Biomechanical projects

Mechanical Plasticity In Locomotion With Age

Joint torques in young (solid) and old (dashed) adults during the stance phase of walking at 1.50 m/s. Old adults used more hip torque and less knee and ankle torque to walk at the same speed as young adults. The sum of the torques, the support torques, were identical in young and old adults. Therefore, old compared to young adults generated a larger proportion of their total torque at the hip joint and smaller proportions at the knee and ankle joints. (from Journal of Applied Physiology, 2000).

Joint powers in young (solid) and old (dashed) adults during the stance phase of walking at 1.50 m/s. Old adults generated more positive work at the hip and less positive and negative work at the knee and ankle joints to walk at the same speed as young adults. The sum of the powers, the total power, was larger in early stance in old adults but smaller in later stance compared to the total power in young adults. Overall, the total positive and negative works were equal between age groups. Therefore, old compared to young adults generated a larger proportion of mechanical work at the hip joint and smaller proportions at the knee and ankle joints. (from Journal of Applied Physiology, 2000).

Mechanical Plasticity In Locomotion With Age (P.I. DeVita): This NIH funded research investigates the fundamental biomechanical hypothesis that healthy human aging involves mechanical plasticity in locomotion that produces a distal to proximal shift in muscle function. This four year project will identify age-related adaptations in lower extremity muscle function in a variety of gait tasks such as level, stairway, and inclined walking. It will identify the developmental pattern of these alterations over the adult lifespan and it will identify the interaction of muscle strength with these adaptations.

The phenomenon of mechanical plasticity with age was first reported as a, “redistribution of joint torques and powers with age,” by Drs. DeVita and Hortobagyi in the year 2000. At self-selected walking speeds, elderly compared with young adults generate decreased joint torques and powers in the lower extremity. These differences may be actual gait-limiting factors and neuromuscular adaptations with age or simply a consciously selected motor pattern to produce a slower gait. The purpose of the study was to compare joint torques and powers of young and elderly adults walking at the same speed. Twelve elderly and fourteen young adults (ages 69 and 21 yr) walked at 1.48 m/s over a force platform while being videotaped. Hip, knee, and ankle torques and powers were calculated from the reaction force and kinematic data. A support torque was calculated as the sum of the three joint torques. Extensor angular impulse during stance and positive work at each joint were derived from the torques and powers. Step length was 4% shorter and cadence was 4% higher in elderly adults (both P , 0.05) compared with young adults. Support angular impulse was nearly identical between groups, but elderly adults had 58% greater angular impulse and 279% more work at the hip, 50% less angular impulse and 39% less work at the knee, and 23% less angular impulse and 29% less work at the ankle compared with young adults (t-test, all P , 0.05). Age caused a redistribution of joint torques and powers, with the elderly using their hip extensors more and their knee extensors and ankle plantar flexors less than young adults when walking at the same speed. Along with a reduction in motor and sensory functions, the natural history of aging causes a shift in the locus of function in motor performance.
Joint torques in young (solid) and old (dashed) adults during the stance phase of walking at 1.50 m/s. Old adults used more hip torque and less knee and ankle torque to walk at the same speed as young adults. The sum of the torques, the support torques, were identical in young and old adults. Therefore, old compared to young adults generated a larger proportion of their total torque at the hip joint and smaller proportions at the knee and ankle joints. (from Journal of Applied Physiology, 2000).
Joint powers in young (solid) and old (dashed) adults during the stance phase of walking at 1.50 m/s. Old adults generated more positive work at the hip and less positive and negative work at the knee and ankle joints to walk at the same speed as young adults. The sum of the powers, the total power, was larger in early stance in old adults but smaller in later stance compared to the total power in young adults. Overall, the total positive and negative works were equal between age groups. Therefore, old compared to young adults generated a larger proportion of mechanical work at the hip joint and smaller proportions at the knee and ankle joints. (from Journal of Applied Physiology, 2000).


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Let’s Be More Positive Than Negative About Muscle Work

Negative, positive and net muscle work in level walking at a constant average velocity. Hip and ankle muscles produced net positive work (i.e. generated energy) while knee muscles produced negative work (i.e. dissipated energy). The net result was that level walking had a net positive amount of muscle work of 16 J per step despite the maintenance of constant average mechanical energy during the stride.

Net muscle work while ascending and descending a 10° ramp and a stair way. Muscle work was 25% and 43% greater in ramp and stair ascent compared to ramp and stair descent. Despite the identical change in total mechanical energy in corresponding ascending and descending gaits, muscles did more work while ascending. We identified three potential causes for the reduced negative vs. positive work in these locomotion tasks: 1) the larger magnitude of the accelerations induced by the larger ground reaction forces in descending compared to ascending gaits elicited greater energy dissipation in non-muscular tissues, 2) the ground reaction force vector was directed closer to the joint centers in ramp and stair descent compared to ascent which reduced the load on the muscular tissues and their energy dissipating response, and 3) despite the need to produce negative muscle work in descending gaits, both ramp and stair descent also had positive muscle work to propel the lower extremity upward and forward into the swing phase movement trajectory.

Muscle work during level walking and ascent and descent ramp and stairway walking was assessed for the purpose of exploring the proposition that muscles perform more positive than negative work during these locomotion tasks. Thirtyfour healthy human adults were tested while maintaining a constant average walking velocity in the five gait conditions. Ground reaction force and sagittal plane kinematic data were obtained during the stance phases of these gaits and used in inverse dynamic analyses to calculate joint torques and powers at the hip, knee, and ankle. Muscle work was derived as the areas under the joint power vs. time curves and was partitioned into positive, negative, and net components. Dependent t-tests were used to compare positive and negative work at each joint in level walking and net joint work between ascent and descent gaits on the ramp and stairs (p<0.010). Total negative and positive work in level walking were –34 J and 50 J, respectively with the difference in magnitude being statistically significantly (p<0.001). Level walking was therefore performed with 16 J of net positive muscle work (see figure 1). The magnitude of the net work in ramp ascent was 25% greater than the magnitude of net work in ramp descent (89 vs. -71 J/m, p<0.010, see figure 2). Similarly, magnitude of the net work in stair ascent was 43% greater than the magnitude of net work in stair descent (107 vs. -75 J/step, p<0.000). We identified three potential causes for the reduced negative vs. positive work in these locomotion tasks: 1) the magnitude of the accelerations induced by the ground reaction forces were large enough to elicit energy dissipation in non-muscular tissues, 2) the direction of the ground reaction force vector was directed closer to the joint centers in ramp and stair descent vs. ascent which reduced the load on the muscular tissues and their energy dissipating response, and 3) despite the need to produce negative muscle work in descending gaits, both ramp and stair descent also had positive muscle work to propel the lower extremity upward and forward into the swing phase movement trajectory. We used these data to formulate two novel hypotheses about human locomotion. First, level walking requires muscles to generate a net positive amount of work per gait cycle to overcome energy losses by other tissues. Second, skeletal muscles generate more mechanical energy in gait tasks that raise the center of mass compared to the mechanical energy they dissipate in gait tasks that lower the center of mass despite equivalent changes in total mechanical energy.


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