the variables in the first column (height, mass, and humerus length) and the variables in the first row (the distances that the innervation zone (IZ) shifted proximally with decreases in elbow joint angle, and the absolute (abs) IZ locations (loc) for each elbow joint angle (mm distances from the acromion process)). Shift from 90° to 120° elbow joint angle
Shift from 120° to 150° elbow joint angle
Shift from 90° to 150° elbow joint angle
Abs IZ Loc at 90°
Abs IZ Loc at 120°
Abs IZ Loc at 150°
Height
Correlation coefficient (r) p-value
0.095 0.658
0.283 0.180
0.108 0.617
0.767 <0.001*
0.765 <0.001*
0.765 <0.001*
Mass
Correlation coefficient (r) p-value
0.008 0.969
0.268 0.205
0.167 0.436
0.696 <0.001*
0.710 <0.001*
0.710 <0.001*
Humerus length
Correlation coefficient (r) p-value
0.046 0.829
0.038 0.858
0.054 0.803
0.717 <0.001*
0.722 <0.001*
0.680 <0.001*
*
Indicates a statistically significant correlation (p < 0.05).
determining the gender differences in absolute IZ locations. Even though there was no mean difference between men and women for humerus length, there were significant positive linear relationships (r = 0.680.77) between the absolute IZ locations and height, mass, and humerus length. Thus, these findings indicated that differences in these anthropometric measures were important for determining the absolute IZ location, and, therefore, expressing the location in relative terms allows for a more precise method of estimating its location. However, with a p-value of 0.052, it is possible that there would have been a significant difference in humerus length between genders with an increased sample size. Additionally, since there were no significant differences between genders for relative IZ locations at the 90° and 120° elbow joint angles, but the men’s IZ location was significantly more distal at the 150° elbow joint angle, it may be more appropriate to use different relative IZ locations for men and women, as well as for each joint angle, rather than a single IZ location. Thus, we propose that IZ location for the biceps brachii be estimated as a percent of humerus length using the results from Table 2. However, like humerus length, the p-values of 0.057, and 0.055 suggest that there may have been significant differences between genders for relative IZ locations at the 90° and 120° elbow joint angles had a greater sample size been used. 4.2. Effects of changes in elbow joint angle Previous studies have shown that the locations of the IZs for different muscles in men moved significantly with changes in elbow (Martin and MacIsaac, 2006), knee (Farina et al., 2001; Rainoldi et al., 2000), and ankle (Farina et al., 2001) joint angles. For example Rainoldi et al. (2000), investigated the effects of changes in knee joint angle on IZ locations for the vastus lateralis and vastus medialis muscles. The results indicated that there were 1 cm proximal shifts in the IZ locations of the vastus lateralis and vastus medialis muscles with increases in knee joint angle from 75° to 165° between the thigh and leg. The effects of changes in joint angle on IZ movement were also examined by Farina et al. (2001) for the vastus lateralis, vastus medialis, rectus femoris, biceps femoris, semitendinosus, tibialis anterior, gastocnemius lateralis, and gastrocnemius medialis muscles. The authors’ (Farina et al., 2001) computer-simulated EMG signals demonstrated a 15 mm shift of the IZ under the skin, while their experimental data revealed shifts of up to 30 mm. More recently, Martin and MacIsaac (2006) used a 16-channel linear electrode array to examine movement of the IZ for the biceps brachii with changes in elbow joint angle. The authors (Martin and MacIsaac, 2006) found that the IZ shifted distally for each subject between 5 and 30 mm as the forearm was extended. As a result, they concluded that the IZ for the biceps brachii shifted beneath the recording electrodes in a predictable manner with changes in joint angle (Martin and MacIsaac, 2006). Supporting these previous studies, the 60° change in elbow joint angle (i.e. 150° to 90° between the arm and forearm) led the IZ location for
the biceps brachii to shift up to 14.4 mm in the proximal direction. Additionally, when compared to these previous investigations, the present study is unique because it describes IZ locations for both men and women. These findings are of practical importance when recording EMG signals during isometric or dynamic contractions because the experimenter(s) should be aware that the IZ for the biceps brachii has different locations based on elbow joint angle and gender. Therefore, in some situations, it may not be appropriate to use a single IZ location for guiding electrode placement, particularly if the experimental protocol requires changes in joint angle. 4.3. Effects of changes in force Additionally, Piitulainen et al. (2008) have found that the IZ for the biceps brachii shifts proximally 0.6 cm with increases in isometric force from 10% to 100% MVC. As mentioned previously, they reported that the IZ effected EMG amplitude, frequency, and conduction velocity estimates. Therefore, IZ estimation may need to be adjusted to account for changes in location that occur due to changes in isometric force. 5. Conclusion In summary, the results from the present study showed that in some cases, the location of the IZ for the biceps brachii was different for men versus women, even when it was expressed as a percentage of humerus length. In addition, the COVs were generally less for the relative IZ locations than for the absolute locations. Thus, these findings suggested that different estimations of IZ location should be used for men and women, and these estimations should be expressed in relative terms. Furthermore, these IZ locations were different for each elbow joint angle. Thus, if the experimental protocol requires changes in elbow joint angle (e.g. dynamic contractions or isometric contractions at multiple joint angles), the investigator(s) should be aware that the IZ for the biceps brachii has a range of movement that should be avoided when placing electrodes. Therefore, we propose that IZ location for the biceps brachii be estimated using the following recommendations: 71.6 ± 4.8 and 66.8 ± 6% for men and women, respectively, at a 90° elbow joint angle, 73.9 ± 5.1 and 69.2 ± 5.3% for men and women, respectively, at a 120° elbow joint angle, and 75.9 ± 5.6 and 70.2 ± 5.8% for men and women, respectively, at a 150° elbow joint angle. Conflict of interest statement None declared. References Biodex System 3 Applications/Operations Manual. Biodex Medical Systems, Inc., Shirley, NY, 1998.
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EMG16 User Manual.16-channels surface electromyographic signal amplifier. LISiN Bioengineering Center Polytechnic of Turin, Department of Electronics Turin, Italy, 2006. Farina D, Merletti R, Nazzaro M, Caruso I. Effect of joint angle on EMG variables in leg and thigh muscles. IEEE Eng Med Biol Mag 2001;20(6):62–71. Li W, Sakamoto K. The influence of location of electrode on muscle fiber conduction velocity and EMG power spectrum during voluntary isometric contraction measured with surface array electrodes. Appl Hum Sci 1996;15(1):25–32. Martin S, MacIsaac D. Innervation zone shift with changes in joint angle in the brachial biceps. J Electromyogr Kinesiol 2006;16(2):144–8. Masuda T, Miyano H, Sadoyama T. The position of innervation zones in the biceps brachii investigated by surface electromyography. IEEE Trans Biomed Eng 1985;32(1):36–42. Masuda T, Sadoyama T. Distribution of innervation zones in the human biceps brachii. J Electromyogr Kinesiol 1991;1(2):107–15. Merletti R, Farina D, Gazzoni M. The linear electrode array: a useful tool with many applications. J Electromyogr Kinesiol 2003;13(1):37–47. Nielsen M, Graven-Nielsen T, Farina D. Effect of innervation zone distribution on estimates of average muscle fiber conduction velocity. Muscle Nerve 2008;37(1):68–78. Piitulainen, H., Rantalainen, T., Linnamo, V., Komi, P., Avela, J. Innervation zone shift at different levels of isometric contraction in the biceps brachii muscle. J Electromyogr Kinesiol 2008. Rainoldi A, Melchiorri G, Caruso I. A method for positioning electrodes during surface EMG recordings in lower limb muscles. J Neurosci Meth 2004;134(1):37–43. Rainoldi A, Nazzaro M, Merletti R, et al. Geometrical factors in surface EMG of the vastus medialis and lateralis muscles. J Electromyogr Kinesiol 2000;10(5):327–36. Roy SH, De Luca CJ, Schneider J. Effects of electrode location on myoelectric conduction velocity and median frequency estimates. J Appl Physiol 1986;61(4):1510–7. Saitou K, Masuda T, Michikami D, Kojima R, Okada M. Innervation zones of the upper and lower limb muscles estimated by using multichannel surface EMG. J Hum Ergol (Tokyo) 2000;29(1–2):35–52. Shiraishi M, Masuda T, Sadoyama T, Okada M. Innervation zones in the back muscles investigated by multichannel surface EMG. J Electromyogr Kinesiol 1995;5(3):161–7. Tokunaga T, Baba S, Tanaka M, et al. Two-dimensional configuration of the myoneural junctions of human masticatory muscle detected with matrix electrode. J Oral Rehabil 1998;25(5):329–34. Yamada M, Kumagai K, Uchiyama A. The distribution and propagation pattern of motor unit action potentials studied by multichannel surface EMG. Electroencephalogr Clin Neurophysiol 1987;67(5):395–401.
Jason M. DeFreitas received a BS (2007) degree in Exercise Science from the University of Connecticut and is currently a Master’s Candidate in Exercise Physiology at the University of Oklahoma. He is also a Certified Strength and Conditioning Specialist through the National Strength and Conditioning Association. His main research interests are noninvasive assessment of muscle function using electromyography and mechanomyography.
Pablo B. Costa received a BS degree in Physical Education from Estácio de Sá University (2004), a Masters degree in Exercise Science from Florida Atlantic University (2007), and is currently working on his Doctorate at the University of Oklahoma. His main research interests include non-invasive assessment of neuromuscular function and the performance effects of stretching.
Eric D. Ryan is currently a doctoral student in the exercise physiology department at the University of Oklahoma under the mentorship of Dr. Joel Cramer. He received a BS (2003) degree in exercise science from Tulane University, New Orleans, Louisiana, and a MS (2005) degree from Florida Atlantic University, Davie, Florida in exercise physiology. He is a member of the American College of Sports Medicine (ACSM) and National Strength and Conditioning Association (NSCA). His primary research interests include the non-invasive assessment of muscle function and the acute effects of different stretching routines on parameters of muscle strength and musculotendinous stiffness.
Trent J. Herda, originally from Iowa, is currently a doctoral student in the exercise physiology department at the University of Oklahoma under the mentorship of Dr. Joel Cramer. He received a BS (2005) degree in exercise science from the University of Sioux Falls, Sioux Falls, South Dakota, and a MS (2007) degree from the University of Oklahoma, Norman, Oklahoma, in exercise physiology. He is a member of the American College of Sports Medicine (ACSM) and National Strength and Conditioning Association (NSCA). His primary research interests include the non-invasive assessment of muscle function and the acute effects of vibration and stretching on mechanical and neural components of muscular contraction.
Joel T. Cramer received a BA (1997) degree in exercise science from Creighton University, Omaha, Nebraska, and MPE (2001) and PhD (2003) degrees from the University of Nebraska–Lincoln under the direction of Dr. Terry J. Housh. He is a member of the American College of Sports Medicine and the National Strength and Conditioning Association. He recently accepted a faculty position at the University of Oklahoma as an assistant professor. He research interests focus on the non-invasive assessment of muscle function using surface electromyography and mechanomyography.
Travis W. Beck received a B.S. (2002) degree in Biology from Doane College, the M.P.E. (2004) degree in Health and Human Performance from the University of Nebraska-Lincoln, and the Ph.D. (2007) degree in Human Sciences from the University of Nebraska-Lincoln. He recently accepted an Assistant Professor position in the Department of Health and Exercise Science at the University of Oklahoma, and his main research interests include evaluation of muscle function using electromyography and mechanomyography.
Contents lists available at ScienceDirect
Journal of Electromyography and Kinesiology journal homepage: www.elsevier.com/locate/jelekin
Innervation zone location of the biceps brachii, a comparison between genders and correlation with anthropometric measurements Jason M. DeFreitas, Pablo B. Costa, Eric D. Ryan, Trent J. Herda, Joel T. Cramer, Travis W. Beck * Biophysics Laboratory, Department of Health and Exercise Science, University of Oklahoma, 1401 Asp Avenue, Norman, OK 73019, USA
a r t i c l e
i n f o
Article history: Received 4 June 2008 Received in revised form 12 September 2008 Accepted 15 September 2008
Keywords: EMG Electrode array Isometric
a b s t r a c t Avoiding the innervation zone (IZ) is important when collecting surface electromyographic data. The purposes of this study were threefold: (1) to examine the precision of two different techniques for expressing IZ location for the biceps brachii, (2) to compare these locations between men and women, and (3) to determine if IZ movement with changes in elbow joint angle is related to different anthropometric measures. Twenty-four subjects (mean ± SD ages = 21.8 ± 3.5 yr) performed isometric contractions of the right forearm flexors at each of three separate elbow joint angles (90°, 120°, and 150° between the arm and forearm). During each contraction, the location of the IZ for the biceps brachii was visually identified using a linear electrode array. These IZ locations were expressed in both absolute (i.e. as a distance (mm) from the acromion process) and relative (i.e. as a percentage of humerus length) terms. The results suggested that the estimations of IZ location were more precise when expressed in relative versus absolute terms, and were generally different for men and women. The shift in IZ location with changes in elbow joint angle was not, however, related to height, weight, or humerus length. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction One of the most important practical issues in surface electromyography (EMG) is determining proper positioning of the electrodes on the surface of the skin. Of particular importance when addressing the issue of electrode location is the innervation zone (IZ) of the muscle. The IZ is a collection of neuromuscular junctions that occupy a relatively small, localized region of a muscle (Masuda et al., 1985). Action potentials originate within this region and travel in opposite directions away from the IZ toward the tendonous regions. Previous studies have shown that when bipolar electrode arrangements were placed over or near the IZ, the absolute EMG amplitude values were lower (Farina et al., 2001; Piitulainen et al., 2008; Rainoldi et al., 2000), the frequency values were higher (Farina et al., 2001; Li and Sakamoto, 1996; Piitulainen et al., 2008; Roy et al., 1986), and conduction velocity estimates were unstable (Nielsen et al., 2008; Roy et al., 1986) when compared to electrode arrangements that were away from the IZ. Thus, it has been suggested (Farina et al., 2001; Nielsen et al., 2008; Piitulainen et al., 2008; Rainoldi et al., 2004; Rainoldi et al., 2000) that the IZ should be avoided when recording surface EMG signals. A multichannel, linear electrode array has been recommended to non-invasively determine the location of the IZ (Merletti et al., 2003). The EMG channel from the array that demonstrates minimal amplitude and phase reversal is then used to estimate the location * Corresponding author. Tel.: +1 405 325 1378; fax: +1 405 325 0594. E-mail address: [email protected] (T.W. Beck). 1050-6411/$34.00 Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jelekin.2008.09.009
of the IZ (Merletti et al., 2003). However, the linear electrode array is a relatively new methodology. Thus, there are many surface EMG laboratories that do not have the necessary resources to record multiple surface EMG signals from a muscle. In these circumstances, it is important to be able to estimate the IZ location such that it can be avoided when positioning the electrodes. Previous studies have also shown that the IZ moves with changes in joint angle. For instance Rainoldi et al. (2000), examined the effects of knee joint angle changes on the IZ locations for the vastus lateralis and vastus medialis muscles. The authors found that when the leg was extended over a 90° joint angle change (75°165°, 180° = full extension of the leg), there were 1 cm proximal shifts in the IZ locations for both muscles. Martin and MacIsaac (2006) also examined IZ location movement with changes in joint angle. These authors (Martin and MacIsaac, 2006) reported a shift of up to 30 mm in IZ location for the biceps brachii with changes in elbow joint angle from 50° to 130° (180° = full extension of the arm). The results from these studies (Martin and MacIsaac, 2006; Rainoldi et al., 2000) suggested that perhaps the location of the IZ should be estimated separately for each joint angle being tested in the experimental protocol. Additionally, it is unclear if the location of the IZ, or the distance that it moves with changes in joint angle, differs on a subject-bysubject basis. Specifically, it is possible that differences in height and/or limb length could cause the absolute location of the IZ (i.e. the distance of the IZ from an anatomical landmark) to differ between subjects. For example, a subject with a relatively long humerus length, and subsequently a long biceps brachii muscle,
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should have an IZ that is located further away from the acromion process than a subject with shorter limbs and muscle lengths. Therefore, it is possible that estimations of IZ locations could be made more precise (i.e. less inter-subject variability) by expressing them as a percentage of limb or muscle length. Rainoldi et al. (2004) provided estimates of IZ locations for 13 different lower limb muscles. However, no studies have given a similar recommendation for IZ location for the biceps brachii. Thus, the purposes of this study were three fold: (1) to examine the precision of two different techniques for expressing IZ location for the biceps brachii, (2) to compare these locations between men and women, and (3) determine if IZ movement with changes in elbow joint angle is related to different anthropometric measures. 2. Methods 2.1. Subjects Seventeen healthy men (mean ± SD age = 21.7 ± 4.1 yr; height 1.80 ± 0.09 m; mass 87.3 ± 16.6 kg) and seven healthy women (age = 21.9 ± 1.5 yr; height 1.61 ± 0.08 m; mass 57.0 ± 4.0 kg) volunteered to participate in this investigation. Each participant completed a pre-exercise health and exercise status questionnaire, which indicated no current or recent (within the past six months) neuromuscular or musculoskeletal problems specific to the shoulder, elbow, or wrist joints. The study was approved by the University Institutional Review Board for Human Subjects, and all participants signed an informed consent form prior to testing. 2.2. Isometric testing The isometric testing was performed on a calibrated Biodex System 3 isokinetic dynamometer (Biodex Medical Systems, Inc., Shirley, NY). The participants were seated with restraining straps over the shoulders and waist, and the elbow was rested on a limb support pad so that the input axis of the dynamometer bisected the longitudinal axis of the shaft of the humerus in accordance with the manufacturer’s instructions (Biodex, 1998). Each participant performed submaximal isometric contractions of the right forearm flexors at randomly ordered joint angles of 90°, 120°, and 150° between the arm and forearm. These submaximal contractions were 6 s in duration and used to provide EMG signals for finding the location of the IZ. The participants were instructed to provide an effort corresponding to approximately 50% of their perceived maximal voluntary contraction (MVC) during each contraction, and the
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location of the IZ was usually identified within three to five trials. Force was not directly measured; however, the dynamometer did guarantee the isometric nature of the effort by not permitting movement of the dynamometer lever arm. Two minutes of rest were allowed between all submaximal contractions, and after testing was completed at the first joint angle, the participant’s forearm was moved to the next randomly ordered joint angle, and the testing was repeated. 2.3. EMG measurements During each contraction, fifteen separate bipolar surface EMG signals were detected from the biceps brachii using a linear 16 electrode array and surface EMG16 data acquisition system (EMG16, LISiN-Prima Biomedical & Sport, Treviso, Italy). The skin over the belly of the biceps brachii was prepared prior to testing by shaving, careful abrading, and cleansing with rubbing alcohol. Conducting gel was then applied to the skin, three reference electrode straps were wrapped around the subject’s wrist to reduce electromagnetic noise as much as possible (EMG16, 2006), and a 15-channel electrode array (1 1 mm prongs, 2.5 mm interelectrode distance, Ottino Bioelettronica, Torino, Italy) was placed over the belly and most prominent part of the muscle to find the location of the IZ. Specifically, the participant was asked to contract the forearm flexors at approximately 50% of his/her perceived MVC, and the location of the IZ was visually identified by the investigator as the EMG channel that demonstrated minimum amplitude and phase reversal (EMG16, 2006; Shiraishi et al., 1995; Tokunaga et al., 1998; Yamada et al., 1987). The IZ location was identified to the nearest 1.25 mm (half of the interelectrode distance). The 15-channel electrode array was then removed, and the location of the IZ at each joint angle was marked on the skin with a permanent marker. In a few cases in which two innervation zones were present, the one that most clearly showed minimum amplitude and phase reversal was marked. In the case of a wide IZ that occupied multiple channels, the middle of the IZ was used. An example of the linear electrode array and its resulting signals are shown in Fig. 1. 2.4. Anthropometric measurements Humerus length (mm), height (m), mass (kg), and the three IZ locations were measured upon completion of the isometric testing. All measurements were taken with the subject in the standing position with their arm and forearm relaxed and hanging at their side. Humerus length was estimated as the distance from the
Fig. 1. (A) A representation of a linear electrode array, (B) an example of a narrow-band innervation zone (IZ), and (C) an example of a wide-band IZ. The two signals came from two separate subjects and show the between subject variability in IZ distribution. The wide-band could be due to either a wide IZ or due to multiple motor units that occupy different IZs in close proximity of each other being recruited at the same time.
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acromion process to the lateral epicondyle of the humerus and recorded to the nearest mm. The distance (mm) of the marked IZ location from the acromion process for each joint angle (90°, 120°, and 150° between the arm and forearm) were then recorded as the absolute IZ locations. In addition, relative IZ locations were calculated by expressing the absolute locations as a percentage of humerus length. All measurements were taken using a cloth tape with a Gulick handle.
imal direction. However, the distances that the IZ shifted with decreases in elbow joint angle were not significantly correlated with height, mass, or humerus length (Table 3). Table 3 also shows that there were significant, positive linear relationships between the absolute IZ locations at all three elbow joint angles and height, mass, and humerus length.
2.5. Statistical analysis
4.1. IZ location
Twelve separate independent-samples t-tests were used to compare men versus women for height, mass, humerus length, absolute and relative IZ locations for each joint angle, and the distances that the IZ shifted proximally with decreases in elbow joint angle from 150° to 120°, 120° to 90°, and 150° to 90°. Additionally, zero-order correlations were used to determine if the distances that the IZ shifted with changes in elbow joint angle, as well as the absolute IZ locations, were related to height, mass, and humerus length. An alpha level of p 6 0.05 was considered to be statistically significant. Coefficients of variation (COVs) were used to examine the precision of the absolute and relative IZ locations and were calculated using the following equation:
These results are important from a practical standpoint because they indicated that when expressed in absolute terms, the location of the IZ for the biceps brachii was different in men compared to women. As stated previously Rainoldi et al. (2004), recently conducted a similar study in which the IZ locations for 13 muscles of the lower limb were examined in men. Like the present investigation Rainoldi et al. (2004), provided reference lines for each muscle and identified where the IZ was located in relation to those lines. Although several studies (Masuda et al., 1985; Masuda and Sadoyama, 1991; Saitou et al., 2000) have examined the distribution patterns of the IZs for the biceps brachii, these investigations did not provide an exact distance from an anatomical landmark that could be used for estimating its location. For example Masuda and Sadoyama (1991) reported that the IZ for the biceps brachii was generally positioned at the belly of the muscle, but its exact location varied considerably between individuals. In addition, the authors (Masuda and Sadoyama, 1991) indicated that the width of the IZ usually ranged from 30–60 mm, but a reference point that could be used to estimate its exact location was not provided. The present study is useful in this regard because it describes these reference points so that the location of the IZ for the biceps brachii may be estimated without the use of a linear electrode array. It is also important to note that for the men in the present study, the COVs for the absolute IZ locations at each elbow joint angle were higher than those for the relative IZ locations. Conversely, the women showed similar COVs between the absolute and relative IZ locations. Although the exact cause for this difference is unclear, it is possible that discrepancies in height, mass, and/or humerus length between men and women were the primary factor
COV ¼
SD 100 Mean
ð1Þ
3. Results Means ± standard deviations (SD) for anthropometric data and absolute and relative IZ locations for the men and women are shown in Tables 1 and 2. There were significant mean differences between genders for height, mass, relative IZ location at a 150° elbow joint angle, and absolute IZ locations for all three elbow joint angles. There were no significant mean differences between genders for humerus length or the relative IZ locations at the 90° and 120° elbow joint angles. With a 60° change in elbow joint angle (i.e. 150° to 90° between the arm and forearm), the mean shifts in IZ locations were 14.4 mm (4.3% of humerus length) for the men and 10.3 mm (3.4% of humerus length) for the women in the prox-
4. Discussion
Table 1 Height (m), mass (kg) and absolute innervation zone (IZ) location for all 24 participants. Absolute IZ location is the distance (mm) from the IZ to the acromion process for each elbow joint angle. Height (m)
Mass (kg)
Absolute IZ location (mm) 90°
*
*
120° *
150° *
254.2 ± 21.2* 8.33
Men n = 17
Mean ± SD COV
1.80 ± 0.09 –
87.3 ± 16.6 –
239.8 ± 20.5 8.54
247.4 ± 20.9 8.44
Women n = 7
Mean ± SD COV
1.61 ± 0.08* –
57.0 ± 4.0* –
207.4 ± 20.7* 9.99
214.7 ± 16.8* 7.82
217.7 ± 17.1* 7.86
<0.001
<0.001
0.002
0.001
0.001
p-values for gender comparisons *
Indicates a statistically significant difference between genders (p < 0.05).
Table 2 Humerus length (mm) and relative innervation zone (IZ) location for all 24 participants. The IZ location is expressed as a percentage of humerus length for each elbow joint angle. Humerus length (mm)
Relative IZ location (%) 90°
120°
150°
Men n = 17
Mean ± SD COV
335.9 ± 30.2 –
71.6 ± 4.8 6.77
73.9 ± 5.1 6.87
75.9 ± 5.6* 7.39
Women n = 7
Mean ± SD COV
310.7 ± 17.2 –
66.8 ± 6.3 9.36
69.2 ± 5.3 7.68
70.2 ± 5.8* 8.31
0.052
0.057
0.055
0.035
p-values for gender comparisons *
Indicates a statistically significant difference between genders (p < 0.05).
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Table 3 Zero-order correlations between the variables in the first column (height, mass, and humerus length) and the variables in the first row (the distances that the innervation zone (IZ) shifted proximally with decreases in elbow joint angle, and the absolute (abs) IZ locations (loc) for each elbow joint angle (mm distances from the acromion process)). Shift from 90° to 120° elbow joint angle
Shift from 120° to 150° elbow joint angle
Shift from 90° to 150° elbow joint angle
Abs IZ Loc at 90°
Abs IZ Loc at 120°
Abs IZ Loc at 150°
Height
Correlation coefficient (r) p-value
0.095 0.658
0.283 0.180
0.108 0.617
0.767 <0.001*
0.765 <0.001*
0.765 <0.001*
Mass
Correlation coefficient (r) p-value
0.008 0.969
0.268 0.205
0.167 0.436
0.696 <0.001*
0.710 <0.001*
0.710 <0.001*
Humerus length
Correlation coefficient (r) p-value
0.046 0.829
0.038 0.858
0.054 0.803
0.717 <0.001*
0.722 <0.001*
0.680 <0.001*
*
Indicates a statistically significant correlation (p < 0.05).
determining the gender differences in absolute IZ locations. Even though there was no mean difference between men and women for humerus length, there were significant positive linear relationships (r = 0.680.77) between the absolute IZ locations and height, mass, and humerus length. Thus, these findings indicated that differences in these anthropometric measures were important for determining the absolute IZ location, and, therefore, expressing the location in relative terms allows for a more precise method of estimating its location. However, with a p-value of 0.052, it is possible that there would have been a significant difference in humerus length between genders with an increased sample size. Additionally, since there were no significant differences between genders for relative IZ locations at the 90° and 120° elbow joint angles, but the men’s IZ location was significantly more distal at the 150° elbow joint angle, it may be more appropriate to use different relative IZ locations for men and women, as well as for each joint angle, rather than a single IZ location. Thus, we propose that IZ location for the biceps brachii be estimated as a percent of humerus length using the results from Table 2. However, like humerus length, the p-values of 0.057, and 0.055 suggest that there may have been significant differences between genders for relative IZ locations at the 90° and 120° elbow joint angles had a greater sample size been used. 4.2. Effects of changes in elbow joint angle Previous studies have shown that the locations of the IZs for different muscles in men moved significantly with changes in elbow (Martin and MacIsaac, 2006), knee (Farina et al., 2001; Rainoldi et al., 2000), and ankle (Farina et al., 2001) joint angles. For example Rainoldi et al. (2000), investigated the effects of changes in knee joint angle on IZ locations for the vastus lateralis and vastus medialis muscles. The results indicated that there were 1 cm proximal shifts in the IZ locations of the vastus lateralis and vastus medialis muscles with increases in knee joint angle from 75° to 165° between the thigh and leg. The effects of changes in joint angle on IZ movement were also examined by Farina et al. (2001) for the vastus lateralis, vastus medialis, rectus femoris, biceps femoris, semitendinosus, tibialis anterior, gastocnemius lateralis, and gastrocnemius medialis muscles. The authors’ (Farina et al., 2001) computer-simulated EMG signals demonstrated a 15 mm shift of the IZ under the skin, while their experimental data revealed shifts of up to 30 mm. More recently, Martin and MacIsaac (2006) used a 16-channel linear electrode array to examine movement of the IZ for the biceps brachii with changes in elbow joint angle. The authors (Martin and MacIsaac, 2006) found that the IZ shifted distally for each subject between 5 and 30 mm as the forearm was extended. As a result, they concluded that the IZ for the biceps brachii shifted beneath the recording electrodes in a predictable manner with changes in joint angle (Martin and MacIsaac, 2006). Supporting these previous studies, the 60° change in elbow joint angle (i.e. 150° to 90° between the arm and forearm) led the IZ location for
the biceps brachii to shift up to 14.4 mm in the proximal direction. Additionally, when compared to these previous investigations, the present study is unique because it describes IZ locations for both men and women. These findings are of practical importance when recording EMG signals during isometric or dynamic contractions because the experimenter(s) should be aware that the IZ for the biceps brachii has different locations based on elbow joint angle and gender. Therefore, in some situations, it may not be appropriate to use a single IZ location for guiding electrode placement, particularly if the experimental protocol requires changes in joint angle. 4.3. Effects of changes in force Additionally, Piitulainen et al. (2008) have found that the IZ for the biceps brachii shifts proximally 0.6 cm with increases in isometric force from 10% to 100% MVC. As mentioned previously, they reported that the IZ effected EMG amplitude, frequency, and conduction velocity estimates. Therefore, IZ estimation may need to be adjusted to account for changes in location that occur due to changes in isometric force. 5. Conclusion In summary, the results from the present study showed that in some cases, the location of the IZ for the biceps brachii was different for men versus women, even when it was expressed as a percentage of humerus length. In addition, the COVs were generally less for the relative IZ locations than for the absolute locations. Thus, these findings suggested that different estimations of IZ location should be used for men and women, and these estimations should be expressed in relative terms. Furthermore, these IZ locations were different for each elbow joint angle. Thus, if the experimental protocol requires changes in elbow joint angle (e.g. dynamic contractions or isometric contractions at multiple joint angles), the investigator(s) should be aware that the IZ for the biceps brachii has a range of movement that should be avoided when placing electrodes. Therefore, we propose that IZ location for the biceps brachii be estimated using the following recommendations: 71.6 ± 4.8 and 66.8 ± 6% for men and women, respectively, at a 90° elbow joint angle, 73.9 ± 5.1 and 69.2 ± 5.3% for men and women, respectively, at a 120° elbow joint angle, and 75.9 ± 5.6 and 70.2 ± 5.8% for men and women, respectively, at a 150° elbow joint angle. Conflict of interest statement None declared. References Biodex System 3 Applications/Operations Manual. Biodex Medical Systems, Inc., Shirley, NY, 1998.
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EMG16 User Manual.16-channels surface electromyographic signal amplifier. LISiN Bioengineering Center Polytechnic of Turin, Department of Electronics Turin, Italy, 2006. Farina D, Merletti R, Nazzaro M, Caruso I. Effect of joint angle on EMG variables in leg and thigh muscles. IEEE Eng Med Biol Mag 2001;20(6):62–71. Li W, Sakamoto K. The influence of location of electrode on muscle fiber conduction velocity and EMG power spectrum during voluntary isometric contraction measured with surface array electrodes. Appl Hum Sci 1996;15(1):25–32. Martin S, MacIsaac D. Innervation zone shift with changes in joint angle in the brachial biceps. J Electromyogr Kinesiol 2006;16(2):144–8. Masuda T, Miyano H, Sadoyama T. The position of innervation zones in the biceps brachii investigated by surface electromyography. IEEE Trans Biomed Eng 1985;32(1):36–42. Masuda T, Sadoyama T. Distribution of innervation zones in the human biceps brachii. J Electromyogr Kinesiol 1991;1(2):107–15. Merletti R, Farina D, Gazzoni M. The linear electrode array: a useful tool with many applications. J Electromyogr Kinesiol 2003;13(1):37–47. Nielsen M, Graven-Nielsen T, Farina D. Effect of innervation zone distribution on estimates of average muscle fiber conduction velocity. Muscle Nerve 2008;37(1):68–78. Piitulainen, H., Rantalainen, T., Linnamo, V., Komi, P., Avela, J. Innervation zone shift at different levels of isometric contraction in the biceps brachii muscle. J Electromyogr Kinesiol 2008. Rainoldi A, Melchiorri G, Caruso I. A method for positioning electrodes during surface EMG recordings in lower limb muscles. J Neurosci Meth 2004;134(1):37–43. Rainoldi A, Nazzaro M, Merletti R, et al. Geometrical factors in surface EMG of the vastus medialis and lateralis muscles. J Electromyogr Kinesiol 2000;10(5):327–36. Roy SH, De Luca CJ, Schneider J. Effects of electrode location on myoelectric conduction velocity and median frequency estimates. J Appl Physiol 1986;61(4):1510–7. Saitou K, Masuda T, Michikami D, Kojima R, Okada M. Innervation zones of the upper and lower limb muscles estimated by using multichannel surface EMG. J Hum Ergol (Tokyo) 2000;29(1–2):35–52. Shiraishi M, Masuda T, Sadoyama T, Okada M. Innervation zones in the back muscles investigated by multichannel surface EMG. J Electromyogr Kinesiol 1995;5(3):161–7. Tokunaga T, Baba S, Tanaka M, et al. Two-dimensional configuration of the myoneural junctions of human masticatory muscle detected with matrix electrode. J Oral Rehabil 1998;25(5):329–34. Yamada M, Kumagai K, Uchiyama A. The distribution and propagation pattern of motor unit action potentials studied by multichannel surface EMG. Electroencephalogr Clin Neurophysiol 1987;67(5):395–401.
Jason M. DeFreitas received a BS (2007) degree in Exercise Science from the University of Connecticut and is currently a Master’s Candidate in Exercise Physiology at the University of Oklahoma. He is also a Certified Strength and Conditioning Specialist through the National Strength and Conditioning Association. His main research interests are noninvasive assessment of muscle function using electromyography and mechanomyography.
Pablo B. Costa received a BS degree in Physical Education from Estácio de Sá University (2004), a Masters degree in Exercise Science from Florida Atlantic University (2007), and is currently working on his Doctorate at the University of Oklahoma. His main research interests include non-invasive assessment of neuromuscular function and the performance effects of stretching.
Eric D. Ryan is currently a doctoral student in the exercise physiology department at the University of Oklahoma under the mentorship of Dr. Joel Cramer. He received a BS (2003) degree in exercise science from Tulane University, New Orleans, Louisiana, and a MS (2005) degree from Florida Atlantic University, Davie, Florida in exercise physiology. He is a member of the American College of Sports Medicine (ACSM) and National Strength and Conditioning Association (NSCA). His primary research interests include the non-invasive assessment of muscle function and the acute effects of different stretching routines on parameters of muscle strength and musculotendinous stiffness.
Trent J. Herda, originally from Iowa, is currently a doctoral student in the exercise physiology department at the University of Oklahoma under the mentorship of Dr. Joel Cramer. He received a BS (2005) degree in exercise science from the University of Sioux Falls, Sioux Falls, South Dakota, and a MS (2007) degree from the University of Oklahoma, Norman, Oklahoma, in exercise physiology. He is a member of the American College of Sports Medicine (ACSM) and National Strength and Conditioning Association (NSCA). His primary research interests include the non-invasive assessment of muscle function and the acute effects of vibration and stretching on mechanical and neural components of muscular contraction.
Joel T. Cramer received a BA (1997) degree in exercise science from Creighton University, Omaha, Nebraska, and MPE (2001) and PhD (2003) degrees from the University of Nebraska–Lincoln under the direction of Dr. Terry J. Housh. He is a member of the American College of Sports Medicine and the National Strength and Conditioning Association. He recently accepted a faculty position at the University of Oklahoma as an assistant professor. He research interests focus on the non-invasive assessment of muscle function using surface electromyography and mechanomyography.
Travis W. Beck received a B.S. (2002) degree in Biology from Doane College, the M.P.E. (2004) degree in Health and Human Performance from the University of Nebraska-Lincoln, and the Ph.D. (2007) degree in Human Sciences from the University of Nebraska-Lincoln. He recently accepted an Assistant Professor position in the Department of Health and Exercise Science at the University of Oklahoma, and his main research interests include evaluation of muscle function using electromyography and mechanomyography.