James J. Dowling

Role of cocontraction in the O2 cost of walking in children with cerebral palsy

UNLABELLED A major movement related limitation for children with spastic cerebral palsy (CP) is the compromised gait pattern, which may explain their excessive energy cost of locomotion. The aims of this study were to determine differences in the O2 cost of locomotion between children with CP (7 males, 2 females; 12.7 +/- 2.8 yr) and able-bodied controls (7 male, 1 female; 13.6 +/- 2.1 yr) and to assess the contribution that cocontraction of agonist and antagonist muscles had upon the elevated O2 cost seen in children with CP versus able-bodied controls. The treadmill submaximal walking protocol consisted of 2 x 4 min intermittent stages at 3 km.h-1 and 90% of the predetermined fastest walking speed (FWS) at 0% grade. Electromyographic data were collected during the final minute of each bout from vastus lateralis and hamstrings (thigh) and tibialis anterior and soleus (lower leg). Significant (P < 0.05) differences were noted at 3 km.h-1 for mass-relative VO2. (CP: 16.6 +/- 6.5 vs control: 10.2 +/- 1.2 ml.kg-1.min-1), % VO2max (CP: 53.5 +/- 26.0 vs CONTROL 22.5 +/- 4.93) and heart rate (CP: 143 +/- 41 vs CONTROL 91 +/- 14 beats.min-1). Thigh and lower leg muscle cocontraction accounted for 51.4% and 42.8%, respectively, of the variability in VO2 for the subjects with CP at 3 km.h-1. These results suggest that cocontraction is a major factor responsible for the higher energy cost of walking seen in children with CP.

Cocontraction in three age groups of children during treadmill locomotion.

This study attempted to assess and compare the amount of cocontraction present in thigh and leg muscles in three groups of children during treadmill walking and running. Thirty children, aged 7-8 (n = 10), 10-12 (n = 10) and 15-16 (n = 10) years, performed 4-min bouts of submaximal treadmill exercise at two walking and four running speeds, assigned in a randomized order. Three seconds of EMG data were collected during the final minute of each bout from the vastus lateralis (VL), hamstrings (H), tibialis anterior (TA) and soleus (S). The processed linear envelopes of VL and H, and likewise of TA and S, were overlapped and a cocontraction index calculated (area of overlap divided by the number of data points) for thigh and leg segments, respectively. Cocontraction was highest for the youngest children and lowest for the oldest, for both thigh and leg, whether expressed in terms of absolute speed or as a percentage of each childs VO(2 max). Larger amounts of cocontraction may help to explain the higher metabolic cost of locomotion for younger children, when compared with adolescents and adults.

Are humans able to voluntarily elicit maximum muscle force

Previous investigators have used electrical stimulation superimposed on voluntary efforts to show that humans are capable of maximum muscle activation. In the present study this notion was tested using the interpolated twitch technique enhanced by triggered averaging and doublet stimulation of the human biceps brachii. It was found that the decline in extra torque evoked by percutaneous stimulation with increasing levels of voluntary effort was nonlinear and that none of the twelve subjects was able to elicit a voluntary effort large enough to prevent extra torque of elbow flexion. The exponential nature of the declining extra torque indicated that an extrapolated maximum muscle force could be considerably larger than that to which subjects were able to elicit voluntarily.

Analysis of body segment parameter differences between four human populations and the estimation errors of four popular mathematical models.

Calculating the kinetics of motion using inverse or forward dynamics methods requires the use of accurate body segment inertial parameters. The methods available for calculating these body segment parameters (BSPs) have several limitations and a main concern is the applicability of predictive equations to several different populations. This study examined the differences in BSPs between 4 human populations using dual energy x-ray absorptiometry (DEXA), developed linear regression equations to predict mass, center of mass location (CM) and radius of gyration (K) in the frontal plane on 5 body segments and examined the errors produced by using several BSP sources in the literature. Significant population differences were seen in all segments for all populations and all BSPs except hand mass, indicating that population specific BSP predictors are needed. The linear regression equations developed performed best overall when compared to the other sources, yet no one set of predictors performed best for all segments, populations or BSPs. Large errors were seen with all models which were attributed to large individual differences within groups. Equations which account for these differences, including measurements of limb circumferences and breadths may provide better estimations. Geometric models use these parameters, however the models examined in this study did not perform well, possibly due to the assumption of constant density or the use of an overly simple shape. Creating solids which account for density changes or which mimic the mass distribution characteristics of the segment may solve this problem. Otherwise, regression equations specific for populations according to age, gender, race, and morphology may be required to provide accurate estimations of BSPs for use in kinetic equations of motion.

Role of mechanical power estimates in the O2 cost of walking in children with cerebral palsy.

UNLABELLED It has been established in able-bodied children that traditional biomechanical descriptors of gait such as stride length or stride frequency do not fully account for the differences seen in the energy cost of locomotion noted with age. Hence, measures of total body mechanical power output have been adopted to explain these differences. PURPOSE The aim of this study was to estimate the ability of this mechanical power calculation to explain the variability in the metabolic energy cost of treadmill walking in children with spastic cerebral palsy (CP). METHODS Thirteen subjects volunteered for the study. One group consisted of eight (6 male, 2 female) children with CP (age 12.2 +/- 2.7 yr). The second group consisted of five (4 male, 1 female) able-bodied controls (age 13.4 +/- 2.8 yr). The treadmill walking protocol consisted of one 4-min stage at 0% grade, 3 km x h(-1). Infrared markers were placed on 12 anatomical landmarks and data were collected using the OPTOTRAK motion analysis system over a 5-s time period during the last 30 s of the 4-min stage. On-line oxygen consumption VO2 measurements were obtained throughout using the Beckman Horizon Metabolic Cart. RESULTS Relative VO2 (mL x kg(-1) x min(-1)) was significantly (P < 0.05) different between the two groups (CP: 16.6 +/- 6.5 vs control: 10.2 +/- 1.2). Simple linear regression analysis demonstrated that mechanical power measurements, incorporating transfers of energy between and within adjacent body segments, accounted for 87.2% of the total variability noted in VO2 for the children with CP, compared with only 2.4% in the able-bodied subjects. CONCLUSIONS The results indicate that mechanical power differences explain the majority of the variability noted in VO2 in children with CP at a submaximal walking speed.

The use of electromyography for the noninvasive prediction of muscle forces. Current issues.

SummarySuitably processed electromyographic (EMG) signals can be combined with Hill-type musculoskeletal models to noninvasively achieve estimations of individual muscle forces. This method has particular advantages over other methods for the assessment of a given performance. The purpose of this review is to report on the current issues facing the human movement scientist who wishes to extend the kinetic information yielded by linked segment models to the kinetics of individual muscles. Such an extension is necessary when considering co-contraction of antagonistic muscles, the role of bi-articular muscles, coordination, movement efficiency or bone-on-bone forces.Currently, linked segment models have not been successfully extended to individual muscle forces for diagnostic purposes by using the EMG approach or any other approach. Most models have been designed for a specific purpose and have only been evaluated over a narrow range of movement conditions. More generalised models will require greater complexity and possibly more extensive calibration or an increased number of specific inputs or greater computational effort. This review shows the promise of the EMG approach and presents the challenges, as well as the strategies, that should enable more general, accurate and precise estimates of individual muscle forces.

Explaining differences in the metabolic cost and efficiency of treadmill locomotion in children

The metabolic cost of locomotion at any given speed, when expressed per kilogram of body mass, is greater for children than for older individuals. Incomplete explanations for the age-related difference motivated this study, which used a multidisciplinary method to examine metabolic, kinematic and electromyographic data from three maturational groups of children. Thirty children aged 7-8 ( n = 10), 10-12 ( n = 10) and 15-16 ( n = 10) years completed 4 min bouts of submaximal treadmill exercise at six speeds - two walking and four running - assigned in random order. Metabolic (net V O 2 ), kinematic (total body mechanical power, energy transfer rates, stride rate) and electromyographic (co-contraction of agonist and antagonist muscles in thigh and leg segments) data were collected. Multiple regression analysis was performed with net V O 2 or efficiency as the dependent variable and mechanical power, thigh and leg co-contraction, stride rate and age as independent variables. It was possible to explain up to 77% of the age-related variance in net V O 2 and 62% of the variation in efficiency using combinations of these variables. Age was the best single predictor of both V O 2 and efficiency. Co-contraction, possibly used to enhance joint stability, was an important component of the observed age-related differences, although mechanical power was not. Additional variance might be explained as specific growth-related factors affecting the metabolic cost of locomotion are identified, as mechanical work models improve, and as methods are developed to measure the effects of stored elastic energy and the metabolic cost of isometric muscle actions.

Body Segment Parameter Estimation of the Human Lower Leg Using an Elliptical Model with Validation from DEXA

Accurate estimates of human body segment parameters (BSPs) are required for kinetic analyses of motion. The purpose of this study was to develop a geometric model of the human lower leg based on the mass distribution properties of the segment. Forty subjects were recruited from 4 human populations. Each population was randomly divided equally into model development (MD) and model validation (MV) groups. Participants underwent frontal and sagittal plane dual energy X-ray absorptiometry (DEXA) scans and anthropometric measurements. Leg BSPs were calculated from the scan information and mass distribution properties in the two planes were determined. Further, a geometric model was developed based on the ensemble averages of the mass distribution information from the MD groups. The model was applied to the MV groups and mean absolute errors were calculated for each BSP and each population. Finally, BSP estimates from literature sources were also determined and compared against DEXA. The model developed produced the lowest errors overall. Additionally, the results showed that the model developed estimated BSPs for all four populations with consistent accuracy whereas the other 4 models tested provided different levels of accuracy depending on the age and gender categories of the group tested. The results of this study present a model that accurately estimates BSPs of the lower leg for individuals varying in age, gender, race, and morphology. This study also presents a modelling technique that may successfully provide similar results for other body segments.

mechanical and Metabolic Work of Persons with Lower-extremity Amputations Walking with Titanium and Stainless Steel Prostheses : a Preliminary Study

A total of 15 subjects with unilateral amputations (8 transfemoral and 7 transtibial) performed treadmill walking with prostheses assembled from titanium and stainless steel components to determine if mass differences had an effect on walking. Standardized components (knees, pylons, adapters, feet) made from each material were added below the level of the socket. Metabolic cost from submaximal oxygen consumption and mechanical power estimates allowing transfers within and between segments were calculated during steady-state walking at self-selected velocities. Results showed that despite significant mechanical power differences, the decreased mass associated with the use of titanium materials did not have a significant effect on the metabolic costs of walking, stride rate, or stride length. Further division of subjects by age and experience walking on a prosthesis suggested that older persons and established walkers benefit most from the use of titanium components, both metabolically and mechanically.

Ability of mechanical power estimations to explain differences in metabolic cost of walking and running among children

The relationship between metabolic cost and three estimates of mechanical power was examined for three groups of children. Thirty children, aged 7–8 (n = 10), 10–12 (n = 10) and 15–16 (n = 10) years, performed 4-min bouts of submaximal treadmill exercise at 6 randomly assigned speeds, 2 walking and 4 running. When metabolic and mechanical power measures were correlated, the two segmental methods were good predictors of VO2 for individuals, but not for groups. When two age groups performed the same absolute speed, VO2 was significantly different between groups, but mechanical power and energy transfer rates were not. The lower metabolic cost of the older children could not be explained by differences in mechanical power or energy transfer rates.