Energy cost and running mechanics during a treadmill run to voluntary exhaustion in humans

The aim of the present study was to examine the physiological and mechanical factors which may be concerned in the increase in energy cost during running in a fatigued state. A group of 15 trained triathletes ran on a treadmill at velocities corresponding to their personal records over 3000m [mean 4.53 (SD 0.28) m · s⁻¹] until they felt exhausted. The energy cost of running (C _R) was quantified from the net O₂ uptake and the elevation of blood lactate concentration. Gas exchange was measured over 1 min firstly during the 3rd–4th min and secondly during the last minute of the run. Blood samples were collected before and after the completion of the run. Mechanical changes of the centre of mass were quantified using a kinematic arm. A significant mean increase [6.9 (SD 3.5)%, P < 0.001] in C _R from a mean of 4.4 (SD 0.4) J · kg⁻¹ · m⁻¹ to a mean of 4.7 (SD 0.4) J · kg⁻¹ · m⁻¹ was observed. The increase in the O₂ demand of the respiratory muscles estimated from the increase in ventilation accounted for a considerable proportion [mean 25.2 (SD 10.4)%] of the increase in C_R. A mean increase [17.0 (SD 26.0)%, P < 0.05] in the mechanical cost (C _M) from a mean of 2.36 (SD 0.23) J · kg⁻¹ · m⁻¹ to a mean of 2.74 (SD 0.55) J · kg⁻¹ · m⁻¹ was also noted. A significant correlation was found between C _R and C _M in the non-fatigued state (r = 0.68, P < 0.01), but not in the fatigued state (r = 0.25, NS). Furthermore, no correlations were found between the changes (from non-fatigued to fatigued state) in C _R and the changes in C _M suggesting that the increase in C _R is not solely dependent on the external work done per unit of distance. Since step frequency decreased slightly in the fatigued state, the internal work would have tended to decrease slightly which would not be compatible with an increase in C _R. A stepwise regressions showed that the changes in C _R were linked (r = 0.77, P < 0.01) to the changes in the variability of step frequency and in the variability of potential cost suggesting that a large proportion of the increase in C _R was due to an increase in the step variability. The underlying mechanisms of the relationship between C _R and step variability remains unclear. Accepted: 15 September 1997     

Healthy older adults have insufficient hip range of motion and plantar flexor strength to walk like healthy young adults

Limited plantar flexor strength and hip extension range of motion (ROM) in older adults are believed to underlie common age-related differences in gait. However, no studies of age-related differences in gait have quantified the percentage of strength and ROM used during gait. We examined peak hip angles, hip torques and plantar flexor torques, and corresponding estimates of functional capacity utilized (FCU), which we define as the percentage of available strength or joint ROM used, in 10 young and 10 older healthy adults walking under self-selected and controlled (slow and fast) conditions. Older adults walked with about 30% smaller hip extension angle, 28% larger hip flexion angle, 34% more hip extensor torque in the slow condition, and 12% less plantar flexor torque in the fast condition than young adults. Older adults had higher FCU than young adults for hip flexion angle (47% vs. 34%) and hip extensor torque (48% vs. 27%). FCUs for plantar flexor torque (both age groups) and hip extension angle (older adults in all conditions; young adults in self-selected gait) were not significantly <100%, and were higher than for other measures examined. Older adults lacked sufficient hip extension ROM to walk with a hip extension angle as large as that of young adults. Similarly, in the fast gait condition older adults lacked the strength to match the plantar flexor torque produced by young adults. This supports the hypothesis that hip extension ROM and plantar flexor strength are limiting factors in gait and contribute to age-related differences in gait.     

A model of human walking energetics with an elastically-suspended load

Elastically-suspended loads have been shown to reduce the peak forces acting on the body while walking with a load when the suspension stiffness and damping are minimized. However, it is not well understood how elastically-suspended loads can affect the energetic cost of walking. Prior work shows that elastically suspending a load can yield either an increase or decrease in the energetic cost of human walking, depending primarily on the suspension stiffness, load, and walking speed. It would be useful to have a simple explanation that reconciles apparent differences in existing data. The objective of this paper is to help explain different energetic outcomes found with experimental load suspension backpacks and to systematically investigate the effect of load suspension parameters on the energetic cost of human walking. A simple two-degree-of-freedom model is used to approximate the energetic cost of human walking with a suspended load. The energetic predictions of the model are consistent with existing experimental data and show how the suspension parameters, load mass, and walking speed can affect the energetic cost of walking. In general, the energetic cost of walking with a load is decreased compared to that of a stiffly-attached load when the natural frequency of a load suspension is tuned significantly below the resonant walking frequency. The model also shows that a compliant load suspension is more effective in reducing the energetic cost of walking with low suspension damping, high load mass, and fast walking speed. This simple model could improve our understanding of how elastic load-carrying devices affect the energetic cost of walking with a load.     

Age-related mechanical work expenditure during normal walking: The Baltimore Longitudinal Study of Aging

The aim of this cross-sectional study was to delineate age-associated kinematic and kinetic gait patterns of normal walking, and to test the hypothesis that older adults exhibit gait patterns that reduce generative mechanical work expenditures (MWEs). We studied 52 adult Baltimore Longitudinal Study of Aging participants (means age 72±9, from 60 to 92 years) who could walk 4 m unaided. Three-dimensional kinematic and kinetic parameters assessed during rotation-defined gait periods were used to estimate MWEs for the rotation of lower extremities about the medial–lateral (ML) and anterior–posterior (AP) axes of proximal joints, which represent MWEs in the AP and ML sides, respectively. Relationships between gait parameters and age were examined using regression analysis with adjustments for walking speed, sex, height, and weight. Older age was associated with slower self-selected walking speed (p<0.001), shorter stride length (p<0.001), and greater propensity of landing flat-footed (p=0.003). With older age, hip generative MWE for thigh rotation was lower about the AP axis (hip abduction and adduction) during stance (p=0.010) and higher about the ML axis (hip extension and flexion) during late stance (p<0.001). Knee absorptive MWE for shank rotation about the AP axis (knee abduction and adduction) during early stance was also lower with older age (p<0.003). These age-related gait patterns may represent a compensatory effort to maintain balance and may also reflect mobility limitations.     

Altered joint kinetic strategies of healthy older adults and individuals with Parkinson’s disease to walk at faster speeds

Individuals with Parkinson’s disease (PD) exhibit poorer walking performance compared to healthy, age-matched adults. Lower extremity joint kinetics may provide insight into this performance deficit but are currently lacking in the PD literature, especially across multiple speeds. The primary purpose of this study was to compare joint kinetics between individuals with PD and healthy older adults at both comfortable and maximal walking speeds. Secondarily, we quantified relationships between joint kinetics and walking speeds within each group. Biomechanical gait analyses were conducted for 13 individuals with PD and 12 age-matched controls during comfortable (CWS) and maximal (MWS) speed walking. Relative contributions to total positive work from the hip, knee, and ankle were compared across groups and speeds. Within each group, relationships between relative joint work and CWS and MWS were also quantified. Significant group by speed interactions indicated that healthy older adults increased hip and decreased ankle relative work at MWS compared to CWS whereas relative work at all joints in PD group remained stable across speeds. In the older group, positive relationships were observed between relative hip work and MWS. In the PD group, negative relationships were observed between relative hip work and CWS and MWS. Healthy older adults disproportionately increased mechanical contributions from the hip at MWS compared to CWS. Individuals with PD did not exhibit similar disproportionate scaling of joint kinetics across speed conditions. Inability to appropriately scale joint kinetics in PD may represent an inflexible neuromuscular system in PD, which may limit walking performance in this population.     

Calf muscle work and segment energy changes in human treadmill walking

The relation between changes in potential and kinetic energy in a seven-segment model of the human body and the work of m. triceps surae was investigated in four subjects walking on a treadmill at speeds between 0.5 and 2.0 m/s. Segment energy levels were determined by means of tachometers attached with strings to various points on the subject's body. Muscle work was assessed by electromyogram to force processing. M. triceps surae is active during stance, first doing negative (eccentric) work and ending with a short period of positive (concentric) work at “push-off”. It turned out that in normal walking these muscles provide the major part of positive work for the initiation of swing at push-off. Only at large step lengths, when push-off starts well before contralateral heel contact, is there a minor pushing forward of the trunk. In the negative work phase, m. triceps surae seem to check the forward speed of the trunk. A related decrease of trunk kinetic energy is not present, however, but this may be obscured by the simultaneous action of m. quadriceps femoris and, in a later stage, by a transfer of energy from the decelerating contralateral (swing) leg to the trunk. Energy of the trunk segment shows a sharp decline in double stance and a more gradual increase in the first half of single stance. Evidence is given that this effect is due to quadriceps action in the knee flexion-extension movement during stance. The presented results are incorporated in a general picture of energy flows in human walking.     

The Predominant Molecular State of Bound Enzyme Determines the Strength and Type of Product Inhibition in the Hydrolysis of Recalcitrant Polysaccharides by Processive Enzymes

Processive enzymes are major components of the efficient enzyme systems that are responsible for the degradation of the recalcitrant polysaccharides cellulose and chitin. Despite intensive research, there is no consensus on which step is rate-limiting for these enzymes. Here, we performed a comparative study of two well characterized enzymes, the cellobiohydrolase Cel7A from Hypocrea jecorina and the chitinase ChiA from Serratia marcescens. Both enzymes were inhibited by their disaccharide product, namely chitobiose for ChiA and cellobiose for Cel7A. The products behaved as noncompetitive inhibitors according to studies using the ¹⁴C-labeled crystalline polymeric substrates ¹⁴C chitin nanowhiskers and ¹⁴C-labeled bacterial microcrystalline cellulose for ChiA and Cel7A, respectively. The resulting observed K_i(obs) values were 0.45 ± 0.08 mm for ChiA and 0.17 ± 0.02 mm for Cel7A. However, in contrast to ChiA, the K_i(obs) of Cel7A was an order of magnitude higher than the true K_i value governed by the thermodynamic stability of the enzyme-inhibitor complex. Theoretical analysis of product inhibition suggested that the inhibition strength and pattern can be accounted for by assuming different rate-limiting steps for ChiA and Cel7A. Measuring the population of enzymes whose active site was occupied by a polymer chain revealed that Cel7A was bound predominantly via its active site. Conversely, the active-site-mediated binding of ChiA was slow, and most ChiA exhibited a free active site, even when the substrate concentration was saturating for the activity. Collectively, our data suggest that complexation with the polymer chain is rate-limiting for ChiA, whereas Cel7A is limited by dissociation.