Static Flexion Research Articles

Increased Lumbar Viscoelastic Tissue Compliance as a Function of Static Passive Trunk Flexion

Increased Lumbar Viscoelastic Tissue Compliance as a Function of Static Passive Trunk Flexion

An Alternative Mechanism of Non-contact Anterior Cruciate Ligament Injury During Jump-landing: In-vitro Simulation

In the United States, an estimated 100,000 anterior cruciate ligament (ACL) injuries occur every year. Despite decades of research, to this date, the mechanism or mechanisms of non-contact ACL injuries are not well understood. This is primarily because trials cannot be conducted on live subjects to understand the injury mechanism, and it is difficult to instrument a live human knee to measure the response of tissues during dynamic activities. In this paper, we present a dynamic knee injury simulator capable of in-vitro modeling of the ACL injury during jump-landing activity. This system was used to simulate jump-landing on cadaveric knees and to successfully test which conditions would result in isolated ACL injury. A restricted flexion of the hip (a hip that flexes minimally or not at all during landing), combined with low quadriceps and hamstring force levels during landing were found to be conducive to ACL injury. Elevated levels of quadriceps force prevented the injury from occurring even under restricted hip flexion conditions. The measured strain rates in the ACL tissue during injury causing activities were over 250%/s.

ESTUDIO DE LAS PROPIEDADES FISICO-MECÁNICAS Y COMPORTAMIENTO EN PROCESOS INDUSTRIALES DE LA MADERA DE KIRI, GREVILLEA, PARAÍSO Y TOONA

Este trabajo tuvo por objetivo determinar las propiedades físicas y mecánicas, así como el comportamiento en distintos procesos industriales, de maderas de las especies: paraíso (Melia azederach), kiri (Paulonia tomentosa), toona (Toona ciliata) y grevillea (Grevillea robusta), cultivadas en la Provincia de Misiones, Argentina. Las propiedades estudiadas fueron peso específico, contracción total, coeficiente de retractabilidad, anisotropía de contracción y flexión estática, bajo las Normas IRAM N° 9544, 9543 y DIN N° 52186. Grevillea, paraíso, toona y kiri presentaron valores medios de pesos específicos aparente de 0,530 g/cm³; 0,600 g/cm³; 0,422 g/cm³ y 0,270 g/cm³ respectivamente. La contracción total en la dirección tangencial de las maderas de paraíso, toona, grevillea y kiri presentaron los valores de 6,82 %; 7,85 %; 5,69 % y 4,51 % respectivamente; en la dirección radial de las mismas maderas presentaron los valores de 5,44 %; 4,11 %; 3,5 % y 1,57 % respectivamente. La madera de paraíso fue la que presentó valores superiores de módulos de rotura y elasticidad 852,89 kg/cm² y 72505 kg/cm², respectivamente. La grevillea presentó valores menores con respecto al paraíso tanto en el módulo de rotura como en el módulo de elasticidad, 521,55 kg/cm² y 45693 kg/cm² respectivamente. La madera de toona presentó un valor medio del módulo de rotura y elasticidad de 468 kg/cm² y 65118 kg/cm², respectivamente. La madera de kiri, como es de esperar, arrojó valores de los módulos de rotura de 256,85 kg/cm² y elasticidad a la flexión estática 25261,52 kg/cm², menores que las especies ya citadas. El rendimiento en el proceso de aserrado de las maderas fue: kiri, 50%; grevillea, 48%; toona, 50% y paraíso, 55%.

Fatigue-induced modulation of the H reflex of soleus muscle in humans

We studied the effect of fatigue of the mm. gastrocnemius-soleus on the H reflex elicited by transcutaneous stimulation of n. tibialis and recorded from the m. soleus; healthy 18-to 34-year-old volunteers were tested. Fatigue was evoked by long-lasting (6 to 9 min) voluntary tonic static sole flexion of the foot (ankle extension) with a force equal to 75% of the maximum voluntary contraction (MVC). The amplitude of H reflex significantly (P < 0.001) decreased to about 60% of the initial value immediately after the period of fatiguing effort. Within 2 to 3 min, it relatively rapidly recovered and reached about 90% of the control, and this was followed by a period of slow recovery to about 96–97% of the initial value 30 min after conditioning fatigue. We suppose that the initial period of suppression of the H reflex results to a considerable extent in an increase in the intensity of presynaptic inhibition of transmission from Ia afferents due to tonic activation of high-threshold (groups III and IV) afferent fibers induced by intensive fatigue-related metabolic changes in the muscles. More long-lasting (tens of minutes) changes are related to slow reverse development of direct effects of fatigue-induced biochemical shifts in the muscle.

Work to rest durations ratios exceeding unity are a risk factor for low back disorder; a feline model

Low back disorders are prominent among the work force engaged in static anterior flexion during the workday. As a continuing part of a long-term research aimed to identify the biomechanical and physiological processes and corresponding risk factors leading to such cumulative trauma disorder (CTD), we ventured to assess the effect of rest and the work-to-rest duration ratios that may prevent CTD. Three groups of the feline model were subjected to three load/rest paradigms: two 30 min loading periods spaced by 10 min rest in Group I, two 30 min loading period spaced by 30 min rest in Group II and one 60 min loading period for Group III. The cumulative loading duration in the three groups was 60 min. Each of the groups were allowed 7 h of rest while monitoring EMG and lumbar viscoelastic tissue creep each hour. The results demonstrate that for two 30 min load periods with a 30 min in between rest, an acute neuromuscular disorder was not present whereas for two 30 min loading with a 10 min rest it was. Similarly, for a 60 min loading with long-term rest, the disorder was present. Post hoc Fisher analysis demonstrated significant differences in the delayed hyperexcitability between the first and second group ( P < 0.0001) and the third and second ( P < 0.0001) group. Statistical difference in the displacement data of the three groups was not present. ANOVA showed a significant effect of time post-loading ( P < 0.0001 and different rest durations ( P < 0.0001) on the EMG data during the 7 h recovery. The new data allow us to conclude that a work-to-rest duration ratio of 1:1 can prevent the development of CTD as long as the work periods are not too long (<60 min). Longer static flexion durations do not respond favorably to rest even if it is of equal or longer duration. It is suggested that appropriate durations of rest may be a viable tool to avert CTD in a certain range whereas long static flexion durations should be avoided at all cost.

Recovery of Muscle Strength after Late Repair of Distal Biceps Brachii Tendon

Ruptures of the distal part of the biceps brachii tendon are rare. The diagnosis is often delayed and only late repair can be considered. In this study, the recovery of muscle strength after late repair of the distal biceps brachii tendon was evaluated. Sixteen patients with a ruptured distal biceps brachii tendon were analysed. The mean delay from the primary trauma to the operation was 35 weeks. The tendon was anatomically re-attached with bone anchors. In three cases a tendon graft was needed. The operated arms were immobilised postoperatively for four weeks, after which mobilisation was allowed. Maximal static flexion and supination strength was measured after an average follow-up time of 124 weeks by using a computer-based isokinetic dynamometer. Patient satisfaction and overall muscle strength recovery were very good. Compared to the non-operated side, average flexion strength recovery was 90% and corresponding recovery of supination strength 78%. Weakness on supination remained in the cases where a tendon graft was used. One patient needed a re-operation for a re-rupture. There were three cases of transient paresthesia of the cutaneous nerve. All patients resumed their previous work. Late anatomical repair of the biceps brachii tendon restores very good flexion and moderate supination strength. This operation should always be considered when the primary diagnosis is delayed.

542 弱屈曲を考慮した膝関節の動的荷重解析(S09-2 関節のバイオメカニズム(2),S09 関節のバイオメカニズム)

Dynamic load applied to human body as daily activities like walking and jumping. In this study, finite element simulations were performed to investigate the impact transmission mechanism in knee joint. Two kind of finite element were prepared which were assumed standing and flexion condition. In the result, it was revealed that the medial and lateral ligaments have support function to restraint femoral displacement during the heavy impact and that the medial displacement decreased related to flexion angle.

Torso Flexion Loads and the Fatigue Failure of Human Lumbosacral Motion Segments

Spine loads associated with lifting a 9-kg weight were estimated at three torso flexion angles (0 degrees, 22.5 degrees, and 45 degrees), and lumbosacral motion segments were cyclically loaded using these loads until failure or to a maximum of 10,020 cycles. To simulate the postures and loads experienced by the lumbar spine during repetitive lifting of moderate weights in different torso flexion postures, and to analyze the fatigue failure response of lumbosacral motion segments. Previous fatigue failure studies of lumbar motion segments have not reproduced the combination of spinal postures, loads, and load rates anticipated in different torso flexion postures during lifting tasks characteristic of those in occupational settings. Twelve fresh human lumbosacral spines were dissected into three motion segments each (L1-L2, L3-L4, and L5-S1). Motion segments within each spine were randomly assigned to a simulated torso flexion angle (0 degrees, 22.5 degrees, or 45 degrees) using a partially balanced incomplete block experimental design. Spinal load and load rate were determined for each torso flexion angle using previously collected data from an EMG-assisted biomechanical model. Motion segments were creep loaded for 15 minutes, then cyclically loaded at 0.33 Hz. Fatigue life was taken as the number of cycles to failure (10 mm displacement after creep loading). Specimens were inspected to determine failure mechanisms. The degree of torso flexion had a dramatic impact on cycles to failure. Motion segments experiencing the 0 degrees torso flexion condition averaged 8,253 cycles to failure (+/-2,895), while the 22.5 degrees torso flexion angle averaged 3,257 (+/-4,443) cycles to failure, and motion segments at the 45 degrees torso flexion angle lasted only 263 cycles (+/-646), on average. The difference was significant at P < 0.0001, and torso flexion accounted for 50% of the total variance in cycles to failure. Fatigue failure of spinal tissues can occur rapidly when the torso is fully flexed during occupational lifting tasks; however, many thousands of cycles can be tolerated in a neutral posture. Future lifting recommendations should be sensitive to rapid development of fatigue failure in torso flexion.

Bone Mineral Content and Fatigue Failure of Lumbar Motion Segments in Simulated Flexed Lifting: Does Specimen Age Influence the Relationship?

This paper describes a comparison of the fatigue failure responses of older versus younger lumbar spine cadaver motion segment specimens. These specimens were repeatedly subjected to loads simulating those experienced by the spine in lifting a 9 kg load in different torso flexion postures (0, 22.5, and 45 degrees of flexion). An older sample of 36 motion segments (average age 81 years + 8 SD) is compared with a younger sample of 18 motion segments (average age: 47 years + 9 SD) with respect to the number of cycles to failure and results of survival analyses evaluating the effects of flexion and bone mineral content with and without the younger data set. Compared to the older sample of spines, the younger sample exhibited many more cycles to fatigue failure in all the torso flexion conditions (10020 versus 8267 average cycles to failure in 0 degrees flexion, 7124 versus 3262 in 22.5 degrees flexion, and 3229 versus 263 cycles to failure in 45 degrees flexion). The increased cycles to failure in young specimens is likely due to the increased bone mineral content (BMC) in younger motion segments (30.7 g + 11.1 g per motion segment versus 27.8 + 9.4 g). Cox regression analyses modeling both flexion effects and BMC indicate that betas are similar when just older spines are modeled and when younger specimens are included in the analysis. Betas for survival analysis models with and without younger specimens generally show little change for flexion (2.716 versus 2.756 for 22.5 degrees [0 degrees referent], 3.926 versus 3.380 for 45 degrees [0 degrees referent]). Bone mineral content shows a similar protective effect in both models (Betas: −0.127 versus −0.087). Overall, comparison of the models indicate that risk ratios are similar for 22.5 degrees flexion, slightly increased for 45 degrees flexion and slightly more protective influence predicted for BMC in the combined sample versus the older sample alone.

Effects of low back disability status on lower back discomfort during sustained and cyclical trunk flexion

A laboratory study was conducted to determine the effects of back disability status on endurance time and perceived discomfort during trunk flexion. Eighty participants (40 with chronic or recurrent low back pain (CRLBP), 40 pain-free) were tested. The trunk was flexed to 15°, 30°, 45° and 60° under three conditions: 1) continuous static flexion; 2) cyclical flexion with 20% rest; and 3) cyclical flexion with 40% rest. Each condition was performed for up to 600 s or until the participant reached his/her pain tolerance limit. Dependent variables included time to distracting discomfort (TDD), total endurance time (TET) and perceived discomfort. For continuous exertions, CRLBP participants had lower TDD (p < 0.001), lower TET (p < 0.001) and greater discomfort (p < 0.001) compared to pain-free controls. In both groups, TDD and TET decreased and perceived discomfort increased as the flexion angle increased. For intermittent exertions, CRLBP participants reported greater discomfort than pain-free participants (p < 0.001). Increasing rest from 20 to 40% reduced discomfort in CRLBP participants, but produced no consistent benefit in pain-free participants. To accommodate persons with CRLBP, consideration should be given to reducing both the magnitude (angle) and duration of trunk flexion required by their jobs.

Soleus and vastus medialis H-reflexes: Similarities and differences while standing or lying during varied knee flexion angles

The H-reflex may be a useful measure to examine the lower extremity muscles activation and inhibition following an injury. Recording the vastus medialis H-reflex amplitudes in healthy subjects while standing or lying during varied knee flexion angles may establish a reference for comparison for patients with ACL injury. Vastus medialis and soleus H-reflexes were recorded from 14 healthy subjects while lying and standing during 0, 30, 45, and 60° knee flexion. EMG unit was used to electrically stimulate the tibial and femoral nerves (using 0.5 ms pulses at 0.2 pps of H-maximum amplitude) and to record four traces of the soleus and vastus medialis H-wave and one trace of the M-wave peak-to-peak amplitudes. Repeated measures three-way ANOVAs were calculated with the global α = 0.05. Results showed that (1) the average soleus H-reflex amplitude was significantly less during standing than lying across all knee flexion conditions, (2) the average vastus medialis H-reflex amplitudes showed no measurable significant differences between neutral standing compared with lying, (3) the average vastus medialis H-reflex amplitudes were significantly greater during standing knee flexion conditions (30, 45, and 60 °) than lying or neutral standing, and (4) there were no differences between soleus and vastus medialis H-reflex amplitudes during lying across all knee flexion conditions. Data from H/M ratio follow the same pattern of H-amplitude. Recording the vastus medialis H-reflex amplitude during standing and knee flexion may be a reflective of the knee function. It is more specific than the soleus H-reflex because it reflects the changes in the excitability of the quadriceps motoneurons acting directly around the knee joint.

Human Lumbar Spine Creep during Cyclic and Static Flexion: Creep Rate, Biomechanics, and Facet Joint Capsule Strain

There is a high incidence of low back pain (LBP) associated with occupations requiring sustained and/or repetitive lumbar flexion (SLF and RLF, respectively), which cause creep of the viscoelastic tissues. The purpose of this study was to determine the effect of creep on lumbar biomechanics and facet joint capsule (FJC) strain. Specimens were flexed for 10 cycles, to a maximum 10 Nm moment at L5-S1, before, immediately after, and 20 min after a 20-min sustained flexion at the same moment magnitude. The creep rates of SLF and RLF were also measured during each phase and compared to the creep rate predicted by the moment relaxation rate function of the lumbar spine. Both SLF and RLF resulted in significantly increased intervertebral motion, as well as significantly increased FJC strains at the L3-4 to L5-S1 joint levels. These parameters remained increased after the 20-min recovery. Creep during SLF occurred significantly faster than creep during RLF. The moment relaxation rate function was able to accurately predict the creep rate of the lumbar spine at the single moment tested. The data suggest that SLF and RLF result in immediate and residual laxity of the joint and stretch of the FJC, which could increase the potential for LBP.

The effect of occupant position in volunteers subjected to whiplash-type rear impacts

The purpose of this study was to examining electromyographically the effects of trunk flexion to the left and right on the cervical muscle response to increasing low-velocity posterolateral impacts. Twenty healthy volunteers were subjected to rear impacts of 4.5, 8.2, 10.6, and 13.9 m/s2 acceleration, offset by 45 degrees to the subject's right with trunk flexed to right and left. At a peak acceleration of 13.9 m/s2, with the trunk flexed to the right, the right trapezius generated 26% and left trapezius 29% of the maximal voluntary contraction electromyogram, while all other muscles generated 20% or less of this variable. When the trunk is flexed to the left, under these same conditions, the right trapezius generated 25% of its maximal voluntary contraction electromyogram, with 30% of the maximal voluntary contraction for the right trapezius, and less than 12% for the remaining muscles. Electromyographic and kinematic variables were significantly affected by the levels of acceleration (P < 0.05). The time to onset and time to peak electromyogram for all muscles progressively decreased with increasing levels of acceleration, for both trunk flexion conditions. Being ȁout-of-positionȁ at the time of impact greatly reduces the EMG response of the cervical muscles compared to what is the known response for impacts with neutral posture. Muscle injury seems less likely under these conditions in low-velocity impacts.

Effects of static flexion–relaxation on paraspinal reflex behavior

Background. Static trunk flexion working postures and disturbed trunk muscle reflexes are related to increased risk of low-back pain. Animal studies conclude that these factors may be related; passive tissue strain in spinal ligaments causes subsequent short-term changes in reflex. Although studies have documented changes in the myoelectric onset angle of flexion–relaxation following prolonged static flexion and cyclic flexion we could find no published evidence related to the human reflex response of the trunk extensor muscles following a period of static flexion–relaxation loading. Methods. Eighteen subjects maintained static lumbar flexion for 15 min. Paraspinal muscle reflexes were elicited both before and after the flexion–relaxation protocol using pseudorandom stochastic force disturbances while recording EMG. Reflex gain was computed from the peak value of the impulse response function relating input force perturbation to EMG response using time-domain deconvolution analyses. Findings. Reflexes showed a trend toward increased gain after the period of flexion–relaxation ( P < 0.055) and were increased with trunk extension exertion ( P < 0.021). Significant gender differences in reflex gain were observed ( P < 0.01). Interpretations. Occupational activities requiring extended periods of trunk flexion contribute to changes in reflex behavior of the paraspinal muscles. Results suggest potential mechanisms by which flexed posture work may contribute to low-back pain. Significant gender differences indicate risk analyses should consider personal factors when considering neuromuscular behavior.

Short rest periods after static lumbar flexion are a risk factor for cumulative low back disorder

The objective of this work was to study the effect of rest periods of various durations applied between six 10-min sessions of static flexion on the development of cumulative low back disorder (CLBD). Three experimental groups of a feline model were used, and the rest duration between sequential static load periods was set to 5, 10, and 20 min, with a corresponding load-to-rest ratio of 2:1, 1:1 and 1:2, respectively. The reflex electromyographic (EMG) activity from the multifidus muscles and supraspinous ligament displacement (creep) were recorded during the flexion periods and over 7 h of rest following the load–rest cycles. It was found that a minor disorder developed in all the groups whereas a severe neuromuscular disorder including a delayed hyperexcitability was observed only in the group subjected to 5 min rest. The two-way ANOVA showed a significant effect of time post loading ( p < 0.001) and rest duration ( p < 0.001) on the Normalized Integrated EMG (NIEMG) recovery data; a significant effect of time post loading on the Displacement data ( p < 0.001) was observed as well. The post hoc Fisher test performed on the NIEMG data during the recovery phase showed a significant difference between the group subjected to 5 min rest and the other two groups ( p < 0.001). These results suggest that while a short rest period of 2:1 load-to-rest ratio leads to CLBD, longer rest at 1:1 and 1:2 load-to-rest ratio are more favorable for preventing or attenuating the development of CLBD. Short rest periods between sessions of static lumbar flexion, therefore, are a risk factor for the development of CLBD.

Mechanically induced disruption of the healthy bovine intervertebral disc.

An intact bovine caudal disc model was used to investigate how combinations of biomechanical parameters influence the severity of disruption during compressive loading. To quantify the combined influence of flex-ion, hydration level, and compressive loading rate on nuclear disruption. The risk of disc pro-lapse is known to increase when the disc is loaded flexed. However, there are few experimental data available quantifying the extent to which loading parameters might interact to produce disruption in the healthy disc. Reproducible states of full and partial hydration were established for 96 isolated caudal discs. These discs were then subjected to compression under combined conditions of high or low hydration, zero or full flexion, and moderate or low loading rate. The extent of disc disruption was assessed macroscopically using a damage weighting procedure. Maximum disruption of the intact, healthy disc occurred under combined conditions of full hydration and flexion. Loading rate, whether at 0.004 MPa/sec or 4 MPa/sec, had little influence when applied without flexion, but there was an increased risk of disruption when the moderate rate was combined with full flexion. The investigation demonstrates that significant levels of disruption can be induced by mechanical loading in intact discs free of degenerative change. Our findings support the hypothesis that mechanical injury to a healthy disc might initiate a process of degeneration.

Longer static flexion duration elicits a neuromuscular disorder in the lumbar spine.

The objective of this study was to assess the impact of two sequential long, static, anterior lumbar flexions on the development of a neuromuscular disorder and to compare it with previously obtained data from a series of short static flexion periods of the same cumulative time (Sbriccoli P, Solomonow M, Zhou BH, Baratta RV, Lu Y, Zhu MP, and Burger EL, Muscle Nerve 29: 300-308, 2004). Static flexions with loads of 20, 40, and 60 N were applied to the lumbar spine over two 30-min periods with a 10-min rest in between. The reflex EMG activity from the multifidus muscles and supraspinous ligament displacement (creep) was recorded during the flexion periods. Creep and EMG were also monitored over 7 h of rest following the work-rest-work cycle. It was found that the creep that developed in the first 30-min flexion period did not recover completely during the following 10 min of rest, giving rise to a large cumulative creep at the end of the work-rest-work session. Spasms were frequently seen within the EMG during the static flexion. Initial and delayed hyperexcitabilities were observed in all of the preparations at any of the three loads explored during the 7-h rest period. ANOVA revealed a significant effect of time (P < 0.0001) on the postloading data. Larger loads elicited larger magnitudes of the initial and delayed hyperexcitabilities, yet were not statistically different. It was concluded that the 3:1 work-to-rest duration ratio resulted in a neuromuscular disorder, regardless of the load magnitude. The conclusions are reinforced in view of the results from a previous study using 60 min of flexion overall but at 1:1 work-to-rest ratio in which only the highest load elicited a delayed hyperexcitability (Sbriccoli et al., Muscle Nerve 29: 300-308, 2004). An optimal dose-to-duration ratio needs to be established to limit, attenuate, or prevent the adverse effects of static load on the lumbar spine while considering the loading duration as a major risk factor.

Static load magnitude is a risk factor in the development of cumulative low back disorder.

Occupations requiring frequent periods of static lumbar flexion are known epidemiologically to be risk factors for the development of cumulative low back disorder. The impact of the load magnitude sustained during a series of short static lumbar flexions followed by an equally long rest period on the development of a cumulative low back disorder was addressed in an in vivo feline model. Static loads of 20, 40, and 60 N were applied over 10 min of flexion followed by 10-min rest sessions that were repeated six times (for a total of 2 h) while monitoring lumbar viscoelastic creep (laxity) and reflex electromyographic (EMG) activity from the multifidus muscles. Creep and EMG were also monitored over 7 h of rest following the six flexion-rest sessions. It was found that the creep developed in the 10-min flexion periods did not recover completely during the following 10 min of rest, giving rise to a large cumulative creep at the end of the work-rest session. Muscle activity demonstrated spasms during the static flexion periods as well as initial and delayed hyperexcitability during the 7-h rest period. Loads of 20 and 40 N did not result in delayed hyperexcitability, whereas loads of 60 N resulted in delayed hyperexcitability. Statistical analysis demonstrated that increased load significantly intensified the magnitude of the hyperexcitabilities (P < 0.05). Thus, repeated periods of static lumbar flexion were found to result in a transient neuromuscular disorder with an intensity directly related to the load magnitude, which should be considered a compounding risk factor.

Modulation of interhemispheric inhibition during passive movement of the upper limb reflects changes in motor cortical excitability.

We investigated modulation of inhibition of motor evoked potentials in the left forearm musculature attributable to changes in corticomotor excitability induced by passive rhythmic movement of the right limb. In the first experiment, eight healthy volunteers pre-activated their left flexor carpi radialis (FCR) in a simple isometric contraction (2.5-7.5% MVC) while their right hand underwent passive wrist flexion-extension. Transcranial magnetic (TMS) or electrical (TES) stimulation was applied to the right motor cortex and responses recorded from the test (left) limb in eight phases of the wrist flexion-extension cycle of the passively driven right limb. In half of the trials TMS conditioning was applied to the left motor cortex. The conditioning stimulus significantly inhibited TMS-evoked responses in the test FCR muscle, whereas TES-evoked responses did not appear to be inhibited. For TMS-evoked responses only, inhibition in the static pre-activated left FCR was modulated such that inhibition was greater when the right wrist was passively flexing than when it was extending. In the second experiment TMS was applied to the right motor cortex, contralateral to the test (left) limb, with the right hand either passively extending or flexing through the neutral position. Conditioning was applied to the left motor cortex at a range of intensities adjusted to threshold for flexion and extension movements. No difference was evident in the maximum magnitude of inhibition between the extension and flexion conditions. We propose there is an increased absolute threshold for recruitment and a decreased gain of inhibitory callosal pathways during extension phases of the wrist flexion-extension cycle.

Biomechanics and electromyography of a cumulative lumbar disorder: response to static flexion

Objective. To assess the mechanical and neurological processes active in the development of a cumulative trauma disorder (CTD) associated with repetitive exposure to periods of static lumbar flexion. Methods. The spine of the feline model was subjected to a series of three 10 min sessions of static lumbar flexion with each session followed by a 10 min rest. A 7 h rest period was implemented after the series of three flexion-rest sessions while monitoring viscoelastic (disks, ligaments, etc.) creep and multifidus EMG. A model was fitted to the experimental data from the flexion-rest period and the 7 h recovery period. Results. The creep developed in each 10 min static flexion period did not fully recovery during the following 10 min rest, resulting in a large cumulative creep at the end of the flexion-rest period. The cumulative creep did not fully recover over the following 7 h rest period. A neuromuscular disorder consisting of reduced muscular activity superimposed by spasms during static flexion periods and hyperexcitability during the 7 h recovery was evident. Comparison of the data to previous tests of continuous static flexion for 20 min reveal that the neuromuscular disorder elicited by the series of three 10 min flexion-rest was substantially attenuated when compared to a single 20 min static flexion although the overall work time was 50% larger. Conclusions. Frequent rest periods are highly beneficial in attenuating the development of a CTD, yet not able to prevent it, as viscoelastic tissues residual creep accumulates and its recovery is of extremely long duration. Relevance The data provides direct biomechanical and physiological evidence that explain the development of a CTD due to prolonged exposure to static lumbar flexion as well as confirms the epidemiological data correlating such work conditions with substantial increase in symptoms of low back disorders. The benefit of frequent rest periods in attenuating the risk of such a disorder is validated as an effective intervention.