The effects of pain and a secondary task on postural sway during standing.

BACKGROUND: Pain impairs available cognitive resources and somatosensory information, but its effects on postural control during standing are inconclusive. The aim of this study was to investigate whether postural sway is affected by the presence of pain and a secondary task during standing. METHODS: Sixteen healthy subjects stood as quiet as possible at a tandem stance for 30s on a force platform at different conditions regarding the presence of pain and a secondary task. Subjects received painful stimulations on the right upper arm or lower leg according to a relative pain threshold [pain 7 out 10 on a Visual Analog Scale (VAS) - 0 representing "no pain" and 10 "worst pain imaginable"] using a computer pressurized cuff. The secondary task consisted of pointing to a target using a head-mounted laser-pointer as visual feedback. Center of Pressure (COP) sway area, velocity, mean frequency and sample entropy were calculated from force platform measures. FINDINGS: Compared to no painful condition, pain intensity (leg: VAS = 7; arm VAS = 7.4) increased following cuff pressure conditions (P < .01). Pain at the leg decreased COP area (P < .05), increased COP velocity (P < .05), mean frequency (P < .05) and sample entropy (P < .05) compared with baseline condition regardless the completion of the secondary task. During condition with pain at the leg, completion of the secondary task reduced COP velocity (P < .001) compared with condition without secondary task. INTERPRETATION: Pain in the arm did not affect postural sway. Rather, postural adaptations seem dependent on the location of pain as pain in the lower leg affected postural sway. The completion of a secondary task affected postural sway measurements and reduced the effect of leg pain on postural sway. Future treatment interventions could benefit from dual-task paradigm during balance training aiming to improve postural control in patients suffering from chronic pain.

Estimation of Spinal Loading During Manual Materials Handling Using Inertial Motion Capture.

Musculoskeletal models have traditionally relied on measurements of segment kinematics and ground reaction forces and moments (GRF&Ms) from marked-based motion capture and floor-mounted force plates, which are typically limited to laboratory settings. Recent advances in inertial motion capture (IMC) as well as methods for predicting GRF&Ms have enabled the acquisition of these input data in the field. Therefore, this study evaluated the concurrent validity of a novel methodology for estimating the dynamic loading of the lumbar spine during manual materials handling based on a musculoskeletal model driven exclusively using IMC data and predicted GRF&Ms. Trunk kinematics, GRF&Ms, L4-L5 joint reaction forces (JRFs) and erector spinae muscle forces from 13 subjects performing various lifting and transferring tasks were compared to a model driven by simultaneously recorded skin-marker trajectories and force plate data. Moderate to excellent correlations and relatively low magnitude differences were found for the L4-L5 axial compression, erector spinae muscle and vertical ground reaction forces during symmetrical and asymmetrical lifting, but discrepancies were also identified between the models, particularly for the trunk kinematics and L4-L5 shear forces. Based on these results, the presented methodology can be applied for estimating the relative L4-L5 axial compression forces under dynamic conditions during manual materials handling in the field.

Four-way-leaning test shows larger limits of stability than a circular-leaning test.

INTRODUCTION: Limits of stability (LOS) have extensive clinical and rehabilitational value yet no standard consensus on measuring LOS exists. LOS measured using a leaning or a circling protocol is commonly used in research and clinical settings, however differences in protocols and reliability problems exist. OBJECTIVE: This study measured LOS using a four-way-leaning test and a circular-leaning test to test which showed larger LOS measurements. Furthermore, number of adaptation trials needed for consistent results was assessed. METHOD: Limits of stability were measured using a force plate (Metitur Good Balance System®) sampling at 50Hz. Thirty healthy subjects completed 30 trials assessing LOS alternating between four-way-leaning test and circular-leaning test. RESULTS: A main effect of methods (ANOVA:F(1,28)=45.86, P<0.01) with the four-way-leaning test showing larger values than the circular-leaning test (NK, P<0.01). An interaction between method×directions was found (ANOVA:F(3, 84)=24.87, P<0.01). The four-way-leaning test showed larger LOS in anterior (NK, P<0.05), right (NK, P<0.01) and left direction (NK, P<0.01). Analysis of LOS for the four-way-leaning test showed a difference between trials (ANOVA:F(14,392)=7.81, P<0.01). Differences were found between trial 1 and 7 (NK, P<0.03), trial 6 and 8 (NK, P<0.02) and trial 7 and 15 (NK, P<0.02). Four-way-leaning test showed high correlation (ICC>0.87) between first and second trial for all directions. CONCLUSION: Four-way-leaning test yields larger LOS in anterior, right and left direction making it more reliable when measuring LOS. A learning effect was found up to the 8th trial, which suggests using 8 adaptation trials before reliable LOS is measured.