Elsevier

Gait & Posture

Volume 53, March 2017, Pages 55-60
Gait & Posture

Full length article
Plantarflexor passive-elastic properties related to BMI and walking performance in older women

https://doi.org/10.1016/j.gaitpost.2017.01.007Get rights and content

Highlights

  • Passive-elastic properties of skeletal muscle may be altered by age and adiposity.

  • Ankle plantarflexor stiffness is lower in overweight and obese older adults than normal weight.

  • Stiffness and work absorption are related to propulsive forces and maximal speed during walking.

  • Muscle activation during walking is greater in obese older adults who possess low plantarflexor stiffness.

Abstract

The objective of this study was to examine the influence of BMI on the passive-elastic properties of the ankle plantarflexors in older women. Twenty-three women, 65–80 yr, were separated into normal weight (NW, BMI < 25.0 kg m−2, n = 11) and overweight-obese (OW, BMI  25.0 kg m−2, n = 12) groups. Resistive torque of the ankle plantarflexors was recorded on an isokinetic dynamometer by passively moving the ankle into dorsiflexion. Stiffness, work absorption, and hysteresis were calculated across an ankle dorsiflexion angle of 10–15°. Maximal plantarflexor strength was assessed, then participants walked at maximal speed on an instrumented gait analysis treadmill while muscle activation (EMG) was recorded. Plantarflexor stiffness was 34% lower in OW (26.4 ± 12.7 Nm rad−1) than NW (40.0 ± 15.7 Nm rad−1, p = 0.032). Neither work absorption nor hysteresis were different between OW and NW. Stiffness per kg was positively correlated to strength (r = 0.66, p < 0.001), peak vertical ground reaction force during walking (r = 0.72, p < 0.001), weight acceptance rate of force (r = 0.51, p = 0.007), push-off rate of force (r = 0.41, p = 0.026), maximal speed (r = 0.61, p = 0.001), and inversely correlated to BMI (r = −0.61, p = 0.001), and peak plantarflexor EMG (r = −0.40, p = 0.046). Older women who are OW have low plantarflexor stiffness, which may limit propulsive forces during walking and necessitate greater muscle activation for active force generation.

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.

References (30)

  • D.P. LaRoche et al.

    Excess body weight and gait influence energy cost of walking in older adults

    Med. Sci. Sports Exerc.

    (2015)
  • A.L. Maslow et al.

    Fitness and adiposity as predictors of functional limitation in adults

    J. Phys. Act. Health

    (2011)
  • M. Ishikawa et al.

    Muscle-tendon interaction and elastic energy usage in human walking

    J. Appl. Physiol. (1985)

    (2005)
  • D.J. Farris et al.

    Human medial gastrocnemius force-velocity behavior shifts with locomotion speed and gait

    Proc. Natl. Acad. Sci. U. S. A.

    (2012)
  • G.S. Sawicki et al.

    It pays to have a spring in your step

    Exerc. Sport Sci. Rev.

    (2009)
  • Cited by (4)

    View full text