Trunk muscle activation and associated lumbar spine joint shear forces under different levels of external forward force applied to the trunk.

High anterior intervertebral shear loads could cause low back injuries and therefore the neuromuscular system may actively counteract these forces. This study investigated whether, under constant moment loading relative to L3L4, an increased externally applied forward force on the trunk results in a shift in muscle activation towards the use of muscles with more backward directed lines of action, thereby reducing the increase in total joint shear force. Twelve participants isometrically resisted forward forces, applied at several locations on the trunk, while moments were held constant relative to L3L4. Surface EMG and lumbar curvature were measured, and an EMG-driven muscle model was used to calculate compression and shear forces at all lumbar intervertebral joints. Larger externally applied forward forces resulted in a flattening of the lumbar lordosis and a slightly more backward directed muscle force. Furthermore, the overall muscle activation increased. At the T12L1 to L3L4 joint, resulting joint shear forces remained small (less than 200N) because the average muscle force pulled backward relative to those joints. However, at the L5S1 joint the average muscle force pulled the trunk forward so that the increase in muscle force with increasing externally applied forward force caused a further rise in shear force (by 102.1N, SD=104.0N), resulting in a joint shear force of 1080.1N (SD=150.4N) at 50Nm moment loading. It is concluded that the response of the neuromuscular system to shear force challenges tends to increase rather than reduce the shear loading at the lumbar joint that is subjected to the highest shear forces.     

No difference in 1RM strength and muscle activation during the barbell chest press on a stable and unstable surface

Exercise or Swiss balls are increasingly being used with conventional resistance exercises. There is little evidence supporting the efficacy of this approach compared to traditional resistance training on a stable surface. Previous studies have shown that force output may be reduced with no change in muscle electromyography (EMG) activity while others have shown increased muscle EMG activity when performing resistance exercises on an unstable surface. This study compared 1RM strength, and upper body and trunk muscle EMG activity during the barbell chest press exercise on a stable (flat bench) and unstable surface (exercise ball). After familiarization, 13 subjects underwent testing for 1RM strength for the barbell chest press on both a stable bench and an exercise ball, each separated by at least 7 days. Surface EMG was recorded for 5 upper body muscles and one trunk muscle from which average root mean square of the muscle activity was calculated for the whole 1RM lift and the concentric and eccentric phases. Elbow angle during each lift was recorded to examine any range-of-motion differences between the two surfaces. The results show that there was no difference in 1RM strength or muscle EMG activity for the stable and unstable surfaces. In addition, there was no difference in elbow range-of-motion between the two surfaces. Taken together, these results indicate that there is no reduction in 1RM strength or any differences in muscle EMG activity for the barbell chest press exercise on an unstable exercise ball when compared to a stable flat surface. Moreover, these results do not support the notion that resistance exercises performed on an exercise ball are more efficacious than traditional stable exercises.     

Shoulder muscular activity during isometric three-point kneeling exercise on stable and unstable surfaces

The purpose of this study was to determine if performing isometric 3-point kneeling exercises on a Swiss ball influenced the isometric force output and EMG activities of the shoulder muscles when compared with performing the same exercises on a stable base of support. Twenty healthy adults performed the isometric 3-point kneeling exercises with the hand placed either on a stable surface or on a Swiss ball. Surface EMG was recorded from the posterior deltoid, pectoralis major, biceps brachii, triceps brachii, upper trapezius, and serratus anterior muscles using surface differential electrodes. All EMG data were reported as percentages of the average root mean square (RMS) values obtained in maximum voluntary contractions for each muscle studied. The highest load value was obtained during exercise on a stable surface. A significant increase was observed in the activation of glenohumeral muscles during exercises on a Swiss ball. However, there were no differences in EMG activities of the scapulothoracic muscles. These results suggest that exercises performed on unstable surfaces may provide muscular activity levels similar to those performed on stable surfaces, without the need to apply greater external loads to the musculoskeletal system. Therefore, exercises on unstable surfaces may be useful during the process of tissue regeneration.     

Co-activation alters the linear versus non-linear impression of the EMG-torque relationship of trunk muscles

The use of electromyographic signals in the modeling of muscle forces and joint loads requires an assumption of the relationship between EMG and muscle force. This relationship has been studied for the trunk musculature and been shown to be predominantly non-linear, with more EMG producing less torque output at higher levels of activation. However, agonist-antagonist muscle co-activation is often substantial during trunk exertions, yet has not been adequately accounted for in determining such relationships. The purpose of this study was to revisit the EMG-moment relationship of the trunk recognizing the additional moment requirements necessitated due to antagonist muscle activity. Eight participants generated a series of isometric ramped trunk flexor and extensor moment contractions. EMG was recorded from 14 torso muscles, and the externally resisted moment was calculated. Agonist muscle moments (either flexor or extensor) were estimated from an anatomically detailed biomechanical model of the spine and fit to: the externally calculated moment alone; the externally calculated moment combined with the antagonist muscle moment. When antagonist activity was ignored, the EMG-moment relationship was found to be non-linear, similar to previous work. However, when accounting for the additional muscle torque generated by the antagonist muscle groups, the relationships became, in three of the four conditions, more linear. Therefore, it was concluded that antagonist muscle co-activation must be included when determining the EMG-moment relationship of trunk muscles and that previous impressions of non-linear EMG-force relationships should be revisited.     

Trunk muscle activation during dynamic weight-training exercises and isometric instability activities

The purpose of this study was to examine the extent of activation in various trunk muscles during dynamic weight-training and isometric instability exercises. Sixteen subjects performed squats and deadlifts with 80% 1 repetition maximum (1RM), as well as with body weight as resistance and 2 unstable calisthenic-type exercises (superman and sidebridge). Electromyographic (EMG) activity was measured from the lower abdominals (LA), external obliques (EO), upper lumbar erector spinae (ULES), and lumbar-sacral erector spinae (LSES) muscle groups. Results indicated that the LSES EMG activity during the 80% 1RM squat significantly exceeded 80% 1RM deadlift LSES EMG activity by 34.5%. The LSES EMG activity of the 80% 1RM squat also exceeded the body weight squat, deadlift, superman, and sidebridge by 56, 56.6, 65.5, and 53.1%, respectively. The 80% 1RM deadlift ULES EMG activity significantly exceeded the 80% 1RM squat exercise by 12.9%. In addition, the 80% 1RM deadlift ULES EMG activity also exceeded the body weight squat, deadlift, superman, and sidebridge exercises by 66.7, 65.5, 69.3, and 68.6%, respectively. There were no significant changes in EO or LA activity. Therefore, the augmented activity of the LSES and ULES during 80% 1RM squat and deadlift resistance exercises exceeded the activation levels achieved with the same exercises performed with body weight and selected instability exercises. Individuals performing upright, resisted, dynamic exercises can achieve high trunk muscle activation and thus may not need to add instability device exercises to augment core stability training.     

Electromyographic comparison of standard and modified closed-chain isometric knee extension exercises

The purpose of this study was to compare electromyographic (EMG) activity during open kinetic chain (OKC) and a modified closed kinetic chain (MCKC) knee extension exercises. Both OKC and closed kinetic chain (CKC) exercises provide benefits when devising conditioning programs; however, there are no exercises that combine the benefits of both exercises. Subjects performed maximum isometric knee extensions for both traditional OKC and MCKC knee extension exercises. Surface electrodes were placed on 8 lower-extremity muscles. One second of integrated EMG activity followed 95% maximal knee extension force. The following muscles demonstrated greater EMG activity during the MCKC vs. the OKC knee extension exercises: vastus medialis, medial hamstring, lateral hamstrings, and gluteus maximus. There was no difference between force output between the 2 conditions. This study demonstrates that modifications to traditional OKC exercises demonstrate some characteristics of CKC exercises, and therefore provide another avenue of rehabilitation or strengthening.     

Contribution of the musculature to rotatory laxity and torsional stiffness at the knee

The relationships between the mean rectified EMG from two muscle groups crossing the knee joint and the rotational stiffness and laxity about the longitudinal axis of the lower leg were investigated. The EMG signals from three of the quadricep muscle group and two of the hamstring muscle group were monitored using surface electrodes. Each subject sustained self-induced muscle activity from specific muscle combinations while the foot was twisted internally and externally by the researcher. Joint rotation was measured using an electrogoniometer. Analyses of the data showed increased joint stiffness with increased numbers of active muscles. The stiffness measurements ranged from 0.16 to 2.54 Nm degree-1 depending upon the combination of active muscles. The stiffness measured in different tests were very repeatable with standard deviations ranging from 0.02 to 0.25 Nm degree-1. Increases in joint stiffness of over 400% by activation of these muscles were measured.     

Direction-specific recruitment of rotator cuff muscles during bench press and row

Recent studies indicate that rotator cuff (RC) muscles are recruited in a reciprocal, direction-specific pattern during shoulder flexion and extension exercises. The main purpose of this study was to determine if similar reciprocal RC recruitment occurs during bench press (flexion-like) and row (extension-like) exercises. In addition, shoulder muscle activity was comprehensively compared between bench press and flexion; row and extension; and bench press and row exercises. Electromyographic (EMG) activity was recorded from 9 shoulder muscles sites in 15 normal volunteers. All exercises were performed at 20, 50 and 70% of subjects’ maximal load. EMG data were normalized to standard maximal voluntary contractions. Infraspinatus activity was significantly higher than subscapularis during bench press, with the converse pattern during the row exercise. Significant differences in activity levels were found in pectoralis major, deltoid and trapezius between the bench press and flexion exercises and in lower trapezius between the row and extension exercises. During bench press and row exercises, the recruitment pattern in each active muscle did not vary with load. During bench press and row exercises, RC muscles contract in a reciprocal direction-specific manner in their role as shoulder joint dynamic stabilizers to counterbalance antero-posterior translation forces.     

Influence of muscle activity on the forces in the femur: An in vivo study

Experiments were performed on two patients with custom-made instrumented massive proximal femoral prostheses implanted after tumour resection. In vivo axial forces transmitted along the prostheses were telemetered during level walking, single- and double-leg stance, and isometric exercises of the hip muscles. These activities varied the lever arms available to the external loads: minimum for double-leg stance and maximum for hip isometric exercises. Kinematic, force plate, EMG and telemetered force data were recorded simultaneously. The force magnification ration (FMR; the ratio of the telemetered axial force to the external force) was calculated. The FMRs ranged from 1.3 (during double-leg stance) to 29.8 (during abductors test), indicating that a major part of the axial force in the long bones is a response to muscle activity, the strength of which depends on the lever arms available to the external loads. From these results, it was shown that the bulk of the bending moment along limbs is transmitted by a combination of tensile forces in muscles and compressive forces in bones, so moments transmitted by the bones are smaller than the limb moments. It was concluded that appropriate simulation of muscle forces is important in experimental or theoretical studies of load transmission along bones.     

Elevation and orientation of external loads influence trunk neuromuscular response and spinal forces despite identical moments at the L5–S1 level

A wide range of loading conditions involving external forces with varying magnitudes, orientations and locations are encountered in daily activities. Here we computed the effect on trunk biomechanics of changes in force location (two levels) and orientation (5 values) in 4 subjects in upright standing while maintaining identical external moment of 15 Nm, 30 N m or 45 Nm at the L5–S1. Driven by measured kinematics and gravity/external loads, the finite element models yielded substantially different trunk neuromuscular response with moderate alterations (up to 24% under 45 Nm moment) in spinal loads as the load orientation varied. Under identical moments, compression and shear forces at the L5–S1 as well as forces in extensor thoracic muscles progressively decreased as orientation of external forces varied from downward gravity (90°) all the way to upward (−25°) orientation. In contrast, forces in local lumbar muscles followed reverse trends. Under larger horizontal forces at a lower elevation, lumbar muscles were much more active whereas extensor thoracic muscle forces were greater under smaller forces at a higher elevation. Despite such differences in activity pattern, the spinal forces remained nearly identical (<6% under 45 Nm moment). The published recorded surface EMG data of extensor muscles trend-wise agreed with computed local muscle forces as horizontal load elevation varied but were overall different from results in both local and global muscles when load orientation altered. Predictions demonstrate the marked effect of external force orientation and elevation on the trunk neuromuscular response and spinal forces and questions attempts to estimate spinal loads based only on consideration of moments at a spinal level.     

The effect of increasing resistance on trunk muscle activity during extension and flexion exercises on training devices

Although progressive resistance training of trunk muscles on devices is very common, today, the effects of increasing resistance on trunk muscle activity during dynamic extension and flexion movements on training devices have not been reported yet. Thirty healthy subjects participated in maximal isometric and submaximal dynamic (at 30%, 50% and 70% of maximum mean torque (MMT)) extension and flexion exercises on Tergumed lumbar training devices. The normalized (as a percentage of maximal voluntary isometric contractions (MVIC)) electromyographic activity of 16 abdominal and back muscles was investigated. The results of the present study indicated that in general, with increasing resistance from 30% MMT to 50% MMT and 70% MMT, the activity of all back muscles during the extension exercises and the activity of all abdominal muscles during the flexion exercises increased significantly. To train strength (>60% of MVIC), low intensities (30% and 50% MMT) appeared sufficient to affect the back muscles, but for the abdominals higher resistance (70% MMT) was required. In contrast to the other back muscles, the lumbar multifidus demonstrated high activity levels during both the extension and the flexion exercises. As the lumbar multifidus is demonstrated to be an important muscle in segmental stabilization of the lumbar spine, this finding may help in understanding the efficacy of rehabilitation programs using specific training devices.     

Evaluation of fatigue of respiratory and lower limb muscles during prolonged aerobic exercise

The respiratory muscles may fatigue during prolonged exercises and thereby become a factor that limits extreme physical activity. The aim of the current study was to determine whether respiratory muscle fatigue imposes a limitation on extreme physical activity of well-trained young men. Electromyography (EMG) signals of respiratory (external intercostal and sternomastoid) and calf muscles (gastrocnemius) were measured (N = 8) during 1 hr of treadmill marching at a speed of 8 km/hr with and without a 15 kg backpack. The root mean square (RMS) and the mean power frequency of the EMG signals were evaluated for calculating fatigue indices. The EMG RMS revealed that the respiratory and calf muscles did not fatigue during the marching without a backpack load. The study did show, however, a significant rise in the EMG values when a backpack was carried with respect to the no-load condition (p < .05), which suggests that respiratory muscles should be trained in military recruits who are required to carry loaded backpacks while marching.     

Validation of a biodynamic model of pushing and pulling.

Pushing and pulling during manual material handling can increase the compressive forces on the lumbar disc region while creating high shear forces at the shoe-floor interface. A sagittal plane dynamic model derived from previous biomechanical models was developed to predict L5/S1 compressive force and required coefficients of friction during dynamic cart pushing and pulling. Before these predictions could be interpreted, however, it was necessary to validate model predictions against independently measured values of comparable quantities. This experiment used subjects of disparate stature and body mass, while task factors such as cart resistance and walking speed were varied. Predicted ground reaction forces were compared with those measured by a force platform, with correlations up to 0.67. Predicted erector spinae and rectus abdominus muscle forces were compared with muscle forces derived from RMS-EMGs of the respective muscle groups, using a static force build-up regression relationship to transform the dynamic RMS-EMGs to trunk muscle forces. Although correlations were low, this was attributed in part to the use of surface EMG on subjects of widely varied body mass. The biodynamic model holds promise as a tool for analysis of actual industrial pushing and pulling tasks, when carefully applied.     

Electromyographic response of the abdominal musculature to varying abdominal exercises

This study examined the electromyographic (EMG) response of the upper rectus abdominis (URA), lower rectus abdominis (LRA), internal obliques (IOs), external obliques (EOs), and the rectus femoris (RF) during various abdominal exercises (crunch, supine V-up, prone V-up on ball, prone V-up on slide board, prone V-up on TRX, and prone V-up on Power Wheel). The subjects (n = 21) performed an isometric contraction of the abdominal musculature while performing these exercises. Testing revealed no statistically significant differences between any of the exercises with respect to the EOs, the URA, or the LRA. However, when examining the IO muscle, the supine V-up exercise displayed significantly greater muscle activity than did the slide exercise. In addition, EMG activity of the RF during the crunch was significantly less than in any of the other 5 exercises. These results indicate that when performing isometric abdominal exercises, non-equipment-based exercises stressed the abdominal muscles similarly to equipment-based exercises. Based on the findings of the current study, the benefit of training the abdominal musculature in an isometric fashion using commercial equipment could be called into question.     

Lumbar muscles recruitment during resistance exercise for upper limbs

The aim of this study was to evaluate the EMG activity of lumbar multifidus (MU), longissimus thoracis (LT) and iliocostalis (IC) muscles during an upper limb resistance exercise (biceps curl). Ten healthy males performed maximal voluntary isometric contraction (MVC) of the trunk extensors, after this, the biceps curl exercise was executed at 25%, 30%, 35% and 40% one repetition maximum during 1 min, with 10 min rest between them. EMG root mean square (RMS) and median frequency (MFreq) were calculated for each lifting and lowering of the bar during the exercise bouts, to calculate slopes and intercepts. The results showed increases in the RMS and decreases in the MFreq slopes. RMS slopes were no different between muscles, indicating similar fatigue process along the exercise irrespective of the load level. MU and LT presented higher RMS irrespective of the load level, which can be related to the specific function during the standing position. On the other hand, IC and MU presented higher MFreq intercepts compared to LT, demonstrating possible differences in the muscle fiber conduction velocity of these muscles. These findings suggest that trunk muscles are differently activate during upper limb exercises, and the fatigue process affects the lumbar muscles similarly.     

Electromyographic normalization procedures for determining exercise intensity of closed chain exercises for strengthening the quadriceps femoris muscles

The purpose of this study was to compare the electromyographic (EMG) amplitudes of the quadriceps femoris (QF) muscles during a maximum voluntary isometric contraction (MVIC) to submaximal and maximal dynamic concentric contractions during active exercises. A secondary purpose was to provide information about the type of contraction that may be most appropriate for normalization of EMG data if one wants to determine if a lower extremity closed chain exercise is of sufficient intensity to produce a strengthening response for the QF muscles. Sixty-eight young healthy volunteers (39 female, 29 male) with no lower extremity pain or injury participated in the study. Surface electrodes recorded EMG amplitudes from the vastus medialis obliquus (VMO), rectus femoris (RF), and vastus lateralis (VL) muscles during 5 different isometric and dynamic concentric exercises. The last 27 subjects performed an additional 4 exercises from which a second data set could be analyzed. Maximum isokinetic knee extension and moderate to maximum closed chain exercises activated the QF significantly more than a MVIC. A 40-cm. lateral step-up exercise produced EMG amplitudes of the QF muscles of similar magnitude as the maximum isokinetic knee extension exercises and would be an exercise that could be considered for strengthening the QF muscles. Most published EMG studies of exercises for the QF have been performed by comparing EMG amplitudes during dynamic exercises to a MVIC. This procedure can lead one to overestimate the value of a dynamic exercise for strengthening the QF muscles. We suggest that when studying the efficacy of a dynamic closed chain exercise for strengthening the QF muscles, the exercise be normalized to a dynamic maximum muscle contraction such as that obtained with knee extension during isokinetic testing.     

Variation of muscle recruitment during exercises performed below horizontal arm elevation that target the lower trapezius: A repeated measures cross-sectional study on asymptomatic individuals

The gold standard exercise for recruitment of the lower trapezius is the Y prone exercise which is performed above 90° of shoulder elevation. However, clinicians often prescribe exercises that avoid high elevation postures during early stages of rehabilitation. Comparatively little data exists on relative muscle recruitment during lower arm elevation exercises. This study examined the EMG activity of four shoulder girdle muscles during four exercises accomplished below 90° of shoulder elevation and compared them to the Y prone while considering sex effects. Variance across exercises of the ratio between upper trapezius and lower trapezius was also explored. 32 healthy participants completed standardized muscle-specific MVCs and two repetitions of each exercise. The side lying external rotation and the wall slide exercises produced the highest peak EMG for the lower trapezius, both 33 and 29% lower than the Y Prone. For the upper trapezius to lower trapezius ratio, the side lying external rotation elicited the lowest value, followed by the Y prone and wall slide (53 and 59% respectively higher). Sex influenced some EMG values, typically interacting with exercise type. Thus, side lying external rotation and the wall slide are recommended for targeting the lower trapezius muscle during early rehabilitation.     

Methodological aspects of SEMG recordings for force estimation – A tutorial and review

Insight into the magnitude of muscle forces is important in biomechanics research, for example because muscle forces are the main determinants of joint loading. Unfortunately muscle forces cannot be calculated directly and can only be measured using invasive procedures. Therefore, estimates of muscle force based on surface EMG measurements are frequently used. This review discusses the problems associated with surface EMG in muscle force estimation and the solutions that novel methodological developments provide to this problem. First, some basic aspects of muscle activity and EMG are reviewed and related to EMG amplitude estimation. The main methodological issues in EMG amplitude estimation are precision and representativeness. Lack of precision arises directly from the stochastic nature of the EMG signal as the summation of a series of randomly occurring polyphasic motor unit potentials and the resulting random constructive and destructive (phase cancellation) superimpositions. Representativeness is an issue due the structural and functional heterogeneity of muscles. Novel methods, i.e. multi-channel monopolar EMG and high-pass filtering or whitening of conventional bipolar EMG allow substantially less variable estimates of the EMG amplitude and yield better estimates of muscle force by (1) reducing effects of phase cancellation, and (2) adequate representation of the heterogeneous activity of motor units within a muscle. With such methods, highly accurate predictions of force, even of the minute force fluctuations that occur during an isometric and isotonic contraction have been achieved. For dynamic contractions, EMG-based force estimates are confounded by the effects of muscle length and contraction velocity on force producing capacity. These contractions require EMG amplitude estimates to be combined with modeling of muscle contraction dynamics to achieve valid force predictions.     

Surface EMG amplitude and frequency dependence on exerted force for the upper trapezius muscle: a comparison between right and left sides

Surface electromyographic (EMG) amplitude and mean power frequency (MPF) were used to study the isometric muscular activity of the right versus the left upper trapezius muscles in 14 healthy right-handed women. The EMG activity was recorded simultaneously with force signals during a 10-15 s gradually increasing exertion of force, up to maximal force. Only one side at a time was tested. On both sides there was a significant increase in EMG amplitude (microV) during the gradually increasing force from 0% to 100% maximal voluntary contraction (MVC). The right trapezius muscle showed significantly less steep slopes for regression of EMG amplitude versus force at low force levels (0%-40% MVC) compared intra-individually with high force levels (60%-100% MVC). This was not found for the left trapezius muscle. At 40% MVC a significantly lower MPF value was found for the right trapezius muscle intra-individually compared with the left. An increase in MPF between 5% and 40% MVC was statistically significant when both sides were included in the test. The differences in EMG activity between the two sides at low force levels could be due to more slow-twitch (type I fibres) motor unit activity in the right trapezius muscles. It is suggested that this is related to right-handed activity.     

Relation between the ankle joint angle and the maximum isometric force of the toe flexor muscles

The purpose of this study was to investigate the relationships between the ankle joint angle and maximum isometric force of the toe flexor muscles. Toe flexor strength and electromyography activity of the foot muscles were measured in 12 healthy men at 6 different ankle joint angles with the knee joint at 90 deg in the sitting position. To measure the maximum isometric force of the toe flexor muscles, subjects exerted maximum force on a toe grip dynamometer while the activity levels of the intrinsic and extrinsic plantar muscles were measured. The relation between ankle joint angle and maximum isometric force of the toe flexor muscles was determined, and the isometric force exhibited a peak when the ankle joint was at 70–90 deg on average. From this optimal neutral position, the isometric force gradually decreased and reached its nadir in the plantar flexion position (i.e., 120 deg). The EMG activity of the abductor hallucis (intrinsic plantar muscle) and peroneus longus (extrinsic plantar muscle) did not differ at any ankle joint angles. The results of this study suggest that the force generation of toe flexor muscles is regulated at the ankle joint and that changes in the length-tension relations of the extrinsic plantar muscle could be a reason for the force-generating capacity at the metatarsophalangeal joint when the ankle joint angle is changed.