The effect of heel lifts on trunk muscle activation during gait: A study of young healthy females

Heel lifts are a treatment option for low back pain (LBP), whilst high-heeled shoes have been linked to LBP development. This study evaluated the effects of in-shoe 20 mm high bilateral heel lifts on trunk muscle activity. Activity of the erector spinae (ErSp), internal oblique and external oblique muscles was evaluated using surface electromyography in 15 young (20.7 ± 0.9 years) healthy female participants. Measures were taken during overground gait, both immediately and following two days habituation to the heel lifts. Immediately following the addition of the heel lifts, levels of ErSp muscle activity in the 5% epoch following heel strike increased by 19.2% (p < 0.05). Following habituation, levels of ErSp muscle activity in the 5% epoch prior to heel strike increased by 24.1% (p < 0.05), and a 14 ms (p < 0.001) earlier onset of ErSp muscle activity prior to heel strike was observed. These results indicate the heel lifts altered muscle activity reactively around heel strike (i.e. greater activity after heel strike) immediately after application and proactively (i.e. earlier onsets and greater activity prior to heel strike) after short term habituation. When put in context of previous research on trunk muscle activity in LBP populations, these changes may be important considerations for the aetiology, treatment and prevention of LBP.     

Effects of two neuromuscular fatigue protocols on landing performance

The purpose of the study was to investigate the effects of two fatigue protocols on landing performance. A repeated measures design was used to examine the effects of fatigue and fatigue protocol on neuromuscular and biomechanical performance variables. Ten volunteers performed non-fatigued and fatigued landings on two days using different fatigue protocols. Repeated maximum isometric squats were used to induce fatigue on day one. Sub-maximum cycling was used to induce fatigue on day two. Isometric squat maximum voluntary contraction (MVC) was measured before and after fatigued landings on each day. During the landings, ground reaction force (GRF), knee kinematics, and electromyographic (EMG) data were recorded. Isometric MVC, GRF peaks, loading rates, impulse, knee flexion at contact, range of motion, max angular velocity, and EMG root mean square (RMS) values were compared pre- and post-fatiguing exercise and between fatigue protocols using repeated ANOVA. Fatigue decreased MVC strength (p ? 0.05), GRF second peak, and initial impulse (p ? 0.01), but increased quadriceps medium latency stretch reflex EMG activity (p ? 0.012). Knee flexion at contact was 5.2° greater (p ? 0.05) during fatigued landings following the squat exercise compared to cycling. Several variables exhibited non-significant but large effect sizes when comparing the effects of fatigue and fatigue protocol. In conclusion, fatigue alters landing performance and different fatigue protocols result in different performance changes.     

Active leg stiffness and energy stored in the muscles during maximal counter movement jump in the aged

The purpose of this study was to examine the effects of age on active leg stiffness adjustment, electromyogram (EMG) activities and energy stored during eccentric and concentric phases in performing a maximal functional task involving stretch-shorten cycle. Ten young (24.3 ± 2 years) and 10 old (68.6 ± 5 years) healthy male subjects were filmed during maximal performance of counter movement jump (CMJ) and squat jump (SJ) on force plate. Integrated EMG (IEMG), ground reaction force (GRF), active leg stiffness, energy stored/returned and active work done by the muscles were compared between two groups on eccentric (ECC) and concentric (CON) phases of CMJ. The GRF, leg stiffness and energy stored in ECC and GRF, IEMG, energy returned and active work in CON were less in the elderly (p < 0.05). These results demonstrate that the neuromuscular function of adjusting active stiffness, storing elastic energy and optimizing the performance may decrease with age during CMJ.     

The influence of whole body vibration on the plantarflexors during heel raise exercise

Whole body vibration (WBV) during exercise offers potential to augment the effects of basic exercises. However, to date there is limited information on the basic physiological and biomechanical effects of WBV on skeletal muscles. The aim of this study was to determine the effects of WBV (40 Hz, 1.9 mm synchronous vertical displacement) on the myoelectrical activity of selected plantarflexors during heel raise exercise. 3D motion capture of the ankle, synchronised with sEMG of the lateral gastrocnemius and soleus, was obtained during repetitive heel raises carried out at 0.5 Hz on 10 healthy male subjects (age 27 ± 5 years, height 1.78 ± 0.04 m, weight 75.75 ± 11.9 kg). During both vibration and non vibration the soleus activation peaked earlier than that of the lateral gastrocnemius. The results indicate that WBV has no effect on the timing of exercise completion or the amplitude of the lateral gastrocnemius activity, however significant increases in amplitudes of the soleus muscle activity (77.5–90.4% MVC P < 0.05). WBV had no significant effect on median frequencies of either muscle. The results indicate that the greatest effect of WBV during heel raise activity is in the soleus muscles during the early phases of heel raise.     

Between-day reliability of a hand-held dynamometer and surface electromyography recordings during isometric submaximal contractions in different shoulder positions

Functional shoulder assessments require the use of objective and reliable standardized outcome measures. Therefore, the aim of this study was to examine the between-day reliability of a hand-held dynamometer when measuring muscle strength during flexion, abduction, and internal and external rotation as well as surface electromyography (EMG) when measuring muscle activity from m. trapezius superior and deltoideus anterior. Twenty-four healthy subjects participated and performed four isometric contractions measured with a hand-held dynamometer and EMG. Both relative and absolute reliability were calculated based on the mean of the last three of the four repetitions. EMG amplitude was assessed calculating both absolute and normalized root-mean-square (RMS) values. The reliability of the hand-held dynamometer was high (LOA = 3.2–7.6% and ICC = 0.89–0.98). The absolute reliability for EMG showed similar results for absolute RMS values (LOA = 20.0–68.4%) and normalized RMS values (LOA = 42.4–66.5%). However, the results concerning the relative reliability showed higher ICC for absolute RMS values (ICC = 0.82–0.92) compared with normalized values (ICC = 0.57–0.72).The outcome measurements of this study with healthy subjects were found reliable and, therefore, have the potential to detect changes in muscle strength and muscle activity.     

Braking characteristics during cutting and pivoting in female soccer players

Most biomechanical studies into changing direction focus on final contact (FC), whilst limited research has examined penultimate contact (PEN). The aim of this study was to explore the kinematic and kinetic differences between PEN and FC of cutting and pivoting in 22 female soccer players (mean ± SD; age: 21 ± 3.1 years, height: 1.68 ± 0.07 m, mass: 58.9 ± 7.3 kg). Furthermore, the study investigated whether horizontal force–time characteristics during PEN were related to peak knee abduction moments during FC. Three dimensional motion analyses of cutting and pivoting on the right leg were performed using Qualysis ‘Proreflex’ infrared cameras (240 Hz). Ground reaction forces (GRF) were collected from two AMTI force platforms (1200 Hz) to examine PEN and FC. Both manoeuvres involved significantly (P < 0.05) greater knee joint flexion angles, peak horizontal GRF, but lower average horizontal GRF during PEN compared to FC. Average horizontal GRF during PEN (R = −0.569, R² = 32%, P = 0.006) and average horizontal GRF ratio (R = 0.466, R² = 22%, P = 0.029) were significantly related to peak knee abduction moments during the FC of cutting and pivoting, respectively. The results indicate PEN during pre-planned changing direction helps reduce loading on the turning leg where there is greater risk of injuries to knee ligaments.     

The use of EMG biofeedback for learning of selective activation of intra-muscular parts within the serratus anterior muscle: A novel approach for rehabilitation of scapular muscle imbalance

Motor control and learning possibilities of scapular muscles are of clinical interest for restoring scapular muscle balance in patients with neck and shoulder disorders. The aim of the study was to investigate whether selective voluntary activation of intra-muscular parts within the serratus anterior can be learned with electromyographical (EMG) biofeedback, and whether the lower serratus anterior and the lower trapezius muscle comprise the lower scapula rotation force couple by synergistic activation. Nine healthy males practiced selective activation of intra-muscular parts within the serratus anterior with visual EMG biofeedback, while the activity of four parts of the serratus anterior and four parts of the trapezius muscle was recorded. One subject was able to selectively activate both the upper and the lower serratus anterior respectively. Moreover, three subjects managed to selectively activate the lower serratus anterior, and two subjects learned to selectively activate the upper serratus anterior. During selective activation of the lower serratus anterior, the activity of this muscle part was 14.4 ± 10.3 times higher than the upper serratus anterior activity (P < 0.05). The corresponding ratio for selective upper serratus vs. lower serratus anterior activity was 6.4 ± 1.7 (P < 0.05). Moreover, selective activation of the lower parts of the serratus anterior evoked 7.7 ± 8.5 times higher synergistic activity of the lower trapezius compared with the upper trapezius (P < 0.05). The learning of complete selective activation of both the lower and the upper serratus anterior of one subject, and selective activation of either the upper or lower serratus anterior by five subjects designates the promising clinical application of EMG biofeedback for restoring scapular muscle balance. The synergistic activation between the lower serratus anterior and the lower trapezius muscle was observed in only a few subjects, and future studies including more subjects are required before conclusions of a lower scapula rotation couple can be drawn.     

History dependence of the electromyogram: Implications for isometric steady-state EMG parameters following a lengthening or shortening contraction

Residual force enhancement (RFE) and force depression (FD) refer to an increased or decreased force following an active lengthening or shortening contraction, respectively, relative to the isometric force produced at the same activation level and muscle length. Our intent was to determine if EMG characteristics differed in the RFE or FD states compared with a purely isometric reference contraction for maximal and submaximal voluntary activation of the adductor pollicis muscle. Quantifying these alterations to EMG in history-dependent states allows for more accurate modeling approaches for movement control in the future. For maximal voluntary contractions (MVC), RFE was 6–15% (P < 0.001) and FD was 12–19% (P < 0.001). The median frequency of the EMG was not different between RFE, FD and isometric reference contractions for the 100% and 40% MVC intensities (P > 0.05). However, root mean square EMG (EMG_RMS) amplitude for the submaximal contractions was higher in the FD and lower in the RFE state, respectively (P < 0.05). For maximal contractions, EMG_RMS was lower for the FD state but was the same for the RFE state compared to the isometric reference contractions (P > 0.05). Neuromuscular efficiency (NME; force/EMG) was lower in the force depressed state and higher in the force enhanced state (P < 0.05) compared to the isometric reference contractions. EMG spectral properties were not altered between the force-enhanced and depressed states relative to the isometric reference contractions, while EMG amplitude measures were.     

Knee joint angle affects EMG–force relationship in the vastus intermedius muscle

It is not understood how the knee joint angle affects the relationship between electromyography (EMG) and force of four individual quadriceps femoris (QF) muscles. The purpose of this study was to examine the effect of the knee joint angle on the EMG–force relationship of the four individual QF muscles, particularly the vastus intermedius (VI), during isometric knee extensions. Eleven healthy men performed 20–100% of maximal voluntary contraction (MVC) at knee joint angles of 90°, 120° and 150°. Surface EMG of the four QF synergists was recorded and normalized by the root mean square during MVC. The normalized EMG of the four QF synergists at a knee joint angle of 150° was significantly lower than that at 90° and 120° (P < 0.05). Comparing the normalized EMG among the four QF synergists, a significantly lower normalized EMG was observed in the VI at 150° as compared with the other three QF muscles (P < 0.05). These results suggest that the EMG–force relationship of the four QF synergists shifted downward at an extended knee joint angle of 150°. Furthermore, the neuromuscular activation of the VI was the most sensitive to change in muscle length among the four QF synergistic muscles.     

Kinematic and electromyographic analysis of the Nordic Hamstring Exercise

The Nordic Hamstring Exercise (NHE) has been introduced as a training tool to improve the efficiency of eccentric hamstring muscle contraction. The aim of this study was to perform a biomechanical analysis of the NHE. Eighteen participants (20.4 ± 1.9 years) performed two sets of five repetitions each of the NHE and maximal eccentric voluntary contraction (MEVC) of the knee flexors on an isokinetic dynamometer whilst knee angular displacement and electrical activity (EMG) of biceps femoris were measured. EMG was on average higher during the NHE (134.3% of the MEVC). During the forward fall of the NHE, the angle at which a sharp increase in downward velocity occurred varied between 47.9 and 80.5 deg, while the peak knee angular velocity (pVelocity) varied between 47.7 and 132.8 deg s^?1. A significant negative correlation was found between pVelocity and peak EMG (r = ?0.62, p < 0.01) and EMG at 45 deg (r = ?0.75, p < 0.01) expressed as a percentage of peak MEVC EMG. Some of the variables analyzed exhibited good to excellent levels of intra- and inter-session reliability. This type of analysis could be used to indirectly monitor the level of eccentric strength of the hamstring muscles while performing the NHE and potentially any training- or injury-related changes.     

Postural reactions following forward platform perturbation in young,middle-age,and old adults

The aim of the study was to examine how individuals of different ages react to forward balance perturbations. Thirty-six volunteers, divided into four groups [young (YA), middle-age (MA₄₀ and MA₅₀), and old (OA) adults], stood on a platform that was either kept stationary, moved backward, or moved forward. EMG onset, EMG time-to-peak, iEMG, and agonist–antagonist co-activation, as well as cumulative angular excursion, maximum center of mass (CM) backward displacement, and CM time-to-reversal were assessed after forward translations. Postural synergies were assessed using principal component analysis (PCA). The results showed that OA activated their muscles later than YA [TA = 25 ms, RF = 17 ms] and OA and MA₅₀ reached the peak of activation later than YA [MA₅₀:TA = 23 ms, RF = 32 ms, OA:TA = 28 ms, RF = 21 ms]. Moreover, OA kept a higher level of activation longer than all younger groups. No differences among groups were observed in co-activation, kinematic, and PCA variables. We conclude that changes in temporal EMG patterns can be seen in the fifth decade. However, such changes have no effect on the CM horizontal displacement and CM time-to-reversal after perturbation, which cannot be justified by the use of different postural synergies, suggesting that temporal aspects of muscle activation could play a minor role in controlling excessive CM displacements after perturbations.     

Spatial reorganisation of muscle activity correlates with change in tangential force variability during isometric contractions

The aim of this study was to quantify the effects of spatial reorganisation of muscle activity on task-related and tangential components of force variability during sustained contractions. Three-dimensional forces were measured from isometric elbow flexion during submaximal contractions (50 s, 5–50% of maximal voluntary contraction (MVC)) and total excursion of the centre of pressure was extracted. Spatial electromyographic (EMG) activity was recorded from the biceps brachii muscle. The centroids of the root mean square (RMS) EMG and normalised mutual information (NMI) maps were computed to assess spatial muscle activity and spatial relationship between EMG and task-related force variability, respectively. Result showed that difference between the position of the centroids at the beginning and at the end of the contraction of the RMS EMG and the NMI maps were different in the medial–lateral direction (P < 0.05), reflecting that muscle regions modulate their activity without necessarily modulating the contribution to the task-related force variability over time. Moreover, this difference between shifts of the centroids was positively correlated with the total excursion of the centre of pressure at the higher levels of contractions (>30% MVC, R² > 0.30, P < 0.05), suggesting that changes in spatial muscle activity could impact on the modulation of tangential forces. Therefore, within-muscle adaptations do not necessarily increase force variability, and this interaction can be quantified by analysing the RMS EMG and the NMI map centroids.     

Repeated bout effect is absent in resistance trained men: An electromyographic analysis

A prior bout of exercise is well known to confer protection from subsequent eccentric bouts (i.e. repeated bout effect; RBE), which may be fostered through neural adaptations, specifically a shift in the frequency content of the surface electromyogram (EMG). It is currently not clear whether chronically resistance trained men are capable of a RBE driven by neural adaptations. Eleven resistance trained men (23.5 ± 3.4 yrs) performed 100 eccentric actions of the barbell bench press exercise, followed by an equivalent bout 14 days later. Indirect markers of muscle damage (i.e. force production, soreness) along with surface EMG were measured before and through 48 h of recovery. Median frequency and maximal isometric force demonstrated time main effects (p > 0.05), but no RBE. A prior bout of eccentric exercise does not confer a RBE for indirect markers of muscle injury or elicit changes in the frequency content of the EMG signal in resistance trained men.     

The upper and lower segments of subscapularis muscle have different roles in glenohumeral joint functioning

Subscapularis muscle is divided into two independent segments, upper and lower (USUB and LSUB), but the role of each segment in glenohumeral functioning is unclear. We compared the electromyographic (EMG) activity of USUB and LSUB during a variety of shoulder movements, with and without an external translation force. Intramuscular electrodes were inserted in USUB and LSUB segments of 20 adults without pathology and EMG activity was measured in stabilization trials (with and without an anterior or posterior directed force at the humerus and isometric rotations) and two shoulder positions (shoulder neutral, abduction). Maximal voluntary isometric contraction (MVIC) trials were performed in abduction, internal and external rotation of the shoulder. In MVIC trials, USUB showed higher activity during internal rotation (p = 0.03), whereas LSUB showed higher activity during external rotation (p < 0.01). In stabilization trials, the interaction effects were significant for muscle segment × condition (p < 0.01), and approached significance for muscle segment × position (p = 0.06). In the neutral position, the pattern of activity for LSUB was similar to USUB. In the abducted position the LSUB, unlike USUB, was more active during external rotation (p = 0.06) and also showed increased activity in response to the posterior directed force at the humerus (p = 0.04). Our results suggest that USUB primarily acts as an agonist for internal rotation. In contrast LSUB was particularly active in external rotation in the abducted position and demonstrated increased EMG activity in response to the posteriorly directed force at the humerus in that position, suggesting more of a role in glenohumeral stabilization.     

Neuromuscular fatigue following low-intensity dynamic exercise with externally applied vascular restriction

This study investigated neuromuscular fatigue following low-intensity resistance exercise with vascular restriction (VR) and without vascular restriction (control, CON). Fourteen males participated in two experimental trials (VR and CON) each separated by 48 h. Each participant performed two isometric maximum voluntary contractions (MVCs) before and after five sets of 20 dynamic constant external resistance leg extension exercises (DCER-EX) at 20% of one-repetition maximum (1-RM). The participants were asked to lift (1.5 s) and lower (1.5 s) the load at a constant velocity. Surface electromyography (EMG) was recorded from the vastus lateralis during MVC and DCER-EX. Twitch interpolation was used to assess the percent of maximal voluntary activation (%VA) during the MVC. During performing five sets of 20 DCER-EX, the increases (p < 0.05) in EMG amplitude and decreases (p < 0.05) in EMG mean power frequency were similar for both VR and CON. However, there were significant differences between VR and CON for MVC force, %VA, and potentiated twitch force and significant interactions for EMG amplitude. VR decreased MVC force, %VA, potentiated twitch force, and EMG amplitude more than CON. Our findings suggest that the VR-induced fatigue may have been due to a combination of peripheral (decreases in potentiated twitch) and central (decreases in %VA and EMG amplitude) fatigue.     

Effect of changes in orientation and position of external loads on trunk muscle activity and kinematics in upright standing

Forces at different heights and orientations are often carried by hands while performing occupational tasks. Trunk muscle activity and spinal loads are likely dependent on not only moments but also the orientation and height of these forces. Here, we measured trunk kinematics and select superficial muscle activity of 12 asymptomatic subjects while supporting forces in hands in upright standing. Magnitude of forces in 5 orientations (−25°, 0°, 25°, 50° and 90°) and 2 heights (20 cm and 40 cm) were adjusted to generate flexion moments of 15, 30 and 45 N m at the L5-S1 disc centre. External forces were of much greater magnitude when applied at lower elevation or oriented upward at 25°. Spinal kinematics remained nearly unchanged in various tasks.Changes in orientation and elevation of external forces substantially influenced the recorded EMG, despite similar trunk posture and identical moments at the L5-S1. Greater EMG activity was overall recorded under larger forces albeit constant moment. Increases in the external moment at the L5-S1 substantially increased EMG in extensor muscles (p < 0.001) but had little effect on abdominals; e.g., mean longissimus EMG for all orientations increased by 38% and 75% as the moment level altered from 15 N m to 30 N m and to 45 N m while that in the rectus abdominus increased only by 2% and 4%, respectively. Under 45 N m moment and as the load orientation altered from 90° to 50°, 25°, 0° and −25°, mean EMG dropped by 3%, 12%, 12% and 1% in back muscles and by 17%, 17%, 19% and 13% in abdominals, respectively. As the load elevation increased from 20 cm to 40 cm, mean EMG under maximum moment decreased by 21% in back muscles and by 17% in abdominals.Due to the lack of EMG recording of deep lumbar muscles, changes in relative shear/compression components and different net moments at cranial discs despite identical moments at the caudal L5-S1 disc, complementary model studies are essential for a better comprehension of neuromuscular strategies in response to alterations in load height and orientation.     

Landing from different heights: Biomechanical and neuromuscular strategies in trained gymnasts and untrained prepubescent girls

The purpose of this study was to examine the biomechanics of the lower limb, during landing in female prepubertal gymnasts and prepubertal untrained girls, aged 9–12 years. Ten healthy participants were included in each group and performed five landings from 20, 40, and 60 cm. Kinematics, ground reaction forces (GRF) and electromyogram (EMG) from the lateral gastrocnemius, tibialis anterior, and vastus lateralis are presented. Gymnasts had higher vertical GRF and shorter braking phase during landing. Compared to untrained girls, gymnasts exhibited for all examined drop heights more knee flexion before and at ground contact, but less knee flexion at maximum knee flexion position. Especially when increasing drop heights the gymnasts activated their examined muscles earlier, and generally they had higher pre- and post landing EMG amplitudes normalized to the peak EMG at 60 cm drop height. Furthermore, gymnasts had lower antagonist EMG for the tibialis anterior compared to untrained girls, especially when landing from higher heights. It is concluded that the landing strategy preferred by gymnasts is influenced by long-term and specialized training and induces a stiffer landing pattern. This could have implications in injury prevention, which requires further investigation.     

The minimum number of contractions required to examine the EMG amplitude versus isometric force relationship for the vastus lateralis and vastus medialis

Studies have demonstrated that the electromyographic (EMG) amplitude versus submaximal isometric force relationship is relatively linear. The purpose of this investigation was to determine the minimum number of contractions required to study this relationship. Eighteen men (mean age = 23 years) performed isometric contractions of the leg extensors at 10–90% of the maximum voluntary contraction (MVC) in 10% increments while surface EMG signals were detected from the vastus lateralis and vastus medialis. Linear regression was used to determine the coefficient of determination, slope coefficient, and y-intercept for each muscle and force combination with successively higher levels included in the model (i.e., 10–30%, … 10–90% MVC). For the slope coefficients, there was a main effect for force combination (P < .001). The pairwise comparisons showed there was no difference from 10–60% through 10–90% MVC. For the y-intercepts, there were main effects for both muscle (vastus lateralis [4.3 μV RMS] > vastus medialis [−3.7 μV RMS]; P = .034) and force combination (P < .001), with similar values shown from 10–50% through 10–90% MVC. The linearity of the absolute EMG amplitude versus isometric force relationship for the vastus lateralis and vastus medialis suggests that investigators may exclude high force contractions from their testing protocol.     

What gives Bolt the edge—A.V. Hill knew it already!

The 100 m is the blue ribboned event of world athletics competitions, with the winner crowned as the fastest human on Earth. Currently that fastest human is Usain Bolt, who covers 100 m in 9.58 s, achieving an average velocity of 10.43 m s^?1. Bolt is a phenomenal athlete, but what is his trick? Using Hills model, relating muscle force and heat liberation to shortening velocity, we propose that Bolt is at an advantage in relative power development and biomechanical efficiency compared to his contemporaries.     

Effect of pain location on spatial reorganisation of muscle activity

IntroductionWe aimed to determine whether the changes in muscle activity (in terms of both gross electromyography (EMG) and motor unit (MU) discharge characteristics) observed during pain are spatially organized with respect to pain location within a muscle which is the main contributor of the task.MethodsSurface and fine-wire EMG was recorded during matched low-force isometric plantarflexion from soleus (from four quadrants with fine-wire EMG and from the medial/lateral sides with surface EMG), both gastrocnemii heads, peroneus longus, and tibialis anterior. Four conditions were tested: two control conditions that each preceded contractions with pain induced in either the lateral (Pain_L) or medial (Pain_M) side of soleus.ResultsNeither the presence (p = 0.28) nor location (p = 0.19) of pain significantly altered gross muscle activity of any location (lateral/medial side of soleus, gastrocnemii, peroneus longus and tibialis anterior). Group data from 196 MUs show redistribution of MU activity throughout the four quadrants of soleus, irrespective of pain location. The significant decrease of MU discharge rate during pain (p < 0.0001; Pain_L: 7.3 ± 0.9–6.9 ± 1.1 Hz, Pain_M: 7.0 ± 1.1 to 6.6 ± 1.1 Hz) was similar for all quadrants of the soleus (p = 0.43), regardless of the pain location (p = 0.98). There was large inter-participant variation in respect to the characteristics of the altered MU discharge with pain.ConclusionResults from both surface and fine-wire EMG recordings do not support the hypothesis that muscle activity is reorganized in a simple systematic manner with respect to pain location.