Increased Trunk Stiffness Research Articles

The effects of prolonged sitting, standing, and an alternating sit-stand pattern on trunk mechanical stiffness, trunk muscle activation and low back discomfort

Sit-stand desks continue to be a popular intervention for office work. While previous studies have reported changes in subjective measures, there is limited understanding of how sit-stand work differs from prolonged sitting or standing work, from a biomechanical standpoint. The objective of this study was to investigate the effects of prolonged sitting, prolonged standing, and a sit-stand paradigm on changes in trunk stiffness, low back discomfort, and trunk muscle activation. Twelve healthy participants performed 2 h of computer-based tasks in each protocol, on three different days. The sit-stand protocol was associated with a significant increase in trunk stiffness and a decrease in muscle activation of lumbar multifidus and longissimus thoracis pars thoracis, compared to both prolonged sitting and standing. Both sitting and standing were associated with increased low back discomfort. These findings may be worth exploring in more detail, for why alternating sit-stand patterns may help alleviate low back pain. Practitioner summary: We explored changes in objective and subjective measures related to low back discomfort following prolonged sitting, standing, and alternating sit-stand patterns. Alternating sit-stand pattern was associated with increased trunk stiffness and decreased back muscle activity. Hence, sit-stand desks may have benefits in terms of preventing/mitigating low back pain. Abbreviations: DOF: degree of freedom; EMG: electromyogram; ILL: iliocostalis lumborum pars lumborum; LTL: longissimus thoracis pars lumborum; LTT: longissimus thoracis pars thoracis; LBP: low back pain; LM: lumbar multifidus; MVEs: maximum isometric voluntary exertions; RANOVA: repeated-measure analysis of variance; RMS: root mean square

Non-specific chronic low back pain elicits kinematic and neuromuscular changes in walking and gait termination

BackgroundChronic low back pain (CLBP) is associated with an increased trunk stiffness and muscle coactivation during walking. However, it is still unclear whether CLBP individuals are unable to control neuromechanically their upper body motion during a sudden termination of gait (GT), which involves a challenging balance transition from walking to standing. Research questionDoes CLBP elicit neuromuscular and kinematic changes which are specific to walking and GT?. MethodsEleven individuals with non-specific CLBP and 11 healthy controls performed walking and sudden GT in response to an external visual cue. 3D kinematic characteristics of thorax, lumbar and pelvis were obtained, with measures of range of motion (ROM) and intra-subject variability of segmental movement being calculated. Electromyographic activity of lumbar and abdominal muscles was recorded to calculate bilateral as well as dorsoventral muscle coactivation. ResultsCLBP group reported greater transverse ROM of the lumbar segment during walking and GT compared to healthy controls. Thorax sagittal ROM was higher in CLBP than healthy participants during GT. Greater overall movement variability in the transverse plane was observed in the CLBP group while walking, whereas GT produced greater variability of lumbar frontal motion. CLBP participants showed higher bilateral lumbar coactivation compared to healthy participants after the stopping stimulus delivery during GT. SignificanceThese results suggest that CLBP can elicit a wider and more variable movement of the upper body during walking and GT, especially in the transverse plane and at lumbar level. Alterations in upper body motor control appeared to depend on task, plane of motion and segmental level. Therefore, these findings should be considered by practitioners when screening before planning specific training interventions for recovery of motor control patterns in CLBP population.

Trunk stiffness decreases and trunk damping increases with experimental low back pain

Movement adaptations to low back pain (LBP) are believed to protect the painful area. Increased trunk stiffness and decreased trunk damping have been shown in people with recurrent LBP. However, no study has examined these properties using external force perturbations to the trunk during acute LBP when protective adaptations might be expected to have most relevance.Adaptations to an acute painful stimulus via unilateral injection of hypertonic saline into the right longissimus muscle were assessed using a trunk force perturbation paradigm and a mass-spring-damper model to describe effective trunk dynamical properties. Equal weights (15% body weight) were connected to the front and back of the trunk via a cable. Either one was dropped at random to perturb the trunk. Effective trunk dynamical properties were estimated in fourteen males (mean (standard deviation) age 25 (6) years) assuming that trunk movement can be modelled as a second order linear system. Effective trunk dynamical properties were compared before, during and after the experimentally induced painful period.Estimates of effective trunk stiffness (K) decreased and damping (B) increased during pain compared to both before ([mean contrast, 95% CI] K: −403 [−651 to −155] Nm−1, B: 28 [9–50] Nms−1) and after (K: −324 [−58 to −591] Nm−1, B: 20 [4–33] Nms−1) the experimentally induced painful period. We interpret our results to show that, when challenged by a step force perturbation, a healthy system adapts to noxious input by controlling trunk velocity rather than trunk displacement, in contrast to observations during remission from recurrent clinical LBP.

Trunk muscle co-activation using functional electrical stimulation modifies center of pressure fluctuations during quiet sitting by increasing trunk stiffness.

BackgroundThe purpose of this study was to examine the impact of functional electrical stimulation (FES) induced co-activation of trunk muscles during quiet sitting. We hypothesized that FES applied to the trunk muscles will increase trunk stiffness. The objectives of this study were to: 1) compare the center of pressure (COP) fluctuations during unsupported and FES-assisted quiet sitting - an experimental study and; 2) investigate how FES influences sitting balance - an analytical (simulation) study.MethodsThe experimental study involved 15 able-bodied individuals who were seated on an instrumented chair. During the experiment, COP of the body projected on the seating surface was calculated to compare sitting stability of participants during unsupported and FES-assisted quiet sitting. The analytical (simulation) study examined dynamics of quiet sitting using an inverted pendulum model, representing the body, and a proportional-derivative (PD) controller, representing the central nervous system control. This model was used to analyze the relationship between increased trunk stiffness and COP fluctuations.ResultsIn the experimental study, the COP fluctuations showed that: i) the mean velocity, mean frequency and the power frequency were higher during FES-assisted sitting; ii) the frequency dispersion for anterior-posterior fluctuations was smaller during FES-assisted sitting; and iii) the mean distance, range and centroidal frequency did not change during FES-assisted sitting. The analytical (simulation) study showed that increased mechanical stiffness of the trunk had the same effect on COP fluctuations as the FES.ConclusionsThe results of this study suggest that FES applied to the key trunk muscles increases the speed of the COP fluctuations by increasing the trunk stiffness during quiet sitting.

Ten-year follow-up of strengthening versus flexibility exercises with or without abdominal bracing in recurrent low back pain.

Randomized prevention trial. To compare the long-term effect of strengthening versus flexibility exercises and to evaluate the additional effect of abdominal bracing in recurrent low back pain (LBP). No conclusions could be made regarding appropriate exercise types or parameters in recurrent LBP. Abdominal bracing increases trunk stiffness; however, its long-term effect has not been evaluated in recurrent LBP yet. Six hundred patients with recurrent LBP participated. They were randomized into 4 groups-150 patients (age: 42.5 ± 12.7) performed strengthening exercises; 150 patients (age: 41.3 ± 11.5) performed flexibility exercises; 150 patients (age: 41.0 ± 13.2) performed strengthening exercises and used abdominal bracing in daily activities/exercises; and 150 patients (age: 40.6 ± 12.3) performed flexibility exercises and used abdominal bracing in daily activities/exercises. At the beginning of the study and at the end of 10 consecutive years were recorded 6 outcomes-frequency, intensity, and duration of pain, as well as frequency, intensity, and duration of exercises. Regarding the first 4 outcomes-all groups showed improvement from the beginning to the second year, but worsening from the second to the 10th year; there was no difference between strengthening and flexibility groups; bracing groups showed better results versus nonbracing groups. Intensity, frequency, and duration of the pain correlated with each other and with frequency of the exercises, but not with exercise duration or intensity. The exercise frequency is more important than the type, duration, or intensity of the exercise. Abdominal bracing adds to the exercise effect. It could be considered as a "preliminary muscle back belt on demand" increasing the trunk stiffness and the frequency of the trunk muscle contractions/cocontractions without interruption of daily activities, which may remind/convince the patients to exercise more frequently. Frequent exercising and bracing seems effective long-term prevention advices in recurrent LBP. 2.

Effects of experimentally increased trunk stiffness on thorax and pelvis rotations during walking

Patients with non-specific low back pain, or a similar disorder, may stiffen their trunk, which probably alters their walking coordination. To study the direct effects of increasing trunk stiffness, we experimentally increased trunk stiffness during walking, and compared the results with what is known from the literature about gait coordination with, e.g., low back pain. Healthy subjects walked on a treadmill at 3 speeds (0.5, 1.0 and 1.5m/s), in three conditions (normal, while contracting their abdominal muscles, or wearing an orthopedic brace that limits trunk motions). Kinematics of the legs, thorax and pelvis were recorded, and relative Fourier phases and amplitudes of segment motions were calculated. Increasing trunk stiffness led to a lower thorax–pelvis relative phase, with both a decrease in thorax–leg relative phase, and an increase in pelvis–leg relative phase, as well as reduced rotational amplitude of thorax relative to pelvis. While lower thorax–pelvis relative phase was also found in patients with low back pain, higher pelvis–leg relative phase has never been reported in patients with low back pain or related disorders. These results suggest that increasing trunk stiffness in healthy subjects causes short-term gait coordination changes which are different from those seen in patients with back pain.

Mechanical coupling between transverse plane pelvis and thorax rotations during gait is higher in people with low back pain

This study investigated whether people with low back pain (LBP) reduce variability of movement between the pelvis and thorax (trunk) in the transverse plane during gait at different speeds compared to healthy controls. Thirteen people with chronic LBP and twelve healthy controls walked on a treadmill at speeds from 0.5 to 1.72m/s, with increments of 0.11m/s. Step-to-step variability of the trunk, pelvis, and thorax rotations were calculated. Step-to-step deviations of pelvis and thorax rotations from the average pattern (residual rotations) were correlated to each other, and the linear regression coefficients between these deviations calculated. Spectral analysis was used to determine the frequencies of the residual rotations, to infer the relation of reduced trunk variability to trunk stiffness and/or damping. Variability of trunk motion (thorax relative to pelvis) was lower (P=0.02), covariance between the residual rotations of pelvis and thorax motions was higher (P=0.03), and the linear regression coefficients were closer to 1 (P=0.05) in the LBP group. Most power of segmental residual rotations was below stride frequency (∼1Hz). In this frequency range, trunk residual rotations had less power than pelvis or thorax residual rotations. These data show that people with LBP had lower variability of trunk rotations, as a result of the coupling of deviations of residual rotations in one segment to deviations of a similar shape (correlation) and amplitude (regression coefficient) in the other segment. These results support the argument that people with LBP adopt a protective movement strategy, possibly by increased trunk stiffness.

The effects of a three-week use of lumbosacral orthoses on trunk muscle activity and on the muscular response to trunk perturbations

BackgroundThe effects of lumbosacral orthoses (LSOs) on neuromuscular control of the trunk are not known. There is a concern that wearing LSOs for a long period may adversely alter muscle control, making individuals more susceptible to injury if they discontinue wearing the LSOs. The purpose of this study was to document neuromuscular changes in healthy subjects during a 3-week period while they regularly wore a LSO.MethodsFourteen subjects wore LSOs 3 hrs a day for 3 weeks. Trunk muscle activity prior to and following a quick force release (trunk perturbation) was measured with EMG in 3 sessions on days 0, 7, and 21. A longitudinal, repeated-measures, factorial design was used. Muscle reflex response to trunk perturbations, spine compression force, as well as effective trunk stiffness and damping were dependent variables. The LSO, direction of perturbation, and testing session were the independent variables.ResultsThe LSO significantly (P < 0.001) increased the effective trunk stiffness by 160 Nm/rad (27%) across all directions and testing sessions. The number of antagonist muscles that responded with an onset activity was significantly reduced after 7 days of wearing the LSO, but this difference disappeared on day 21 and is likely not clinically relevant. The average number of agonist muscles switching off following the quick force release was significantly greater with the LSO, compared to without the LSO (P = 0.003).ConclusionsThe LSO increased trunk stiffness and resulted in a greater number of agonist muscles shutting-off in response to a quick force release. However, these effects did not result in detrimental changes to the neuromuscular function of trunk muscles after 3 weeks of wearing a LSO 3 hours a day by healthy subjects.

Oblique abdominal muscle activity in response to external perturbations when pushing a cart

Cyclic activation of the external and internal oblique muscles contributes to twisting moments during normal gait. During pushing while walking, it is not well understood how these muscles respond to presence of predictable (cyclic push-off forces) and unpredictable (external) perturbations that occur in pushing tasks. We hypothesized that the predictable perturbations due to the cyclic push-off forces would be associated with cyclic muscle activity, while external perturbations would be counteracted by cocontraction of the oblique abdominal muscles. Eight healthy male subjects pushed at two target forces and two handle heights in a static condition and while walking without and with external perturbations. For all pushing tasks, the median, the static (10th percentile) and the peak levels (90th percentile) of the electromyographic amplitudes were determined. Linear models with oblique abdominal EMGs and trunk angles as input were fit to the twisting moments, to estimate trunk stiffness. There was no significant difference between the static EMG levels in pushing while walking compared to the peak levels in pushing while standing. When pushing while walking, the additional dynamic activity was associated with the twisting moments, which were actively modulated by the pairs of oblique muscles as in normal gait. The median and static levels of trunk muscle activity and estimated trunk stiffness were significantly higher when perturbations occurred than without perturbations. The increase baseline of muscle activity indicated cocontraction of the antagonistic muscle pairs. Furthermore, this cocontraction resulted in an increased trunk stiffness around the longitudinal axis.

Comparison of trunk stiffness provided by different design characteristics of lumbosacral orthoses

Background Lumbosacral orthoses (LSOs) are class I medical devices that are used in conservative and postoperative management of low back pain. The effectiveness of LSOs depends on their design aimed at enhancing trunk stiffness. Therefore, the purpose of this study was to compare two lumbar supports: extensible (made of neoprene and lycra) and non-extensible (made of polyester and nylon). Methods Trunk stiffness and damping was estimated from trunk displacement data in response to a quick force release in trunk flexion, extension, and lateral bending. Fourteen male and 6 female subjects performed five trials at each experimental condition: (1) No LSO, (2) extensible LSO, (3) non-extensible LSO, (4) non-extensible LSO with a small rigid front panel, and (5) non-extensible LSO with a large rigid front panel. Testing order was randomized and the LSOs were cinched to a pressure of 70 mmHg (9.4 kPa) measured between posterior aspect of the iliac crest and the orthosis. Findings The non-extensible LSO reduced trunk displacement by 14% and increased trunk stiffness by 14% ( P < 0.001). The extensible LSO did not result in any significant change in trunk displacement or stiffness. The addition of rigid front panels to the non-extensible LSO did not improve its effectiveness. The trunk damping did not differ between the LSO conditions. Interpretation A non-extensible LSO is more effective in augmenting trunk stiffness and limiting trunk motion following a perturbation than an extensible LSO. The rigid front panels do not provide any additional trunk stiffness most likely due to incongruence created between the body and a brace.

Trunk antagonist co-activation is associated with impaired neuromuscular performance

The goal of this paper was to determine if trunk antagonist activation is associated with impaired neuromuscular performance. To test this theory, we used two methods to impair neuromuscular control: strenuous exertions and fatigue. Force variability (standard deviation of force signal) was assessed for graded isometric trunk exertions (10, 20, 40, 60, 80% of max) in flexion and extension, and at the start and end of a trunk extensor fatiguing trial. Normalized EMG signals for five trunk muscle pairs (RA rectus abdominis, EO external oblique, IO internal oblique, TE thoracic erector spinae, and LE lumbar erector spinae) were collected for each graded exertion, and at the start and end of a trunk extensor fatiguing trial. Force variability increased for more strenuous exertions in both flexion (P < 0.001) and extension (P < 0.001), and after extensor fatigue (P < 0.012). In the flexion direction, both antagonist muscles (TE and LE) increased activation for more strenuous exertions (P < 0.001). In the extension direction, all antagonist muscles except RA increased activation for more strenuous exertions (P < 0.05) and following fatigue (P < 0.01). These data demonstrate a strong relationship between force variability and antagonistic muscle activation, irrespective of where this variability comes from. Such antagonistic co-activation increases trunk stiffness with the possible objective of limiting kinematic disturbances due to greater force variability.

The influence of artificially increased trunk stiffness on the balance recovery after a trip

Falls occur frequently in the growing population of elderly. Since trunk control is critical for maintaining balance, the higher trunk stiffness in elderly people compared to the general population has been associated with their increased fall-risk. Theoretically, trunk stiffness may be beneficial for balance recovery in walking, i.e. after a trip. A stiff joint may provide a torque that restricts the perturbation effects and thereby reduces the probability of a fall. The aim of this study was to test whether trunk stiffness impaired or assisted balance recovery after a trip during walking. An orthopedic corset was used to simulate trunk stiffness in 11 young male adults. Subjects walked over a platform, with or without the corset on, and were occasionally tripped over a hidden obstacle. Kinematics of the tripping reaction were measured. Initial trunk accelerations were significantly attenuated by the corset, which indicates a positive effect of the stiffening corset. However, no subsequent effects on peak trunk inclination and on the peak moment arm of gravity on the trunk were found. The pattern of trunk motion allowed ample time for triggered or voluntary muscle responses to be generated, before a substantial inclination occurred. It appears that such active responses were sufficient in the young subjects tested to obtain a similar net effect with or without the increased trunk stiffness induced by the corset.

Active trunk stiffness increases with co-contraction

Trunk dynamics, including stiffness, mass and damping were quantified during trunk extension exertions with and without voluntary recruitment of antagonistic co-contraction. The objective of this study was to empirically evaluate the influence of co-activation on trunk stiffness. Muscle activity associated with voluntary co-contraction has been shown to increase joint stiffness in the ankle and elbow. Although biomechanical models assume co-active recruitment causes increase trunk stiffness it has never been empirically demonstrated. Small trunk displacements invoked by pseudorandom force disturbances during trunk extension exertions were recorded from 17 subjects at two co-contraction conditions (minimal and maximal voluntary co-contraction recruitment). EMG data were recorded from eight trunk muscles as a baseline measure of co-activation. Increased EMG activity confirms that muscle recruitment patterns were different between the two co-contraction conditions. Trunk stiffness was determined from analyses of impulse response functions (IRFs) of trunk dynamics wherein the kinematics were represented as a second-order behavior. Trunk stiffness increased 37.8% ( p < 0.004) from minimal to maximal co-activation. Results support the assumption used in published models of spine biomechanics that recruitment of trunk muscle co-contraction increases trunk stiffness thereby supporting conclusions from those models that co-contraction may contribute to spinal stability.

Evidence for a role of antagonistic cocontraction in controlling trunk stiffness during lifting

Activity of the abdominal muscles during symmetric lifting has been a consistent finding in many studies. It has been hypothesized that this antagonistic coactivation increases trunk stiffness to provide stability to the spine. To test this, we investigated whether abdominal activity in lifting is increased in response to destabilizing conditions. Ten healthy male subjects lifted 35 l containers containing 15 l of water (unstable condition), or ice (stable condition). 3D-kinematics, ground reaction forces, and EMG of selected trunk muscles were recorded. Euler angles of the thorax relative to the pelvis were determined. Inverse dynamics was used to calculate moments about L5S1. Averaged normalized abdominal EMG activity was calculated to express coactivation and an EMG-driven trunk muscle model was used to estimate the flexor moment produced by these muscles and to estimate the L5S1 compression force. Abdominal coactivation was significantly higher when lifting the unstable load. This coincided with significant increases in estimated moments produced by the antagonist muscles and in estimated compression forces on the L5S1 disc, except at the instant of the peak moment about L5S1. The lifting style was not affected by load instability as evidenced by the absence of effects on moments about L5S1 and angles of the thorax relative to the pelvis. The data support the interpretation of abdominal cocontraction during lifting as subserving spinal stability. An alternative function of the increased trunk stiffness due to cocontraction might be to achieve more precise control over the trajectory of lifted weight in order to avoid sloshing of the water mass in the box and the consequent perturbations.

Trunk stiffness increases with steady-state effort

Trunk stiffness was measured in healthy human subjects as a function of steady-state preload efforts in different horizontal loading directions. Since muscle stiffness increases with increased muscle activation associated with increasing effort, it is believed that coactivation of muscles helps to stiffen and stabilize the trunk. This paper tested whether increased steady-state preload effort increases trunk stiffness. Fourteen young healthy subjects each stood in an apparatus with the pelvis immobilized. They were loaded horizontally at directions of 0, 45, 90, 135 and 180° to the forward direction via a thoracic harness. Subjects first equilibrated with a steady-state load of 20 or 40% of their maximum extension effort. Then a sine-wave force perturbation of nominal amplitude of 7.5 or 15% of maximum effort and nominal period of 250 ms was applied. Both the applied force and subsequent motion were recorded. Effective trunk mass and trunk-driving point stiffness were estimated by fitting the experimental data to a second-order differential equation of the trunk dynamic behavior. The mean effective trunk mass was 14.1 kg (s.d.=4.7). The trunk-driving point stiffness increased on average 36.8% (from 14.5 to 19.8 N/mm) with an increase in the nominal steady-state preload effort from 20 to 40% ( F 1,13=204.96, p<0.001). There was a smaller, but significant variation in trunk stiffness with loading direction. The measured increase in trunk stiffness probably results from increased muscle stiffness with increased muscle activation at higher steady-state efforts.

Trunk kinematics and trunk muscle activity during a rapidly applied load

This study investigated the trunk kinematics and electromyographic (EMG) activity of eight trunk muscles when “expected” and “unexpected” loads were applied directly to the torso. Twenty individuals (mean age: 25.1 yr; range 20–33 yr) participated in this mixed model study in which gender was the between-subjects factor, and expectancy and symmetry of the applied load were within-subject factors. The sudden load was delivered to the subject via a cable attached to a thoracic harness and motion was restricted to the lumbar spine by strapping the pelvis to a rigid fixation apparatus. Surface EMG was recorded bilaterally from the longissimus thoracis (LGT), erector spinae (ERS), rectus abdominis (RAB) and the external obliques (EXO). Trunk kinematics were measured with a Lumbar Motion Monitor™. During expected loading conditions, the peak muscle activity was reduced for the RAB and EXO bilaterally, and for the ERS(R) ( p<0.01) relative to the unexpected conditions. Conversely, the normalized area of EMG activity prior to the onset of load was increased for the ERS and EXO bilaterally, and for the RAB(R) ( p<0.05) during an expected loading event. Trunk motion in the sagittal and frontal planes was reduced during expected loading. Activation of the trunk muscles just prior to a rapid loading event increases trunk stiffness, decreasing trunk displacement and peak muscle activity.