Mobility impairment in the elderly: challenges for biomechanics research

Age differences in abilities to perform tandem stand and walk tasks of graded difficulty

We quantified the abilities of 12 young (mean age 23.3 years) and 12 old (mean age 72.0 years) healthy women to tandem stand and tandem walk on a set of six beams, ranging from 15 cm down to 2.5 cm in width. Tandem stand tasks were conducted with eyes both open and closed. Tandem walk tasks were conducted with eyes open, and with both heel-to-toe and comfortable-distance step types. Significant age differences were found in the mean rate of successful performance for both tandem stand and tandem walk (P<0.001, rm-ANOVA). For both age groups, significant main effects for beam width were found for both tandem stand and tandem walk tasks. During tandem stand with eyes closed, young women were always successful on the widest beam while elderly women had an average success rate of only 62% and demonstrated an increased dependence on vision on narrower beams. Copyright 1998 Elsevier Science B.V. All rights reserved

Real-time feedback of body center of gravity for postural training of elderly patients with peripheral neuropathy

A system that estimates and displays, in real-time, the location of the center of gravity of the human body relative to the feet was developed and then used in a biofeedback training program for improving the postural instability caused by deterioration of the proprioceptive system in elderly patients with significant diabetic sensory neuropathy during perturbations of a support platform. Training sessions were scheduled three times a week for four weeks and consisted of continuous perturbations of the support platform for the standing subject. Postural stability of each subject was evaluated before, during and after training on a weekly basis. After twelve sessions of training, subjects who received visual feedback showed greater reduction in total number of falls and in fall magnitude defined by force in the body suspension cord, and longer delay in time between perturbation and fall than NFB group.

Stepping over an obstacle increases the motions and moments of the joints of the trailing limb in young adults

Tripping over obstacles is the most frequently mentioned causes of falls. Thus, this study was performed to test the hypotheses that when crossing obstacles, toe-obstacle clearance and the three-dimensional motions and moments at the hip, knee, and ankle of the trailing limb (limb crossing the obstacle last) increase with obstacle height. Data were collected using an optoelectronic digitizing system and force platform. Fourteen healthy young adults were tested during unobstructed level walking and when stepping over obstacles of 51, 102, 153, or 204 mm heights. Toe-obstacle clearances of the trailing foot increased from 31 mm during unobstructed gait to an average of 146 mm when stepping over obstacles of any of these heights. Obstacle height was not found to affect toe-obstacle clearance. When the toe of the trailing limb was over the obstacle, the flexion angles of the hip and knee increased linerly with obstacle height. Compared to flexion of the hip or ankle, flexion of the knee appears to be of primary importance when crossing obstacles with the trailing limb. The maximum extension moment at the hip joint during late stance decreased linearly with obstacle height. At the knee joint, the maximum flexion moment during early stance and the maximum adduction moment during late stance increased linearly with obstacle height. At the ankle joint, the maximum dorsiflexion moment during late stance increased linearly with obstacle height. These greater demands on motions and moments may affect the abilities of those elderly having decreased muscle strengths to step over obstacles.

Altered fractal dynamics of gait: reduced stride-interval correlations with aging and Huntington's disease

Fluctuations in the duration of the gait cycle (the stride interval) display fractal dynamics and long-range correlations in healthy young adults. We hypothesized that these stride-interval correlations would be altered by changes in neurological function associated with aging and certain disease states. To test this hypothesis, we compared the stride-interval time series of 1) healthy elderly subjects and young controls and of 2) subjects with Huntington's disease and healthy controls. Using detrended fluctuation analysis we computed alpha, a measure of the degree to which one stride interval is correlated with previous and subsequent intervals over different time scales. The scaling exponent alpha was significantly lower in elderly subjects compared with young subjects (elderly: 0.68 +/- 0.14; young: 0.87 +/- 0.15; P < 0.003). The scaling exponent alpha was also smaller in the subjects with Huntington's disease compared with disease-free controls (Huntington's disease: 0.60 +/- 0.24; controls: 0.88 +/-0.17; P < 0.005). Moreover, alpha was linearly related to degree of functional impairment in subjects with Huntington's disease (r = 0.78, P < 0.0005). These findings demonstrate that strike-interval fluctuations are more random (i.e., less correlated) in elderly subjects and in subjects with Huntington's disease. Abnormal alterations in the fractal properties of gait dynamics are apparently associated with changes in central nervous system control.

Postural control in young and elderly adults when stance is perturbed: dynamics

Responses in maintaining or restoring standing balance were measured in 24 healthy young and 15 healthy elderly adults (mean ages 26 and 72) under four task conditions: two involving self-generated motions and two involving imposed disturbances. The two primary objectives of the study were to quantify the whole-body dynamics of these responses and to identify any age related differences in those dynamics. Response dynamics were analyzed using a seven-link biomechanical model. In terms of approximate population-mean values, maximum whole-body center of mass (CM) excursions ranged to 3 cm, maximum center of support-surface reaction (CR) excursions ranged to 8 cm, vertical reaction force changes ranged to 50 N, anteroposterior support surface reactions ranged to 30 N, maximum joint torques used per side ranged to 20 Nm and peak angular momenta about a transverse axis through the ankles ranged to 6 kg m2 s-1. The elderly adults, compared to the young tended to exhibit higher-frequency oscillations in excursions and larger horizontal excursions of their CM and CR, tended to develop larger support surface reactions and use larger response joint torques, and tended to arrest less of their angular momentum in their first cycle of response during the two imposed-disturbance tasks. Only some of these tendencies proved statistically significant. The results suggest that healthy elderly subjects with no apparent musculoskeletal or neurological impairments differ from healthy young adult subjects in their responses to modest perturbations of upright stance. However, the differences are generally not large and their magnitudes are perturbation-specific.

Aging of human muscle: structure, function and adaptability

With increasing age, human skeletal muscles gradually decrease in volume, mainly due to a reduced number of motor units and muscle fibers, and a reduced size of type 2 fibers. As a result, progressive weakening and impaired mobility occur. High-resistance strength training is beneficial, even in the very old, and could possibly reverse some of the detrimental effects of age-related weakness. The importance of exercise for older people affords an excellent opportunity for the medicine community as a major source of information and promotion of physical activity for this rapidly growing segment of the population. In this review, we summarize the current knowledge of the effects of aging on the human neuromuscular system, describe some of the major underlying mechanisms of the aging atrophy and focus on the importance of strength training to improve muscle function in older people.

Predicting the kinematics and kinetics of gait based on the optimum trajectory of the swing limb

An algorithm was developed to predict the minimum energy consumption trajectory of the swing limb. The method of dynamic programming, a multistage optimization method, was applied to generate the optimum trajectory of the swing ankle which minimized the mechanical energy required to generate the moments of the joints of the lower extremities during the single support phase of gait. Predictions and measurements of gait were compared for six healthy subjects. The predicted hip and knee flexion angles of the swing limb were not significantly different from those experimentally measured except for hip flexion at times greater than 75% of the swing period. The predicted ground reaction forces were not significantly different from the measured ground reaction forces. Furthermore, the moments about the joints were not significantly different from those computed using the measured ground reaction forces and kinematics of the limbs. The results of this study support the hypothesis that human gait is energy efficient.

A comparison of grip strength and selected psychomotor performance measures in healthy and frail elderly females

Researchers have examined the relationships between strength and psychomotor skills among healthy male subjects far more thoroughly than among female subjects, especially elderly females. The purpose of the present investigation was to compare healthy and frail elderly females on grip strength and psychomotor performance measures. Nineteen healthy elderly females (M = 75.7 years of age) and 20 frail elderly females (M = 80.5 years of age) performed simple aiming movements in order for the investigators to obtain psychomotor measures (e.g., reaction time, movement time, kinematic movement characteristics). In addition, strength measures were obtained in the dominant and nondominant hands. Significant differences were found between elderly females on grip strength and psychomotor performance measures. In general, healthy subjects were stronger and exhibited superior control of movements (e.g., greater impulse, fewer movement adjustments).

Optimal control of non-ballistic muscular movements: a constraint-based performance criterion for rising from a chair

To understand how humans perform non-ballistic movements, we have developed an optimal control model to simulate rising from a chair. The human body was modeled as a three-segment, articulated, planar linkage, with adjacent links joined together by frictionless revolutes. The skeleton was actuated by eight musculotendinous units with each muscle modeled as a three-element entity in series with tendon. Because rising from a chair presents a relatively ambiguous performance criterion, we chose to evaluate a number of different performance criteria, each based upon a fundamental dynamical property of movement; muscle force. Through a quantitative comparison of model and experiment, we found that neither a minimum-impulse nor a minimum-energy criterion is able to reproduce the major features of standing up. Instead, we introduce a performance criterion based upon an important and previously overlooked dynamical property of muscle: the time derivative of force. Our motivation for incorporating such a quantity into a mathematical description of the goal of a motor task is founded upon the belief that non-ballistic movements are controlled by gradual increases in muscle force rather than by rapid changes in force over time. By computing the optimal control solution for rising from a static squatting position, we show that minimizing the integral of a quantity which depends upon the time derivative of muscle force meets an important physiological requirement: it minimizes the peak forces developed by muscles throughout the movement. Furthermore, by computing the optimal control solution for rising from a chair, we demonstrate that multi-joint coordination is dictated not only by the choice of a performance criterion but by the presence of a motion constraint as well.

Body Segment Moments of Inertia of the Elderly

The segment principal moments of inertia of a sample of 7 elderly males and 12 elderly females were estimated using a model based on stacked elliptical cylinders at 2-cm intervals. Apart from the thigh, all male parameters were larger than female parameters. The largest differences were for the lower trunk and hand and for the forearm. The inertia parameters of the thigh for the males were about 12% smaller than the females. Nonlinear estimations of segment principal moments were then determined. The effect of the differences was tested by cross validating cadaver results against the elliptical model results. The regressions were then cross validated using an independent sample of 6 subjects. The standard errors of fit given as a percentage of the mean, Sf, were smaller than the cross validation results for the cadaver regressions and the differences were attributed to differences between cadavers and living subjects.