Mechanical Energy Expenditure Research Articles

Analysis of nutritional status and influence factors in PICU children with mechanical ventilation

Objective To investigate the nutritional status of children with mechanical ventilation, explore the influencing factors of nutritional status, and provide a basis for clinical nutrition support for children with mechanical ventilation. Methods The children with mechanical ventilation who had ventilated for more than 5 days during the period from February 1, 2017 to October 31, 2017 were selected as subjects.According to the mechanical ventilation time, they were divided into group A (5-16 d) and group B(17-29 d). We assessed malnutrition, screened nutritional risk, and measured albumin both on the admission and withdrawal of the ventilator, respectively.We also calculated the average caloric intake value (EI) and fluid volume during mechanical ventilation.Measured resting energy expenditure (MREE)was calculated by using the Schofield-HTWT formula.Feeding status during mechanical ventilation was assessed by average EI/MREE. Results The incidence of malnutrition at admission was 36.1%(13/36), and the incidence of malnutrition was 55.6%(20/36) when evacuated from the ventilator.There was a significant difference(P<0.05). The incidence of malnutrition was similar in group A and group B on admission, but the incidence of malnutrition in group B(62.5%) was significantly higher than that in group A (53.6%) when weaning.The average calorie in group B[(32.25±7.36)kcal/(kg·d)]was significantly lower than that in group A[(40.11±6.00)kcal/(kg·d)]. The mechanical ventilation time of group B[(540.63±89.66)h]was significantly longer than that of group A[(224.46±64.06)h](P<0.05). Conclusion Children with mechanical ventilation in the PICU have a higher incidence of malnutrition and the nutritional status deteriorates further during hospitalization, which may be related to calorie supply, duration of mechanical ventilation, fluid volume, and primary disease. Key words: Mechanical ventilation; Nutritional status; Malnutrition assessment; Nutritional risk screening

Quantifying the physical intensity of construction workers, a mechanical energy approach

Construction workers typically undertake highly demanding physical tasks involving various types of stresses from awkward postures, using excessive force, highly repetitive actions, and excessive energy expenditure, which increases the likelihood of unsafe actions, productivity loss, and human errors. Biomechanical models have been developed to estimate joint loadings, which can help avoid strenuous physical exertion, potentially enhancing construction workforce productivity, safety, and well-being. However, the models used are mainly in 2D, or to predict static strength ignored their velocity and acceleration or using marker-based method for dynamic motion data collection. To address this issue, this paper proposes a novel framework for investigating the mechanical energy expenditure (MEE) of workers using a 3D biomechanical model based on computer vision-based techniques. Human 3D Pose Estimation algorithm based on 2D videos is applied to approximate the coordinates of human joints for working postures, and smart insoles are used to collect foot pressures and plantar accelerations, as input data for the biomechanical analyses. The results show a detailed MEE rate for the whole body, at which joints the maximum and minimum values were obtained to avoid excessive physical exertion. The proposed method can approximate the total daily MEE of construction tasks by summing the assumed cost of individual tasks (such as walking, lifting, and stooping), providing suggestions for the design of a daily workload that workers can sustain without developing cumulative fatigue.

Efficiency and perceived exertion of novel wheelchair wheels: A comparative study

ABSTRACTNew geared wheelchair wheels have been developed allowing the user to pull backwards on the rims to move forward. Theoretically, this utilizes larger muscle groups compared to conventional propulsion preventing repetitive stress injuries. This study compared the gross mechanical efficiency (GME), perceived effort and energy expenditure of propelling the wheelchair using two types of these geared wheels and standard forward propulsion wheels. A within participants repeated measures design was used. Participants’ energy expenditure and perceived exertion were measured. GME was calculated based on power output and energy expenditure values. Ten able-bodied participants were enrolled in the study. Standard wheels required the least energy expenditure and were perceived as the lightest to propel for nine of the participants; while the low-geared wheels were perceived as the hardest to propel. Results favored the standard wheels under these conditions. Research is needed to explore the effect of training and different slope angles.

In situ behavioral plasticity as compensation for weather variability: implications for future climate change

While climatic effects on species biogeographic distributions are well documented, less mobile species must compensate for climate change in situ via behavioral plasticity. Despite this being a critical mechanism, behavioral plasticity is rarely modeled explicitly. Here, we use novel accelerometer and active-RFID transponder technology to quantify weather-driven modification of activity, mechanical energy expenditure, and ranging behavior, using the European badger as a model species. We then examine how these behaviors could respond to future climate change. From multi-model inference, activity was promoted significantly by a quadratic relationship with temperature, but inhibited by a quadratic relationship with humidity, and the amount of solar radiation. Drier conditions also encouraged more movement. Modeled against IPCC SRES low and high emissions climate change scenarios, milder and drier conditions projected for the next century would likely produce a change to badgers’ current phenology, with elevated levels of activity being maintained into the winter. This increased activity could necessitate up to a 15% increase in energy expenditure. Furthermore, conditions projected under the high emissions scenario may also lead to substantially increased movement, with implications for road traffic mortality rates. We contend that behavioral adaptation must be better incorporated into conservation strategies, versus the assumption of non-adaptive failure.

On upstream fish passage in standard box culverts: interactions between fish and turbulence

ABSTRACTThe challenge to understanding the fluid mechanics of fish swimming is knowing exactly what the water is doing where the fish swims. Recent field and laboratory observations in box culvert barrel showed that fish tend to swim preferentially close to the channel sidewalls, in regions of slow velocity and often high turbulence intensity. An analogy with human swimming is developed herein. Fish minimise their energy expenditure by swimming in inter-connected low-velocity zones (LVZs) and minimising acceleration-deceleration amplitudes. In a box culvert barrel, the mechanical energy expenditure is drastically reduced in sidewall and corner flow regions, characterised by low velocities and secondary current motion. These regions were “sweet spots” used by small bodied fish to minimise their rate of work. Both bed and sidewall roughness must be scaled to the fish dimensions. More generally, the methodology brings rigorous scientific insights into why certain culvert designs, possibly equipped with baffles and apertures, are more efficient in promoting fish passage. One may foresee an evolution of the scientific approach towards using advanced physics-based theory supported by high-quality data sets. The results also raise questions on limitations of current fish swim tunnel tests, and matching swimming performance data to hydrodynamic measurements.

Comfort Evaluation Model for Gesture Motion based on Mechanical Energy Expenditure and Efficiency

As an advanced interaction mode, the gesture has been widely used for the human-computer interaction (HCI).The paper proposes a comfort evaluation model based on the Mechanical Energy Expenditure (MEE) and the Mechanical Efficiency (ME) to predict the comfort of gestures. The proposed comfort evaluation model takes nineteen muscles and seven degrees of freedom into consideration based on the data of muscles and joints, and is capable of simulating the MEE and the ME of both static and dynamic gestures. The comfort scores (CS) can be therefore calculated by normalizing and assigning different decision weights to the MEE and the ME. Compared with the traditional comfort prediction methods based on measurement, on the one hand, the proposed comfort evaluation model makes it possible for providing a quantitative value for the comfort of gestures without using electromyography (EMG) or other measuring devices; on the other hand, from the ergonomic perspective, the results provide an intuitive indicator to predict which act has the higher risk of fatigue or injury for joints and muscles. Experiments are conducted to validate the effectiveness of proposed model. According to the comparison result among the proposed comfort evaluation model, the model based on the range of motion (ROM) and the model based on the MMGA, a difference can be found due to the ignorance of dynamic gestures and the relative kinematics characteristics during the movements of dynamic gestures. Therefore, the research can be applied to the optimization of gesture design while considering the feedback of perceived effects and gesture recognition rate in HCI.

Analysis of Energy-Based Metrics for Laparoscopic Skills Assessment.

The complexity of minimally invasive surgery (MIS) requires that trainees practice MIS skills in numerous training sessions. The goal of these training sessions is to learn how to move the instruments smoothly without damaging the surrounding tissue and achieving operative tasks with accuracy. In order to enhance the efficiency of these training sessions, the proficiency of the trainees should be assessed using an objective assessment method. Several performance metrics have been proposed and analyzed for MIS tasks. The differentiation of various levels of expertise is limited without the presence of an external evaluator. In this study, novel objective performance metrics are proposed based on mechanical energy expenditure and work. The three components of these metrics are potential energy, kinetic energy, and work. These components are optimally combined through both one-step and two-step methods. Evaluation of these metrics is accomplished for suturing and knot-tying tasks based on the performance of 30 subjects across four levels of experience. The results of this study show that the one-step combined metric provides 47 and 60 accuracy in determining the level of expertise of subjects for the suturing and knot-tying tasks, respectively. The two-step combined metric provided 67 accuracy for both of the tasks studied. The results indicate that energy expenditure is a useful metric for developing objective and efficient assessment methods. These metrics can be used to evaluate and determine the proficiency levels of trainees, provide feedback and, consequently, enhance surgical simulators.

The Effects of Squats and Jump Squats on Mechanical Work and Energy Expenditure

PURPOSE: To investigate the effects of two non-ballistic squat and two ballistic jump squat protocols performed over multiple sets on the total mechanical work performed and oxygen uptake. METHODS: In a counterbalanced cross-over design, 11 resistance-trained men (age: 21.9 ± 1.8 years; height: 1.79 ± 0.05 m; mass: 87.0 ± 7.4 kg) attended four testing sessions during a three week period where they performed multiple sets of squats and jump squats with a load equivalent to 30% 1-repeititon maximum under one of the following conditions: 1) three sets of four non-ballistic repetitions (30N-B), 2) three sets of four non-ballistic repetitions with a 3-second pause between the eccentric and concentric phases (30PN-B), 3) three sets of four ballistic repetitions (30B), 4) three sets of four ballistic repetitions with a 3-second pause between the eccentric and concentric phases (30PB). Force plates and a 3-D motion analysis system were used to determine the total mechanical work performed during each session while a portable gas analysis system was used to collect expired gases. RESULTS: Total mechanical work performed during each set was significantly greater during 30B compared to 30N-B (mean difference [MD]: 7,792 J, p<0.001, effect size [ES]: 1.88) and 30PN-B (MD: 7,749 J, p<0.001, ES: 1.89), while that during 30PB was significantly greater than that during both 30N-B (MD: 7,488 J, p<0.001, ES: 1.89) and 30PN-B (MD: 7,445 J, p<0.001, ES: 1.90). Energy expenditure during each set was significantly greater during the 30B condition compared to the 30N-B (MD: 13,983 J, p<0.001, ES: 1.08) and the 30PN-B (MD: 11,326 J, p=0.001, ES: 0.92). Energy expenditure during 30PB was also significantly greater than that during 30N-B (MD: 12,615 J, p=0.001, ES: 1.04) and 30PN-B (MD: 9,958 J, p=0.006, ES: 0.86). Furthermore, energy expenditure during set 1 was significantly greater than that during set 2 (MD: 6,840 J, p<0.001, ES: 0.64) and set 3 (MD: 8,070 J, p<0.001, ES: 0.75). CONCLUSION: Ballistic resistance training exercises may represent a more effective metabolic stimulus compared to traditional resistance training exercises and a pause inserted between the eccentric and concentric phases has little effect.

Experimental study and modelling the evolution of viscoelastic hysteresis loop at different frequencies in myocardial tissue.

Our work involved experimental study of the influence of actomyosin complexes and the main structural components of the myocardial tissue - connective tissue collagen framework and cardiomyocytes - on the characteristics of viscoelastic hysteresis at different frequencies. In this paper a new method was introduced for the analysis of the viscoelastic characteristics of the force hysteresis in the isolated myocardial preparation for the assessment of mechanical energy expenditure in the tension-compression cycle. We established that basic myocardial structures have an impact on the to the characteristics of the viscoelastic hysteresis in many ways. It was shown that in rat's myocardium cardiomyocytes one main factor that define the stiffness and viscosity of the myocardium in the physiological range of deformations, while binding of calcium ions with EGTA and calcium removal of sarcoplasmic reticulum with caffeine reduces viscoelasticity by ~30% and collagen framework is responsible for about 10% of viscoelasticity. It was revealed that in the physiological range of the hysteresis frequencies (3 to 7 Hz) expenditure of mechanical energy per unit of time increases linearly with increasing frequency. We proposed the structural and functional model that adequately describes the characteristics of the viscoelastic hysteresis in myocardial preparation in the range of strains and frequencies being under study.

Medial gastrocnemius muscle-tendon interaction and architecture change during exhaustive hopping exercise

Background: Previous literature has shown in vivo changes in muscle-tendon interaction during exhaustive stretch-shortening cycle (SSC) exercise. It is unclear whether these changes in muscle-tendon length during exhaustive SSC exercise are associated with changes in mechanical efficiency (ME). The purpose of the study was to investigate whether changes in platarflexor contractile component (CC) length, tendon length, and changes in plantarflexor muscle activity could explain reduction in ME during exhaustive SSC exercise. Methods: Eight males participated in an exhaustive hopping task to fatigue. Mechanical work and energy expenditure were calculated at different time-points during the hopping task. Furthermore, hopping kinetics and kinematics, medial gastrocnemius (MG) muscle activity, and in vivo ultrasound of the MG were also collected at different time-points throughout the hopping task. Results: ME did not change during the hopping protocol despite shorter tendon and longer CC lengths as subjects approached exhaustion. Percent decreases in pennation angle and muscle thickness were most strongly correlated to time to exhaustion (r=0.94, p⩽0.05; r=0.87, p⩽0.05; respectively). Percent changes in CC length change and pennation angle were strongly correlated to percent decrease in maximal voluntary isometric plantarflexion (MVIP) force (r=−0.71, p⩽0.04; r=0.70, p⩽0.05; respectively). Braking/push-off EMG ratio increased from initial pre-fatigue values to all other time points showing neuromuscular adaptations to altered muscle lengths. Conclusion: Findings from the current study suggest that changes in CC and tendon lengths occur during repetitive hopping to exhaustion, with the amount change strongly related to time to exhaustion. ME of hopping remained unchanged in the presence of altered CC and tendon lengths.

Disassembly task evaluation by muscle fatigue estimation in a virtual reality environment

Today, disassembly operations play a very important role during the initial design phase of industrial products considering the role played by these operations throughout the product life cycle. Current simulation platforms do not offer the necessary information and versatility required for a complete disassembly process simulation, including human/operator physiological data management. The paper deals with a new method for disassembly sequence evaluation. It is based on metabolic energy expenditure and muscle fatigue estimation. For this purpose, the analytical model for mechanical energy expenditure is proposed. In this model, the required mechanical work is used as a parameter that allows comparing the relationships among fatigue levels when performing disassembly sequences. Then, the fatigue levels are evaluated by analyzing the recorded electromyography signal on an operator’s arm. The proposed method is validated by a set of experimental disassembly tests performed in a virtual reality environment. The comparison of the analytical and experimental results has shown good correlation between them. The main result of this study is the proposed model for assessing muscle fatigue and its validation by experimental procedure. The proposed method provides the feasibility to integrate human muscle fatigue into disassembly sequence evaluation via mechanical energy expenditure when performing disassembly operation simulations.

Evidence for subjective values guiding posture and movement coordination in a free-endpoint whole-body reaching task.

When moving, humans must overcome intrinsic (body centered) and extrinsic (target-related) redundancy, requiring decisions when selecting one motor solution among several potential ones. During classical reaching studies the position of a salient target determines where the participant should reach, constraining the associated motor decisions. We aimed at investigating implicit variables guiding action selection when faced with the complexity of human-environment interaction. Subjects had to perform whole body reaching movements towards a uniform surface. We observed little variation in the self-chosen motor strategy across repeated trials while movements were variable across subjects being on a continuum from a pure ‘knee flexion’ associated with a downward center of mass (CoM) displacement to an ‘ankle dorsi-flexion’ associated with an upward CoM displacement. Two optimality criteria replicated these two strategies: a mix between mechanical energy expenditure and joint smoothness and a minimization of the amount of torques. Our results illustrate the presence of idiosyncratic values guiding posture and movement coordination that can be combined in a flexible manner as a function of context and subject. A first value accounts for the reach efficiency of the movement at the price of selecting possibly unstable postures. The other predicts stable dynamic equilibrium but requires larger energy expenditure and jerk.

Mobility Diversity-Assisted Wireless Communication for Mobile Robots

Mobile robots that wish to communicate wirelessly often suffer from fading channels. They need to devise an energy-efficient strategy to search for a high-channel-gain position in a near vicinity from which to begin communications. Such a strategy has recently been introduced through the mobility diversity with multithreshold algorithm (MDMTA). In this paper, we establish the theoretical framework for a generalized version of the MDMTA. This allows improved wireless communications in fading channels for mobile robots via intelligent robotic motion with low mechanical energy expenditure.

Energy flow analysis of the lower extremity during gait in persons with chronic stroke

BackgroundA decline in walking capacity and high energy cost can limit mobility following stroke. Mechanical energy exchange between lower limb and trunk segments can reflect gait inefficiencies, but reveals little about active energy flow between adjacent segments through muscle actions. This study evaluated mechanical energy expenditures (MEEs) during walking in stroke and healthy groups to understand movement control and explore the impact of walking speed on mechanical energy exchanges. MethodsThirteen adults with hemiparesis and six healthy controls walked at self-selected speed. Power curves for each lower limb joint were segmented into concentric and eccentric sources of muscle power and transfer/no-transfer modes to calculate MEEs during stance. FindingsMEEs were lower in the stroke group on the affected side compared to the less affected side and compared to controls. Specifically, the affected plantarflexors transferred less energy distally via concentric action in late stance compared to the less affected side. However, the stroke group generated greater energy at the ankle in the absence of transfer compared to controls. Less concentrically transferred energy through midstance and absorbed in late stance was evident by the knee extensors bilaterally in stroke. At the hip, the total energy (no transfer) was reduced on the affected side. Classifying stroke subjects by walking speed (<.6m/s, >.6m/s) revealed disruptions in harnessing energy through motion and transfer energy across segments in the slower group. InterpretationThe limited ability of those with stroke to exploit intersegmental energy transfer to optimize efficiency may limit endurance and functional independence.

Metabolic Cost, Mechanical Work, and Efficiency During Normal Walking in Obese and Normal-Weight Children

Purpose: This study aimed to investigate the influence of childhood obesity on energetic cost during normal walking and to determine if obese children choose a walking strategy optimizing their gait pattern. Method: Sixteen obese children with no functional abnormalities were matched by age and gender with 16 normal-weight children. All participants were asked to walk along a nearly circular track 30 m in length at a self-selected speed. Spatiotemporal data, kinematics, and ground reaction force were collected during walking using a three-dimensional motion analysis system. Metabolic cost was collected by a portable gas analyzer simultaneously. Results: The mechanical energy expenditure (MEE) was 72.7% higher in obese children than in normal-weight children. The net metabolic cost was 65.7% higher in obese children. No difference was found in the metabolic rate (J·kg− 1·m− 1), normalized MEE (J·kg− 1·m− 1) and mechanical efficiency between the obese and normal-weight groups. The obese children walked with a 0.15 m/s slower walking speed, 10.0% shorter cadence, and 30.9% longer double-support phase compared with normal-weight children. In addition, no differences were found in the mediolateral and vertical body center of mass displacement. Conclusion: Body mass played a dominant role in the total metabolic and mechanical cost per stride. Obese children may adopt a walking strategy to avoid an increase in the metabolic cost and the mechanical work required to move their excess body mass.

Mechanical Analysis of Single Myocyte Contraction in a 3-D Elastic Matrix

BackgroundCardiac myocytes experience mechanical stress during each heartbeat. Excessive mechanical stresses under pathological conditions cause functional and structural remodeling that lead to heart diseases, yet the precise mechanisms are still incompletely understood. To study the cellular and molecular level mechanotransduction mechanisms, we developed a new ‘cell-in-gel’ experimental system to exert multiaxial (3-D) stresses on a single myocyte during active contraction.MethodsIsolated myocytes are embedded in an elastic hydrogel to simulate the mechanical environment in myocardium (afterload). When electrically stimulated, the in-gel myocyte contracts while the matrix resists shortening and broadening of the cell, exerting normal and shear stresses on the cell. Here we provide a mechanical analysis, based on the Eshelby inclusion problem, of the 3-D strain and stress inside and outside the single myocyte during contraction in an elastic matrix.Results(1) The fractional shortening of the myocyte depends on the cell’s geometric dimensions and the relative stiffness of the cell to the gel. A slender or softer cell has less fractional shortening. A myocyte of typical dimensions embedded in a gel of similar elastic stiffness can contract only 20% of its load-free value. (2) The longitudinal stress inside the cell is about 15 times the transverse stress level. (3) The traction on the cell surface is highly non-uniform, with a maximum near its ends, showing ‘hot spots’ at the location of intercalated disks. (4) The mechanical energy expenditure of the myocyte increases with the matrix stiffness in a monotonic and nonlinear manner.ConclusionOur mechanical analyses provide analytic solutions that readily lend themselves to parametric studies. The resulting 3-D mapping of the strain and stress states serve to analyze and interpret ongoing cell-in-gel experiments, and the mathematical model provides an essential tool to decipher and quantify mechanotransduction mechanisms in cardiac myocytes.

Mechanical energy expenditure of subject with multiple sclerosis engaged in daily activities: a case study

Multiple sclerosis (MS) is an acquired inflammatory, demyelinating central nervous system disorder associated with a high prevalence of co-morbidity due to recurrent episodes of neurologic dysfunct...

Exercise modality and metabolic efficiency in children

Current exercise prescription guidelines for children recommend at least 60 min of moderate-to-vigorous physical activity every day. However, little is known about the efficacy of different cardiorespiratory exercise modalities prescribed to the pediatric cohort. Sixteen healthy children (8-12 years) completed 5-min trials of treadmill walking, cycling, and elliptical training in a randomized order. The treadmill walking speed was determined from measurements collected during a self-selected walking trial. The workloads for treadmill walking, cycling, and elliptical training were matched (40.3 W). Mechanical efficiency (ME%), perceived exertion (RPE), oxygen uptake, metabolic equivalents, and net energy expenditure were measured. ME% in walking was significantly higher than in cycling (P = 0.001) and elliptical training (P < 0.001), and cycling was significantly higher than elliptical training (P = 0.003). RPE in walking was significantly lower than in elliptical training (P = 0.006) but not from cycling (P = 0.314), and cycling resulted in significantly lower RPE than elliptical training (P = 0.021). Treadmill walking appears to be the most efficacious exercise prescription for otherwise healthy children; however, longitudinal studies need to be implemented to investigate the long-term benefits of each exercise modality.

Mechanical Efficiency of Walking with Spring-loaded Axillary Crutches

ABSTRACT The objective of this study was to investigate the effect of using spring-loaded crutches on mechanical efficiency in comparison to the use of standard crutches. Seven healthy male participants were recruited for this experiment. They were instructed to walk around a track on two types of crutches for five minutes under four controlled conditions. Ground reaction force and motion trajectories were recorded, which were used to calculate mechanical energy expenditure. Simultaneously, portable gas analysis machinery was used to collect the metabolic parameters of participants, in order to determine mechanical efficiency, the ratio of mechanical energy expenditure to metabolic energy expenditure. It was found that the mechanical efficiency was significantly higher when participants walked with spring-loaded crutches than when they walked with standard crutches. There were no significant differences in metabolic energy expenditure between the spring-loaded crutch conditions and standard crutch conditions.

Humans, geometric similarity and the Froude number: is ‘‘reasonably close’’ really close enough?

SummaryUnderstanding locomotor energetics is imperative, because energy expended during locomotion, a requisite feature of primate subsistence, is lost to reproduction. Although metabolic energy expenditure can only be measured in extant species, using the equations of motion to calculate mechanical energy expenditure offers unlimited opportunities to explore energy expenditure, particularly in extinct species on which empirical experimentation is impossible. Variability, either within or between groups, can manifest as changes in size and/or shape. Isometric scaling (or geometric similarity) requires that all dimensions change equally among all individuals, a condition that will not be met in naturally developing populations. The Froude number (Fr), with lower limb (or hindlimb) length as the characteristic length, has been used to compensate for differences in size, but does not account for differences in shape.To determine whether or not shape matters at the intraspecific level, we used a mechanical model that had properties that mimic human variation in shape. We varied crural index and limb segment circumferences (and consequently, mass and inertial parameters) among nine populations that included 19 individuals that were of different size. Our goal in the current work is to understand whether shape variation changes mechanical energy sufficiently enough to make shape a critical factor in mechanical and metabolic energy assessments.Our results reaffirm that size does not affect mass-specific mechanical cost of transport (Alexander and Jayes, 1983) among geometrically similar individuals walking at equal Fr. The known shape differences among modern humans, however, produce sufficiently large differences in internal and external work to account for much of the observed variation in metabolic energy expenditure, if mechanical energy is correlated with metabolic energy. Any species or other group that exhibits shape differences should be affected similarly to that which we establish for humans. Unfortunately, we currently do not have a simple method to control or adjust for size–shape differences in individuals that are not geometrically similar, although musculoskeletal modeling is a viable, and promising, alternative. In mouse-to-elephant comparisons, size differences could represent the largest source of morphological variation, and isometric scaling factors such as Fr can compensate for much of the variability. Within species, however, shape differences may dominate morphological variation and Fr is not designed to compensate for shape differences. In other words, those shape differences that are “reasonably close” at the mouse-to-elephant level may become grossly different for within-species energetic comparisons.