Stephen H.M. Brown

Creep response of the lumbar spine to prolonged full flexion.

The time course of full lumbar flexion under a prolonged flexion moment, lasting 20 min, was documented in 27 male and 20 female subjects. Peak flexion increased by 5.5° over the 20 min. The flexion-creep data was fitted with a first-order step input response having a time constant of 9.4 min. Maximum flexion was also documented over the recovery phase, lasting 30 min, indicating that subjects regained approximately 50% of their resting joint stiffness within 2 min of resuming relaxed lordosis, although full recovery took longer than the flexion-creep, indicating the presence of viscoelastic hysteresis. For this reason it may be prudent to advise those who experience prolonged full flexion postures (as might a seated warehouse shipper/receiver, gardener, or construction worker) to stand and walk for a few minutes prior to performing demanding manual exertions. Indeed, temporary joint flexion laxity, following a bout of full flexion, may increase the risk of hyperflexion injury to certain tissues.

Predicting conduct problems : Can high-risk children be identified in kindergarten and Grade 1?

Externalizing behavior symptoms (EBS) in childhood are a strong predictor of future conduct problems. This study evaluated their predictive accuracy using logistic regression and receiver operating characteristic curve techniques. EBS, alone and in combination with other child and familial risk factors, were used to predict conduct problems 30 months later in a nonclinic population of kindergartners and Grade 1 children. The sensitivity (Sn) and positive predictive value (PPV) of EBS alone were below preset criteria of > or = 50% for each (prevalence < or = 15%). Sn and PPV increased when other child and familial factors were combined with symptoms but did not exceed the preset criteria. From a developmental perspective, substantial stability of EBS exists over time. However, from the perspective of prevention science, significant levels of misclassification will occur when EBS are used to designate high-risk status under the low-prevalence conditions of normal populations.

The intrinsic stiffness of the in vivo lumbar spine in response to quick releases: Implications for reflexive requirements

Torso muscles contribute both intrinsic and reflexive stiffness to the spine; recent modeling studies indicate that intrinsic stiffness alone is sometimes insufficient to maintain stability in dynamic situations. The purpose of this study was to experimentally test this idea by limiting muscular reflexive responses to sudden trunk perturbations. Nine healthy males lay on a near-frictionless apparatus and were subjected to quick trunk releases from the neutral position into flexion or right-side lateral bend. Different magnitudes of moment release were accomplished by having participants contract their musculature to create a range of moment levels. EMG was recorded from 12 torso muscles and three-dimensional lumbar spine rotations were monitored. A second-order linear model of the trunk was employed to estimate trunk stiffness and damping during each quick release. Participants displayed very limited reflex responses to the quick load release paradigms, and consequently underwent substantial trunk displacements (>50% flexion range of motion and >70% lateral bend range of motion in the maximum moment trials). Trunk stiffness increased significantly with significant increases in muscle activation, but was still unable to prevent the largest trunk displacements in the absence of reflexes. Thus, it was concluded that the intrinsic stiffness of the trunk was insufficient to adequately prevent the spine from undergoing potentially harmful rotational displacements. Voluntary muscular responses were more apparent than reflexive responses, but occurred too late and of too low magnitude to sufficiently make up for the limited reflexes.

A comparison of ultrasound and electromyography measures of force and activation to examine the mechanics of abdominal wall contraction

BACKGROUNDnUltrasound imaging is a valuable tool which, when applied appropriately, has the potential to provide information regarding the mechanics of abdominal muscle contraction. Typically, changes in muscle thickness are obtained and interpreted. However, the link between ultrasound measures of muscle thickening and EMG measures of activation is not clear.nnnMETHODSnFive healthy males performed a series of abdominal muscle contractions while surface EMG and trunk posture were monitored and ultrasound images of the internal oblique and external oblique were captured both at relaxation and upon contraction. Ramped isometric flexor and extensor moment contractions were also assessed and compared between EMG and ultrasound.nnnFINDINGSnNo definitive relationship between increases in muscle activation and corresponding measures of thickening was observed. Correlations between the two measures, across all contraction types, were 0.14 for internal oblique and -0.22 for external oblique.nnnINTERPRETATIONnThe lack of clear association between abdominal muscle activation and thickening may be due to the composite laminate-like structure of the abdominal wall, with force being transmitted between obliquely oriented muscle layers. Thus, ultrasound alone may not be a valid measure of muscle activation or force in the unique architecture of the abdominal wall.

The Active Straight Leg Raise Test and Lumbar Spine Stability

To determine the utility of the active straight leg raise (ASLR) test as a screen of lumbar spine stability and abdominal bracing (AB) ability.

Concerning the estimation of biological age.

The conditions which must be satisfied by an index of biological age are discussed, and a high correlation with chronological age is shown to be neither a necessary nor a sufficient condition to be satisfied by any such index.The conditions which must be satisfied by an index of biological age are discussed, and a high correlation with chronological age is shown to be neither a necessary nor a sufficient condition to be satisfied by any such index.

The relationship between trunk muscle activation and trunk stiffness: examining a non-constant stiffness gain.

The relationship between muscle activation, force and stiffness needs to be known to interpret the stability state of the spine. To test the relationship between these variables, a quick release approach was used to match quantified torso stiffness with an EMG activation-based estimate of individual muscle stiffnesses. The relationship between activation, force and stiffness was modelled as , where k, F and l are muscle stiffness, force and length, respectively, and q is the dimensionless stiffness gain relating these variables. Under the tested experimental scenario, the ‘stiffness gain’, q, which linked activation with stiffness, demonstrated a decreasing trend with increasing levels of torso muscle activation. This highlights the likelihood that the choice of a single q value may be over simplistic to relate force to stiffness in muscles that control the spine. This has implications for understanding the potential for spine instability in situations requiring high muscular demand.

An ultrasound investigation into the morphology of the human abdominal wall uncovers complex deformation patterns during contraction.

The abdominal wall components, specifically muscle and connective tissue, must meet and accommodate a wide range of force demands for torso movement, spine stabilization, and respiration. It has a composite laminate nature that may lend itself to facilitating the required tissue responses. The purpose of this exploratory study was to examine the deformations of the abdominal wall connective tissues, with a special focus on both the internal oblique aponeurosis and the tendinous intersections of the rectus abdominis, using ultrasound imaging, during relatively simple contractions of the abdominal musculature. There were two main findings of this study: (1) deformations occurred in nearly 50% of contractions that would be characterized by a simultaneous expansion in multiple planes; (2) the laterally generated forces of the oblique and transverse muscles transfer a great deal of force across the rectus abdominis muscle and sheath, leading to a lateral movement of the rectus muscle during abdominal contraction.

Strength limitations to proper child safety seat installation: Implications for child safety

A majority of child safety restraints are misused in some manner, often leading to an increased risk of serious injury or death. It is possible that at least some instances of misuse are the result of biomechanical limitations during the installation process. Twenty-seven adult participants were trained and then monitored in three stages of child safety seat installation. All installations were done with an identical restraint system in the rear bench seat of a mocked-up minivan. EMG of 10 muscles, as well as trunk, shoulder, and wrist postures were analyzed. Peak maximum efforts were often required of the trunk extensor, forearm, and anterior shoulder muscles during the installation process. Routing and tightening of the seatbelt, as well as placing and securing the child into the seat were observed to be particularly difficult tasks. Many portions of the child safety seat installation process were found to be very physically demanding; some individuals may not be capable of performing these tasks correctly, thereby putting the child at greater risk in the motor vehicle.

The effect of reducing the number of EMG channel inputs on loading and stiffness estimates from an EMG-driven model of the spine.

Electromyography (EMG)-driven models of the spine routinely require between ten and 14 EMG channels to estimate joint load and stiffness variables. This study was designed to determine the sensitivity of common EMG-driven model outputs to the removal of individual EMG channels, and to test two adapted models driven from eight channels. A total of 11 male participants performed a variety of static exertions designed to resist either an applied trunk flexion or right side trunk lateral bend moment. In this study, 14 channels of EMG were recorded and used to drive a biomechanical model of the spine to predict L4-L5 joint load and stiffness values. The model was subsequently re-run after the removal of individual pairs of bilateral EMG channels, and again with eight-channel models in which the rectus abdominus, latissimus dorsi and multifidus EMG-channels were eliminated. Results showed that the eight-channel model provided estimates for the majority of output variables that did not differ substantially from the 14-channel model, except in instances in which muscle force output was ramped to resist flexion moments. Estimates of the output variables were, in general, improved when multifidus fascicles were re-added to the model and driven from the lumbar erector spinae EMG sites.