Energy expenditure during basic mobility and approaches to energy conservation

Author(s): Prateek Grover, MD, PhD, Oksana Volshteyn MD

Originally published:08/22/2016

Last updated:08/22/2016

1. 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

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 gait energetics, muscles are considered as the primary drivers of locomotion. They utilize metabolic energy to derive mechanical energy for segmental force generation to facilitate motion.1 As a corollary, decreased metabolic energy, reflected by reduced skeletal muscle mitochondrial oxidative capacity has been found to be associated with reduced walking speed.2

How well metabolic energy is converted into mechanical energy is defined as Metabolic Efficiency. This is calculated as the ratio of mechanical work (approximated by sum of external and internal work, and assessed using kinetics) and metabolic cost of muscles1, and has been estimated to be approximately 20-25%.3 The rest of the energy is converted into heat.1

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.1,3 Energy expenditure definitions based upon these measures are presented in Table 1.

Table 1. Energetics of Locomotion – Definitions1,3

Physiologic State Oxygen consumed

(ml/kg/min)

Calories Consumed

(kJ/m2/hr)

At Rest Resting oxygen consumption Basal Metabolic Rate, BMR
For a given activity Oxygen Consumption, VO2 Metabolic Equivalent, MET*
With maximal exertion Maximal Aerobic Capacity, VO2Max**  

*MET is a multiple of BMR.

**VO2max  dictates functional ability, and is normally achieved within 2-3 minutes of exercise. Metabolism switches from aerobic to anaerobic at 55-65% of VO2 max for untrained subjects.

Definitions specific to locomotion 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

Other measures to calculate energy expenditure include CO2 production, doubly labeled water and heart rate.

Energy conservation

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.4 This principle suggests that efficient locomotion, including preferred walking speed 5 is designed to minimize energy expenditure.4 As a motor skill, walking is acquired through repetition, and is efficient, automatic, and goal-directed.

As a corollary, deviations in gait, explained by biomechanical and neuromuscular factors5, lead to increased energy expenditure. Hence, gait training, by a repetitive, task-specific, goal directed approach5, promotes energy minimization, stability4 and maneuverability.5

2. RELEVANCE TO CLINICAL PRACTICECUTTING EDGE/UNIQUE CONCEPTS/EMERGING ISSUES

ENERGY EXPENDITURE DURING BASIC MOBILITY

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%).7

ii. Walking and Running
Ox
ygen 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.8

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).9

iii. Balance
Maintaining coronal plane stability while progressing in the sagittal plane carries an energy cost.10

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.11 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).11 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.11

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.6

In the elderly, the physiologic cost of walking is increased. This can be explained by abnormal lower limb kinematics and kinetics, timing of gait phase transitions, as well as reduced neural control and uncoordinated muscle activation patterns.5

Literature is divided over the influence of gender on oxygen consumption, varying from male or female predominance to no difference.6

b. Pathologic Gait

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

For instance, in stroke, in addition to deconditioning, impaired motor control, muscle spasticity, abnormal motor strategies, altered kinetics and kinematics6, and impaired balance12 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 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 are more energy intensive.13 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).14

APPROACHES TO ENERGY CONSERVATION

a. Minimize Gait Parameter Costs

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. 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).15

b. Provide External support

External support minimizes energy cost. Handrail use in treadmill walking decreased energy consumption by 16% in chronic stroke patients.12  External stabilization (modeled by external springs attached to subjects) reduced energy cost in both non-pathologic (6% decrease) 10 and pathologic gait (10% decrease in SCI).16 Shoe design and inserts, by impacting gait kinetics and kinematics, can also impact energy expenditure.6

c. 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 could help improve upon inefficient compensatory gait strategies and reduce energy expenditure.6

d. Assisted gait

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 may, in fact, destabilize gait and prevent recovery during falls, especially in case of weakness, impaired neuromotor control, and excessive postural excursions.17 Opinion is divided regarding metabolic cost associated with assistive aid use.17

Cane support in the unaffected hand during level ground walking resulted in an 8% decrease in energy requirement in aid-dependent ambulators (n=12). However, unassisted ambulators (n=12) who used a cane experienced a 6% increase in energy expenditure, hypothesized to be due to increased cognitive demands of learning a new task, namely, using a cane.12

With crutches, swing through gait 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 rolling walker minimizing energy cost.17 Manual Wheelchair propulsion techniques as well as design parameters (weight, component positions and proportions, etc.), by influencing biomechanics and energy expenditure, can impact clinical outcomes, such as development of rotator cuff pathology.13

e. Orthoses

Conventional limb and spinal orthoses add extra weight to the appendicular and axial skeleton and hence can increase energy expenditure. However, limb orthoses also 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).18 In paraplegic patients, compared with conventional mechanical orthoses, power and hybrid orthoses (mechanical orthoses combined with FES) reduced metabolic cost.19

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.20

3. RELEVANCE TO CLINICAL PRACTICECUTTING EDGE/UNIQUE CONCEPTS/EMERGING ISSUES

ENERGY EXPENDITURE DURING BASIC MOBILITY

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%).7

ii. Walking and Running
Ox
ygen 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.8

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).9

iii. Balance
Maintaining coronal plane stability while progressing in the sagittal plane carries an energy cost.10

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.11 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).11 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.11

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.6

In the elderly, the physiologic cost of walking is increased. This can be explained by abnormal lower limb kinematics and kinetics, timing of gait phase transitions, as well as reduced neural control and uncoordinated muscle activation patterns.5

Literature is divided over the influence of gender on oxygen consumption, varying from male or female predominance to no difference.6

b. Pathologic Gait

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

For instance, in stroke, in addition to deconditioning, impaired motor control, muscle spasticity, abnormal motor strategies, altered kinetics and kinematics6, and impaired balance12 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 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 are more energy intensive.13 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).14

APPROACHES TO ENERGY CONSERVATION

a. Minimize Gait Parameter Costs

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. 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).15

b. Provide External support

External support minimizes energy cost. Handrail use in treadmill walking decreased energy consumption by 16% in chronic stroke patients.12  External stabilization (modeled by external springs attached to subjects) reduced energy cost in both non-pathologic (6% decrease)10and pathologic gait (10% decrease in SCI).16 Shoe design and inserts, by impacting gait kinetics and kinematics, can also impact energy expenditure.6

c. 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 could help improve upon inefficient compensatory gait strategies and reduce energy expenditure.6

d. Assisted gait

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 may, in fact, destabilize gait and prevent recovery during falls, especially in case of weakness, impaired neuromotor control, and excessive postural excursions.17 Opinion is divided regarding metabolic cost associated with assistive aid use.17

Cane support in the unaffected hand during level ground walking resulted in an 8% decrease in energy requirement in aid-dependent ambulators (n=12). However, unassisted ambulators (n=12) who used a cane experienced a 6% increase in energy expenditure, hypothesized to be due to increased cognitive demands of learning a new task, namely, using a cane.12

With crutches, swing through gait 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 rolling walker minimizing energy cost.17 Manual Wheelchair propulsion techniques as well as design parameters (weight, component positions and proportions, etc.), by influencing biomechanics and energy expenditure, can impact clinical outcomes, such as development of rotator cuff pathology.13

e. Orthoses

Conventional limb and spinal orthoses add extra weight to the appendicular and axial skeleton and hence can increase energy expenditure. However, limb orthoses also 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).18 In paraplegic patients, compared with conventional mechanical orthoses, power and hybrid orthoses (mechanical orthoses combined with FES) reduced metabolic cost.19

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.20

4. CUTTING EDGE/UNIQUE CONCEPTS/EMERGING ISSUES

Our understanding of the field of mobility energetics is evolving. An experimental approach is being used to supplement older models of energy conservation such as the gait determinant theory with additional information, such as provide 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.

5. 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. Owing to the vastness and the evolving nature of this field, the authors acknowledge that this chapter has been limited to presenting energy expenditure for normal and assisted locomotion only. Concepts from kinetics and muscle energetics have been included only where they improve the explanation of the primary topic, energy expenditure in mobility and conservation.

References

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  2. Coen PM, Jubrias SA, Distefano G, Amati F, Mackey DC, Glynn NW, Manini TM, Wohlgemuth SE, Leeuwenburgh C, Cummings SR, Newman AB, Ferrucci L, Toledo FG, Shankland E, Conley KE, Goodpaster BH. Skeletal muscle mitochondrial energetics are associated with maximal aerobic capacity and walking speed in older adults. J Gerontol A Biol Sci Med 2013 Apr;68(4):447-55.
  3. Smith LK, Weiss EL, Lehmkull LD. Brunnstrom’s Clinical Kineseology, 5th Ed, Philadelphia, PA, FA Davis Company.
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  5. VanSwearingen JM, Studenski SA. Aging, motor skill, and the energy cost of walking: implications for the prevention and treatment of mobility decline in older persons. J Gerontol A Biol Sci Med 2014 Nov;69(11):1429-36.
  6. Waters RL, Mulroy The energy expenditure of normal and pathologic gait. Gait & Posture 1999 Jul;9(3):207-31
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  8. Farris DJ, Sawicki GS. The mechanics and energetics of human walking and running: a joint level perspectiv JR Soc Interface. 2012 Jan 7;9(66):110-8.
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  10. Ijmker T, Houdijk H, Lamoth CJ, Beek PJ, van der Woude LH. Energy cost of balance control during walking decreases with external stabilizer stiffness independent of walking speed. J Biomech. 2013 Sep 3;46(13):2109-1
  11. Voloshina AS, Ferris DP. Biomechanics and energetics of running on uneven terrain. J ExpB 2015 Mar;218(Pt 5):711-9.
  12. Ijmker T, Houdijk H, Lamoth CJ, Jarbandhan AV, Rijntjes D, Beek PJ, van der Woude LH. Effect of balance support on the energy cost of walking after strok Arch Phys Med Rehabil. 2013 Nov;94(11):2255-61.
  13. Cifu D Braddom’s Physical Medicine and Rehabilitation, 5th Ed, Philadelphia, PA: Elsevier.
  14.  Schmalz T, Blumentritt S, Jarasch Energy expenditure and biomechanical characteristics of lower limb amputee gait: the influence of prosthetic alignment and different prosthetic components. Gait & Posture. 2002 Dec;16(3):255-63.
  15. Bertram JE, Hasaneini SJ. Neglected losses and key costs: tracking the energetics of walking and runnin J Exp Biol. 2013 Mar 15;216(Pt 6):933-8.
  16. Matsubara JH, Wu M, Gordon KE. Metabolic cost of lateral stabilization during walking in people with incomplete spinal cord inju Gait & Posture. 2015 Feb;41(2):646-51
  17. Bateni H, Maki BE. Assistive devices for balance and mobility: benefits, demands, and adverse consequenc Arch Phys Med Rehab. 2005 Jan;86(1):134-45.
  18. Takahashi KZ, Lewek MD, Sawicki GS, Neuromechanics-based powered ankle exoskeleton to assist walking post-stroke: a feasibility study. J Neuroeng. Rehabil. 2015 Feb 25;12:23.
  19. Arazpour M, Samadian M, Bahramizadeh M, Joghtaei M, Maleki M, Ahmadi Bani M, Hutchins The efficiency of orthotic interventions on energy consumption in paraplegic patients: a literature review. Spinal Cord 2015 53;168–175
  20. Marconi V, Hachez H, Renders A, Docquier PL, Detrembleur C. Mechanical work andenergy consumption in children with cerebral palsy after single-event multilevel surger Gait & Posture. 2014 Sep;40(4):633-9.

Author Disclosure

Prateek Grover, MD, PhD
Nothing to Disclose

Oksana Volshteyn MD
Nothing to Disclose

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