Energy Expenditure During Basic Mobility and Approaches to Energy Conservation

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. Mechanics of energy expenditure, energy conservation

Energy Conservation is one goal of human locomotion

The human body can be biomechanically modeled as body segments linked at joints. Human locomotion can then 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 

This principle suggests that efficient locomotion, including preferred walking speed ² is designed to minimize energy expenditure.¹ As a motor skill, walking is acquired through repetition, and is intended to be efficient, automatic, and goal-directed. Rehabilitation principles of mobility training and assistive device use help achieve these goals for 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 the section II. APPROACHES TO ENERGY CONSERVATION d. Assistive Devices Utilization for more details.

As a corollary, deviations in gait, explained by biomechanical and neuromuscular factors⁵, such as muscle weakness, joint deformity, and inappropriate bracing lead to increased energy expenditure.  Hence, gait training combined with appropriate assistive devices and bracing promotes energy minimization, stability¹ and maneuverability²  by a repetitive, task-specific, goal directed approach.²  

Basics of Energy Expenditure relevant to mobility

As part of gait energetics, muscles are considered to be primary drivers of locomotion. They utilize metabolic energy to derive mechanical energy for segmental force generation to facilitate motion.³ As a corollary, decreased metabolic energy, reflected by reduced skeletal muscle mitochondrial oxidative capacity, has been found to be associated with reduced walking speed.⁴

Metabolic Efficiency defines how well metabolic energy is converted into mechanical energy; and is calculated as the ratio of mechanical work (approximated by sum of external and internal work, and assessed using kinetics) and metabolic cost of muscles³. Metabolic efficiency is estimated to be approximately 20-25%⁵, with the rest of the energy converted into heat.³

Energy expenditure is most commonly estimated by (a) oxygen consumed by muscles (ml/kg/min) (b) heat produced in conversion of metabolic (aerobic and anaerobic) energy to mechanical energy.^3,5 Energy expenditure parameters are presented in Table 1.

Table 1. Energetics – Basic Definitions^,3,5

Energy expenditure definitions specific to mobility include Physiologic cost of gait or transport, defined as VO₂/walking speed (ml/kg/m), and Physiologic efficiency of gait, defined as energy expenditure [(subject)/ (normal gait)].⁶ 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 unit energy expenditure per unit distance and/or unit time.

Measuring Energy Expenditure (EE)

Total Energy Expenditure (TEE) can be calculated for a 24-hour period as the sum of Resting Energy expenditure (REE), Activity Energy expenditure (AEE), and thermic effect of food (TEF). ⁷

1. The gold standard method is the Doubly Labeled Water (DLW) Method, where the elimination of ingested ²H₂¹⁸O is measured to estimate TEE using the modified Wier formula.

TEE (kcal/day) = 22.4 × (3.9 × [rCO2/FQ] + 1.1 × rCO₂) where rCO2 is rate of CO2 production, and FQ is food quotient.

2. Direct calorimetry measures the rate of heat loss using a device called a calorimeter, but restricts mobility. Indirect calorimetry uses consumed O₂ (VO₂) and produced CO₂ (VCO₂) values of these gases collected using devices such as facemasks to measure EE.

REE (kcal/day) = (3.941 × VO₂ [L/min] + 1.106 × VCO₂ [L/min]) × 1,440.

Accelerometry utilizes triplanar data to approximate energy expenditure using predictive equations.  Heart Rate (HR) monitors similarly utilize predictive equations relating measured HR with VO₂.

3. Self-reported measures help understand the individual perspective and activity patterns but may not provide quantitative data. ⁸

Relevance to Clinical Practice

I. 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

a. Physiologic Gait

i. 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, highest and lowest energy consumption in the gait cycle are both seen in double limb support, in the first and second phases, respectively. Hip extensors (40%) consume more energy than ankle plantarflexors (27%), and knee extensors require the least energy (18%).⁹

ii. 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 m/s, running requires greater metabolic power.¹⁰

In terms of mechanical efficiency, however, 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 m/s).¹¹

iii. Balance
Maintaining coronal plane stability while progressing in the sagittal plane carries an energy cost.¹²

Uneven terrain creates demands on balance, and hence 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 increase metabolic cost.¹³ Healthy individuals (n=11) walking and running over uneven terrain (modeled by wooden blocks covered with foam, attached to a treadmill) showed a corresponding 27% and 5% increase in metabolic cost, when compared with even surfaces (models as foam only, and no foam).¹³

The type of terrain itself influences energy expenditure. Walking on sand is more energetically expensive than on grass. The highest sand/grass ratio (1.65) was seen at the intermediate walking speed of 1.4 m/s. Overall, energy expenditure increased with speed (0.8-1.7 m/s) with both terrains.¹³

iv. Age and gender
At self-selected speed of walking⁶, children have the highest physiologic cost and lower physiologic efficiency, associated with a slower velocity, shorter step length, and increased cadence. Children also demonstrate a higher HR. Physiologic cost decreases over the teen years into adulthood as the gait pattern matures for velocity, step length and cadence .⁶

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 decrease cadence, timing of gait phase transitions, reduced neural control and uncoordinated muscle activation patterns.² Additionally, aerobic capacity reduction over age 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.^14,15

b. Pathologic Gait

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

For instance, in stroke, in addition to deconditioning, impaired motor control, muscle spasticity, abnormal motor strategies, altered kinetics and kinematics⁶, and impaired balance¹⁶ increase the cost of locomotion.

In Cerebral palsy, spasticity and associated abnormal (crouch) gait, balance and motor control result in increased energy expenditure.⁶

In 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.⁶

In amputees, both non-prosthetic and prosthetic ambulation impose significant metabolic demands. It is well known that dysvascular, higher level, and bilateral lower limb amputation gait is energy intensive.¹⁷ In addition, prosthesis design and alignment influence energy cost. In traumatic amputees, knee design, but not foot (at slow to moderate walking speeds) significantly changed energy consumption, with the hydraulic knee being superior to the single axis knee. The metabolic cost of joint misalignment was much greater in transfemoral amputees (single-axis knee aligned 2 cm posterior) compared with transtibial amputees (ankle joint dorsiflexion and plantarflexion).¹⁸

II. Approaches to Energy Conservation

Energy conservation is most relevant for pathologies where the cardiopulmonary reserve is limited, such as for congestive heart failure and severe chronic obstructive pulmonary disease, for neuromuscular disorders such as multiple sclerosis and amyotrophic lateral sclerosis, and for generalized cachexia and fatigue as seen with malignancy. By utilizing the techniques and the principles outlined above, 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

a. 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, fatigue management by pharmacologic measures such as Amantadine, Pemoline and Modafinil for multiple sclerosis, and Carnitine and Donepezil in malignancy has some low-level evidence.¹⁷ Sleep and mood management as well as education for risk factor modification such as smoking serve as essential adjuncts for energy conservation.¹⁹

At the activity level, strategies highlighted 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. ²⁰ At the same time, it is important to stress upon daily exercise. Aerobic conditioning programs have demonstrated reduced oxygen consumption during unassisted and assisted walking, in multiple pathologies as well as with ageing. Strengthening and focused gait training help improve inefficient compensatory gait strategies and reduce energy expenditure.⁶

At the participation level, ergonomic modifications are a key part of maximizing work and social ability, as is social support to help with tasks ²⁰.  Educational interventions regarding energy conservation strategies for activities of daily and mobility already part of many diagnosis-specific clinical programs using in-person as well as distance (e.g. web-based, phone-based) formats. Yoga, Tai Chi, Breathing exercises, and meditation have also been proposed as self-management methods.

b. 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 the center of mass (COM) displacement to minimize energy expenditure. Newer models propose that additional factors have to be involved in energy conservation.

As an illustration of this concept, COM vertical displacement is reduced by shorter steps and flat gait, but overall metabolic expenditure is still increased. This is likely due to increased swing and stance phase costs, respectively. In addition, more steps per unit distance and increased cadence to maintain speed are both very inefficient. Hence, energy-conservative gait is one that 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).²¹

c. 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 the waist belt worn by subjects reduced energy cost in both non-pathologic (6% decrease) ¹² and pathologic gait (10% decrease in SCI)²². Another example is handrail use in treadmill walking decreasing energy consumption by 16% in chronic stroke patients.¹⁶

 d. Assistive Devices 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 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 is especially true in case of weakness, impaired neuromotor control, and excessive postural excursions.²³ In general, metabolic cost associated with assistive aid use has to be balanced with benefits of balance and mobility specific to the indication of use²¹.

Cane use has been found to increase energy expenditure in healthy young²⁴ as well as assistive-device-independent users, hypothesized to be due to increased cognitive demands of learning a new task, namely, using a cane in the latter.¹⁴. In contrast, and healthy older subjects did not show this increase²⁴, while aid-dependent ambulators demonstrated decreased energy requirements with cane use¹⁴

Crutches promote swing through gait that is more energy intensive but also more efficient (due to increased speed) when compared with reciprocating gait.⁶

The design of the walker influences energy cost, with a rollator minimizing energy cost compared with a non-wheeled walker.²³

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 can also result in decreased performance and increased energy expenditure.¹⁷

e. Footwear and Orthoses Use

Shoe design and inserts, by influencing gait kinetics and kinematics, can also impact energy expenditure.⁶ As an example, after toe amputation, while heavier diabetic shoes can increase energy expenditure, toe fillers with a footplate can compensate positively.

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 can hence reduce the overall cost of walking.⁶

Powered orthoses to facilitate locomotion are a new field of study, with feasibility studies documenting positive outcomes. An Exo-ankle assisting plantarflexion in stance showed decreasing metabolic cost trends in a sample of stroke patients (n=15).²⁵ In paraplegic patients, compared with conventional mechanical orthoses, power and hybrid orthoses (mechanical orthoses combined with FES) reduced metabolic cost.²⁶

f. Surgical management

Appropriately selected and performed surgery in certain pathologies, such as single event multilevel surgery in CP (n=10), has potential to reduce energetic cost of long-term postsurgical locomotion (at 1 year), attributable indirectly to decreased effort required of muscles to maintain posture.²⁷

Cutting Edge/ Unique Concepts/ Emerging Issues

Our understanding of the field of mobility energetics is evolving. Experimental approaches are currently being used to supplement older models of energy conservation such as the gait determinant theory with additional information such as provided by the dynamic gait model.⁴

Rehabilitation strategies for cost-efficient gait are also evolving, especially with evolution in technology. Conventional power wheelchairs minimize energy expenditure at the cost of cardiovascular conditioning. Newer solutions include standing wheelchairs, power wheels with variable assistance, and exoskeletons, minimize this pitfall. Translational research on mobility energetics can be instrumental in refining these devices for ubiquitous everyday use.

Gaps in Knowledge/ Evidence Base

The definition of mobility, from a functional perspective, can range from walking to an instrumented activity such as driving. Locomotion, then, is only a subset of mobility; energetics is one subset of locomotion, and energy expenditure a subset of energetics. Concepts from kinetics and muscle energetics have been included only where they improve the explanation of the primary topic, namely, energy expenditure in mobility and conservation. Owing to the vastness and the evolving nature of the field, this write-up has focused primarily on normal and assisted locomotion, with a brief section on impairment, daily activities and participation to supplement holistic understanding of management of concepts relevant to energy expenditure and conservation

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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. Published 8/22/2016

Author Disclosure

Prateek Grover, MD, PhD, MHA
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