Biomechanical projects
Gait analysis in obese adults and children
There are several clinical conditions that modify the human walking pattern. One such condition is obesity, being overweight. As the rate of obesity increases in children and adults the potential for life-long exposure to large body mass also increases. Because body mass has been related to the development of degenerative joint diseases, especially to knee osteoarthritis, it is important to understand how increased body mass modifies mechanical loads in lower extremity joints. Preliminary work from our laboratory indicate that healthy obese adults free of knee osteoarthritis compared with healthy lean adults adopt a different distribution of joint torques in their lower extremity (DeVita and Hortobagyi 2003). Joint torques and powers were statistically identical at the hip and knee but were 88% and 61% higher (both p < 0.000) at the ankle in obese compared to lean participants. Obese adults alter their gait biomechanics and despite their greater mass, they have less or equal knee torque and power as lean individuals. Our recent data in children ages 10-12 suggest that a knee-sparing adaptation in gait may occur at a young age, coupled with a compensatory increase in ankle loads. We plan to expand this work in the future by systematically examining gait adaptations and its clinical correlates across the lifespan, including the gait pattern of patients after weight loss.
Joint torques during the stance phase of walking. Obese subjects (BMI of 42, dashed and solid lines) reveal same absolute joint moments at the hip and knee and increased joint moments at the ankle compared with lean individuals (BMI of 27, gray dotted line). These data suggest that obese adults modify their gait and joint torques do not increase in a simple linear fashion at all joints during gait in these individuals.
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Mechanical Plasticity In Locomotion With Age
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
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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Neurophysiology projects
Age-related increase in agonist and antagonist muscle co-activation
Cortical stimulation approach
Advancing age alters the control of human voluntary movement. One manifestation of altered motor control with age is a modification in the balance between pairs of muscles around a joint. Young individuals tend to activate their flexor and extensor muscle pairs in an alternating pattern.
In contrast, old compared with young individuals activate their flexor and extensor muscles with more overlap.
This overlap is also called “co-activation”, that is the agonist and antagonist muscles fire not in an alternating fashion but simultaneously. The mechanism of this age-related change in activation pattern of agonist and antagonist muscles is unknown.
We have addressed this issue using transcranial magnetic brain stimulation. We found that old compared with young individuals exhibited reduced amount of cortical reciprocal inhibition (Hortobágyi et al. 2006b). We also found that age also modified the timing and the magnitude of cortical activity associated with the antagonist wrist extensors during rapid wrist flexion (Hortobágyi et al. 2006a).
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Coherence approach
Another approach to determining the mechanism of the age-related increase in agonist and antagonist muscle co-activation is coherence analysis that measures the similarity or correlation between two EMG signals in the frequency domain. Previous work in individuals with spinal cord injury showed that coherence between pairs of muscles may occur in the alpha band of 5 to 18 Hz, suggesting that this lower band is spinal in origin (Norton et al. 2004). In addition, EMG/EMG coherence in the higher-frequency beta band of 24-40 Hz is reduced after stroke and spinal cord injury, suggesting that it is supraspinally mediated (Farmer et al. 1993). We are applying coherence analysis to EMG data from pairs of muscles in young and old individuals as they walk on a treadmill at different speeds and inclines. The coherence project is a collaboration between researchers in the Biomechanics Laboratory at ECU and Drs. Peter Lin, Ou Bai, and Mark Hallett at the National Institutes of Health, Bethesda, MD.
Coherence analysis (Fig. 2) in a young and an old adult during treadmill walking at 3 miles per hour. Note that there is little coherence (vertical axis) between the EMG activity of the gastrocnmeius and tibialis anterior muscles in young subject as there are no coherence peaks appearing above the horizontal line denoting significance, compared with the old adult. The horizontal axis is frequency of EMG activity.
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Interhemispheric plasticity in human
According to traditional neuroanatomy, the left primary motor cortex (M1) controls voluntary movement produced by muscles on the right side of the body. However, recent research, including data from the Biomechanics Laboratory, suggests that there is substantial activity in the “non-involved”, left M1 as well (Hortobágyi et al. 2003). Indeed, when individuals exercise a finger of the right hand for several weeks using a particular task the skill or strength of the same finger in the left hand also improves. We suspect that there is communication between the two hemispheres that mediate such transfer of force or skill from one side to the other. One mechanism involved in such transfer of function is the modulation of the amount of inhibition that is normally present between pairs of motor cortical areas. In collaboration with researchers at the National Institutes of Health we found that the amount of functional transfer correlates with the reduction in interhemispheric inhibition, measured with transcranial magnetic brain stimulation.
We are in the process of examining the task specificity associated with such transfers because our previous work suggests that the amount of transfer is larger when subjects exercise with lengthening compared with shortening contractions (Hortobágyi et al. 1997). Such differences in transfer may be related to the intrinsic differences in cortical control of muscles during shortening and lengthening contractions.
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Bibliography
- DeVita P, Hortobágyi T (2003) Obesity is not associated with increased knee joint torque and power during level walking. J Biomech 36: 1355-1362
- Farmer SF, Swash M, Ingram DA, Stephens JA (1993) Changes in motor unit synchronization following central nervous lesions in man. J Physiol 463: 83-105
- Hortobágyi T, del Olmo FM, Rothwell JC (2006a) Age alters the magnitude and timing of cortical control of antagonist muscle in humans. In: 5th World Congress of Biomechanics, vol S34. Journal of Biomechanics, Münich, p 8
- Hortobágyi T, Del Olmo MF, Rothwell JC (2006b) Age reduces cortical reciprocal inhibition in humans. Exp Brain Res 171: 322-329
- Hortobágyi T, Hill JP, Lambert NJ (1997) Greater cross education following training with muscle lengthening than shortening. Med. Sci. Sports Exerc. 29: 107-112
- Hortobágyi T, Taylor JL, Russell G, Petersen N, Gandevia SC (2003) Changes in segmental and motor cortical output with contralateral muscle contractions and altered sensory inputs in humans. J Neurophysiol 90: 2451-2459
- Norton JA, Wood DE, Day BL (2004) Is the spinal cord the generator of 16-Hz orthostatic tremor? Neurology 62: 632-634
