Norman Heglund

The role of gravity in human walking: pendular energy exchange, external work and optimal speed

1 During walking on Earth, at 1.0 g of gravity, the work done by the muscles to maintain the motion of the centre of mass of the body (Wext) is reduced by a pendulum‐like exchange between gravitational potential energy and kinetic energy. The weight‐specific Wext per unit distance attains a minimum of 0.3 J kg−1 m−1 at about 4.5 km h−1 in adults. 2 The effect of a gravity change has been studied during walking on a force platform fixed to the floor of an aircraft undergoing flight profiles which resulted in a simulated gravity of 0.4 and 1.5 times that on Earth. 3 At 0.4 g, such as on Mars, the minimum Wext was 0.15 J kg−1 m−1, half that on Earth and occurred at a slower speed, about 2.5 km h−1. The range of walking speeds is about half that on Earth. 4 At 1.5 g, the lowest value of Wext was 0.60 J kg−1 m−1, twice that on Earth; it was nearly constant up to about 4.3 km h−1 and then increased with speed. The range of walking speeds is probably greater than that on Earth. 5 A model is presented in which the speed for an optimum exchange between potential and kinetic energy, the ‘optimal speed’, is predicted by the balance between the forward deceleration due to the lift of the body against gravity and the forward deceleration due to the impact against the ground. 6 In conclusion, over the range studied, gravity increases the work required to walk, but it also increases the range of walking speeds.

Mechanical work and muscular efficiency in walking children

SUMMARY The effect of age and body size on the total mechanical work done during walking is studied in children of 3–12 years of age and in adults. The total mechanical work per stride (Wtot) is measured as the sum of the external work, Wext (i.e. the work required to move the centre of mass of the body relative to the surroundings), and the internal work, Wint (i.e. the work required to move the limbs relative to the centre of mass of the body, Wint,k, and the work done by one leg against the other during the double contact period, Wint,dc). Above 0.5 m s–1, both Wext and Wint,k, normalised to body mass and per unit distance (J kg–1 m–1), are greater in children than in adults; these differences are greater the higher the speed and the younger the subject. Both in children and in adults, the normalised Wint,dc shows an inverted U-shape curve as a function of speed, attaining a maximum value independent of age but occurring at higher speeds in older subjects. A higher metabolic energy input (J kg–1 m–1) is also observed in children, although in children younger than 6 years of age, the normalised mechanical work increases relatively less than the normalised energy cost of locomotion. This suggests that young children have a lower efficiency of positive muscular work production than adults during walking. Differences in normalised mechanical work, energy cost and efficiency between children and adults disappear after the age of 10.

Biomechanics of locomotion in Asian elephants

SUMMARY Elephants are the biggest living terrestrial animal, weighing up to five tons and measuring up to three metres at the withers. These exceptional dimensions provide certain advantages (e.g. the mass-specific energetic cost of locomotion is decreased) but also disadvantages (e.g. forces are proportional to body volume while supportive tissue strength depends on their cross-sectional area, which makes elephants relatively more fragile than smaller animals). In order to understand better how body size affects gait mechanics the movement of the centre of mass (COM) of 34 Asian elephants (Elephas maximus) was studied over their entire speed range of 0.4-5.0 m s−1 with force platforms. The mass-specific mechanical work required to maintain the movements of the COM per unit distance is ~0.2 J kg−1 m−1 (about 1/3 of the average of other animals ranging in size from a 35 g kangaroo rat to a 70 kg human). At low speeds this work is reduced by a pendulum-like exchange between the kinetic and potential energies of the COM, with a maximum energy exchange of ~60% at 1.4 m s−1. At high speeds, elephants use a bouncing mechanism with little exchange between kinetic and potential energies of the COM, although without an aerial phase. Elephants increase speed while reducing the vertical oscillation of the COM from about 3 cm to 1 cm.

Effect of an increase in gravity on the power output and the rebound of the body in human running

SUMMARY The effect of an increase in gravity on the mechanics of running has been studied by using a force platform fixed to the floor of an aircraft undergoing flight profiles, resulting in a simulated gravity of 1.3 g. The power spent to maintain the motion of the centre of mass of the body is ∼1.3 times greater than on Earth, due to a similar increase of both the power spent against gravity and to sustain the forward speed changes. This indicates that the average vertical displacement per unit distance and the average direction of the push are unchanged. The increase in power is mainly due to an increase in step frequency rather than to an increase in the work done at each step. The increase in step frequency in turn is mainly due to a decreased duration of the effective aerial phase (when the vertical force is less than body weight), rather than an increase in the stiffness of the bouncing system. The maximal speed where step frequency can match the resonant frequency of the bouncing system is increased by ∼5 km h–1 at 1.3 g. These results suggest a similar running mechanics at higher gravity, maintained at the expense of greater energy expenditure.

Walking on Mars

Sometime in the near future humans may walk in the reduced gravity of Mars. Gravity plays an essential role in walking. On Earth, the body uses gravity to ‘fall forwards’ at each step and then the forward speed is used to restore the initial height in a pendulum-like mechanism. When gravity is reduced, as on the Moon or Mars, the mechanism of walking must change. Here we investigate the mechanics of walking on Mars onboard an aircraft undergoing gravity-reducing flight profiles. The optimal walking speed on Mars will be 3.4 km h−1 (down from 5.5 km h−1 on Earth) and the work done per unit distance to move the centre of mass will be half that on Earth.

The double contact phase in walking children

SUMMARY During walking, when both feet are on the ground (the double contact phase), the legs push against each other, and both positive and negative work are done simultaneously. The work done by one leg on the other (Wint,dc) is not counted in the classic measurements of the positive muscular work done during walking. Using force platforms, we studied the effect of speed and age (size) on Wint,dc. In adults and in 3-12-year-old children, Wint,dc (J kg-1 m-1) as a function of speed shows an inverted U-shaped curve, attaining a maximum value that is independent of size but that occurs at higher speeds in larger subjects. Normalising the speed with the Froude number shows that Wint,dc is maximal at about 0.3 in both children and adults. Differences due to size disappear for the most part when normalised with the Froude number, indicating that these speed-dependent changes are primarily a result of body size changes. At its maximum, Wint,dc represents more than 40% of Wext (the positive work done to move the centre of mass of the body relative to the surroundings) in both children and adults.

Biomechanical analysis of running in weightlessness on a treadmill equipped with a subject loading system

One countermeasure used during long-duration spaceflight to maintain bone and muscle mass is a treadmill equipped with a subject loading system (SLS) that simulates gravity. To date, little is known about the biomechanics of running in weightlessness on such a treadmill-SLS system. We have designed an instrumented treadmill/force plate to compare the biomechanics of running in weightlessness to running on Earth. Gravity was simulated by two pneumatic pistons pulling downward on a subject’s harness, with a force approximately equal to body weight on Earth. Four transducers, mounted under the treadmill, measured the three components of the reaction force exerted by the tread belt under the foot. A high-speed video camera recorded the movements of limb segments while the electromyography of the four lower limb muscles was registered. Experiments in weightlessness were conducted during the European Space Agency parabolic flight campaigns. Control experiments were performed on the same subjects on Earth. When running on the treadmill with an SLS, the bouncing mechanism of running is preserved. Depending on the speed of progression, the ground reaction forces, contact and aerial times, muscular work and bone stress differed by a maximum of ±5–15% during running on the treadmill with an SLS, as compared to that on Earth. The movements of the lower limb segments and the EMG patterns of the lower limb muscles were also comparable. Thus, the biomechanics of running on Earth can reasonably be duplicated in weightlessness using a treadmill with an SLS that generates a pull-down force close to body weight on Earth.

Effect of stretching on undamped elasticity in muscle fibres from Rana temporaria

Muscle stiffness was measured from the undamped elastic recoil taking place when the force attained during ramp stretches of muscle fibres, tetanized on the plateau of the tension–length relation, was suddenly reduced to the isometric value developed before the stretch, T0. Sarcomere elastic recoil was measured on a tendon-free segment of the fibre by means of a striation follower. After small ramp stretches, stiffness increases to a value 1.33×greater than that measured during release from a state of isometric contraction to 0.9T0. While the relative increase in stiffness is equal to that reported for fibres of Rana esculenta (Piazzesi et al., 1992), the absolute value of stiffness measured during release from isometric contraction is just over half. As stretch amplitude is increased, on the plateau of the force–length relation, stiffness decreases toward the isometric value. This finding shows that the decrease in stiffness with large stretches cannot be due to a decrease in myofilament overlap (as may be the case when stretching occurs on the descending limb of the tension–length relation, Sugi & Tsuchiya, 1988), but must be due to an effect of the ramp stretch per se. For a given stretch amplitude, the after- stretch transient shortening against T0 taking place after the elastic recoil (which is expression of the work enhancement induced by stretching, Cavagna et al., 1986, 1994) is similar in fibres with very different stiffness of their undamped elastic elements. This suggests that this work enhancement is not due to the recoil of damped elastic structures recruited during stretching because of sarcomere length inhomogenity, a condition which would result in a decrease in stiffness (Morgan et al., 1996).

Energy transfer during stress relaxation of contracting frog muscle fibres

1 A contracting muscle resists stretching with a force greater than the force it can exert at a constant length, To. If the muscle is kept active at the stretched length, the excess tension disappears, at first rapidly and then more slowly (stress relaxation). The present study is concerned with the first, fast tension decay. In particular, it is still debated if and to what extent the fast tension decay after a ramp stretch involves a conservation of the elastic energy stored during stretching into cross‐bridge states of higher chemical energy. 2 Single muscle fibres of Rana temporaria and Rana esculenta were subjected to a short ramp stretch (∼15 nm per half‐sarcomere at either 1.4 or 0.04 sarcomere lengths s−1) on the plateau of the force‐length relation at temperatures of 4 and 14°C. Immediately after the end of the stretch, or after discrete time intervals of fixed‐end contraction and stress relaxation at the stretched length (Δtisom= 0.5–300 ms), the fibre was released against a force ∼To. Fibre and sarcomere stiffness during the elastic recoil to To (phase 1) and the subsequent transient shortening against To (phase 2), which is expression of the work enhancement by stretch, were measured after different Δtisom and compared with the corresponding fast tension decay during Δtisom. 3 The amplitude of fast tension decay is large after the fast stretch, and small or nil after the slow stretch. Two exponential terms are necessary to fit the fast tension decay after the fast stretch at 4°C, whereas one is sufficient in the other cases. The rate constant of the dominant exponential term (0.1–0.2 ms−1 at 4°C) increases with temperature with a temperature coefficient (Q10) of ∼3. 4 After fast stretch, the fast tension decay during Δtisom is accompanied in both species and at both temperatures by a corresponding increase in the amplitude of phase 2 shortening against To after Δtisom: a maximum of ∼5 nm per half‐sarcomere is attained when the fast tension decay is almost complete, i.e. 30 ms after the stretch at 4°C and 10 ms after the stretch at 14°C. After slow stretch, when fast tension decay is small or nil, the increase in phase 2 shortening is negligible. 5 The increase in phase 2 work during fast tension decay (ΔWout) is a constant fraction of the elastic energy simultaneously set free by the recoil of the undamped elastic elements. 6 ΔWout is accompanied by a decrease in stiffness, indicating that it is not due to a greater number of cross‐bridges. 7 It is concluded that, during the fast tension decay following a fast ramp stretch, a transfer of energy occurs from the undamped elastic elements to damped elements within the sarcomeres by a temperature‐dependent mechanism with a dominant rate constant consistent with the theory proposed by A. F. Huxley and R. M. Simmons in 1971 .

The mechanics of head-supported load carriage by Nepalese porters.

ABSTRACT In the Everest valley of Nepal, because of the rugged mountain terrain, roads are nothing more than dirt paths and all material must be conveyed on foot. The Nepalese porters routinely carry head-supported loads, which often exceed their body mass, over long distances up and down the steep mountain footpaths. In Africa, women transport their loads economically thanks to an energy-saving gait adaptation. We hypothesized that the Nepalese porters may have developed a corresponding mechanism. To investigate this proposition, we measured the mechanical work done during level walking in Nepalese porters while carrying different loads at several speeds. Our results show that the Nepalese porters do not use an equivalent mechanism as the African women to reduce work. In contrast, the Nepalese porters develop an equal amount of total mechanical work as Western control subjects while carrying loads of 0 to 120% of their body mass at all speeds measured (0.5–1.7 m s−1), making even more impressive their ability to carry loads without any apparent mechanically determined tricks. Nevertheless, our results show that the Nepalese porters have a higher efficiency, at least at slow speeds and high loads. Highlighted Article: The low energy consumption of Nepalese porters while carrying load cannot be explained by a reduction of their muscular mechanical work.