Evidence for an exposure-response relationship between trunk flexion and impairments in trunk postural control

Prolonged trunk flexion alters passive and active trunk tissue behaviors, and exposure-response relationships between the magnitude of trunk flexion exposure and changes in these behaviors have been reported. This study assessed whether similar exposure-response relationships exist between such exposures and impairments in trunk postural control. Twelve participants (6 M, 6 F) were exposed to three distinct trunk flexion conditions (and a no-flexion control condition), involving different flexion durations with/without an external load, and which induced differing levels of passive tissue creep. Trunk postural control was assessed prior to and immediately following trunk flexion exposures, and during 10 min of standing recovery, by tracking center of pressure (COP) movements during a seated balance task. All COP-based sway measures increased following each flexion exposure. In the anteroposterior direction, these increases were larger with increasing exposure magnitude, whereas such a relationship was not evident for mediolateral sway measures. All measures were fully recovered following 10 min of standing. The present results provide evidence for an exposure-response relationship between trunk flexion exposures and impairments in trunk postural control; specifically, larger impairments following increased exposures (i.e., longer flexion duration and presence of external load). Such impairments in trunk postural control may result from some combination of reduced passive trunk stiffness and altered/delayed trunk reflex responses, and are generally consistent with prior evidence of exposure-dependent alterations in trunk mechanical and neuromuscular behaviors assessed using positional trunk perturbations. Such evidence suggests potential mechanistic pathways through which trunk flexion exposures may contribute to low-back injury risk.     

Trunk muscle responses to suddenly applied loads: Do individuals who develop discomfort during prolonged standing respond differently?

Individuals with low back pain or injury (LBP/LBI) have been shown to display altered muscle responses to trunk perturbations; however it is unclear whether these observations are a cause or a result of the LBP/LBI. In this study, a 6.78 kg load was suddenly applied to the hands to perturb the trunk prior to and following a 2-h standing period, during which trunk and hip electromyography (EMG) and centre of pressure (CoP) at the feet were recorded. Seven of the 13 participants developed substantial low back discomfort (LBDiscomfort) during the standing period. These individuals, both pre- and post-standing, showed a greater average number of responsive extensor muscles (3.8 compared to 3.1 in those who did not develop discomfort) and a greater occurrence of extensor muscle response (95–100% of trials) as compared to those who did not develop LBDiscomfort (73–86% of trials). Also, after discomfort developed, these individuals displayed an increased response in their abdominal muscles. This overall increase in trunk musculature activity could either be detrimental by potentially increasing spinal loading leading to LBDiscomfort, or beneficial in that this increased musculature responsiveness may reduce one’s likelihood of developing a future LBI through a pathway of increased spine stability. In either case, these responses indicate motor control characteristics that can distinguish the likelihood of an individual developing LBDiscomfort during common tasks such as prolonged standing.     

Reflex delays of the trunk muscles in response to postural perturbations: A reliability study

It has been reported that altered neuromuscular control of the trunk is associated with lower back pain. In this context reflex delays of the trunk muscles have often been assessed but the reliability of the tests has not been well established. The aim of this study was to test the reliability of measuring reflex delays of the trunk muscles after two types of postural perturbations. 24 Healthy subjects participated in the intra-session study and 13 of them repeated the test protocol within 1–3 weeks, to determine inter-session reliability. Postural reflex delays to unexpected loading and unloading of the arms were assessed in a standing unrestrained position. Each subject performed 40 trials of each test in order to evaluate muscle responses of 5 trunk muscles using surface electromyography. Overall reliability increased with higher number of the averaged trials. Good intra-session (ICC_3,1>0.75) and moderate (ICC_3,1>0.60) inter-session reliability were reached in most of the monitored trunk muscles. Within the performed number of trials we did not observe any significant systematic intra- or inter-session bias effect. Averaging a higher number of consecutive trials would be recommended in future research and clinical practice.     

Assessment of unloaded and loaded squat jump performance with a force platform: Which jump starting threshold provides more reliable outcomes?

This study aimed to explore the influence of different onset thresholds on the between-session reliability and magnitude of squat jump (SJ) performance. Twenty men were tested on two sessions separated by 48 h against external loads of 0.5, 30 and 60 kg. The initiation of the jump was defined as the first instant in which the vertical ground reaction force exceeded system weight by 10 N (10 N), 50 N (50 N), 1% of system weight (1%SW), 10% of system weight (10%SW) and five standard deviation of system weight minus 30 ms (5SDSW). The following variables were calculated from the force-time signal collected on a force platform: mean, peak and time to peak values of force, power and velocity, average rate of force development, peak rate of force development, rate of force development index, impulse, jump height, and push-off time. The 50 N, 10%SW and 5SDSW thresholds generally revealed a higher reliability than the 10 N and 1%SW thresholds (97 and 21 out of 252 comparisons for the coefficient of variation and intraclass correlation coefficient, respectively). The magnitude of most of the variables calculated using the 50 N and 10%SW thresholds significantly differed with respect to the 10 N, 1%SW and 5SDSW thresholds (P < 0.05). These results suggest that both the reliability and magnitude of SJ performance variables are influenced by the jump starting threshold. The 50 N, 10%SW and 5SDSW thresholds maximise the reliability of SJ performance variables, while the 5SDSW should be recommended since it considers more force signal than the 50 N and 10%SW thresholds.     

Comparing the local dynamic stability of trunk movements between varsity athletes with and without non-specific low back pain

The local dynamic stability of trunk movements, quantified using the maximum Lyapunov exponent (λ_max), can provide important information on the neuromuscular control of spine stability during movement tasks. Although previous research has displayed the promise of this technique, all studies were completed with healthy participants. Therefore the goal of this study was to compare the dynamic stability of spine kinematics and trunk muscle activations, as well as antagonistic muscle co-contraction, between athletes with and without low back pain (LBP). Twenty interuniversity varsity athletes (10 LBP, 10 healthy controls) were recruited to participate in the study. Each participant completed a repetitive trunk flexion task at 15 cycles per minute, both symmetrically and asymmetrically, while trunk kinematics and muscular activity (EMG) were monitored. The local dynamic stability of low back EMG was significantly higher (lower λ_max) in healthy individuals (p=0.002), whereas the dynamic stability of kinematics, the dynamic stability of full trunk system EMG, and the amount of antagonistic co-contraction were significantly higher when moving asymmetrically (p<0.05 for all variables). Although non-significant, kinematic and trunk system EMG stability also tended to be impaired in LBP participants, whereas they also tended to co-contract their antagonist muscles more. This study provides evidence that Lyapunov analyses of kinematic and muscle activation data can provide insight into the neuromuscular control of spine stability in back pain participants. Future research will repeat these protocols in patients with higher levels of pain, with hopes of developing a tool to assess impairment and treatment effectiveness in clinical and workplace settings.     

Criterion validity and between-day reliability of an inertial-sensor-based trunk postural stability test during unstable sitting

IntroductionAdequate neuromuscular control of the lumbar spine is required to prevent lumbar injuries. A trunk postural stability test has been proposed earlier, using a chair wobbling on a central pivot and four springs with adjustable positions to modulate task difficulty. An inertial sensor is fixed on the chair to measure postural sway. The aim of this study is to assess the criterion validity and between-day reliability of the calibration and testing components.MethodsThirty six subjects (with and without low back pain) followed a calibration procedure, four practice trials and three 60-s trials on 2 days. The criterion validity of the inertial sensor was tested against an optoelectronic system and a force platform. The reliability of 38 body sway measures obtained from the inertial sensor angular measures was estimated.ResultsThe inertial sensor led to valid estimates of postural sway. The reliability of the calibration procedure was moderate. Practically no learning effect was detected except for a few body sway measures in patients with CLBP. Three 60-s trials provided acceptable reliability for approximately half of the body sway measures, although this is more difficult to achieve in patients with CLBP.DiscussionThe use of an easy to use inertial sensor led to valid measures of postural sway. A number of body sway measures were identified as reliable tools for individual follow-ups but inter-subject comparisons were anticipated as more difficult when patients with CLBP are involved.     

The role of trunk muscles in sitting balance control in people with low back pain

The purpose of this study was to examine the muscular activities and kinetics of the trunk during unstable sitting in healthy and LBP subjects. Thirty-one healthy subjects and twenty-three LBP subjects were recruited. They were sat on a custom-made chair mounted on a force plate. Each subject was asked to regain balance after the chair was tilted backward at 20°, and then released. The motions of the trunk and trunk muscle activity were examined. The internal muscle moment and power at the hip and lumbar spine joints were calculated using the force plate and motion data. No significant differences were found in muscle moment and power between healthy and LBP subjects (p > 0.05). The duration of contraction of various trunk muscles and co-contraction were significantly longer in the LBP subjects (p < 0.05) when compared to healthy subjects, and the reaction times of the muscles were also significantly reduced in LBP subjects (p < 0.05). LBP subjects altered their muscle strategies to maintain balance during unstable sitting, but these active mechanisms appear to be effective as trunk balance was not compromised and the internal moment pattern remained similar. The changes in muscle strategies may be the causes of LBP or the result of LBP with an attempt to protect the spine.     

不同干预措施对兔抗谐振后血清标志物的影响

目的:探讨在4 Hz谐振频率下不同干预方式对兔抗谐振后血清代谢标志物特异性碱性磷酸酶(BALP)、6-酮前列腺素1α(6-keto-PGF 1α)及丙二醛(MDA)含量的影响。方法:75只新西兰兔随机均分为正常对照组、模型对照组及3个谐振干预实验组,每组15只,分别于实验的2、4、6周从每组随机抽取5只在清晨空腹状态下经耳缘静脉穿刺抽取5 m L静脉血,测定血清BALP、6-keto-PGF 1α及MDA含量并观察其动态变化。结果:与正常对照组和谐振干预3组比较,模型对照组、谐振干预1、2组血清BALP、6-keto-PGF 1α第4周开始升高,MDA含量从第6周开始升高且差异显著(P0.05)。与模型对照组相比,正常对照组、谐振干预3组血清BALP、6-keto-PGF 1α和MDA的含量在第4、6周降低且谐振干预1、2组与谐振干预3组同期相比差异显著(P0.05)。结论:不同干预对谐振引起的下腰痛早期防治具有重要参考价值,尤以PEP加工成大小不同空心棉球组成表面凹凸不平的座垫对于防治下腰痛发生具有重要意义。     

A comparison of a maximum exertion method and a model-based, sub-maximum exertion method for normalizing trunk EMG

The problem with normalizing EMG data from patients with painful symptoms (e.g., low back pain) is that such patients may be unwilling or unable to perform maximum exertions. Furthermore, the normalization to a reference signal, obtained from a maximal or sub-maximal task, tends to mask differences that might exist as a result of pathology. Therefore, we presented a novel method (GAIN method) for normalizing trunk EMG data that overcomes both problems. The GAIN method does not require maximal exertions (MVC) and tends to preserve distinct features in the muscle recruitment patterns for various tasks. Ten healthy subjects performed various isometric trunk exertions, while EMG data from 10 muscles were recorded and later normalized using the GAIN and MVC methods. The MVC method resulted in smaller variation between subjects when tasks were executed at the three relative force levels (10%, 20%, and 30% MVC), while the GAIN method resulted in smaller variation between subjects when the tasks were executed at the three absolute force levels (50 N, 100 N, and 145 N). This outcome implies that the MVC method provides a relative measure of muscle effort, while the GAIN-normalized data gives an estimate of the absolute muscle force. Therefore, the GAIN-normalized data tends to preserve the differences between subjects in the way they recruit their muscles to execute various tasks, while the MVC-normalized data will tend to suppress such differences. The appropriate choice of the EMG normalization method will depend on the specific question that an experimenter is attempting to answer.