The effects of motion on force control abilities.

OBJECTIVE: To investigate the effects of motion on submaximal force control abilities in the knee extensors. DESIGN: Analysis of a force control task in a quasi-experimental design. BACKGROUND: Measuring strength control rather than strength capacity may provide insights in therapeutic intervention. METHODS: Twenty younger and 19 older community dwelling healthy adult males were instructed to maintain knee extensor strength at two levels of strength (20% and 60% of the maximal voluntary contraction) within a bandwidth (+/-6.10 N m) with visual feedback in isometric and isokinetic force control conditions. The effects of motion, force level, and age on force control ability were investigated. RESULTS: The addition of motion to the force control task decreased performance for both groups at both force levels. Isokinetic force control, compared to the isometric, increased force variability at both force levels. In isokinetic force control, as the force level increased, performance decreased for both groups. For the 60% maximal voluntary contraction isokinetic force control task, the older adults, compared to the younger, had an increased bias, increased percent time in bandwidth, and decreased number of correctional instances. CONCLUSIONS: This study has demonstrated that in the presence of motion, healthy younger and older adults experienced a decreased ability to maintain submaximal forces and their ability decreased even further with an increase in force level. Age group differences became more apparent by adding motion and increasing the force level. Further, we have successfully adapted the methodology used to assess isometric force control abilities to isokinetic force control testing in the lower extremities.

Upper extremity control in adults post stroke with mild residual impairment.

Motor control deficits in the upper extremity (UE) ipsilateral to the side of brain damage persist after stroke, but it is not known if the presence of these deficits is related to impairment of the contralateral UE. The purpose of this study was to investigate whether motor deficits are present in the ipsilateral UE when contralateral UE impairment is mild in adults with chronic stroke. Right-handed adults (10 controls, 10 right stroke, 10 left stroke) performed rapid continuous aiming movements to small and large targets. Using kinematic analysis, temporal measures of the movement were defined, including movement time (MT) and the three components of MT: acceleration, deceleration, and dwell time (i.e., time on target). Participants with right stroke had prolonged MT only with the left UE, primarily due to longer dwell times. Participants with left stroke had prolonged MT with both UEs as a result of longer dwell times. The results indicate that control deficits of the ipsilateral UE are evident in individuals with left but not right brain damage who have minimal impairment of the contralateral UE. These findings are consistent with the role of the left hemisphere in the control of both UEs.

Effects of age on balance assessment using voluntary and involuntary step tasks.

BACKGROUND: Taking a step while standing and modifying a step while walking are two strategies often used to maintain balance when balance disturbances are encountered during activities of daily living. This study investigated whether performance on an involuntary step task, which is assumed to be a surrogate for fall recovery abilities, was comparable to performance on a voluntary step task. METHODS: The performance of a voluntary and an involuntary step task was measured in healthy young adult (mean age 21 years) and healthy elderly adult (mean age 68 years) female subjects. Subjects stepped as fast as possible in the direction of a minimally destabilizing lateral waist pull (voluntary step task), or they responded naturally to a large destabilizing lateral waist pull (involuntary step task). The effects of age, task, and their interaction on the primary outcome variables of step foot liftoff time, landing time, step length, and step height were examined. RESULTS: In the voluntary step task, the older adults, compared to the young, required significantly more time to lift their foot (Young: 307 msec; Elderly: 424 msec). In the involuntary step task, the elderly were as quick as the young in lifting their foot (Young: 322 msec; Elderly: 335 msec). The young lifted their foot at about the same time for the two tasks. The elders, on the other hand, lifted their foot significantly earlier in the involuntary step task, compared to the voluntary step task (Vol: 424 msec; Invol: 335 msec). CONCLUSIONS: A voluntary step task underestimates the ability of healthy elderly adults to respond quickly when large destabilizing balance disturbances are encountered.

Stepping responses of young and old adults to postural disturbances: kinematics.

OBJECTIVES: When large disturbances of upright stance occur, balance must usually be restored by taking a step. We undertook this study to examine the biomechanics of stepping responses to sudden backward pulls at the waist. Primarily, response differences between young and old healthy adults were sought. DESIGN: A controlled laboratory study. SUBJECTS: Two groups of healthy and physically-fit adult females, 12 of mean age 22 (Young) and 12 of mean age 73 years (Old). MEASUREMENTS: Response kinematics were measured. From them, the stepping strategies of the subjects were derived, including the timing, length, and height of the first step taken and the rotations of major body segments and at major body joints that occurred. RESULTS: In response to sufficiently large backward pull forces, all subjects responded by taking one or more steps backwards. No significant age group difference appeared in the smallest disturbance for which subjects sometimes used a step response. A significant age group difference appeared in the smallest disturbance at which subjects began consistently to use step responses, and that disturbance was larger for the Old than for the Young. Distinct age group differences were found in stepping strategy. At large disturbance levels, the Young mostly responded by taking a single step, whereas the Old mostly responded by taking multiple steps. The steps taken by the Old, compared with those of the Young, were significantly shorter, had significantly smaller heights, and were taken significantly earlier in the responses. Body segment and joint rotations were generally modest, and few significant age group differences were found in these kinematics. CONCLUSIONS: In restoration of perturbed balance by step-taking, the responses of the healthy, physically-fit young and old adults studied here were similar in many respects, but they differed in some important features. Joint range of motion (ROM) limitations are unlikely to explain age group differences in stepping responses to postural disturbances among healthy subjects because the ROM actually used in any of the responses observed were substantially smaller than the ROM available.

A model for studies of the deformable rib cage.

An earlier model for the study of rib cage mechanics was modified so that rib deformity in scoliosis could be better represented. The rigid ribs of that model were replaced by five-segment deformable ribs. Literature data on cadaver rib mechanical behavior were used to assign stiffnesses to the new individual model ribs so that experimental and model rib deflections agreed. Shear and tension/compression stiffnesses had little effect on individual rib deformation, but bending stiffnesses had a major effect. Level-to-level differences in mechanical behavior could be explained almost exclusively by level to level differences in the rib shape. The model ribs were then assembled into a whole rib cage. Computer simulations of whole rib cage behaviors, both in vivo and in vitro, showed a reasonable agreement with the measured behaviors. The model was used to study rib cage mechanics in two scolioses, one with a 43 degrees and the other with a 70 degrees Cobb angle. Scoliotic rib cage deformities were quantified by parameters measuring the rib cage lateral offset, rib cage axial rotation, rib cage volume and rib distortion. Rib distortion was quantified both in best-fit and simulated computer tomography (CT) scan planes. Model rib distortion was much smaller in best-fit planes than in CT planes. The total rib cage volume changed little in the presence of the scolioses, but it became asymmetrically distributed.

Motions and loads within the human pelvis: a biomechanical model study.

The pelvis, a major load carrying component of the musculoskeletal system, is sometimes suspected as a site of mechanically provoked low back pain. Thus, studies of its basic biomechanics seem warranted. This study used biomechanical model simulations to examine how pelvic geometry and joint soft tissue properties influence relative motions among its bones and internal loadings of its joints. A biomechanical model of a pelvis was loaded by forces of up to 1000 N and moments of up to 25 Nm. Its resulting motions and joint loadings were determined. The effects on these responses due to pelvic geometry changes of 20% in lengths and 10 degrees in angles, and soft tissue stiffness changes, most of which were 25%, were also examined. For all situations examined, model pelvis relative displacements were at most a few mm, and relative rotations were at most a few degrees. Internal forces and moments were always less than 530 N and 30 Nm, respectively.