Karen L. Troy

Attention demanding tasks during treadmill walking reduce step width variability in young adults

BackgroundThe variability of step time and step width is associated with falls by older adults. Further, step time is significantly influenced when performing attention demanding tasks while walking. Without exception, step time variability has been reported to increase in normal and pathologically aging older adults. Because of the role of step width in managing frontal plane dynamic stability, documenting the influence of attention-demanding tasks on step width variability may provide insight to events that can disturb dynamic stability during locomotion and increase fall risk. Preliminary evidence suggests performance of an attention demanding task significantly decreases step width variability of young adults walking on a treadmill. The purpose of the present study was to confirm or refute this finding by characterizing the extent and direction of the effects of a widely used attention demanding task (Stroop test) on the step width variability of young adults walking on a motorized treadmill.MethodsFifteen healthy young adults walked on a motorized treadmill at a self-selected velocity for 10 minutes under two conditions; without performing an attention demanding task and while performing the Stroop test. Step width of continuous and consecutive steps during the collection was derived from the data recorded using a motion capture system. Step width variability was computed as the standard deviation of all recorded steps.ResultsStep width decreased four percent during performance of the Stroop test but the effect was not significant (p = 0.10). In contrast, the 16 percent decrease in step width variability during the Stroop test condition was significant (p = 0.029).ConclusionThe results support those of our previous work in which a different attention demanding task also decreased step width variability of young subjects while walking on a treadmill. The decreased step width variability observed while performing an attention demanding task during treadmill walking may reflect a voluntary gait adaptation toward a more conservative gait pattern emphasizing frontal plane control of the trunk. Extension of the experimental paradigm to older adults and mechanistic approaches to link step width variability to dynamic stability, and falls, in a cause-effect manner are necessary.

The prevalence of sarcopenia in patients with respiratory failure classified as normally nourished using computed tomography and subjective global assessment.

BACKGROUND Declines in nutrition status and adverse body composition changes frequently occur in the critically ill. The objective of this cross-sectional study was to examine the prevalence of sarcopenia and its occurrence in patients classified as normal nourished using subjective global assessment (SGA). METHODS Exploiting diagnostic CT images, skeletal muscle mass at the L3 region was quantified and used to determine sarcopenia and its association with normal nutrition status in 56 patients with respiratory failure. Sarcopenia was defined as an L3 skeletal muscle index of ≤38.5 cm(2)/m(2) for women and ≤52.4 cm(2)/m(2) for men. CT imaging and SGA classifications completed within 14, 10 and 7 days of each other were analyzed to assess sarcopenia and the influence of time between scans on misclassification (ie, normal nourished and sarcopenic). Descriptive statistics were conducted. RESULTS The average patient was 59.2 (± 15.6) years old, admitted with sepsis/infection, an APACHE II score of 26 (± 8.0), and BMI of 28.3 (± 5.8). Sarcopenia and sarcopenic obesity were prevalent in a minimum of 56% and 24% of patients, respectively, depending on the number of days between CT imaging and SGA assessment. Misclassified individuals were predominantly male, minority and overweight or obese. Controlling for age, no significant differences were noted for patients classified as normal nourished vs malnourished by SGA for lumbar muscle cross-sectional, whole-body lean mass, or skeletal muscle index. CONCLUSIONS Sarcopenia is highly prevalent among patients with respiratory failure requiring mechanical ventilation (MV) and not readily detected in patients classified as normal nourished using SGA.

Task-specific Training Reduces Trip-related Fall Risk in Women

PURPOSE The potential of task-specific training as a fall-prevention intervention was studied. The primary purpose of the study was to determine the extent to which a task-specific training protocol decreased the number of falls by middle-age and older women after a laboratory-induced trip. Secondary purposes were to explore the ability of trunk kinematics during the initial recovery step and the length of the initial recovery step to correctly classify the trip outcome and to quantify the extent to which the training protocol affected these variables. METHODS Healthy community-dwelling women (n = 52) were assigned to either a training group or a control group that received no training. Training group women participated in an individually tailored, task-specific training protocol during which forward-directed stepping responses were necessary to avoid a fall after treadmill-delivered postural disturbances. Following the protocol, the ability to avoid a fall after a laboratory-induced trip was assessed. The primary outcome variable was the success (recover) or failure (fall) of the posttrip stepping response. RESULTS Compared with the control group, there were fewer falls by the trained women after the laboratory-induced trip (P < 0.001; odds ratio = 0.13). Using logistic regression, falls and recoveries after the trip were sensitively classified by trunk flexion angle at the completion of the initial recovery step and the length of the initial recovery step (sensitivity = 0.67, specificity = 0.98), the former of which improved as a result of the task-specific training protocol. CONCLUSIONS The task-specific training protocol significantly reduced the number of falls after a laboratory-induced trip. Prospective study is required to determine whether this task-specific training reduces falls in the community and, consequently, may complement currently used exercise-based fall prevention intervention methods.

Exploitation of diagnostic computed tomography scans to assess the impact of nutrition support on body composition changes in respiratory failure patients.

BACKGROUND Assessment of nutritional status in intensive care unit (ICU) patients is limited. Computed tomography (CT) scans that include the first to fifth lumbar region completed for diagnostic purposes measures fat and lean body mass (LBM) depots and are frequently done in ICU populations and can be used to quantify fat and LBM depots. The purpose of this study was to assess if these scans could measure change in skeletal muscle (SKT), visceral adipose (VAT), and intermuscular adipose (IMAT) tissue and to examine the association between the amount of energy and protein received and changes in these depots. METHODS Cross-sectional area of SKT, VAT, and IMAT from CT scans at the third lumbar region was quantified at 2 time points (CT1 and CT2). Change scores between CT1 and CT2 for each of these depots and the percentage of estimated energy/protein needs received were determined in 33 adults that with acute respiratory failure. Descriptive statistics and multiple regression was used to evaluate the influence of baseline characteristics and the percentage energy/protein needs received between CT1 and CT2 on percentage change/day between CT1 and CT2 on SKM, IMAT, and VAT. RESULTS Participants were on average (SD) 59.7 (16) years old, received 41% of energy and 57% of protein needs. The average time between CT1 and CT2 was 10 (5) days. SKM declined 0.49%/day (men P = .07, women P = .09) and percentage of energy needs received reduced loss (β = 0.024, P = .03). No change in VAT or IMAT occurred. CONCLUSIONS CT scans can be exploited to assess change in body composition in ICU patients and may assist in detecting the causal link between nutritional support and outcomes in future clinical trials.

Finite element prediction of surface strain and fracture strength at the distal radius

To better understand the mechanisms underlying distal radius fracture we have developed finite element models to predict radius bone strain and fracture strength under loading conditions simulating a fall. This study compares experimental surface strains and fracture loads of the distal radius with specimen-specific finite element models to validate our model-generating algorithm. Five cadaveric forearms were instrumented with strain gage rosettes, loaded non-destructively to 300 N, and subsequently loaded until failure. Finite element models were created from computed tomography data; three separate density-elasticity relationships were examined. Fracture strength was predicted for three specimens that failed at the distal radius using six different failure theories. The density-elasticity relationship providing the strongest agreement between measured and predicted strains had a correlation of r=0.90 and a root mean squared error 13% of the highest measured strain. Mean absolute percent error (11.6%) between measured and predicted fracture loads was minimized with Coulomb-Mohr failure theory and a tensile-compressive strength ratio of 0.5. These results suggest that our modeling method is a suitable candidate for the in vivo assessment of distal radius bone strain and fracture strength under fall type loading configurations.

The presence of an obstacle influences the stepping response during induced trips and surrogate tasks

Falling is a frequent cause of serious injury in older adults and trips are a dominant cause of falls in this rapidly growing population. Although there are few laboratory protocols that induce actual trips, there are many protocols that utilize surrogate tasks. These surrogate tasks, which are time-critical but do not involve an obstacle, appear to share a number of biomechanical characteristics with stepping responses following a trip. However, although rapid and safe negotiation of the obstacle and restoration of dynamic equilibrium are common requisites for success, we expected that stepping response kinematics during a successful recovery from a trip over a previously unseen obstacle would be substantially different than those of surrogate tasks without an obstacle. Unexpected trips were induced in 13 older men and women by an obstacle, the presence of which the subjects were previously unaware. Selected kinematics of the leading and trailing limb stepping responses during recovery from the induced trip were compared to those of two surrogate tasks that did not involve an obstacle. Multivariate analysis of variance (MANOVA) revealed that step height, step length, peak horizontal velocity, and peak vertical velocity of the leading and trailing limbs were significantly different during recovery from the induced trip compared to the surrogate tasks. These between-task performance differences may limit the extent to which performance of the surrogate tasks accurately and precisely reflect the potential to recover dynamic equilibrium following a trip. Therefore, these findings may be applicable in the design of new or modification of existing interventions to reduce falls in older adults.

Modifiable performance domain risk-factors associated with slip-related falls

Falls are a major source of injury in older adults. Many falls occur after slipping. This study determined performance-related factors that both contribute to slip-related falls and that may be effectively and efficiently modified through targeted intervention. Thirty-five young adults and 21 healthy older adults (age: 70.9+/-5.1 years) were slipped in a laboratory using a slippery surface. The biomechanics of the 18 older adults who fell and the 30 younger adults who recovered following slips were analyzed. A set of potentially modifiable variables, initially based on significant between-groups differences, was further analyzed using stepwise discriminant analysis and logistic regression. The discriminant analysis correctly categorized 93.8% of the falls and recoveries based on two variables; the velocity of the slipping foot relative to the velocity of the whole body center of mass (COM), and the lateral placement of the recovery foot relative to the COM. The logistic regression determined the expected change in the odds of a recovery following a slip given a hypothesized intervention-induced improvement of these variables. Decreased velocity of the slipping foot relative to the COM, or decreased lateral placement of the recovery foot relative to the COM to zero, increased the odds of recovery by 17% and 27%, respectively. This suggests that intervention targeted at improving these specific lower extremity control variables following the onset of a slip has the potential to significantly decrease slip-related fall risk.

Exercise-based fall prevention: can you be a bit more specific?

Trip-specific perturbation training reduces trip-related falls after laboratory-induced trips and, prospectively, in the community. Based on an emerging body of evidence, we hypothesize that using task-specific perturbation training as a stand-alone approach or in conjunction with conventional exercise-based approaches will improve the effectiveness of fall prevention interventions significantly.

The mechanical consequence of actual bone loss and simulated bone recovery in acute spinal cord injury

INTRODUCTION Spinal cord injury (SCI) is characterized by rapid bone loss and an increased risk of fragility fracture around regions of the knee. Our purpose was to quantify changes in torsional stiffness K and strength Tult at the proximal tibia due to actual bone loss and simulated bone recovery in acute SCI. METHODS Computed tomography scans were acquired on ten subjects with acute SCI at serial time points separated by a mean of 3.9months (range 3.0 to 4.8months). Reductions in bone mineral were quantified and a validated subject-specific finite element modeling procedure was used to predict changes in K and Tult. The modeling procedure was subsequently used to examine the effect of simulated hypothetical treatments, in which bone mineral of the proximal tibiae were restored to baseline levels, while all other parameters were held constant. RESULTS During the acute period of SCI, subjects lost 8.3±4.9% (p<0.001) of their bone mineral density (BMD). Reductions in K (-9.9±6.5%; p=0.002) were similar in magnitude to reductions in BMD, however reductions in Tult (-15.8±13.8%; p=0.005) were some 2 times greater than the reductions in BMD. Owing to structural changes in geometry and mineral distribution, Tult was not necessarily recovered when bone mineral was restored to baseline, but was dependent upon the degree of bone loss prior to hypothetical treatments (r≥0.719; p≤0.019). CONCLUSIONS Therapeutic interventions to halt or attenuate bone loss associated with SCI should be implemented soon after injury in an attempt to preserve mechanical integrity and prevent fracture.

Reduction in Proximal Femoral Strength in Patients With Acute Spinal Cord Injury

Bone loss after spinal cord injury (SCI) is associated with an increased risk of fracture resulting from minor trauma. Proximal femoral fractures account for approximately 10% to 20% of the fractures in this population and are among the most serious of injuries. Our purpose was to quantify changes to proximal femoral strength in patients with acute SCI. Thirteen subjects received dual‐energy X‐ray absorptiometry (DXA) and clinical computed tomography (CT) scans at serial time points during acute SCI separated by a mean of 3.5 months (range 2.6 to 4.8 months). Areal bone mineral density (aBMD) at the proximal femur was quantified from DXA, and proximal femoral strength was predicted using CT‐based finite element (FE) modeling in a sideways fall configuration. During the acute period of SCI, femoral neck and total proximal femur aBMD decreased by 2.0 ± 1.1%/month (p < 0.001) and 2.2 ± 0.7%/month (p < 0.001), respectively. The observed reductions in aBMD were associated with a 6.9 ± 2.0%/month (p < 0.001) reduction in femoral strength. Thus, changes in femoral strength were some 3 times greater than the observed changes in aBMD (p < 0.001). It was interesting to note that in just 3.5 months of acute SCI, reductions in strength for some patients were on the order of that predicted for lifetime declines owing to aging. Therefore, it is important that therapeutic interventions are implemented soon after SCI in an effort to halt bone loss and decrease fracture risk. In addition, clinicians utilizing DXA to monitor bone health after SCI should be aware of the potential discrepancy between changes in aBMD and strength.