187,099 research outputs found
Step frequency in walking
No full textIncreasing the step length and decreasing the step frequency at a given speed of walking requires a greater power to lift and accelerate the centre of mass of the body (Ẇ(ext)), whereas decreasing the step length and increasing the step frequency requires a greater power to accelerate the limbs relative to the centre of mass (Ẇ(int)). The sum Ẇ(tot) = Ẇ(ext) + Ẇ(int) reaches a minimum at a step frequency which is 20%-30% less than that freely chosen during walking at that speed. The step frequency at which Ẇ(tot) is minimum increases with speed similarly to the natural step frequency during normal walking
Mechanics of competition walking
No full text1. The work done at each step to lift and accelerate the centre of mass of the body has been measured in competition walkers during locomotion from 2 to 20 km/hr. 2. Three distinct phases characterize the mechanics of walking. From 2 to 6 km/hr the vertical displacement during each step, Sv, increases to a maximum (3.5 vs. 6 cm in normal walking) due to an increase in the amplitude of the rotation over the supporting leg. 3. The transfer, R, between potential energy of vertical displacement and kinetic energy of forward motion during this rotation, reaches a maximum at 4-5 km/hr (R = 65%). From 6 to 10 km/hr R decreases more steeply than in normal walking, indicating a smaller utilization of the pendulum-like mechanism characteristic of walking. 4. Above 10 km/hr potential and kinetic energies vary during each step because both are simultaneously taken up and released by the muscles with almost no transfer between them (R = 2-10%). Above 13-14 km/hr an aerial phase (25-60 msec) takes place during the step. 5. Speeds considerably greater than in normal walking are attained thanks to a greater efficiency of doing positive work. This is made possible by a mechanism of locomotion allowing an important storage and recovery of mechanical energy by the muscles
The determinants of the step frequency in walking in humans
No full textThe mechanical power spent during walking in lifting and accelerating the centre of mass, Wext, has been measured at three given speeds maintained at different step frequencies: at any given speed, Wext is smaller the greater the step frequency used. The mechanical power spent in accelerating the limbs relative to the centre of mass during walking at a given speed, but with different step frequencies, Wint, was calculated from previous data obtained during free walking (Cavagna & Kaneko, 1977). At a given walking speed, Wint increases with the step frequency. The total power, Wtot = Wext + Wint, reaches a minimum at a step frequency which is 20-30% less than the step frequency freely chosen at the same period. The step frequency at which Wtot is minimum increases with speed in a similar way to the natural step frequency during free walking
The mechanics of walking in children
No full textThe work done at each step, during level walking at a constant average speed, to lift the centre of mass of the body, to accelerate it forward, and to increase the sum of both gravitational potential and kinetic energies, has been measured at various speeds on children of 2-12 years of age, with the same technique used previously for adults (Cavagna, 1975; Cavagna, Thys & Zamboni, 1976). The pendulum-like transfer between potential and kinetic energies (Cavagna et al. 1976) reaches a maximum at the speed at which the weight-specific work to move the centre of mass a given distance is at a minimum ('optimal' speed). This speed is about 2 X 8 km/hr at 2 years of age and increases progressively with age up to 5 km/hr at 12 years of age and in adults. The speed freely chosen during steady walking at the different ages is similar to this 'optimal' speed. At the 'optimal' speed, the time of single contact (time of swing) is in good agreement with that predicted, for the same stature, by a ballistic walking model assuming a minimum of muscular work (Mochon & McMahon, 1980). Above the 'optimal' speed, the recovery of mechanical energy through the potential-kinetic energy transfer decreases. This decrease is greater the younger the subject. A reduction of this recovery implies a greater amount of work to be supplied by muscles: at 4 X 5 km/hr the weight-specific muscular power necessary to move the centre of mass is 2 X 3 times greater in a 2-year-old child than in an adult
Scour around circular piers: espermental test on the existence of a final equilibiurm situation
No full textThe two power limits conditioning step frequency in human running
No full text1. At high running speeds, the step frequency becomes lower than the apparent natural frequency of the body's bouncing system. This is due to a relative increase of the vertical component of the muscular push and requires a greater power to maintain the motion of the centre of gravity, Wext. However, the reduction of the step frequency leads to a decrease of the power to accelerate the limbs relatively to the centre of gravity, Wint, and, possibly, of the total power Wtot = Wext + Wint. 2. In this study we measured Wext using a force platform, Wint by motion picture analysis, and calculated Wtot during human running at six given speeds (from 5 to 21 km h-1) maintained with different step frequencies dictated by a metronome. The power was calculated by dividing the positive work done at each step by the duration of the step (step-average power) and by the duration of the positive work phase (push-average power). 3. Also in running, as in walking, a change of the step frequency at a given speed has opposite effects on Wext, which decreases with increasing step frequency, and Wint, which increases with frequency; in addition, a step frequency exists at which Wtot reaches a minimum. However, the frequency for a minimum of Wtot decreases with speed in running, whereas it increases with speed in walking. This is true for both the step-average and the push-average powers. 4. The frequency minimizing the step-average power equals the freely chosen step frequency at about 13 km h-1: it is higher at lower speeds and lower at higher speeds. The frequency minimizing the push-average power approaches the freely chosen step frequency at high speeds (around 22 km h-1 for our subjects). 5. It is concluded that the increase of the vertical push does reduce the step-average power, but that a limit is set by the increase of the push-average power. Between 13 and 22 km h-1 the freely chosen step frequency is intermediate between a frequency minimizing the step-average power, eventually limited by the maximum oxygen intake (aerobic power), and a frequency minimizing the push-average power, set free by the muscle immediately during contraction (anaerobic power). The first need prevails at the lower speed, the second at the higher speed
Bilateral choroiditis as the only sign of persistent Mycobacterium intracellulare infection following haematogenous spread in an immunocompromised patient
No full textAn immunocompromised patient had positive blood cultures for Mycobacterium intracellulare and no identifiable organ seeding was started on treatment. One month later, the patient was clinically well with negative blood cultures but drug-induced myelotoxicity had developed. Ocular fundus examination at this time revealed bilateral choroidal granulomas which changed patient management
The bounce of the body in running, trotting and hopping
No full textThe bouncing mechanism of running, trotting and hopping has never been substantiated by an analysis of the vertical motion of the centre of mass of the body during the stride. In this study, the stride period was divided, as in a simple harmonic motion, into four quadrants delimited by the instants of zero and maximal (upward and downward) vertical velocity of the centre of mass. During most speeds of trotting and at low speeds of running, the stride is symmetric, i.e. the duration of the two lower quadrants of the vertical oscillation equals that of the two upper quadrants. Stride frequency and natural frequency of the bouncing system coincide. During hopping and at high speeds of running, the stride is asymmetric, i.e. the duration of the two upper quadrants is greater than the duration of the two lower quadrants and the stride frequency is smaller than the natural frequency of the system. At a given speed, the asymmetric mechanism requires a greater power to sustain the motion of the centre of mass. It is possibly adopted to contain the power necessary to reset the limbs
- …
