JEPonline
Journal of 
Exercise Physiologyonline

ISSN 1097-9751

An International Electronic
Journal for Exercise Physiologists

Vol 1 No 1 April 1998

Metabolic Responses To Exercise
The oxygen cost of walking with an artificially immobilized knee with and without a shoe-lift

LARRY BIRNBAUM and CHAD HEDLUND

Department of Exercise Physiology, The College of St. Scholastica, Duluth, MN

BIRNBAUM, L., C. HEDLUND. The oxygen cost of walking with an artifically immobilized knee with and without a shoe-lift. JEPonline Vol. 1. No. 1, 1998. The purpose of this study was to determine the oxygen cost (VO2) of walking with and without a shoe-lift on the contralateral foot of an immobilized extended knee. Cardiac output (Q) and related cardiorespiratory measurements were also analyzed to determine the effect of a shoe-lift on central (heart rate, HR; stroke volume, SV) and peripheral (arteriovenous oxygen difference, a-vO2 diff) components of VO2. Seven (six female, one male) subjects participated in the study. None of the subjects were on medication, and they had no known cardiopulmonary or musculoskeletal disorders. The Medical Graphics CPX/D metabolic analyzer was used to determine breath-by-breath VO2 in ml/kg/min, which was converted to oxygen cost (ml/kg/m). The shoe-lift had no significant (p>0.05) effect on oxygen rate (i.e., VO2 l/min or ml/kg/min) or oxygen cost (ml/kg/m). There were no significant differences in VCO2, RER, Ve, Vt, and Fb. There were no statistically significant differences in HR, SV, Q, a-vO2 diff, and SVR between the two walking sessions with and without a shoe-lift. The results of the study indicate that a shoe-lift added to the contralateral foot of an immobilized extended knee has no effect on oxygen cost during walking and, therefore, does not improve walking economy.

Key Words: OXYGEN COST, ECONOMY, CARDIAC OUTPUT, STROKE VOLUME, HEART RATE, EFFICIENCY

Introduction
Injuries and diseases that limit the ability to flex the knee are relatively common. Knee injuries may be treated with casting or orthotics that immobilize the knee. A question of walking economy with an immobilized knee has been raised by some investigators (1,2,3). According to three reports, oxygen cost increases 18% to 23% when the knee is immobilized in a fully extended position compared to normal walking . One strategy to improve walking economy for subjects with an immobilized extended knee is to wear a shoe-lift on the opposite foot. The rationale for the shoe lift is that it should improve the gait of the subject. However, this belief may not be based on scientific evidence.

A literature search produced only one report on the effect of a shoe-lift added to the contralateral foot of an immobilized extended knee (4). The investigators compared the oxygen cost of walking with and without a shoe-lift on the contralateral foot of an immobilized extended knee to normal walking and found a significant improvement with the use of a shoe-lift compared to normal walking. They did not compare walking economy with a shoe-lift on the contralateral foot of an immobilized extended knee to walking economy without a shoe-lift on the contralateral foot of an immobilized extended knee. In other words, the question should be whether or not a shoe-lift improves walking economy for persons with an immobilized extended knee.

The purpose of the present study was to repeat the study by Abdulhadi et al.(4) to support or refute the findings. The oxygen cost of walking with and without a shoe-lift added to the contralateral foot of the immobilized extended knee was determined as well as the effect on central (heart rate, HR; stroke volume, SV) and peripheral (arteriovenous oxygen difference, a-vO2 diff) components of VO2.

Methods
Six female and one male agreed to participate in this study. An informed consent was obtained from each subject. Six subjects were 21 years old and one was 23 years old. The height of the subjects ranged from 160-173 cm, and the weight range was 50-79.5 kg. Two walking sessions were employed in this study. In one session, a one-inch rubber shoe-lift was strapped to the sole of the left shoe with athletic tape. The shoe-lift was not worn in the other session. The order in which the shoe-lilft was worn was randomized. All subjects wore an external knee immobilizer applied unilaterally to keep the right knee in full extension throughout the gait cycle in both sessions.

Six subjects walked on a Biodex treadmill at 3.5 mph at 0% grade for 20 minutes during both walking sessions. One subject walked at 3.1 mph during both sessions. During each session, steady-state oxygen consumption (VO2) and related respiratory measures were continuously monitored throughout the 20 minute period via a Medical Graphics CardiO2 System. Heart rate was monitored during the last 10 seconds of each minute of data collection, using the Physio-Control LifePak 9 with a 3-lead electrocardiographic configuration. The HR data were averaged across minutes 5 through 9 and 15 through 19. The subjects were previously familiarized with the treadmill, and six agreed that 3.5 mph was a comfortable walking speed (CWS). One subject felt most comfortable at 3.1 mph. The gait of the subjects was maintained throughout the walking sessions.

Steady-state oxygen cost was calculated by dividing oxygen consumption (VO2 in ml/min) by the subject’s body weight to yield ml/kg/min, which was then divided by the distance traveled per minute (m/min). The steady-state carbon dioxide production (VCO2) was calculated in the same manner. All other respiratory parameters, including respiratory exchange ratio (VCO2/VO2), were continuously monitored by the Medical Graphics CardiO2 System.

Cardiac output (Q) was determined during the 10th and 19th minutes of both walking sessions using the CO2 rebreathing (equilibrium) technique (5). Arterial CO2 (PaCO2) was derived from the end-tidal pulmonary CO2 (PETCO2). Mixed venous pulmonary CO2 (PvCO2) was derived from the CO2 rebreathing (bag) procedure. Arterial CO2 and mixed venous contents were calculated from arterial CO2 tension and PvCO2, respectively, using a standard oxygenated CO2 dissociative curve (6,7). The Medical Graphics CardiO2 System displayed the CO2 signal graphically to ensure the PvCO2 equilibrium.

Immediately after cessation of the 20-minute walking session, systolic and diastolic blood pressures were measured by auscultation of the left brachial artery using a standard mercury sphygmomanometer. Systolic blood pressure (SBP) was determined as the point of appearance of Kortkoff sounds, and diastolic blood pressure (DBP) the point of disappearance of these sounds. Mean arterial pressure (MAP) was calculated by adding one third (0.32) of the pulse pressure (the difference between SBP and DBP) to the diastolic pressure. Systemic vascular resistance (SVR) was estimated by dividing MAP by Q. Stroke volume (SV) was estimated by dividing Q by HR. Arteriovenous oxygen difference (a-vO2 diff, ml/100ml) was calculated by dividing VO2 (i.e., O2 rate) by Q.

Results are given as means±SD; p<0.05 was taken to represent statistical significance (Tables 1 and 2). A repeated measures ANOVA was performed to compare the two walking sessions with and without a shoe-lift.

Results
Tables 1 and 2 illustrate the cardiorespiratory responses while walking with and without a one-inch shoe-lift on the contralateral foot of the immobilized extended knee. Wearing the shoe-lift had no significant (p>0.05) effect on VO2 (l/min or ml/kg/min) or O2 cost (ml/kg/m). No significant differences in VCO2, RER, Ve, Vt, and Fb were observed. Likewise, the shoe-lift produced no significant differences in HR, SV, Q, a-vO2 diff, and SVR.

Discussion
The results of this study indicate that a shoe-lift on the contralateral foot of an immobilized extended knee does not enhance walking efficiency as measured by O2 cost. Consequently, it is unlikely that a person with an immobilized extended knee would benefit from a contralateral shoe-lift. The present study therefore disagrees with the study by Abdulhadi et al. (5). However, Abdulhadi and colleagues (5) reported a lower O2 cost when walking with a shoe-lift versus normal walking rather than comparing O2 cost differences between walking with and without a shoe-lift on the contralateral foot of an immobilized extended knee. They reported a significantly lower O2 cost of 12% above normal walking while wearing a one-inch shoe-lift on the contralateral foot versus 20% above normal walking without the shoe-lift. This may seem impressive, but the issue is not whether a shoe-lift lowers O2 cost compared to normal walking. The question is whether a shoe-lift improves walking efficiency for subjects with an immobilized extended knee. Thus, the comparison should have been made between walking with (one session) and without (another session) a shoe-lift on the contralateral foot with an immobilized extended knee in both walking sessions.

Relative (ml/kg/min) and absolute (l/min) differences in VO2 between walking with (treatment) and without a shoe-lift (control) on the contralateral foot of the immobilized extended knee reinforce the conclusion that the shoe-lift did not improve walking economy. Only a 2% mean difference in VO2 rate (18.71 ml/kg/min without a shoe-lift vs. 18.27 with a shoe-lift), and thus O2 cost (0.202 ml/kg/m without a shoe-lift vs. 0.198 with a shoe-lift), was observed between the treatment and the control. Similarly, the absolute mean difference was only 0.02 lmin (1.22 l/min without the shoe-lift vs. 1.20 l/min with the shoe-lift). If the difference had been statistically significant, it would have been too small to be relevant.

Table 1. Effect of a contralateral shoe-life on oxygencost and ventilatory
responses of walking with an artifically immobilized knee

Variable With 
Shoe-Lift		 Without 
Shoe-Lift		 F-ratio 
& Prob
O2 Rate
l/min		 1.20 
± .29		 1.22 
± .28		 1.10 
& .33
O2 Rate
ml/kg/min		 18.3 
± 1.4		 18.7 
± 1.5		 1.99 
& .20
O2 Cost 
ml/kg/m		 .198 
± .015		 .202 
± .017		 1.09 
& .34
VCO2
l/min		 1.11 
± .29		 1.10 
± .29		 .40 
& .55
VCO2
ml/kg/min		 16.9 
± 1.7		 16.7 
± 2.1		 .33 
& .59
VCO2
ml/kg/m		 .184 
± .017		 .181 
± .021		 .42 
& .54
RER .93 
± .03		 .89 
± .05		 4.57 
& .08
Ve
l/min		 36 
± 12		 36 
± 10		 .00 
& 1.0
Vt
ml/breath		 1074 
± 240		 1076 
± 259		 .00 
& 1.0
Fb
breaths/min		 35 
± 9		 35 
± 6		 .00 
& .96

The shoe-lift had no effect on other respiratory parameters or on central and peripheral components of O2 rate. There was no significant change in Ve, which is related to the insignificant change in O2 rate, nor did the subjects’ rate (Fb) and depth (Vt) of breathing change, a reflection of the subjects’ ventilatory response. Similarly, the lack of change in central (HR and SV) and peripheral (a-vO2 diff) components of VO2 also correspond to the insignificant change in VO2. With respect to the equation where VO2 is the product of O2 transport (Q = HR x SV) and O2 utilization (a-vO2 diff), the insignificant change in VO2 was due to the insignificant change in the subjects’ central (Q) and peripheral (a-vO2 diff) adjustments. The cardiac output (Q) response was due to the insignificant differences in HR and SV, respectively, with and without a shoe-lift. Given the strong correlation (r = .90) between HR and myocardical oxygen consumption (8), this finding also indicates that the heart’s requirement for O2 is not affected by the contralateral shoe-lift. The subjects’ HR was the same with and without a shoe-lift.

Table 2. Effect of a contralateral shoe-lift on thecentral (HR, SV, Q) and peripheral (a-vO2 diff)
componentsof oxygen cost of walking with an artifically immobilized knee

Variable With 
Shoe-Lift		 Without 
Shoe-Lift		 F-ratio 
& Prob
HR
beats/min		 140 
± 27		 142 
± 26		 1.62 
& .25
SV
ml		 96 
± 25		 95 
± 35		 .09 
& .77
Q
l/min		 13.20 
± 3.16		 13.01 
± 3.39		 .07 
& .80
a-vO2 diff
ml/100 ml		 9.26 
± 1.79		 9.66 
± 2.05		 1.22 
& .31
SVR
mmHg/l/min		 7.7 
± 1.4		 8.1 
± 1.8		 .46 
& .52

Conclusion
This study indicates that a one-inch shoe-lift worn on the contralateral foot does not lower the oxygen cost of walking with an immobilized extended knee. Furthermore, if the VO2 is converted to heat production rate with a shoe-lift (5.51 Kcal/hr/kg) and without a shoe-lift (5.55 Kcal/hr/kg), the 0.7% difference in power output (414 watts vs. 417 watts) has no practical value (9). In consideration of these findings, additional studies are recommended before concluding that patients with an immobilized extended knee will benefit from a contralateral shoe-lift.

References
1. Inman, V.T., Ralston, H., and Todd, F. Human walking. Baltimore: Williams and Williams, 1984.
2. Mattsson, E. and Brostrom, L-A. The increase in energy cost of walking with an immobilized knee or an unstable ankle. Scand J Rehab Med, 1990;22:51-53.
3. Perry, J. Gait analysis: normal and pathological function. Thorofare (NJ): Slack, Inc., 1992.
4. Abdulhadi, H. M., Kerrigan, D. C., and LaRaia, P. J. Contralateral shoe-lift: effect on oxygen cost of walking with an immobilized knee. Arch Phys Med Rehabil, 1996;77:670-672.
5. Heigenhauser, B. andJones, N. Measurement of cardiac output by carbon dioxide rebreathing methods. Chest 1989;10:255-264.
6. Jones, N. andRebuck, A. Rebreathing equilibrium of CO2 during exercise. J Appl Physiol, 1973;35:538-541.
7. Collier, C. R. Determination of mixed venous CO2 tensions by rebreathing. J Appl Physiol, 1956;9:25-29.
8. Kitmura, K., Jorgensen, C. R., Gobel, F. L., Taylor, H. L., and Wang, Y. Hemodynamic correlates of myocardial oxygen consumption during upright exercise. J Appl Physiol, 1972:32;516-522.
9. Robergs, R.A., Roberts, S.O. Exercise physiology: exercise, performance, and clinical applications. St. Louis (MI): Mosby-Year Book, Inc., 1977.

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