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Trunk muscle activation during moderate- and high-intensity running David G. Behm, Dario Cappa, and Geoffrey A. Power
Abstract: Time constraints are cited as a barrier to regular exercise. If particular exercises can achieve multiple training functions, the number of exercises and the time needed to achieve a training goal may be decreased. It was the objective of this study to compare the extent of trunk muscle electromyographic (EMG) activity during running and callisthenic activities. EMG activity of the external obliques, lower abdominals (LA), upper lumbar erector spinae (ULES), and lumbosacral erector spinae (LSES) was monitored while triathletes and active nonrunners ran on a treadmill for 30 min at 60% and 80% of their maximum heart rate (HR) reserve, as well as during 30 repetitions of a partial curl-up and 3 min of a modified Biering-Sørensen back extension exercise. The mean root mean square (RMS) amplitude of the EMG signal was monitored over 10-s periods with measures normalized to a maximum voluntary contraction rotating curl-up (external obliques), hollowing exercise (LA), or back extension (ULES and LSES). A main effect for group was that triathletes had greater overall activation of the external obliques (p < 0.05), LA (p = 0.01), and LSES (p < 0.05) than did nonrunners. Main effects for exercise type showed that the external obliques had less EMG activity during 60% and 80% runs, respectively, than with the curl-ups (p = 0.001). The back extension exercise provided less ULES (p = 0.009) and LSES (p = 0.0001) EMG activity than the 60% and 80% runs, respectively. In conclusion, triathletes had greater trunk activation than nonrunners did while running, which could have contributed to their better performance. Back-stabilizing muscles can be activated more effectively with running than with a prolonged back extension activity. Running can be considered as an efficient, multifunctional exercise combining cardiovascular and trunk endurance benefits. Key words: abdominals, back, EMG, erector spinae, external obliques, core exercises. Re´sume´ : On mentionne souvent le manque de temps comme obstacle a` la pratique re´gulie`re de l’activite´ physique. Pour passer moins de temps a` la poursuite d’objectifs, il devient pertinent de trouver un exercice produisait sur l’organisme un vaste e´ventail de be´ne´fices sur le plan de l’entraıˆnement. Cette e´tude se propose de comparer l’EMG (activite´ myoe´lectrique) des muscles du tronc au cours d’une se´ance de course a` l’EMG observe´e au cours d’exercices de gymnastique. On enregistre donc l’activite´ EMG des obliques externes, des abdominaux infe´rieurs (LA), des e´recteurs du rachis lombaire supe´rieur (ULES) et des e´recteurs du rachis lombo-sacre´ (LSES) chez des triathloniens et des personnes actives non entraıˆne´es a` la course au cours de deux se´ances de course sur tapis roulant sollicitant respectivement 60 % et 80 % de la fre´quence cardiaque (HR) de re´serve, au cours d’un exercice de demi-redressement partiel re´pe´te´ 30 fois et au cours d’un exercice d’extension du dos (Biering-Sorensen modifie´) d’une dure´e de 3 min. On enregistre sur une pe´riode de 10 s l’amplitude moyenne des valeurs quadratiques moyennes (rms) du signal EMG en standardisant les mesures en fonction de la contraction maximale volontaire (MVC) lors d’un mouvement de demi-redressement en rotation (obliques externes), d’un exercice de creusement de l’abdomen (LA) et d’une extension du dos (ULES et LSES). L’analyse de variance re´ve`le une plus grande activation des obliques externes (p < 0,05), des LA (p = 0,01) et des LSES (p < 0,05) chez les triathloniens comparativement aux personnes non entraıˆne´es a` la course. L’analyse de variance re´ve`le aussi une moins grande activite´ EMG des obliques externes au cours des se´ances de course aux deux niveaux d’intensite´ que durant les exercices de demiredressement (p = 0,001). L’exercice d’extension du dos sollicite moins les ULES (p = 0,009) et les LSES (p = 0,0001) sur le plan de l’activite´ EMG que les se´ances de course aux deux niveaux d’intensite´. En conclusion, on observe une plus grande activation des muscles du tronc chez les triathloniens que chez les personnes non entraıˆne´es a` la course, ce qui pourrait contribuer a` leur meilleure performance. On peut activer davantage les muscles stabilisateurs du dos durant la course qu’au cours d’un exercice prolonge´ d’extension du dos. On peut de`s lors conside´rer la course comme un exercice multifonctionnel combinant les bienfaits cardiovasculaires et l’endurance des muscles du tronc. Mots-cle´s : abdominaux, dos, EMG, e´recteurs du rachis, obliques externes, exercices de base. [Traduit par la Re´daction]
Received 20 March 2009. Accepted 12 August 2009. Published on the NRC Research Press Web site at apnm.nrc.ca on 4 November 2009. D.G. Behm.1 School of Human Kinetics and Recreation, Memorial University of Newfoundland, St. John’s, NL A1C 5S7, Canada. D. Cappa. Dr. Jorge E. Coll Institute of Physical Education, Mendoza, Argentina. G.A. Power. School of Kinesiology, Faculty of Health Sciences, The University of Western Ontario, London ON N6A 5B8, Canada. 1Corresponding
author (e-mail:
[email protected]).
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doi:10.1139/H09-102
Published by NRC Research Press
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Introduction The development of strength and endurance of the trunk muscles has been emphasized as an important component for general musculoskeletal health and sports conditioning programs in addition to active rehabilitation programs for individuals with low back pain (LBP) (Alaranta et al. 1995; Biering-Sørensen 1984). A lack of back muscle endurance is strongly associated with LBP (Nourbakhsh and Arab 2002). However, the most effective and efficient method of training the trunk muscles remains unclear, whether it is for LBP, maintenance of musculoskeletal health, or athletic performance. Numerous exercises are prescribed and performed while supine or prone with and without devices to promote dorsal and ventral trunk activation (Axler and McGill 1997; Parfrey et al. 2008). Typical prone and supine callisthenictype exercises for the trunk can isolate and activate the trunk muscles (Parfrey et al. 2008) but they do not follow the concept of action or training specificity (Behm 1995). Recently, exercises with the use of instability devices have been shown to increase the activation of trunk muscles to a greater degree than similar exercises performed under more stable conditions (Anderson and Behm 2005; Behm and Anderson 2006; Behm et al. 2003, 2005). However, these instability exercises are performed primarily while seated, supine, or prone. Furthermore, a number of supine and prone exercises for the abdominals and back muscles, such as situps and back extensions, can result in injury because of excessive compressive and shear forces on the vertebrae (McGill 2001). As a bipedal locomotion animal, humans do most of their activities while upright, which raises the question as to whether it is possible to achieve high trunk activation while exercising erect. Hamlyn et al. (2007) demonstrated that dorsal and ventral trunk activation was higher with squats and dead lifts using 80% of their 1-repetition maximum than with common callisthenic-style exercises for the trunk performed on unstable devices. Other popular forms of exercise may also effectively activate the trunk. Running involves a series of unilateral hip flexion and extension movements that can place considerable destabilizing torques on the trunk (Schache et al. 1990). To run efficiently and smoothly, the trunk muscles must stabilize the upper body from the moments and reaction forces of the lower limbs (Dintiman and Ward 2003). There has been considerable attention paid in the literature to the pattern of muscle activation with respiration during running (Abe et al. 1996; Abraham et al. 2002; Gazendam and Hof 2007; Saunders et al. 2004). Less research has focused on how other trunk muscles are used during running. Studies have demonstrated that abdominal muscles fatigue with prolonged submaximal cycling exercise (Fuller et al. 1996; Taylor et al. 2006). Other studies have reported an increase in abdominal muscle activity with increased running speed (Cappellini et al. 2006; Saunders et al. 2005). No studies have compared the extent of trunk muscle activation during running with typical supine and prone trunk-strengthening callisthenic activities. Whereas the compressive and shear forces with a number of supine and prone trunk callisthenic exercises have been found to be high (McGill 2001), the risk of LBP with jogging has been found to be low (Morris 2006). If running is an effective and safe method of activating dorsal and ventral trunk musculature, then additional trunk-specific callisthenic
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exercises such as sit-ups and back extensions may not be necessary. Because lack of time has been cited as a major impediment to regular exercise (Canadian Society for Exercise Physiology 2003), the identification of exercises, such as running, that serve multiple functions (i.e., cardiovascular and trunk muscle endurance) would encourage more consistent activity and would benefit the health of the general population. Therefore, it was the objective of this study to ascertain the extent of dorsal and ventral trunk muscle electromyographic (EMG) activation during 2 intensities of running and to compare the extent of activation to typical trunk-specific exercises (i.e., curl-up and back extension) in run-trained and non-run-trained individuals.
Materials and methods Experimental design Seventeen subjects participated in 3 experimental sessions. The participants were monitored for the mean root mean square (RMS) amplitude of the EMG activity of ventral and dorsal trunk musculature external obliques, lower abdominals (LA), lumbosacral erector spinae (LSES), and upper lumbar erector spinae (ULES) for 10-s periods while (i) running on a treadmill at 60% of their maximal heart rate (HR) reserve (Karvonen et al. 1957) for 30 min; (ii) running on a treadmill at 80% of their maximal HR reserve for 30 min; (iii) performing 30 curl-ups (Canadian Society for Exercise Physiology 2003); and (iv) holding a modified Biering-Sørensen isometric back extension posture for 180 s (Canadian Society for Exercise Physiology 2003). EMG activity was normalized to a resisted maximum isometric rotating curl-up, abdominal hollowing, and modified Biering-Sørensen isometric back extension posture (Pitcher et al. 2007, 2008). Subjects Seventeen subjects participated in the study. Seven of the participants were highly trained triathletes from Mendoza, Argentina (age, 26.4 ± 5.1 years; height, 1.76 ± 0.62 m; mass, 74.2 ± 7.9 kg; 60% HR reserve, 139.6 ± 3.4 beatsmin–1; 80% HR reserve, 165.6 ± 4.1 beatsmin–1), who trained an average of 4–5 times per week, running, swimming, and cycling. The remaining 10 participants were highly active non-run-trained (i.e., 7 subjects competed or were involved with rock climbing, rugby, cycling, soccer, basketball, hockey, and (or) curling) and recreationally active (3 subjects resistance trained 3 or more times per week) Canadian university students (age, 23.4 ± 2.4 years; height, 1.74 ± 0.33 m; mass, 70.0 ± 9.9 kg; 60% HR reserve, 143.8 ± 2.9 beatsmin–1; 80% HR reserve, 170.2 ± 3.2 beatsmin–1). All subjects completed a Physical Activity Readiness Questionnaire (PAR-Q) form (Canadian Society for Exercise Physiology 2003), which identifies the existence of significant health problems. Exclusion criteria included any individual with acute or chronic back pain, past or present hernias, or positive responses to the PAR-Q form, or any who were aerobically unfit to complete the designated activities. The trunk exercise interventions (curl-ups and back extensions) were designed to represent good to very good health scores on the Canadian Physical Activity, Published by NRC Research Press
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Fitness and Lifestyle Appraisal (CPAFLA) tests for 15- to 29-year-old males (Canadian Society for Exercise Physiology 2003). Each subject was required to read and sign a consent form prior to participating in the study. The study was approved by the Memorial University of Newfoundland (St. John’s, N.L.) Human Investigations Committee. Dependent variables Electromyography Surface EMG electrodes were used to measure signals from the LA, external obliques, LSES, and ULES muscle groups. All electrodes were placed on the right side of the body. Skin surfaces were shaved, abraded, and cleansed with alcohol to improve the conductivity of the EMG signal. Electrodes (Kendall Medi-trace 100 series, Kendall Health Care Products, Chicopee, Mass.) were placed 2 cm lateral to the L5-S1 spinous processes for the LSES and 6 cm lateral to the L1-L2 spinous processes for the ULES muscles. Additional electrodes were placed immediately superior to the inguinal ligament and medial to the anterior superior iliac spine (ASIS) for the LA. Electrodes to monitor the external obliques were placed midway between the ASIS and the 10th rib’s articulation with the costal cartilage. General descriptive (i.e., LSES and ULES), rather than specific (i.e., multifidus, longissimus), trunk muscle terminology is used in this paper because of the conflicting findings of similar studies. A number of studies have used a similar L5-S1 electrode placement to measure the EMG activity of the multifidus (Danneels et al. 2001; Hermann and Barnes 2001; Hodges and Richardson 1996; Ng et al. 1998). In contrast, Stokes et al. (2003) reported that accurate measurement of the multifidus requires intramuscular electrodes. Thus, the EMG detected by the electrodes in the present study is referred to as LSES muscle activity. According to anatomic nomenclature, erector spinae muscles include both superficial (spinalis, longissimus, iliocostalis) and deep (multifidus) vertebral muscles (Martini and Nath 2008). Back muscles have also been described as local and global stabilizing muscles, based on their role in stabilizing the trunk (Berkmark 1989). The multifidus is described as a component of the local stabilizing system, whereas the longissimus contributes to the global stabilizing system. The ULES EMG electrode positioning was more lateral than the lower back EMG positioning to diminish the detection of multifidus activity and thus emphasize the measurement of global stabilizing muscles (longissimus). The LSES electrode positioning would represent more of the local stabilizing functions. Additional electrodes were placed superior to the inguinal ligament and medial to the ASIS for the LA. McGill et al. (1996) reported that surface electrodes adequately represent the EMG amplitude of the deep abdominal muscles within a 15% RMS difference. However, Ng et al. (1998) indicated that electrodes placed medial to the ASIS would receive competing signals from the external obliques and transverse abdominus with the internal obliques. Based on these findings, the EMG signals obtained from this abdominal location are described in the present study as the LA, which would be assumed to include EMG information from both the transverse abdominus and internal obliques. The transverse abdominus and internal obliques are
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also considered to contribute to the local stabilizing system (Berkmark 1989). Similar electrode arrangements and nomenclature have been published previously (Anderson and Behm 2005; Behm et al. 2005; Hamlyn et al. 2007; Wahl and Behm 2008) The EMG signals were amplified (Biopac Systems MEC 100 amplifier, Santa Barbara, Calif.), monitored, and directed through an analog-digital converter (Biopac MP100) to be stored on the computer (Hewlett Packard, Cerritos, Calif.) for further analysis. EMG activity was sampled at 2000 Hz, with a Blackman –61 dB band-pass filter between 10 and 500 Hz, amplified (Biopac Systems MEC bi-polar differential 100 amplifier, input impedance = 2M, common mode rejection ratio ‡ 110 dB min (50–60 Hz), gain 1000, noise ‡ 5 mV), and analog to digitally converted (12 bit). Using the AcqKnowledge software program (AcqKnowledge III, Biopac Systems Inc., Holliston, Mass.), the EMG signal was filtered (10–500 Hz) and smoothed (averaged over 10 samples) and the mean amplitude of the RMS EMG signal was calculated over the duration of the activity. ULES and LSES EMG activity was normalized to a modified Biering-Sørensen maximum isometric back extension posture (Pitcher et al. 2007, 2008). LA and external obliques EMG activity was normalized to abdominal hollowing maximum voluntary contractions (MVC) (Behm et al. 2005) and an MVC isometric rotating curl-up (modified CPAFLA protocol), respectively. Heart rate HR was monitored during the tests using a Polar HR monitor (Polar Electro, Oy, Finland) with the data recorded manually. HR, rather than relative aerobic capacity (V_ O2), was used to monitor running intensity because there was no metabolic cart available in Argentina. Furthermore, because most of the general population has access to HR monitors, there would be an easier application of the protocol for the nonscientist population. Independent variables There were 3 randomly allocated experimental sessions. All sessions were separated by at least 24 h and conducted at approximately the same time of day to minimize any possible diurnal variations. One session began with the normalization procedures, which included 2 maximum isometric rotating curl-ups as a measure of external oblique activation. The position adopted corresponded to the CPAFLA curl-up protocol (knees flexed at 908, feet not secured, hands on floor with trunk raised until hands move 10 cm) (Canadian Society for Exercise Physiology 2003). The only difference from the CPAFLA protocol was that the participant’s contralateral shoulder was manually resisted while he or she attempted to rotate across his or her body while performing the curl-up. Karst and Willett (2004) have illustrated the effectiveness of a curl-up with rotation to activate the obliques. Two abdominal hollowing trials were performed to determine the highest LA activation (Behm et al. 2005). Subjects were requested to contract their abdominal muscles (the researchers used the navel as a physical cue) up and back towards the spine (Karst and Willett 2004), and another verbal cue had the subjects attempt to pull their ASIS together (Behm et al. 2005). Participants were also provided Published by NRC Research Press
Behm et al.
visual feedback from the computer monitor, which illustrated the EMG activity during the hollowing trials. Two modified Biering-Sørensen manually resisted maximum isometric back extension trials were used to identify maximum dorsal trunk (LSES, ULES) activation (Behm et al. 2005; Pitcher et al. 2007, 2008). Intraclass correlation coefficients as a measure of reliability of the Biering-Sørensen test for force as well as LSES and ULES EMG activity have been reported to be high in healthy individuals (Pitcher et al. 2008). Manually resisted MVCs have been shown to be valid and reliable measures of force (Boe et al. 2008) and have been used previously in published studies from this laboratory (Hamlyn et al. 2007). The average EMG activities of the 2 exercises were used as the normalization value. The isometric contractions were held for 5 s and the EMG activity was analyzed from the middle 3 s (2–4 s) of activity. Subjects then performed a random allocation of (i) 30 repetitions of the CPAFLA-style curl-up and (ii) a modified Biering-Sørensen isometric back extension posture maintained for 180 s (Canadian Society for Exercise Physiology 2003). Both tests correspond to ‘‘good’’ on the CPAFLA category of abdominal and back health for 15- to 29-yearold males (Canadian Society for Exercise Physiology 2003). EMG activity was recorded during the curl-up at 1–3, 10– 13, 20–23, and 27–30 repetitions and at 5–15, 50–60, 110– 120, and 170–180 s during the modified Biering-Sørensen back extension. The average mean amplitude of the RMS EMG activity of the 3 repetitions of the curl-up or 10 s of the back extension was used for analysis. The other sessions involved trunk activation with moderateto higher-intensity treadmill running. The running was conducted on a treadmill for 30 min at either 60% or 80% of the participant’s maximum HR reserve (Karvonen et al. 1957). A 5-min warm-up treadmill run was allocated to achieve the prescribed HR by progressively increasing the treadmill velocity. If the HR during the 30-min run exceeded the prescribed intensity, the treadmill velocity was decreased to maintain the intended percentage of HR. This type of exercise corresponds to the recommended duration of time (20–30 min) to achieve cardiovascular benefits when performed on a regular basis (Canadian Society for Exercise Physiology 2003). EMG activity was recorded at 10-s intervals at 5–15 s, 9 min 50 s to 10 min, 19 min 50 s to 20 min, and 29 min 20 s to 29 min 30 s. Figure 1 shows a typical EMG recording over a 10 s interval illustrating the clarity of the signal and the lack of impact artifacts. Statistical analysis For each muscle group, data were analyzed with separate 3-way analyses of variance (ANOVAs) (2 4 4) with repeated measures. The 3 levels included trained state (nonrun-trained participants vs. elite triathletes), type of activity (CPAFLA trunk tests, moderate-intensity (60% of maximum HR reserve) and high-intensity (80% of maximum HR reserve) running), and time (4 measures throughout each test). Where significant differences were detected (p < 0.05), a Bonferroni (Dunn’s) correction factor was used to identify the individual differences among the exercises. The means and standard deviation (SD) are illustrated in the figures.
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Results Significant main effects and interactions were noted for trained state and types of activity; however, there was a lack of significant main effects for time. Main effects for trained state A main effect for trained state indicated that triathletes had greater activation of the external obliques, LA, and LSES than the active non-run-trained subjects. Overall, nonrunner athletes had 23.4% (p < 0.05), 55.6% (p = 0.01), and 38.6% (p < 0.05) less EMG activity associated with their external obliques, LA, and LSES, respectively (Fig. 2). Main effects for type of activity The external obliques had 64.8% and 45.8% less EMG activity during 60% and 80% runs, respectively, than with the curl-ups (p = 0.001) (Table 1). The back extension exercise provided 63.1% and 68.9% less ULES EMG activity than the 60% and 80% runs, respectively (p = 0.009) (Fig. 3). Similarly, the back extension exercise provided 55.2% and 59.9% less LSES EMG activity than the 60% and 80% runs, respectively (p = 0.0001) (Table 1). Interactions External oblique EMG activity during all curl-up measures (1–3, 10–13, 20–23, and 27–30 repetitions) was greater (p < 0.0001) than during all 60% run measures (5–15 s, 9 min 50 s to 10 min, 19 min 50 s to 20 min, and 29 min 20 s to 29 min 30 s) (Fig. 4). In addition, external oblique activity at 10–13, 20–23, and 27–30 repetitions of the curlup exceeded the EMG activity during the 80% run at 5– 15 s, 9 min 50 s to 10 min, and 19 min 50 s to 20 min. (p < 0.0001) (Fig. 4). There were no significant interactions for LA EMG activity. Back extension measures at 2 and 3 min had lower ULES EMG activity than with the 80% run at 29 min 20 s to 29 min 30 s (p < 0.0001) (Fig. 3). All back extension measures (5–15 , 50–60, 110–120, and 170–180 s) had less LSES EMG activity than during the 60% run at 5–15 s and the 80% run at 5–15 s and 9 min 50 s to 10 min (p < 0.0001) (Fig. 5). Running speed At 60% of maximum HR reserve, the triathletes ran at an average of 10.4–13.0 kmh–1, whereas at 80% of maximum HR reserve, the triathletes ran between 14.2 and 18.1 kmh–1 Non-run-trained participants were significantly (p < 0.01) slower, running at an average of 8.3–9.2 kmh–1 and 10.2– 11.7 kmh–1 for the 60% and 80% of maximum HR reserve conditions, respectively.
Discussion The most important findings of the present study were that (i) triathletes had greater trunk activation (external obliques, LA, and LSES) than non-run-trained subjects; (ii) moderate- and high-intensity running provided greater activation of the back stabilizer muscles than prolonged unresisted back extension; (iii) curl-ups provided higher activation of the external obliques than running; and (iv) the LA are activated equally with running and repetitive curl-ups. Published by NRC Research Press
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Fig. 1. A typical electromyographic (EMG) tracing from a 10-s segment of treadmill running at 80% of maximal heart rate reserve. The first to fourth rows of EMG represent the external obliques, lower abdominal stabilizers, upper lumbar erector spinae, and lumbosacral erector spinae muscles, respectively. RMS, root mean square.
Fig. 2. Main effects for trained state (non-run-trained participants and triathletes) are illustrated for the external obliques (EO), lower abdominal stabilizers (LAS), and lumbosacral erector spinae (LSES). Data on the y axis represent normalized electromyographic (EMG) values to a resisted maximum voluntary contraction (MVC) isometric oblique curl-up for the EO, an MVC hollowing maneuver (LAS), or an MVC resisted isometric back extension (LSES). *, Significant differences (p < 0.05) between groups.
Table 1. Main effect data for type of activity.
Muscles EO ULES LLES
60% HR reserve run 0.214±0.164 0.341±0.362 0.323±0.184
80% HR reserve run 0.329±0.229 0.404±0.28 0.362±0.193
Curl-ups (EO)– back extensions (ULES, LLES) 0.609±0.304* 0.125±0.056* 0.145±0.066*
Note: Data represent the normalized EMG of the specified muscle during the respective activity. For example, 0.214 indicates that the external obliques during the 60% of HR reserve run averaged 21.4% of the EMG activity compared with the normalization activity (maximum isometric resisted oblique curl-up). HR, heart rate; EO, external obliques; ULES, upper lumbar erector spinae; LLES, lower lumbar erector spinae. *Significantly different normalized electromyographic activity compared with the other 2 activities (60% and 80% HR reserve runs).
The greater activation of the external obliques, LA, and LSES by the triathletes may have contributed to their enhanced running performance, which could be attributed partially to a greater absorption by the trunk muscles of disrupting torques generated by the lower limbs. Nagy et al. (2004) reported that triathletes were more stable (had better balance) than controls, similar to the findings of the present study. Running is a series of unilateral hip flexion and extension movements that can place considerable destabilizing torques on the trunk (Schache et al. 1990). Core strength training has been shown to improve 5000-m run perform-
ance in runners (Sato and Mokha 2009), as well as other athletic performance measures such as pitching speed and accuracy (Marsh et al. 2004) and golf club head speed (Thompson et al. 2007). Considerable research has demonstrated that force and power exerted from an unstable base is impaired (Anderson and Behm 2004; Behm and Anderson 2006; Behm et al. 2002; Drinkwater et al. 2007). In addition, an unstable base can reduce the velocity and range of movement (Drinkwater et al. 2007) and can contribute to greater cocontractile activity (Behm et al. 2002). Therefore, less-activated trunk muscles would not provide as solid a base for the absorption of high limb torques exerted during higher-speed running (Behm and Anderson 2006). Efficient runners attempt to exert their propulsive forces such that their body is moved in a linear manner (Dintiman and Ward 2003). Because a less-activated trunk would not absorb as efficiently the disruptive torques of the unilateral reactive running moments, the trunk and body could tend to rotate in reaction to the limb-induced moments. Limb forces Published by NRC Research Press
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Fig. 3. Interactive effects (type of activity time) are illustrated for upper lumbar erector spinae muscle activation. Data on the y axis represent normalized electromyographic (EMG) values to a maximum voluntary contraction (MVC) resisted isometric back extension (ext). *, Significant (p < 0.0001) differences compared with all back extension measures; {, the 80% heart rate reserve run at 30 min was significantly (p < 0.0001) different from both back extensions at 2 and 3 min.
Fig. 4. Interactive effects (type of activity time) are illustrated for external oblique activation. Data on the y axis represent normalized electromyographic (EMG) values to a resisted maximum voluntary contraction (MVC) isometric rotating curl-up. *, Significant differences (p < 0.0001) between all 60% HR reserve runs and all curl-up measures. Horizontal bars indicate that the normalized EMG activity during the 80% HR reserve measures situated below the bar was significantly (p < 0.0001) lower than the curl-up measures located below that horizontal bar.
would then be exerted at angles that would divert the runner from the intended path (i.e., a straight line). A related question to the finding of higher trunk activation in triathletes is how to most efficiently and effectively train the trunk musculature to attain optimal running performance and musculoskeletal trunk health. A second major finding of the study was that moderate- and high-intensity running provided greater activation of the back stabilizer muscles (ULES and LSES) compared with prolonged (3 min) unresisted back extension. Cappellini et al. (2006) reported increased thoracic and lumbar erector spinae EMG activity with increased running speed, as compared with walking and slower running. Saunders et al. (2005) found bursts of trunk muscle EMG activity during running, ranging from 10% to 100% of MVC. Hence, in the present study, destabilizing torques associated with a unilateral activity like running were more effective for activating the LSES and ULES than a stable back extension exercise. It has been suggested that the erector spinae activity during run-
ning is used to control trunk forward rotation as the trunk decelerates during the foot strike (Winter and Yack 1987). Another possibility, offered by Shiavi (1990), is that erector spinae contractions are related to the mass transference between limbs and the reversal in direction of thoracic and pelvic rotation. Because the compressive and shearing forces on the spine can be lower with running–jogging (Morris 2006) than with sit-ups and back extensions (McGill 2001), running may be both an effective and a safe activity for providing training-induced stress to the LSES and ULES. ULES and LSES EMG activity was substantially lower during the stable back endurance test compared with running. Because the muscles tested have a high stabilizing responsibility, the stable nature of the modified Biering-Sørensen back endurance test may not effectively promote the activity of these muscles. Furthermore, Pitcher et al. (2007) demonstrated that muscle synergism contributes to the ability to sustain the modified Biering-Sørensen back posture. Motor unit substitution during fatigue protocols has been reported for a number of trunk muscles (Westgaard and de Luca 1999). Kouzaki and Shinohara (2006) reported that subjects with more frequent alternate muscle activity experience less muscle fatigue. Muscle substitution protects postural muscles from excessive fatigue when there is a demand for sustained low-level muscle activity (Westgaard and de Luca 1999). It is suspected that at higher intensities, such as with an MVC, there is less time for implementing a motor control strategy that coordinates load sharing across synergistic muscles and thus less opportunity to employ alternative recruitment strategies. Pitcher et al. (2007) highlighted one particular subject in whom it was evident that during the modified Biering-Sørensen test, a load-sharing strategy between lumbar extensors and biceps femoris alternating bursts of activity in each muscle group created ‘‘microrest periods’’. This case highlights the notion that motor control strategies may be able to attenuate the increase in ULES and LSES activity during prolonged back extension exercises, whereas the more unstable nature of running necessitates higher levels of activity. It might be argued that the lower ULES and LSES EMG activity during isometric back extensions may be attributed Published by NRC Research Press
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Fig. 5. Interactive effects (type of activity time) are illustrated for lumbosacral erector spinae muscle activation. Data on the y axis represent normalized electromyographic (EMG) values to a maximum voluntary contraction (MVC) resisted isometric back extension (ext). *, Significant differences (p < 0.0001) with all back extension measures.
to the muscle wisdom effect (Jones et al. 1979), which is not as pronounced during dynamic activities (Griffin et al. 2000). Although the running activity was dynamic, the LSES and ULES muscles act primarily as stabilizers during this activity and thus would be primarily isometric contractions. There would not have been substantial flexion or extension of the trunk during running and thus no substantial change in length while these muscles were active. The greater extent of activation would be expected to provide a more potent training stimulus for these muscles. Although the extent of activation achieved was only approximately 40% of a maximal effort, a number of authors have stated that lower-load, endurance-type activities are more protective of the back than high-load, strength-type activities (McGill 1998; Gibbons and Comerford 2001; Barr et al. 2005). It has been suggested that back muscle contractions as low as 25% of MVC are able to provide maximal joint stiffness (Cresswell et al. 1994). Furthermore, because lumbar stabilizing muscles such as the multifidus are mainly composed of type I muscle fibres (Thortstensson and Carlson 1987), relatively low loads of 30%–40% of MVC are needed to improve their effectiveness (McGill 1998). In addition, because most human daily activities are performed while upright, the use of moderate- and high-intensity run training to train the back stabilizer muscles would follow the training specificity principle to a greater degree. The training specificity concept indicates that the more a training stimulus deviates from the performance or testing activity, the fewer the training gains will be accrued (Behm 1995). Thus, if the predominant form of activity is upright, training the back stabilizer muscles from stable prone positions may not provide the optimal transfer of training adaptations. The activation, training, or rehabilitation of deep abdominal stabilizers such as the transversus abdominus and internal obliques (represented by the LA in the present study) often involves supine callisthenic activities such as the hollowing maneuver (Behm et al. 2005; Hamlyn et al. 2007; Parfrey et al. 2008). In the present study, running and curlup activities involved approximately 40%–60% of the LA EMG of an MVC hollowing technique. There was no significant difference in LA activation between repetitive curl-ups and running. Two studies have reported increases in internal oblique (Cappellini et al. 2006; Saunders et al. 2005) and
transversus abdominus (Saunders et al. 2005) EMG activity with greater running speeds. There was no similar trend in the present study. However, the lowest range of treadmill speeds in the present study (9–12 kmh–1) was similar to the top speeds in the aforementioned studies. It has been hypothesized that the increased EMG was associated with increased lumbo-pelvic motion in all planes (Saunders et al. 2005). Lumbo-pelvic motion may not have differed significantly with the range of higher treadmill speeds used in the present study. Because the transverses abdominus aids in compressing the abdomen, and the internal obliques also compress the abdomen and depress the ribs (Martini and Nath 2008), a portion of the LA activity may also have been derived from forced expiration during the run. Whereas neither exercise provides full activation of the LA, 40%– 60% of full LA activation while running for 30 min would be expected to provide an endurance training stimulus for these abdominal muscles. There is evidence to suggest that improved endurance of the LA is beneficial for LBP individuals because they aid in lumbar spine stabilization (Karst and Willett 2004). In opposition to the other trunk muscles tested in the present study, the external obliques were less activated with running compared with a callisthenic trunk activity such as repetitive curl-ups. External oblique EMG activity ranged from approximately 20% to 40% of MVC while running, whereas curl-up activity was approximately 60% of MVC (Fig. 2). The EMG associated with running was similar to the findings of Abraham et al. (2002), who illustrated external oblique EMG activity while cycling that approximated 30%–40% of peak EMG activity. Although the external obliques can play a trunk-stabilizing role, especially with abdominal compression, to increase intra-abdominal pressure, they are important mobilizing muscles, involved with flexion and rotation of the spine or trunk and with active expiration (Martini and Nath 2008). Both Saunders et al. (2005) and Cappellini et al. (2006) reported greater external oblique activity with running than with walking but they did not compare the extent of run-induced EMG activity with other activities. Because the primary function of the external obliques is not stability, they may not be stimulated to a similar extent with run-induced instability of the trunk or lumbopelvic area or active expiration as compared with their trunk Published by NRC Research Press
Behm et al.
flexion functions during repetitive curl-ups. These findings are similar to those of Anderson and Behm (2005), who found greater activation of the postural soleus muscle with greater degrees of unstable squats but no significant difference with a primary mover such as the quadriceps. Hence, higher activation or more intensive training of the external obliques may be better achieved with curl-ups than with 30 min of moderate- or higher-intensity running.
Conclusion Thus, as was found in previously published instability studies (Anderson and Behm 2005; Behm and Anderson 2006; Behm et al. 2003, 2005), we found that an instabilityinducing exercise such as running, which involves unilateral hip flexion and extension movements, provides activation to trunk-stabilizing muscles that is greater than (LSES, ULES) or similar to (LA) that of callisthenic exercises such as curlups and isometric back extensions but is not as effective as a prime mover of the trunk (external obliques). Furthermore, highly trained runners such as triathletes demonstrated greater trunk activation than non-run-trained participants. Their prolonged run training may specifically train the trunk stabilizers, contributing to their greater running performance. The findings of this study suggest that additional callisthenic exercises for trunk stabilizers (LSES, ULES, LA) may not be necessary with moderate- or high-intensity run training, which may help counter time constraints as a barrier to exercise. However, greater activation and training of the external obliques as a prime mover may be augmented with trunk callisthenic exercises such as curl-ups. Running may be considered a safe, effective, and efficient multifunctional training activity for cardiovascular and trunk muscle endurance benefits.
Acknowledgements This research was funded by the National Science and Engineering Research Council (NSERC) of Canada.
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