Full length articlePlantarflexor passive-elastic properties related to BMI and walking performance in older women
Introduction
Older adults who are overweight or obese (OW) have preferred and maximal walking speeds that are slower than older adults who are normal weight (NW) placing them at greater risk for mobility disability [1], [2]. Poor mobility and functional limitation in OW are related to low muscle strength per kilogram of body mass, low aerobic capacity, heightened muscle activation, and a mass-specific energy cost of walking that is 20% greater than NW [1], [3], [4], [5].
During walking and running, lower-extremity propulsive forces result from both active muscle force generation and passive-elastic energy (work absorption) stored in the muscle-tendon unit (MTU) [6], [7]. With each step, the muscles that work at the hip, knee, and ankle absorb work to conserve energy so that active muscle force generation and the energy cost of walking are minimized [8]. The ability to absorb work during deformation of the MTU is proportional to its stiffness, that is, stiffer muscle has the potential to absorb and return more elastic energy. However, modeling suggests that MTU stiffness has an optimal range of values that maximize the efficiency of walking [9]. The author theorizes that in OW excess fat mass arithmetically reduces the ratio of MTU stiffness to body mass to suboptimal levels. This may result in an attenuation of lower-extremity forces, greater motor unit recruitment for active force generation, and an elevated metabolic energy cost that contribute to walking difficulty in older adults with obesity.
Continuous overloading of muscles and tendons by excess body weight could lead to tissue degeneration if the microtraumas of repetitive stress exceed tendon regeneration. This, in addition to dyslipidemia and the systemic, chronic, low-grade inflammation that accompanies obesity, may lead to the reduction of collagen fibrils, disorganization of tendon architecture, and intratendinous lipid deposition shown in obese animal models [10]. These histological changes likely diminish the quality of the MTU and could further reduce its capacity to store and return elastic energy. Studying the impact of body weight on MTU passive-elastic properties is important as observational studies demonstrate that obesity increases the risk for tendinopathy, particularly Achilles tendinopathy and plantar fasciopathy [11], [12]. In fact, the incidence of Achilles tendon rupture has increased by 15% over 20 yr, largely driven by a doubling of injury rate in people over the age of 50 yr [13].
Despite the clinical and functional importance of MTU passive-elastic properties, it is currently unclear how they are affected by adiposity in older adults [14]. Therefore, the purpose of this study was to examine the association of body mass index (BMI) with ankle plantarflexor stiffness, work absorption, and hysteresis and relate these passive-elastic properties to walking performance in older women. It was hypothesized that plantarflexor muscle stiffness and work absorption would be lower in OW than in NW, and hysteresis higher in OW. Second, it was hypothesized that plantarflexor stiffness and work absorption would be positively related to walking speed and vertical ground reaction forces (vGRF), and inversely related to plantarflexor muscle activation.
Section snippets
Participants
Twenty-three older women, between the ages of 65–80 yr were recruited to the study by newspaper advertisement and were separated by BMI into a NW group (BMI < 25 kg m−2, n = 11) and an OW group (BMI ≥ 25 kg m−2, n = 12, of which one participant had BMI ≥ 30 kg m−2). Participants were included if they were able to walk independently and had no physical limitations that prevented participation. The study was approved by the University of New Hampshire Intuitional Review Board and all participants gave their
Results
OW were significantly younger, heavier, had a higher BMI and percent body fat, and had slower self-selected maximal walking speeds than NW (Table 1). Plantarflexor stiffness (Fig. 2A, slope) was 34% lower in OW (26.4 ± 12.7 Nm rad−1) than NW (40.0 ± 15.7 Nm rad−1, p = 0.032) and stiffness per kg (Fig. 2B) was 47% lower in OW (0.37 ± 0.16 vs. 0.70 ± 0.28 Nm rad−1 kg−1, p = 0.002). Work absorption (Fig. 2, shaded area under curve) was not different between OW (0.36 ± 0.16 Nm rad) and NW (0.48 ± 0.29 Nm rad, p = 0.31) nor was
Passive-elastic MTU properties
The results of this study support the hypothesis that there is a disproportionately low ratio of plantarflexor MTU stiffness to body mass in OW, but failed to show differences in work absorption and hysteresis between groups. In fact, plantarflexor stiffness was inversely related to adiposity as both body fat percentage and BMI each explained 37% of its variance. Faria and colleagues showed an elevated stiffness in subjects with obesity when expressed in absolute terms, but only a trend toward
Conclusions
OW demonstrated lower MTU stiffness than NW that hinders their capacity to passively store elastic energy. Low plantarflexor stiffness and work absorption were associated with adiposity, low strength, diminished lower-extremity forces and rates during walking, slower walking speed, and elevated muscle activation. This study presents preliminary evidence that the passive-elastic properties of muscle differ between OW and NW and that these properties are related to the maximal walking performance
Conflict of interest
The author reports no conflict of interest.
Acknowledgements
D.P. LaRoche was supported by the National Center for Advancing Translational Sciences via NIH Grant L30 TR000588. The National Institutes of Health had no involvement in the study design, in the collection, analysis and interpretation of data; in the writing of the manuscript; or in the decision to submit the manuscript for publication.
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