Isometric axial rotation of the trunk in the neutral posture

Lumbar muscle adaptations to external perturbations are modulated by trunk posture

PURPOSE

To investigate if the recruitment of different regions within the lumbar extensor muscles in response to unexpected perturbations depends on trunk posture.

METHODS

In a semi-seated posture, healthy adult participants experienced unexpected posterior-anterior trunk perturbations in three different postures: neutral, trunk flexion and left trunk rotation. High-density surface electromyography was used to identify the regional distribution of activation within the lumbar erector spinae muscles. The effect of posture and side (left vs right) on muscle activity and centroid coordinates was investigated at baseline and in response to perturbations.

RESULTS

Higher muscle activity was observed in trunk flexion compared to neutral and rotation postures at baseline (multiple p < 0.001) and in response to the perturbation (multiple p < 0.01). At baseline, the centroid of the electromyographic amplitude distribution was localized more medially in trunk flexion compared to trunk neutral posture (p = 0.003), while activation was localized more laterally in response to the perturbation (multiple p < 0.05). When the trunk was rotated, the electromyographic amplitude distribution was localized more cranially on the left than the right side, both at baseline (p = 0.001) and in response to the perturbation (p = 0.001). Finally, a more lateral location of the centroid on the left side in rotation compared to neutral posture was observed in response to the perturbation (multiple p < 0.001).

CONCLUSIONS

Regional differences in the distribution of electromyographic amplitude indicate that different muscle regions were recruited in different trunk postures and in response to perturbations, possibly based on regional mechanical advantage of the erector spinae muscle fibers.

Angular velocity affects trunk muscle strength and EMG activation during isokinetic axial rotation

OBJECTIVE

To evaluate trunk muscle strength and EMG activation during isokinetic axial rotation at different angular velocities.

METHOD

Twenty-four healthy young men performed isokinetic axial rotation in right and left directions at 30, 60, and 120 degrees per second angular velocity. Simultaneously, surface EMG was recorded on external oblique (EO), internal oblique (IO), and latissimus dorsi (LD) bilaterally.

RESULTS

In each direction, with the increase of angular velocity, peak torque decreased, whereas peak power increased. During isokinetic axial rotation, contralateral EO as well as ipsilateral IO and LD acted as primary agonists, whereas, ipsilateral EO as well as contralateral IO and LD acted as primary antagonistic muscles. For each primary agonist, the root mean square values decreased with the increase of angular velocity. Antagonist coactiviation was observed at each velocity; however, it appears to be higher with the increase of angular velocity.

CONCLUSION

Our results suggest that velocity of rotation has great impact on the axial rotation torque and EMG activity. An inverse relationship of angular velocity was suggested with the axial rotation torque as well as root mean square value of individual trunk muscle. In addition, higher velocity is associated with higher coactivation of antagonist, leading to a decrease in torque with the increase of velocity.

EMG in rotation-flexion of the torso

The objective of this study was to determine the magnitude and phasic relationship of the torso muscles in rotation-flexion of varying degree of asymmetries of the trunk. Nineteen normal young subjects (7 males and 12 females) were stabilized on a posture stabilizing platform and instructed to assume a flexed and right rotated posture. A combination 20°, 40° and 60° of rotation and 20°, 40° and 60° of flexion resulted in nine postures. These postures were assumed in a random order. The subjects were asked to exert their maximal voluntary isometric contraction (MVC) in the plane of rotation of the posture assumed for a period of 5s. The surface EMG from the external and internal obliques, rectus abdominis, latissimus dorsi and erector spinae at the 10th thoracic and 3rd lumbar vertebral levels was recorded. The abdominal muscles had the least response at 40° of flexion, the dorsal muscles had the highest magnitude. With increasing right rotation, the left external oblique continued to decrease its activity. The ANOVA revealed that rotation and muscles had a significant main effect on normalized peak EMG (p<0.02) in both genders. There was a significant interaction between rotation and flexion in both genders (p<0.02) and rotation and muscle in females. The erector spinae activity was highest at 40° flexion, due to greater mechanical disadvantage and having not reached the state of flexion-relaxation. The abdominal muscle activity declined with increasing asymmetry, due to the decreasing initial muscle length. The EMG activity was significantly affected by rotation than flexion (p<0.02).

Analysis of trunk muscle activity in the side medicine-ball throw

The purpose of this study was to compare the electromyogram (EMG) activity of the trunk musculature for long throwers and short throwers during the side medicine-ball throw. The study consisted of 2 sessions. Thirty athletes performed the side medicine-ball throw (S-MBT) in the first session. The top 5 subjects (long throwers) and the worst 5 subjects (short throwers) in the performance of the S-MBT proceeded to the second session. These subjects performed the fast side medicine-ball throw (FS-MBT) and the isometric maximal trunk rotation torque (IMTRT) test after applying surface electrodes bilaterally to the following muscles: pectoralis major, rectus abdominis, external oblique, and latissimus dorsi. The value of IMTRT in the left turn in long throwers was significantly greater (p < 0.05) than that recorded for short throwers, whereas no difference was observed in the right turn. The velocities of the medicine ball using 2-kg, 4-kg, and 6-kg medicine balls were significantly different (p < 0.01-0.001) between long throwers and short throwers The examination of EMG activity in each muscle group for long throwers and short throwers revealed that the major difference between the 2 groups is EMG activity of the left external oblique. These results suggest that the EMG activity of the external oblique on either side is an important factor for FS-MBT.

Trunk rotational strength asymmetry in adolescents with idiopathic scoliosis: an observational study

Background

Recent reports have suggested a rotational strength weakness in rotations to the concave side in patients with idiopathic scoliosis. There have been no studies presenting normative values of female adolescent trunk rotational strength to which a comparison of female adolescents with idiopathic scoliosis could be made. The purpose of this study was to determine trunk rotational strength asymmetry in a group of female adolescents with AIS and a comparison group of healthy female adolescents without scoliosis.

Methods

Twenty-six healthy adolescent females served as the healthy group (HG) (average age 14 years) and fourteen otherwise healthy adolescent females with idiopathic scoliosis served as the idiopathic scoliosis group (ISG) (average age 13.5 years, average Cobb 28°). Participant's isometric trunk rotational strength was measured in five randomly ordered trunk positions: neutral, 18° and 36° of right and left pre-rotation. Rotational strength asymmetry was compared within each group and between the two groups using several different measures.

Results

The HG showed strength asymmetry in the 36° pre-rotated trunk positions when rotating towards the midline (p < 0.05). The ISG showed strength asymmetry when rotating towards the concavity of their primary curve from the neutral position (p < 0.05) and when rotating towards the concavity from the 18° (p < 0.05) and 36° (p < 0.05) concave pre-rotated positions. The ISG is significantly weaker than the HG when rotating away from the midline toward the concave (ISG)-left (HG) side from the concave/left pre-rotated 18° (p < 0.05) and 36° (p < 0.05) positions.

Conclusion

The AIS females were found to be significantly weaker when contracting toward their main curve concavity in the neutral and concave pre-rotated positions compared to contractions toward the convexity. These weaknesses were also demonstrated when compared to the group of healthy female adolescent controls. Possible mechanisms for the strength asymmetry in ISG are discussed.

Relationship between side medicine-ball throw performance and physical ability for male and female athletes

The purpose of the study was to examine the factors contributing to performance of a side medicine-ball throw (S-MBT) and a fast side medicine-ball throw (FS-MBT) and to analyze some of the factors which account for the difference in side medicine ball throw performance between the sexes. Sixteen males and ten females were evaluated by S-MBT, FS-MBT, isometric maximal trunk rotation torque (IMTRT), One repetition maximum of Parallel Squat (1RM(PS)) and Bench Press (1RM(BP)), Bench Press peak power (BP(PP)), Static Squat Jump peak power (SSJ(PP)) and vertical jump height. Males demonstrated significantly greater scores than females in all measurements. Significant correlations were observed in males, but not in females, between the distances during S-MBT and the IMTRT values (r = 0.596-0.739, P < 0.05-0.01) and the 1RM(PS) values (r = 0.683-0.725, P < 0.01). In FS-MBT performance, significant correlations were observed in males, but not in females, between the ball velocity values during FS-MBT and the IMTRT values (r = 0.611-0.687, P < 0.05-0.01), 1RM(BP) values (r = 0.596-0.655, P < 0.05-0.01) and 1RM(PS) values (r = 0.679-0.718, P < 0.01). These results suggested that the contributing factors of S-MBT and FS-MBT performance were deferent in males and females. Hence, the side medicine-ball throw test would be useful to examine the trunk rotation power of male athletes, but may have a limited potential as a predictor of trunk rotation power for female athletes.

Postural muscle tone in the body axis of healthy humans

Across the entire human body, postural tone might play its most critical role in the body's axis because the axis joins the four limbs and head into a single functioning unit during complex motor tasks as well as in static postures. Although postural tone is commonly viewed as low-level, tonic motor activity, we hypothesized that postural tone is both tonically and dynamically regulated in the human axis even during quiet stance. Our results describe the vertical distribution of postural muscle tone in the neck, trunk, and hips of standing human adults. Each subject stood blindfolded on a platform that axially rotated the neck, trunk, or pelvis at 1 degrees /s and +/-10 degrees relative to the neutral position (i.e., facing forward). The measured resistance to axial rotation was highest in the trunk and lowest in the neck and was characterized by several nonlinear features including short-range stiffness and hysteresis. In half of the subjects, axial muscle activity was relatively constant during axial rotation, and in the other half, muscle activity was modulated by lengthening and shortening reactions, i.e., decreasing activity in lengthening muscles and increasing activity in shortening muscles, respectively. Axial resistance to rotation was reduced in subjects whose muscle activity was modulated. The results indicate that axial tone is modulated sensitively and dynamically, this control originates, at least in part, from tonic lengthening and shortening reactions, and a similar type of control appears to exist for postural tone in the proximal muscles of the arm.

Static load repetition is a risk factor in the development of lumbar cumulative musculoskeletal disorder

STUDY DESIGN

In vivo feline model subjected to variable number of repetitions of a short static lumbar flexion followed by an equally long rest period.

OBJECTIVES

The purpose of this study was to determine the influence of the number of repetitions as a risk factor in promoting a cumulative low back disorder in the feline model.

SUMMARY OF BACKGROUND DATA

Epidemiologic data point out that the increased number of repetitions of static lumbar loading is a major risk factor in the development of cumulative low back disorder. Biomechanical and physiologic confirmation of the epidemiology is lacking. Recent work demonstrated that repetitive static loading results in accumulation of creep in the lumbar viscoelastic tissues, resulting in a neuromuscular disorder consisting of spasms during loading and hyperexcitability of lumbar muscles during following rest. It was also shown that the load magnitude is a major risk factor. It is hypothesized that increased number of repetitions of static load periods result in increased severity of the resulting neuromuscular disorder.

METHODS

Static lumbar flexion of 10 minutes duration followed by 10 minutes rest was repeated three times in one experimental group, six times in the second, and nine times in the third group. In all groups, the creep developing in the lumbar viscoelastic tissues as well as the reflexive EMG from the multifidus were monitored during the flexion/rest periods and throughout a 7-hour recovery period after the repetitions.

RESULTS

Creep developed and accumulated during each of the flexion/rest periods in the three experimental protocols, with larger residual creep at the end of the nine repetitions. A residual creep was still present at the end of the 7 hours of recovery allowed in each of the three groups. During the flexion/rest sessions, EMG spasms were present, and the presence of an initial hyperexcitability was detected during the 7 hours of recovery in all the preparations. The presence of a delayed hyperexcitability was revealed only in the group subjected to nine flexion/rest periods, while it was not observed in the groups subjected to three and six flexion/rest repetitions. The statistical analysis (post hoc Fisher test) performed on the normalized integrated EMG and displacement data during the recovery phase showed a significant difference between the nine repetitions group and the other two groups (P < 0.0001). The two-way ANOVA analysis revealed a significant effect of time (P < 0.005) and number of repetitions (P < 0.0001) on all considered parameters.

CONCLUSIONS

It was concluded that a cumulative neuromuscular disorder develops because of repetition of static lumbar flexion, and the severity of the disorder provoked is magnified by the number of repetitions. Despite the highly favorable 1:1 work-to-rest ratio and the 7-hour post loading rest period, a full recovery of creep was not obtained in this study.

Differential activity of regions of transversus abdominis during trunk rotation

The role of the abdominal muscles in trunk rotation is not comprehensively understood. This study investigated the electromyographic (EMG) activity of anatomically distinct regions of the abdominal muscles during trunk rotation in six subjects with no history of spinal pain. Fine-wire electrodes were inserted into the right abdominal wall; upper region of transversus abdominis (TrA), middle region of TrA, obliquus internus abdominis (OI) and obliquus externus abdominis (OE), and lower region of TrA and OI. Surface electrodes were placed over right rectus abdominis (RA). Subjects performed trunk rotation to the left and right in sitting by rotating their pelvis relative to a fixed thorax. EMG activity was recorded in relaxed supine and sitting, and during an isometric hold at end range. TrA was consistently active during trunk rotation, with the recruitment patterns of the upper fascicles opposite to that of the middle and lower fascicles. During left rotation, there was greater activity of the lower and middle regions of contralateral TrA and the lower region of contralateral OI. The upper region of ipsilateral TrA and OE were predominately active during right rotation. In contrast, there was no difference in activity of RA and middle OI between directions (although middle OI was different between directions for all but one subject). This study indicates that TrA is active during trunk rotation, but this activity varies between muscle regions. These normative data will assist in understanding the role of TrA in lumbopelvic control and movement, and the effect of spinal pain on abdominal muscle recruitment.

Force Maintenance With Submaximal Fatiguing Contractions

Whereas many definitions of fatigue include externally measurable decrements in force or performance, fatigue can be present with no change in the external output of the muscle. The maintenance of submaximal forces can be considered a compromise between neuromuscular force enhancement and competing inhibitory influences. An example of a muscle facilitatory process includes postactivation potentiation that results in an increased sensitivity to Ca++. The neuromuscular system copes with metabolic disruption and subsequent loss of force by recruiting additional motor units and increasing the firing frequency. If the contraction persists, firing frequency may decrease so as to optimize the stimulus rate with the prolonged duration of the muscle fibre action potential (muscle wisdom). The insertion of additional neural impulses into the train of stimuli can result in force potentiation (catch-like properties). Furthermore, there is evidence of neural potentiation and a dissociation of muscle activity with submaximal fatigue. Conversely, inhibition may be derived supraspinally or at the spinal level. While there may be some evidence of intrinsic motoneuronal fatigue, inhibitory afferent influences from chemical, tensile, pressure, and other factors play an important role in the competing influences on force output. Key words: postactivation potentiation, recruitment, rate coding, inhibition, catch-like properties

Ergonomics and biology of spinal rotation

Spinal rotation, though being a very common motion of the body, is poorly understood. Furthermore, this motion and the extent of its development is unique to the human. Beyond the extent of its need in common activities, spinal rotation is a destabilizating motion for an inherently unstable structure. Spinal rotation has been argued to be an essential feature for an efficient bipedal gait. Also, it provides leverage to the upper extremities in delivering a forceful impact. An artificial restriction/elimination of spinal rotation resulted in significantly shorter stride length, slower walking velocity, and higher energy consumption in walking (p < 0.05). Spinal rotation also decreases the amount of force the spinal muscles can generate (to 25% of spinal extension). However, its extensive employment in industrial activities has been associated with 60.4% of back injuries. It is further stated that the amount of scientific information currently available is inadequate to biomechanically model the spinal response in a working environment. For example, when the spine is pre-rotated, a further rotation in the direction of pre-rotation decreases the force production significantly (p < 0.01) and increases the EMG activity significantly (p < 0.01) but the pattern changes with effort in the opposite direction. This and other properties (described in the paper) render biomechanical models inadequate. Muscle activation pattern and neuromotor behaviour of spinal muscles in flexion/extension and rotation of the spine are significantly different from each other (p < 0.01). The localized fatigue in different spinal muscles in the same contraction is significantly different and has been called differential fatigue. Finally, the trunk rotation, being pivotal for bipedal locomotion has brought many back problems to the human race.