Author(s): Richard D Zorowitz, MD, Matthew A Statton, MD
Originally published: August 22, 2016 Last updated: December 28, 2023
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Overview and Description

Introduction

Physiatrists are well versed in visual assessment of gait deviations that accompany musculoskeletal pathology, such as arthritis, and neurologic pathology, such as stroke. What is more difficult to appreciate is the energy expenditure associated with locomotion and how it varies with changes in gait characteristics and deviations.

Energy conservation is one goal of human locomotion

The human body can be modeled biomechanically as body segments linked at joints. Human locomotion then can be studied in terms of relative motion between these segments (kinematics), segmental forces and moments enabling this motion (kinetics), and the energy expenditure of this motion (energetics). As part of energetics, the principle of energy conservation has formed a cornerstone in many models of human bipedal locomotion, such as the gait determinants theory, inverted pendulum theory, and dynamic walking theory.1 

The energy conservation theory suggests that efficient locomotion, including preferred walking speed,2 cadence,3 and step length4 all are tuned to minimize energy expenditure.1 As a motor skill, walking is acquired through repetition and reinforcement, and is intended to be efficient, automatic, and goal-directed. Deviations in gait, explained by biomechanical and/or neuromuscular factors such as muscle weakness, joint deformity, or inappropriate bracing often lead to increased energy expenditure.5 Rehabilitation principles of mobility training and assistive device use help achieve the goal of improved energy efficiency in patients with mobility impairments. For example, an assistive device can reduce energy expenditure by providing support with muscle weakness, or a rocker bottom shoe can decrease energy cost of mobility due to improving fluidity in walking and re-establishing a more normal gait pattern. Please refer to the section, Approaches to Energy Conservation, Assistive Devices Utilization for more details.

Basics of energy expenditure relevant to mobility

Energy metabolism involves the production of energy from the combustion of fuel such as carbohydrates, protein, or fat. In doing so, oxygen is consumed and carbon dioxide is produced.6 During movement, muscles convert metabolic energy to mechanical energy to generate forces and facilitate motion.7

Metabolic Efficiency defines how well metabolic energy is converted into mechanical energy. Metabolic efficiency is calculated as the ratio of mechanical work to metabolic cost7. Metabolic efficiency in human motion tends to be approximately 20-25%,5 with the rest of the energy converted and lost via heat.7

Energy expenditure is commonly estimated by (a) oxygen consumed by muscles (VO2, measured in ml/kg/min) and/or (b) heat produced in conversion of metabolic to mechanical energy.5,7 Energy expenditure parameters are presented in Table 1.

Table 1. Energetics – Basic Definitions5,7

Energy-Expenditure-Table-1

Energy expenditure definitions specific to mobility include physiologic cost of transport, defined as VO2/walking speed (ml/kg/m), and physiologic efficiency of gait, defined as energy expenditure in relation to “normal gait.8”Training and equipment can change the efficiency of gait. This is an important consideration for energy conservation. For example, using a walker or a rollator can decrease the cost of transport and increase the efficiency of gait.

Measuring Total Energy Expenditure (TEE)

Total energy expenditure (TEE) can be calculated as the sum of resting energy expenditure (REE), activity energy expenditure (AEE), and thermic effect of food (TEF).9

In general, total energy expenditure can be estimated using either indirect or direct calorimetry, which measure oxygen consumption or heat production, respectively.6 The gold standard approach to assess total energy expenditure remains the Doubly Labeled Water Technique, a form of indirect calorimetry which measures the elimination (commonly urine) of ingested 2H218O. However, this technique is used less commonly due to the need for sophisticated lab-based equipment. Instead, the most common technique involves a form of indirect calorimetry, using a facemask and metabolic cart to measure respiratory oxygen consumption and carbon dioxide production, as there is a direct, linear relationship between oxygen consumption and energy produced (i.e., for each liter of O2 consumed, approximately 5 kcal of energy is used).6

There are other techniques to approximate total energy expenditure when more sophisticated equipment is not available

  • Accelerometry utilizes triplanar data to approximate energy expenditure using predictive algorithms.
  • Heart Rate (HR) monitors similarly utilize predictive algorithms relating measured HR and VO2.
  • Self-reported ratings of perceived exertion (e.g., Borg scale) during acute activity also correlate well with both heart rate10 and energy expenditure11 when other technology is not readily available.

Relevance to Clinical Practice

Energy expenditure during basic mobility

Energy expenditure for physiologic and pathologic gait is presented in the following sections. Table 2 contrasts activities and subtypes with higher and lower energy expenditure.

Table 2 Activities and subtypes with higher and lower energy expenditure

Energy-Expenditure-Table-2

Physiologic Gait

  • Gait cycle mechanics
    Stance phase is approximately three times as energy intensive as the swing phase. Single and double limb support are responsible for 37% and 44% of the total muscle energy expenditure. Overall, the highest and lowest energy consumption in the gait cycle both are seen in double limb support, in the first and second phases, respectively. Hip extensors consume more energy (40%) than ankle plantar flexors (27%), and knee extensors require the least energy (18%).12
  • Walking and Running
    Oxygen consumption, VO2 (ml/kg/min), increases linearly with walking velocity (m/min) for normal walking speeds (0.67 – 1.67 m/s), and non-linearly with higher walking speeds. At the transition velocity from walking to running of approximately 2.0 m/s, running requires greater metabolic power as we progress from anaerobic to aerobic activity.13

    In terms of mechanical efficiency, running is superior to walking. Efficiency increases linearly with running speeds (2.5 – 8.4 m/s), while walking is most efficient at intermediate velocity (1.4 – 2.0 m/s).14
  • Balance
    Maintaining coronal plane stability while progressing in the sagittal plane carries an energy cost.15

    Uneven terrain creates demands on balance and thus increases energy expenditure. Associated biomechanical and neuromuscular factors such as increased step variability and increased muscle work/co-activation to stabilize joints, respectively, stabilize the individual but also increase metabolic cost.16 For instance, Voloshina and Ferris (2015) demonstrated that healthy individuals asked to walk or run over uneven terrain (modeled by wooden blocks covered with foam, attached to a treadmill) exhibited corresponding 27% and 5% increases in metabolic cost when compared with those on even terrain.16

    The type of terrain itself influences energy expenditure as well. For example, walking on sand is more energetically costly than grass.16
  • Age and gender
    At self-selected speeds of walking,8 children have higher physiologic cost and lower physiologic efficiency, associated with a slower velocity, shorter step length, and increased cadence. Physiologic cost decreases over the teen years into adulthood as the gait pattern and underlying physiology mature.8

    In the elderly, the physiologic cost of walking is increased. This can be explained by abnormal lower limb kinematic changes such as decreased hip extension, spatiotemporal changes such as decreased cadence, timing of gait phase transitions, reduced neural control, and uncoordinated muscle activation patterns.2 Additionally, decreased aerobic capacity in aging reduces energy available for activity.
    Female gender is associated with a lower Total Daily Energy Expenditure (TDEE). While the exercise phase involves higher fat oxidation, the postprandial phase shows lower fat utilization compared with men, suggested to be related to estrogen.17,18

Pathologic Gait

Increased energy expenditure in most neuromusculoskeletal pathologies is multifactorial. Locomotor strategies that promote stability, whether self-learned or taught, may impose additional cost.

For instance, after a stroke, impaired motor control, muscle spasticity, abnormal motor strategies, altered kinetics and kinematics,8 and impaired balance15 increase the cost of locomotion.

In cerebral palsy (CP), spasticity and associated abnormal (e.g., crouch) gait, balance, and motor control result in increased energy expenditure.8

After a spinal cord injury (SCI), reduced lower extremity strength and trunk control impose significant metabolic demands on the upper extremities to facilitate assistive aid enabled ambulation. The weight of orthotics can add to the energy demands.8

In Parkinson’s disease, tremor, rigidity, hypokinesia, and postural instability contribute increased energy expenditure during walking, independent of walking speed.19

In amputees, both non-prosthetic and prosthetic ambulation impose significant metabolic demands. Generally, dysvascular, higher level, and bilateral lower limb amputation gait is more energy intensive.20 Specifically, energy expenditure while walking can increase by least 25% after traumatic transtibial (TT), at least 40% after vascular TT, at least 68% after traumatic transfemoral (TF), and at least 100% after vascular TF amputations.21

In addition, prosthesis design and alignment influence energy cost. In traumatic amputees, knee design, but not foot (at slow to moderate walking speeds) significantly changes energy consumption, with a hydraulic knee being superior to a single axis knee. The metabolic cost of joint misalignment is much greater in transfemoral amputees (single-axis knee aligned 2 cm posterior) compared with transtibial amputees (ankle joint dorsiflexion and plantarflexion).22

Approaches to energy conservation

Energy conservation becomes important especially in pathologies where the cardiopulmonary reserve is limited such as congestive heart failure or severe chronic obstructive pulmonary disease, for neuromuscular disorders such as multiple sclerosis or amyotrophic lateral sclerosis, and for generalized cachexia and fatigue associated with malignancy. Rehabilitation professionals have adopted a multimodal approach to energy conservation across levels of care (inpatient, ambulatory, and community) to minimize the daily occurrence of symptoms such as shortness of breath, pain, and fatigue, presented below. Table 3 summarizes interventions that impact energy expenditure with ambulation.

Table 3. Energy conservation strategies and examples of interventions

Energy-Expenditure-Table-3

Energy Conservation Training – Rehabilitation, Education, and Activity Modification

The WHO International Classification of Functioning Model can be used to understand energy conservation training at the impairment, activity, and participation levels. At the impairment level, pharmacologic management of fatigue using amantadine, pemoline, and modafinil in multiple sclerosis, and carnitine and donepezil in malignancy is represented only by low-level evidence.20 Sleep and mood management as well as education for risk factor modifications such as smoking serve as essential adjuncts for energy conservation.23

At the activity level, strategies include organization to maximize access and minimize distance traveled, time management to separate out events temporally, task simplification, avoiding extremes of temperature, good body posture, and sitting.24 At the same time, it is important to encourage daily exercise. Aerobic conditioning programs have demonstrated reduced oxygen consumption during unassisted and assisted walking in multiple pathologies as well as in healthy aging. Strengthening and focused gait training help improve inefficient compensatory gait strategies as well as reduce energy expenditure.8

At the participation level, ergonomic modifications as well as social support to help with tasks are a key part of maximizing work and social ability.24 Educational interventions regarding energy conservation strategies for activities of daily living and mobility already are a part of many diagnosis-specific clinical programs using in-person as well as distance (e.g., web or phone-based) formats. Yoga, Tai Chi, breathing exercises, and meditation have also been proposed as self-management methods.

Minimize Gait Parameter Costs by Gait Training

Older “recovery” locomotion theories propose that energy exchange is maximized within the system in order to reuse available energy and have at their core the principle of minimizing center of mass (COM) displacement to minimize energy expenditure.

As an illustration of this concept, COM vertical displacement is reduced by shorter steps and flat gait, but overall metabolic expenditure is still increased. In addition, more steps per unit distance and increased cadence to maintain speed are both very inefficient. Hence, energy-conservative gait balances and minimizes (a) COM vertical displacement, (b) stance phase energetic cost (optimizing timing of gait events), and (c) swing phase energy costs (optimizing step length).25

Environmental Modification (Ergonomics)

Stabilization in environmental design can help to improve energy expenditure and ergonomics. For example, external stabilization created by attaching an external frame positioned laterally in the coronal plane to a waist belt worn by subjects reduced energy cost in both non-pathologic (6% decrease)15 and pathologic gait (10% decrease in SCI).26 Similarly, handrail use in treadmill walking was shown to decrease energy consumption by 16% in chronic stroke patients.15

Assistive Device Utilization

Mobility aids such as canes, crutches, and walkers facilitate locomotion and reduce falls. Biomechanical benefits include a larger base of support, reduced lower limb loads, augmented gait initiation and stopping, and increased somatosensory input. However, they also have the potential to impose significant attentional and upper extremity load demands and can destabilize gait and prevent recovery during falls. This especially is true in cases of weakness, impaired neuromotor control, and excessive postural excursions.27 In general, metabolic costs associated with assistive aid use should be balanced with the benefits of balance and mobility training specific to the indications of use.25

Cane use has been found to increase energy expenditure in healthy, young28 users, hypothesized to be due to increased cognitive demands of learning a new task.17 In contrast, aid-dependent ambulators demonstrated decreased energy requirements with cane use.17

Crutches promote swing-through gait that is more energy intensive but also more efficient (due to increased speed) compared with normal, reciprocating gait8.

The design of the walker influences energy cost, with a rollator minimizing energy cost compared with a non-wheeled walker.27

Manual Wheelchair propulsion techniques as well as design parameters (weight, component positions and proportions, etc.) can influence energy expenditure. Clinical complications of manual wheelchair propulsion such as rotator cuff pathology and other overuse injuries also can result in decreased performance and increased energy expenditure.20

Footwear and Orthoses Use

Shoe design and inserts can also impact energy expenditure.8 For example, after a toe amputation, toe fillers with a footplate can compensate positively while heavier diabetic shoes can increase energy expenditure.

Conventional limb and spinal orthoses add extra weight to the appendicular and axial skeleton and hence can increase energy expenditure. However, limb orthoses such as the spring leaf AFO can improve gait kinematics and thus reduce the overall cost of walking.8

Powered orthoses to facilitate locomotion are a new field of study, with feasibility studies documenting positive outcomes. An Exo-ankle assisting plantarflexion in stance exhibited decreased metabolic cost trends in a sample of stroke patients (n = 15).29 In paraplegic patients, power and hybrid orthoses (mechanical orthoses combined with functional electrical stimulation) reduced metabolic cost when compared with conventional mechanical orthoses.30

Surgical Management

Studies have shown mixed effects of surgical intervention on energy expenditure in pathologic gait, but there may be benefit in selecting and performing surgery appropriately in certain pathologies. For instance, single event multilevel surgeries in patients with CP have the potential to reduce energetic cost of long-term postsurgical locomotion.31 However, other studies of these patients have shown limited effects on the energetic cost of locomotion following interventions such as selective dorsal rhizotomy, despite decreases in spasticity following surgery.32

Cutting Edge/Unique Concepts/ Emerging Issues

Our understanding of the field of mobility energetics and its implications for rehabilitation is evolving. Recent studies have aimed to leverage the energy conservation theory to develop new treatment protocols for patients. For instance, one recent study demonstrated that stroke survivors with hemiparetic, asymmetric walking patterns could learn to walk more symmetrically when the new gait pattern reduced the cost of transport.33

While energy conservation remains the most prevalent theory, it also has been suggested that human gait may be optimized to balance several other factors, such as muscle activity, stability, ground-reaction forces, and joint ligament use,34,35 sometimes at the expense of energy efficiency. These other factors may explain in part why certain patient populations tend to default to movement patterns with increased energetic cost.36

Rehabilitation strategies for cost-efficient gait also are evolving, especially with emerging technologies. In addition to the traditional assistive devices and methods described above, there is a growing body of research involving newer technologies such as complete or partial exoskeletons. Such devices have the potential to improve mobility and independence in non-ambulatory people while minimizing energy cost.37 However, the practical use of these technologies is limited by factors including cost and design constraints such as size/weight of the devices.38 Ongoing translational research on mobility energetics can be instrumental in refining these devices for ubiquitous everyday use.

Finally, researchers are developing novel strategies increasingly to understand how the nervous system optimizes energetic cost of movement, including mechatronic39 and biofeedback33-based systems that assess how people optimize their gait patterns in novel environments.

Gaps in Knowledge/Evidence Base

From a functional perspective, the definition of mobility can range from crawling to walking to an instrumented activity such as driving. Thus, locomotion is only a subset of mobility, energetics is one subset of locomotion, and energy expenditure a subset of energetics. For the purposes of this article, concepts from kinetics and muscle energetics have been included only where they improve the explanation of energy expenditure in mobility and conservation. Owing to the vastness and evolving nature of the field, this article has focused primarily on normal and assisted locomotion, including brief sections on impairment, activities, and participation that supplement the holistic understanding of concepts relevant to energy expenditure and conservation.

References

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Original Version of the Topic

Prateek Grover, MD, PhD, Oksana Volshteyn MD. Energy expenditure during basic mobility and approaches to energy conservation. 8/22/2016

Previous Revision(s) of the Topic

Prateek Grover, MD, PhD, MHA, Oksana Volshteyn MD. Energy expenditure during basic mobility and approaches to energy conservation. 12/15/2020

Author Disclosure

Richard D Zorowitz, MD
Brainq, Research Grant, Principal Investigator
Ipsen, Research Grant, Principal Investigator
Ipsen, Honorarium, Advisory Board
Spr, Therapeutics, Honorarium Data, Safety Monitor

Matthew A Statton, MD
Nothing to Disclose