Kinetics and mechanical work done to move the body centre of mass along a curve.

When running on a curve, the lower limbs interact with the ground to redirect the trajectory of the centre of mass of the body (CoM). The goal of this paper is to understand how the trajectory of the CoM and the work done to maintain its movements relative to the surroundings (Wcom) are modified as a function of running speed and radius of curvature. Eleven participants ran at different speeds on a straight line and on circular curves with a 6 m and 18 m curvature. The trajectory of the CoM and Wcom were calculated using force-platforms measuring the ground reaction forces and infrared cameras recording the movements of the pelvis. To follow a circular path, runners overcompensate the rotation of their trajectory during contact phases. The deviation from the circular path increases when the radius of curvature decreases and speed increases. Interestingly, an asymmetry between the inner and outer lower limbs emerges as speed increases. The method to evaluate Wcom on a straight-line was adapted using a referential that rotates at heel strike and remains fixed during the whole step cycle. In an 18 m radius curve and at low speeds on a 6 m radius, Wcom changes little compared to a straight-line run. Whereas at 6 m s-1 on a 6 m radius, Wcom increases by ~25%, due to an augmentation in the work to move the CoM laterally. Understanding these adaptations provides valuable insight for sports sciences, aiding in optimizing training and performance in sports with multidirectional movements.

Modification of the locomotor pattern when deviating from the characteristic heel-to-toe rolling pattern during walking.

PURPOSE: Humans are amongst few animals that step first on the heel, and then roll on the ball of the foot and toes. While this heel-to-toe rolling pattern has been shown to render an energetic advantage during walking, the effect of different foot contact strategies, on the neuromuscular control of adult walking gaits has received less attention. We hypothesised that deviating from heel-to-toe rolling pattern affects the energy transduction and weight acceptance and re-propulsive phases in gait along with the modification of spinal motor activity. METHODS: Ten subjects walked on a treadmill normally, then placed their feet flat on the ground at each step and finally walked on the balls of the feet. RESULTS: Our results show that when participants deviate from heel-to-toe rolling pattern strategy, the mechanical work increases on average 85% higher (F = 15.5; p < 0.001), mainly linked to a lack of propulsion at late stance. This modification of the mechanical power is related to a differential involvement of lumbar and sacral segment activation. Particularly, the delay between the major bursts of activation is on average 65% smaller, as compared to normal walking (F = 43.2; p < 0.001). CONCLUSION: Similar results are observable in walking plantigrade animals, but also at the onset of independent stepping in toddlers, where the heel-to-toe rolling pattern is not yet established. These indications seem to bring arguments to the fact that the rolling of the foot during human locomotion has evolved to optimise gait, following selective pressures from the evolution of bipedal posture.

Neuromuscular Age-Related Adjustment of Gait When Moving Upwards and Downwards.

Locomotor movements are accommodated to various surface conditions by means of specific locomotor adjustments. This study examined underlying age-related differences in neuromuscular control during level walking and on a positive or negative slope, and during stepping upstairs and downstairs. Ten elderly and eight young adults walked on a treadmill at two different speeds and at three different inclinations (0°, +6°, and -6°). They were also asked to ascend and descend stairs at self-selected speeds. Full body kinematics and surface electromyography of 12 lower-limb muscles were recorded. We compared the intersegmental coordination, muscle activity, and corresponding modifications of spinal motoneuronal output in young and older adults. Despite great similarity between the neuromuscular control of young and older adults, our findings highlight subtle age-related differences in all conditions, potentially reflecting systematic age-related adjustments of the neuromuscular control of locomotion across various support surfaces. The main distinctive feature of walking in older adults is a significantly wider and earlier activation of muscles innervated by the sacral segments. These changes in neuromuscular control are reflected in a reduction or lack of propulsion observed at the end of stance in older adults at different slopes, with the result of a delay in the timing of redirection of the centre-of-mass velocity and of an unanticipated step-to-step transition strategy.

Mechanical work as a (key) determinant of energy cost in human locomotion: recent findings and future directions.

NEW FINDINGS: What is the topic of this review? This narrative review explores past and recent findings on the mechanical determinants of energy cost during human locomotion, obtained by using a mechanical approach based on König's theorem (Fenn's approach). What advances does it highlight? Developments in analytical methods and their applications allow a better understanding of the mechanical-bioenergetic interaction. Recent advances include the determination of 'frictional' internal work; the association between tendon work and apparent efficiency; a better understanding of the role of energy recovery and internal work in pathological gait (amputees, stroke and obesity); and a comprehensive analysis of human locomotion in (simulated) low gravity conditions. ABSTRACT: During locomotion, muscles use metabolic energy to produce mechanical work (in a more or less efficient way), and energetics and mechanics can be considered as two sides of the same coin, the latter being investigated to understand the former. A mechanical approach based on König's theorem (Fenn's approach) has proved to be a useful tool to elucidate the determinants of the energy cost of locomotion (e.g., the pendulum-like model of walking and the bouncing model of running) and has resulted in many advances in this field. During the past 60 years, this approach has been refined and applied to explore the determinants of energy cost and efficiency in a variety of conditions (e.g., low gravity, unsteady speed). This narrative review aims to summarize current knowledge of the role that mechanical work has played in our understanding of energy cost to date, and to underline how recent developments in analytical methods and their applications in specific locomotion modalities (on a gradient, at low gravity and in unsteady conditions) and in pathological gaits (asymmetric gait pathologies, obese subjects and in the elderly) could continue to push this understanding further. The recent in vivo quantification of new aspects that should be included in the assessment of mechanical work (e.g., frictional internal work and elastic contribution) deserves future research that would improve our knowledge of the mechanical-bioenergetic interaction during human locomotion, as well as in sport science and space exploration.

Intra-limb and muscular coordination during walking on slopes.

PURPOSE: Intra-limb and muscular coordination during gait are the result of the organisation of the neuromuscular system, which have been widely studied on a flat terrain. Environmental factors, such as the inclination of the terrain, is a challenge for the postural control system to maintain balance. Therefore, we hypothesised that the central nervous system flexibly modifies its control strategies during locomotion on slopes. METHODS: Ten subjects walked on an inclined treadmill at different slopes (from - 9° to + 9°) and speeds (from 0.56 to 2.22 m s-1). Intra-limb coordination was investigated via the Continuous Relative Phase, whereas muscular coordination was investigated by decomposing the coordinated muscle activation profiles into Basic Activation Patterns. RESULTS: A greater stride to stride variability of kinematics was observed during walking on slopes, as compared to walking on the level. On positive slopes, the stride period and width present a greater variability without modification of the time-pattern of the muscular activation and of the variability of intersegmental coordination. On negative slopes, the stride width is larger, the variability of the stride period and of the inter-segmental coordination is greater and the basic activation patterns become broader, especially at slow speeds. CONCLUSION: Our findings suggest that the control strategy of downhill walking corresponds to a more conservative gait pattern, which could be adopted to lower the risk of falling at the cost of a greater energy consumption. In uphill walking, where metabolic demands are high, the strategy adopted may be planned to minimise energy expenditure.

Human Locomotion in Hypogravity: From Basic Research to Clinical Applications.

We have considerable knowledge about the mechanisms underlying compensation of Earth gravity during locomotion, a knowledge obtained from physiological, biomechanical, modeling, developmental, comparative, and paleoanthropological studies. By contrast, we know much less about locomotion and movement in general under sustained hypogravity. This lack of information poses a serious problem for human space exploration. In a near future humans will walk again on the Moon and for the first time on Mars. It would be important to predict how they will move around, since we know that locomotion and mobility in general may be jeopardized in hypogravity, especially when landing after a prolonged weightlessness of the space flight. The combination of muscle weakness, of wearing a cumbersome spacesuit, and of maladaptive patterns of locomotion in hypogravity significantly increase the risk of falls and injuries. Much of what we currently know about locomotion in hypogravity derives from the video archives of the Apollo missions on the Moon, the experiments performed with parabolic flight or with body weight support on Earth, and the theoretical models. These are the topics of our review, along with the issue of the application of simulated hypogravity in rehabilitation to help patients with deambulation problems. We consider several issues that are common to the field of space science and clinical rehabilitation: the general principles governing locomotion in hypogravity, the methods used to reduce gravity effects on locomotion, the extent to which the resulting behavior is comparable across different methods, the important non-linearities of several locomotor parameters as a function of the gravity reduction, the need to use multiple methods to obtain reliable results, and the need to tailor the methods individually based on the physiology and medical history of each person.

Pendular energy transduction within the step during human walking on slopes at different speeds.

When ascending (descending) a slope, positive (negative) work must be performed to overcome changes in gravitational potential energy at the center of body mass (COM). This modifies the pendulum-like behavior of walking. The aim of this study is to analyze how energy exchange and mechanical work done vary within a step across slopes and speeds. Ten subjects walked on an instrumented treadmill at different slopes (from -9° to 9°), and speeds (between 0.56 and 2.22 m s-1). From the ground reaction forces, we evaluated energy of the COM, recovery (i.e. the potential-kinetic energy transduction) and pendular energy savings (i.e. the theoretical reduction in work due to this recovered energy) throughout the step. When walking uphill as compared to level, pendular energy savings increase during the first part of stance (when the COM is lifted) and decreases during the second part. Conversely in downhill walking, pendular energy savings decrease during the first part of stance and increase during the second part (when the COM is lowered). In uphill and downhill walking, the main phase of external work occurs around double support. Uphill, the positive work phase is extended during the beginning of single support to raise the body. Downhill, the negative work phase starts before double support, slowing the downward velocity of the body. Changes of the pendulum-like behavior as a function of slope can be illustrated by tilting the 'classical compass model' backwards (uphill) or forwards (downhill).

Motor Control of Landing from a Jump in Simulated Hypergravity.

On Earth, when landing from a counter-movement jump, muscles contract before touchdown to anticipate imminent collision with the ground and place the limbs in a proper position. This study assesses how the control of landing is modified when gravity is increased above 1 g. Hypergravity was simulated in two different ways: (1) by generating centrifugal forces during turns of an aircraft (A300) and (2) by pulling the subject downwards in the laboratory with a Subject Loading System (SLS). Eight subjects were asked to perform counter-movement jumps at 1 g on Earth and at 3 hypergravity levels (1.2, 1.4 and 1.6 g) both in A300 and with SLS. External forces applied to the body, movements of the lower limb segments and muscular activity of 6 lower limb muscles were recorded. Our results show that both in A300 and with SLS, as in 1 g: (1) the anticipation phase is present; (2) during the loading phase (from touchdown until the peak of vertical ground reaction force), lower limb muscles act like a stiff spring, whereas during the second part (from the peak of vertical ground reaction force until the return to the standing position), they act like a compliant spring associated with a damper. (3) With increasing gravity, the preparatory adjustments and the loading phase are modified whereas the second part does not change drastically. (4) The modifications are similar in A300 and with SLS, however the effect of hypergravity is accentuated in A300, probably due to altered sensory inputs. This observation suggests that otolithic information plays an important role in the control of the landing from a jump.

Does an instrumented treadmill correctly measure the ground reaction forces?

Since the 1990s, treadmills have been equipped with multi-axis force transducers to measure the three components of the ground reaction forces during walking and running. These measurements are correctly performed if the whole treadmill (including the motor) is mounted on the transducers. In this case, the acceleration of the treadmill centre of mass relative to the reference frame of the laboratory is nil. The external forces exerted on one side of the treadmill are thus equal in magnitude and opposite in direction to the external forces exerted on the other side. However, uncertainty exists about the accuracy of these measures: due to friction between the belt and the tread-surface, due to the motor pulling the belt, some believe that it is not possible to correctly measure the horizontal components of the forces exerted by the feet on the belt. Here, we propose a simple model of an instrumented treadmill and we demonstrate (1) that the forces exerted by the subject moving on the upper part of the treadmill are accurately transmitted to the transducers placed under it and (2) that all internal forces - including friction - between the parts of the treadmill are cancelling each other.

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 speed on the energy cost of walking in unilateral traumatic lower limb amputees.

In this work, the effect of walking speed on the energy expenditure in traumatic lower-limb amputees was studied. The oxygen consumption was measured in 10 transfemoral amputees, 9 transtibial amputees and 13 control subjects, while they stood and walked at different speeds from 0.3 m s(-1) to near their maximum sustainable speed. Standing energy expenditure rate was the same in lower-limb amputees and in control subjects (approximately 1.85 W kg(-1)). On the contrary, during walking, the net energy expenditure rate was 30-60% greater in transfemoral amputees and 0-15% greater in transtibial amputees than in control subjects. The maximal sustainable speed was about 1.2 m s(-1) in transfemoral amputees and 1.6 m s(-1) in transtibial amputees, whereas it was above 2 m s(-1) in control subjects. Among these three groups, the cost of transport versus speed presented a U-shaped curve; the minimum cost increased with the level of amputation, and the speed at which this minimum occurred decreased.

Energetics of load carrying in Nepalese porters.

Nepalese porters routinely carry head-supported loads equal to 100 to 200% of their body weight (Mb) for many days up and down steep mountain footpaths at high altitudes. Previous studies have shown that African women carry head-supported loads of up to 60% of their Mb far more economically than army recruits carrying equivalent loads in backpacks. Here we show that Nepalese porters carry heavier loads even more economically than African women. Female Nepalese porters, for example, carry on average loads that are 10% of their Mb heavier than the maximum loads carried by the African women, yet do so at a 25% smaller metabolic cost.