1,720,991 research outputs found
Force platforms as ergometers
No full textWalking and running on the level involves external mechanical work, even when speed averaged over a complete stride remains constant. This work must be performed by the muscles to accelerate and/or raise the center of mass of the body during parts of the stride, replacing energy which is lost as the body slows and/or falls during other parts of the stride. External work can be measured with fair approximation by means of a force plate, which records the horizontal and vertical components of the resultant force applied by the body to the ground over a complete stride. The horizontal force and the vertical force minus the body weight are integrated electronically to determine the instantaneous velocity in each plane. These velocities are squared and multiplied by one-half the mass to yield the instantaneous kinetic energy. The change in potential energy is calculated by integrating vertical velocity as a function of time to yield vertical displacement and multiplying this by body weight. The total mechanical energy as a function of time is obtained by adding the instantaneous kinetic and potential energies. The positive external mechanical work is obtained by adding the increments in total mechanical energy over an integral number of strides
The two asymmetries of the bouncing step
No full textThe increase of the push on the ground with increasing running speed improves the “elastic” rebound of the body by privileging the role of tendons relative to muscle within muscle-tendon units
The bounce of the body in hopping, running and trotting : different machines with the same motor
Full text linkThe bouncing mechanism of human running is characterized by a shorter duration of the brake after ‘landing’ compared with a longer duration of the push before ‘takeoff ’. This landing – takeoff asymmetry has been thought to be a consequence of the force – velocity relation of the muscle, resulting in a greater force exerted during stretching after landing and a lower force developed during shortening before take-off. However, the asymmetric lever system of the human foot during stance may also be the cause. Here, we measure the landing – takeoff asymmetry in bouncing steps of running, hopping and trotting animals using diverse lever systems. We find that the duration of the push exceeds that of the brake in all the animals, indicating that the different lever systems comply with the basic property of muscle to resist stretching with a force greater than that developed during shortening. In addition, results show both the landing – takeoff asymmetry and the mass-specific vertical stiffness to be greater in small animals than in large animals. We suggest that the landing – takeoff asymmetry is an index of a lack of elasticity, which increases with increasing the role of muscle relative to that of tendon within muscle – tendon units
An analysis of the mechanics of phonation
No full textThe mechanical work done by the chest in phonation has been measured together with the sound intensity level. The regulation of the sound intensity is done by regulating the intrapulmonary pressure. This is achieved at high intensity levels through the activity of the respiratory muscles that, together with the elastic recoil of the chest, sustain the work of phonation. At sound intensities below a critical level an additional mechanism for changing the intensity is given by a fine regulation of the opening of the glottis, thus allowing more air to escape without contributing to sound production. The contribution of the respiratory muscles, of the chest elasticity, and of the opening of the glottis to phonation at different intensity levels depend on the degree of inflation of the chest. The efficiency of phonation, as of sound production in mechanical models, seems to increase with increasing intensity and pitch.voice production; work done by chest during phonation; mechanical models of glottis generator; subglottic pressure as a function of sound level; air flow through glottis during phonation; efficiency changes of sound production; variation of sound intensity by regulating opening of glottis; variations of the area of glottis depending on extent of elastic recoil of chestSubmitted on February 10, 196
Efficiency of vertebrate locomotory muscles
No full textWe have examined the efficiency of vertebrate striated muscle at two different organizational levels: whole animals and isolated muscles. Terrestrial locomotion is used as a model of 'normal' muscular contraction; animal size and running speed are used as independent variables in order to change either the metabolic requirements of the muscles or the mechanical power production by the muscles over a wide range of values. The weight-specific metabolic power input to an animal increases nearly linearly with speed and increases with decreasing body size, while the weight-specific mechanical power output increases curvilinearly with speed and is independent of size. Consequently, the efficiency of the muscles in producing positive work increases with speed and the peak efficiency increases with increasing body size, attaining values of over 70% in large animals, but only 7% in small ones. The isolated muscle experiments were performed on frog muscle, and rat 'fast' and 'slow' muscles. We measured the work done, the oxygen consumed during recovery from the stimulation, and calculated the efficiency and the 'economy' (the cost of maintaining tension). The muscles were made to: (i) emulate the contractions seen during locomotion, i.e. shorten after a pre-stretch; or (ii) shorten at the same velocity and from the same muscle length as in (i), but without the pre-stretch. It was found that in mammalian muscles the peak efficiency with a pre-stretch attained high values, approaching the peak efficiencies in large animals. The maximum efficiency (attained at 1 length s-1 in fast muscle and at 0.5 lengths s-1 in slow muscle) did not differ much in the two muscles, whereas economy was greater in the slow muscle than in the fast muscle
Physiological aspects of legged terrestrial locomotion: the motor and the machine
Full text linkThis book offers a succinct but comprehensive description of the mechanics of muscle contraction and legged terrestrial locomotion. It describes on the one hand how the fundamental properties of muscle tissue affect the mechanics of locomotion, and on the other, how the mechanics of locomotion modify the mechanism of muscle operation under different conditions. Further, the book reports on the design and results of experiments conducted with two goals. The first was to describe the physiological function of muscle tissue (which may be considered as the “motor”) contracting at a constant length, during shortening, during lengthening, and under a condition that occurs most frequently in the back-and-forth movement of the limbs during locomotion, namely the stretch-shortening cycle of the active muscle. The second objective was to analyze the interaction between the motor and the “machine” (the skeletal lever system) during walking and running in different scenarios with respect to speed, step frequency, body mass, gravity, age, and pathological gait. The book will be of considerable interest to physiology, biology and physics students, and provides researchers with stimuli for further experimental and analytical work
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