The transition from quadrupedal knuckle walking to striding bipedalism was a distinctive evolutionary event that marked the divergence of hominin lineage from other apes [
Independent of foraging efficiency, which has been progressively losing its relevance over time, walking still continues to be an important function in contemporary humans. It is critical to maintain independence in activities of daily living, to enjoy social relationships, and to retain good emotional vitality, all of which are main determinants of quality of life, particularly in older persons [
Although human walking has been extensively investigated in its multifaceted biomechanical, physiological, and pathological aspects [
Understanding whether lower limb length and body proportions affect the energy cost of overground walking in older persons is a relevant issue. In fact, the ability to walk a certain distance depends on whether the amount of oxygen that can be delivered to muscles by the cardiopulmonary system meets the amount of metabolic energy required for walking, and, intuitively, for a given level of cardiopulmonary capacity, the lower the metabolic cost of walking, the longer the expected walked distance. Interestingly, findings from a recent study on younger adults walking on a treadmill using articulated stilts [
Thus, in this study we tested the above hypothesis in a selected sample of older patients who performed the sixminute walk test (6 mWT) at the end of their cardiac rehabilitation program while wearing a system for direct calorimetry.
Participants were enrolled among patients admitted to our rehabilitation centre for an intensive postacute threeweek rehabilitation program after cardiac surgery over a sixmonth lapse of time. All patients aged 65 years and over who, at the end of the cardiac rehabilitation program, walked on the 6 mWT a distance included within the 95%CI of the normative data for nondisabled communitydwelling older persons according to sex and age strata [
Weight, in kilograms, height, in centimeters, and lower limb length (from greater trochanter to the ground), in centimeters, were measured unshod using commercially available tools. To obtain a measure of body proportions, we also calculated the lower limb length to height ratio, in percentage.
At the end of their cardiac rehabilitation program, patients performed the 6 mWT according to the recommendations of the American Thoracic Society [
We measured the walked distance, in meters, the number of steps, the resting metabolic rate before the test, in joules min^{−1}, and the cumulative gross energy expenditure throughout the test, in joules. Then, we calculated the mean walking speed (walked distance/360), in meters sec^{−1}, the mean kinetic energy (1/2
Statistical analysis was performed using the STATA 7.0 software, from Stata Corporation (College Station, TX, USA). The association of the energy cost of overground walking with anthropometric measures was tested using a multivariable backward regression model with the energy cost of walking as the dependent variable and with lower limb length and lower limb length to height ratio as independent variables, along with age and sex. Height was not entered into the model as it was used to calculate the lower limb length to height ratio, thus introducing structural collinearity among independent variables. Weight was also not entered into the model as it was used to calculate the cost of locomotion. Type 1 error was set at the twosided 0.05 level. Finally, as variables entered into the model were expressed in different units, standardized beta coefficients, that is, the
Ancillary analyses aimed at assessing the effects of 1 cm increase in lower limb length on the lower limb length to height ratio and on the energy cost of overground walking were conducted by creating first two dummy variables, “lower limb length plus one” and “lower limb length plus one to height plus one ratio.” Then, we calculated the energy cost of walking following 1 cm increase in lower limb length, “cost of locomotion to kinetics energy ratio plus one,” according to the multivariable regression model. Finally, we compared the measured cost of locomotion to kinetics energy ratio with the calculated “cost of locomotion to kinetics energy ratio plus one.”
Table
General characteristics of the study sample and results from the sixminute walk test (
General characteristics  
Age, years (mean ± SD)  69.1 ± 5.4 
Female sex, 
12 (19) 
Weight, kg (mean ± SD)  72.3 ± 13.0 
Height, cm (mean ± SD)  169.8 ± 7.7 
Lower limb length, cm (mean ± SD)  87.7 ± 5.6 
Lower limb length to height ratio, % (mean ± SD)  51.7 ± 1.9 
Results from the sixminute walk test  
Distance walked, m (mean ± SD)  424 ± 63 
Mean walking speed, m sec^{−1} (mean ± SD)  1.2 ± 0.2 
Mean kinetic energy, joule (mean ± SD)  51.6 ± 18.4 
Step number, 
310 ± 42 
Step cadence, sec^{−1} (mean ± SD)  0.86 ± 0.12 
Step length, m (mean ± SD)  1.38 ± 0.19 
Step number per meter, m^{−1} (mean ± SD)  0.74 ± 0.10 
Resting metabolic rate, joule min^{−1} (mean ± SD)  5,312 ± 958 
Gross energy expenditure throughout the test, joule (mean ± SD)  129,362 ± 30,213 
Net energy expenditure throughout the test, joule (mean ± SD)  97,489 ± 25,617 
Cost of locomotion, joule kg^{−1} sec^{−1} (mean ± SD)  3.73 ± 0.64 
Cost of locomotion to kinetic energy ratio, kg^{−1} sec^{−1} (mean ± SD)  79.9 * 10^{−3} ± 26.5 * 10^{−3} 
Multivariable regression model testing the association of the energy cost of overground walking with anthropometric measures.
Model: Obs = 62; 


Cost of locomotion to kinetic energy ratio (kg^{−1} sec^{−1}) 


Standardized 
Lower limb length (cm)  −3.72 * 10^{−3} ± 0.74 * 10^{−3}  <0.001  −0.78 
Lower limb length to height ratio (%)  6.61 * 10^{−3} ± 2.14 * 10^{−3}  0.003  0.48 
The initial model also included age and sex that were removed by backward selection (
Ancillary analyses showed that in our series 1 cm increase in lower limb length increased lower limb length to height ratio by 0.284 (95%CI 0.279–0.288) and that, altogether, 1 cm increase in lower limb length reduced the energy cost of overground waking by 2.566% (95%CI 2.345–2.785).
In this study we tested the hypothesis that lower limb length and body proportions might affect the energy cost of overground walking in older persons. In a sample of older patients who completed their cardiac rehabilitation program and who were functionally comparable to nondisabled communitydwelling older persons according to age and sex, we found that the energy cost of overground walking was inversely related to lower limb length and directly related to lower limb length to height ratio.
To the best of our knowledge, these findings have never been reported before, so that direct comparisons with the existing literature are not feasible. However, a few comments are warranted.
First, normalized measures of walking energy expenditure, such as the “cost of locomotion” (joules kg^{−1} sec^{−1}) or the “cost of transport” (joules kg^{−1} m^{−1}) [
Second, previous studies have already shown that lower limb length actually affects the energy cost of walking [
Third, for the first time we have shown that lower limb length to height ratio, a measure of body proportion, is also directly related to the energy cost of overground walking in older persons. Although the interpretation of this finding is, indeed, a difficult task, a possible explanation might be as follows. Lower limb to height ratio can be somehow considered as a proxy of the position of the center of mass with respect to the ground. In fact, if two individuals show the same lower limb length and different height, then the center of mass of the individual with a higher ratio of lower limb length to height, that is, the shorter individual, will be closer to the ground than that of the taller one. Interestingly, Abe et al. [
Finally, our ancillary analyses showed that in spite of the opposite effects of lower limb length and lower limb length to height ratio, the combined effect of 1 centimeter increase in lower limb length, and the consequent increase in lower limb to height ratio, altogether reduced by ~2.5% the energy cost of overground walking in older persons. This finding offers a new insight into exercise physiology as it suggests that changes of few centimeters in the distance from the great trochanter to the ground will result in appreciable reductions in the energy cost of overground walking. This might be particularly relevant for older persons and for patients with severely limited cardiopulmonary capacity, in whom a relatively small reduction of the energy cost of walking might dramatically improve their mobility and, consequently, their independence in the activities of daily living and their quality of life.
Contrary to previous studies, in which the effects of lower limb length on energy expenditure were assessed in children and younger participants walking on a treadmill at preestablished walking speeds, in this study we assessed the effects of lower limb length and lower limb length to height ratio on energy expenditure in older persons freely walking overground along a corridor at their selfselected speed, which is a more natural task and shows close similarity to the activities of daily living. This aspect represents the strength of our study. Nevertheless, some inherent limitations need to be considered. First, our sample size was relatively small (62 subjects), which might have affected the statistical power of our calculations; however, post hoc statistical power of the multivariable regression model was >0.99. Second, our multivariable regression model showed a relatively small adjusted
In conclusion, although our findings should be taken with caution, nevertheless the message to take home from this study is that lower limb length actually affects the energy cost of overground walking and that small changes in lower limb length might substantially affect walking economy. In operational terms, this means that the energy cost of overground walking might be reduced by simply “manipulating” lower limb length (e.g., by using assistive devices/orthosis, such as slightly thicker shoe soles and insoles, properly manufactured to preserve the physiological function of the anklefoot complex), thus overcoming, at least in part, the limitation to physical performance imposed by the cardiopulmonary system. The clinical relevance of these findings is that such a relatively small reduction in the energy cost of walking has the potential to dramatically improve mobility in older persons, or in patients with severely limited cardiopulmonary capacity, and, consequently, to improve their independence in the activities of daily living and their quality of life, which is an important target for cardiopulmonary and geriatric rehabilitation.
However, future studies are needed to confirm our findings and to verify, through an appropriate study design, whether experimentally induced increases in lower limb length actually reduce the energy cost of overground walking in older persons.
The paper submitted does not contain information about medical device(s)/drug(s). Institutional funds were received in support of this work. No benefit in any form has been or will be received from a commercial party related directly or indirectly to the subject of this paper. This paper represents original work and no part of the submitted work has been published or it is not under consideration for publication elsewhere. All authors meet the criteria for authorship and have been involved in the conception and design of the study, or the analysis of the data. All authors have seen and approved the submitted study.
No conflict of interests has been reported by the authors or by any individuals in control of the content of this paper.
This work was partly supported by the European Union within the CYBERLEGs Project FP7/20072013 under Grant Agreement 287894.