Biology subjects

Hosseini-Yazdi, S.-S.

Publications and source records attributed to Hosseini-Yazdi, S.-S..

11 recordsLinked to original sources

bioRxiv2024

The energetic cost of human walking as a function of uneven terrain amplitude

Humans expend more energy walking on uneven terrain, but the exact cost varies across terrains. Few experimental characterizations exist, each describing terrain qualitatively without any relation to others or flat ground. This precludes mechanistic explanation of the energy costs. Here we show that energy cost varies smoothly and approximately quadratically as a function of terrain amplitude. We tested this with healthy adults (N=10) walking on synthetic uneven terrain with random step heights of parametrically controlled maximum amplitude (four conditions 0 - 0.045 m), and at four walking speeds (0.8 - 1.4 m {middle dot} s-1). Both net metabolic rate and the rate of positive work increased approximately with amplitude squared and speed cubed (R2 = 0.74, 0.82 respectively), as predicted by a simple walking model. The model requires work to redirect the body center of mass velocity between successive arcs described by pendulum-like legs, at proportional metabolic cost. Humans performed most of the greater work with terrain amplitude early in the single stance phase, and with faster walking late in stance during push-off. Work and energy rates changed with approximately linear proportionality, with a ratio or delta efficiency of 49.5% (R2 = 0.68). The efficiency was high enough to suggest substantial work performed passively by elastic tendon and not only by active muscle. Simple kinematic measures such as mid-swing foot clearance also increased with terrain amplitude (R2 = 0.65), possibly costing energy as well. Nevertheless, most of the metabolic cost of walking faster or on more uneven terrain can be explained mechanistically by the work performed. Summary statementHumans perform more work and expend more energy on uneven terrain, increasing with the square of terrain amplitude and the cube of walking speed.

bioengineering↗

bioRxiv2024

Center of Mass Work Components May Offer Estimates for Preferred Walking Speeds for Any Walking Conditions

This study investigates the role of optimal push-off impulses in minimizing the total mechanical work dissipation per step, aiming to achieve passive single support work performance for a variety of walking conditions. By optimizing push-offs to cover the entire steps energy demands, the single support work may become only storage and release of mechanical energy through tendons and tissues. Our simulations indicate that for each walking speed, there is an optimal push-off impulse that ensures energy balance without the need for additional energy input or dissipation during the subsequent single support phase when the soft tissue dissipation is negligible. For this circumstance, the step total dissipation is minimum. For level ground (even terrain), the estimated preferred walking speeds align closely with literature values for young adult ([~] 1.2 m.s-1)and older adult ([~] 1.0 m.s-1) self-selected speeds, with a reduction observed for the restricted view of oncoming irregularities in the substrate terrain (15%). For walking on uneven surfaces, terrain amplitude was shown to impact walking costs quadratically, with optimal speeds declining by approximately 20% per unit increase in terrain amplitude. Key findings include evidence that net single support work remains near zero when push-off optimally covers collision and gravity work, confirming the passive nature of single support work under this condition. We also observed that the preferred speeds for older adults tend to be 12-15% lower than for younger adults, likely due to biomechanical adaptations. Beyond certain terrain amplitudes, no preferred walking speed allowed fully passive single support work, highlighting a possible biomechanical threshold where ankle push-off alone becomes insufficient and hip torque compensation may be necessary. This approach provides a framework for estimating preferred walking speeds across different terrain amplitudes (continuous parameter), varying conditions and demographics, with potential applications in designing assistive devices and gait rehabilitation protocols that reduce metabolic cost through optimal mechanical work management.

bioengineering↗

bioRxiv2024

The Energetics of Maintaining the Lateral Balance Are Terrain-Specific; a Normal Lookahead Significantly Reduces Active Balance Maintenance Work

Humans must actively control their lateral balance through frontal plane work or adjusting lateral foot placement. With constant muscle efficiency and considering the energetic consequences of Center of Mass (COM) work variability, we estimated the metabolic cost of lateral balance maintenance and compared it with the Workman model. Like the Workman model, we found that lateral balance energetics were mainly associated with terrain amplitude. Increased walking speeds effect on step transition work might be offset by reduced step width. A significant rise in lateral work magnitude (+157.1%) with restricted lookahead was potentially linked to wider steps. Comparing mechanical work with the Workman model, we found significant differences in magnitudes, suggesting that the Workman model included additional costs such as force rate generation, muscle coactivation, or posture maintenance not reflected in the COM lateral work and its variability. Practitioner SummaryLateral balance maintenance during walking requires active regulation, yet it is not studied mechanically. Our study found that lateral control costs are terrain-specific and increase with restricted lookahead. With age, only variability increases. We assume associated energetics rise with both work and variability.

bioengineering↗

bioRxiv2024

The Powered Simplest Walking Model Explains the Different Vertical Ground Reaction Force Amplitudes at Elevated Walking Speeds

Understanding the vertical ground reaction force (vGRF) profile offers important insight into how humans regulate mechanical work during walking. Although the characteristic double-hump vGRF pattern is well documented, the mechanical factors underlying asymmetry in peak amplitudes and midstance trough timing remain unclear. Using a simple powered walking model and an inverted pendulum simulation with constant hip torque, we examined how step-transition work--collision and push-off--shapes the vGRF trajectory. We further compared these predictions to empirical data spanning walking speeds from 0.8-1.4 m. s-1. The simple walking model predicted symmetric vGRF profiles across speeds because collision and push-off impulses were equal, resulting in passive single-support motion. In contrast, adding hip torque within the pendular model produced stance-phase asymmetries, shifting the vGRF trough earlier when torque added energy and later when torque dissipated energy. Empirical analysis revealed that collision and push-off impulses were generally unequal except at one speed, producing asymmetric vGRF peaks. At low speeds, push-off exceeded collision; at high speeds, the reverse occurred, consistent with a need for compensatory single-support positive work. These mechanical imbalances predicted systematic shifts in trough timing toward the dominant impulse. Therefore, we propose the Vertical GRF Trough Timing Index (vGRF-TTI), combined with collision and push-off peak amplitudes, as a clinically meaningful outcome capturing the balance of step-transition work. Earlier troughs with elevated collision peaks indicate impaired push-off or constrained gait conditions, whereas later troughs with larger push-off peaks reflect compensatory or enhanced propulsion. These metrics provide sensitive, mechanism-based indicators of gait efficiency and neuromotor control.

bioengineering↗

bioRxiv2024

The Significance of Uneven Walking Transitory Modulations in Walking Momentum Regulation and Active Work

Uneven walking presents challenges such as balance maintenance and increased energetics. Beyond average step parameter variations, humans may employ transient strategies to conserve mechanical work or enhance momentum with minimal metabolic costs. Thus, we quantified mid-flight positive mechanical work induction, step length, and effective leg length modulation from four steps before to four steps after a specific encounter for young and older adults with normal and restricted lookaheads. Simulations were also conducted to assess the impact of step length or effective leg length changes on step-to-step transition or post-transition speed. We observed that young adults mid-flight energy inducements were focused around the encounter, influenced by lookahead state affecting modulation based on feedback or feedforward control. While with the restricted lookahead, older adults showed similar regulation, their anticipatory modulation was poorer and extended over the evaluation interval. With normal lookahead, young adults reduced step length just before the encounter, potentially increasing momentum with less heel-strike energy dissipation. Step length changes around and after the perturbation may have been passive. Older adults exhibited longer modulation periods than young adults. We also noted active leg length modulation, likely influenced by tactile sensory information. For up-steps, effective leg lengths were shorter, while for down-steps, they were longer, potentially compensatory actions to minimize COM vertical fluctuations and associated work against gravity. The amplitude of leg length modulation may have been constrained by the flexed legs walking energetic cost or and ankle range of motion. With restricted lookahead, older adults showed larger leg length modulation amplitudes.

bioengineering↗

bioRxiv2024

Analysis of Ground Reaction Force Impulses During Uneven Walking for Young and Older Adults

Humans navigate various terrains by exerting forces to direct the Center of Mass (COM) and maintain balance. During walking, humans transition from one stance leg to the next by exerting impulses during the step-to-step transition. Studying these transition impulses may provide insight into how humans traverse uneven terrains. When walking speed increased (constant terrain amplitude), the average braking and propulsive impulses (posterior/anterior) increased comparably (-0.0270 m {middle dot} s-1 {middle dot} g-1 v-1versus 0.0252 m s-1{middle dot} g-1 v-1). In the vertical direction, while the collision impulse remained constant, the push-off impulse declined by -0.0535 m {middle dot} s-1 {middle dot} g-1 {middle dot} v-1. The interaction of age and speed also increased the collision impulse (0.0202 m {middle dot} s-1 {middle dot} g-1 {middle dot} v-1). With the rise of terrain amplitude (constant speed), the braking and propulsive impulses rose by -0.0607 m {middle dot} s-1 {middle dot} g-1 {middle dot} m-1 and 0.0701 m {middle dot} s-1 {middle dot} g-1 {middle dot} m-1, respectively. Thus, we could infer that the propulsive impulse also contributed to the gait mechanical energy. While the collision impulse increased with terrain amplitude (0.1775 m {middle dot} s-1 {middle dot} g-1 {middle dot} m-1) and the interaction of age and terrain amplitude (0.1058 m {middle dot} s-1 {middle dot} g-1 {middle dot} m-1), the push-off impulse declined (-0.2700 m {middle dot} s-1 {middle dot} g-1 {middle dot} m-1 and -0.1473 m {middle dot} s-1 {middle dot} g-1 {middle dot} m-1). We also observed the push-off as a fraction of the total vertical impulse declined. Therefore, we detected a mechanical energy deficit in the step-to-step transition that must have been compensated for during the mid-flight phase. Considering the portion of push-off occurring after the subsequent heel-strike as delayed push-off, while it increased with walking speed, it declined with terrain amplitude. Thus, during uneven walking, the push-off exertion must have been interrupted, indicating the demand for further mechanical energy infusion after the transition.

bioengineering↗

bioRxiv2024

Optimum Push-off During Uneven Walking for Just-in-Time Strategy; Delayed Push-off Exertion is Mechanically Costly

It is shown that step mechanical work roughly describes walking energetics, and optimal walking economy is achieved by pre-emptive step work. We suggest this is also true for uneven walking. Using a simple powered walking model, we estimated the preferred pre-emptive push-offs to cover the entire step energy. The maximum push-off is exerted when the subsequent heel-strike dissipation is zero, setting an upper bound for step-up amplitude achievable with pre-emptive push-off. For instance, at a walking speed of 1.4 m {middle dot} s-1, the maximum step-up is 0.106 m. Conversely, for any step-up amplitude, there is a minimum walking speed. For a step-up height ({Delta}h) of 0.06 m, the minimum walking speed is 1.06 m {middle dot} s-1. The importance of pre-emptive push-off and optimal timing of push-off and collision is widely discussed. However, there are cases where this timing is undermined, such as during uneven walking, necessitating post-transition mechanical energy compensation. The ankle (via delayed push-off) or hip can provide mid-flight energy, but no mechanical determinant prefers one source over the other. Our modeling demonstrates that delayed push-off entails mechanical energy waste, likely converted to heat by stretching the stance leg. This stretch may also release energy stored during the heel-strike (e.g., in the Achilles tendon), exacerbating the required mechanical work performance in the subsequent step transition. Hence, we propose that during the double support phase, when the stance leg is switched, hip actuation becomes mechanically preferable. Physiological observations also support our proposition.

bioengineering↗

bioRxiv2024

Uneven Walking Momentum Regulation Exhibit Different Strategies based on Age and State of Lookahead

Uneven terrains enforce challenges to continuously regulate the step parameters. Since forward momentum contributes materially to the walking balance, its step-to-step regulation plays an important role to traverse terrain complexities. Here, we exhibited that young and older adults modulation were affected by the state of lookahead. With a normal lookahead, they demonstrated anticipatory control. The young adults encountered terrain irregularities similar to step-up mounting and dismounting in which the total mechanical work was minimum. On the other hand, the older adults might have put more emphasis on foot placement instead, as they slowed down before the encounters. Additionally, their control was extended beyond of young adults. On the other hand, with the restricted lookahead, the control was based on feedback. Since young and older adults momentum regulations were very similar, it also supported suggestions that older adults might inherently rely on feedback to control their gait.

bioengineering↗

bioRxiv2024

Uneven Walking is Challenging: step-ups or extended steps that is the question

Uneven terrain presents significant challenges for walkers, resulting in increased energy expenditures. Given that Center of Mass (COM) work reflects this energy demand, it's reasonable to assume that individuals also seek strategies to minimize mechanical work. One such strategy involves deciding between extending step length to avoid terrain irregularities or simply traversing over them. Each approach carries its own mechanical cost, leading to the adoption of the less costly option. To investigate this, we conducted a simulation focusing on COM mechanical work under the assumption that gait energy is entirely provided through pre-emptive push-off. We examined the COM work required for step length extension, ranging from nominal to twice its magnitude, and compared it with the mechanical work needed for step-ups from zero to 0.05 m. The simulation revealed a critical threshold for a given walking velocity and perturbation amplitude: below it, extending step length was more favorable, while beyond it, landing atop perturbations became the preferred choice. As perturbation amplitude rose, the magnitude of the threshold also increased.

bioengineering↗

bioRxiv2024

The Powered Simplest Walking Model Modifies the Rate of Recovery Definition

An exchange between potential and kinetic energy over the step has long been considered a key feature in the energetic effectiveness of human walking. However, it is difficult to identify mechanisms responsible for limiting such an exchange in human walking. This study proposes a modified definition of center-of-mass (COM) energy recovery (Rc) that quantifies the proportion of mechanical energy transferred from one step to the next while accounting for total step dissipation. Simulations show that Rc decreases nearly linearly with walking speed on level ground, indicating no preferred speed. This behavior arises from analytical formulations that neglect active work during single support (pendular motion). In contrast, empirical data reveal consistently lower Rc, likely due to elevated collision losses or negative net single-support work not captured by the analytical model. When both single- and double-support phases are considered analytically, Rc exhibits a maximum of 59.4% at 1.21 m.s-1, coinciding with minimal active muscle work over the step. We further show that the Rc trajectory is asymmetric, contrary to prior assumptions, and is governed by total step dissipation. Accordingly, challenging walking conditions associated with higher metabolic cost, such as restricted visual lookahead, are predicted to reduce Rc (maximum 58.5%).

bioengineering↗

bioRxiv2024

Calibration of instrumented treadmills using an instrumented pole; a modified version to use relatively smaller forces

The instrumented treadmills quality of the generated Ground Reaction Forces (GRF) may degrade over time, as the original calibration matrix may not accurately represent the exerted forces. A cost-effective alternative to manufacturer recalibration is to use an instrumented pole for calibration. Collins et al. presented a simple method to collect multiple data points by exerting forces in various directions. The sensor on the instrumented pole provides instantaneous force magnitudes, while motion capture records the poles instantaneous directions. They recommended a relatively large force magnitude (1000N), requiring at least two individuals. Using an optimization method, the new calibration may be estimate by relating the exerted forces (pole) to the treadmill signals. Here, we attempted to simplify the process further, allowing a single individual to perform force exertion with additional force exertion direction. Thus, the calibrating forces were reduced to one-third of the prior recommendation. This maintained the structural integrity of the pole and helped avoid inducing bending moments that could affect calibration results. The cross-validation score for test data (prediction score) was at least 0.92. Additionally, comparing GRFs in posterior/anterior and vertical directions with a benchmark treadmill for even walking revealed an average cross-correlation of 0.97.

bioengineering↗