Fig 1.
Conceptual model for how age-related differences in ankle muscle-tendon unit function can increase the vulnerability to unexpected balance disturbances.
A) The onset of a balance perturbation elicits a rapid change in joint position and thus muscle-tendon unit (MTU) lengthening. B) Lesser Achilles tendon stiffness (kAT) in older adults delays the transmission between ΔMTU length and muscle stretch (i.e., ΔLm), thereby delaying afferent detection of the perturbation compared to younger adults. C) Lesser plantarflexor muscle strength (FPF) in older adults reduces force responsiveness during perturbation recovery. D) Age-related decreases in kAT and FPF predispose older adults to increased vulnerability to unexpected balance disturbances.
Fig 2.
Plantarflexor strength and Achilles tendon stiffness for older and younger adults.
Group mean data for older (OA, solid line, n = 21) and younger (YA, dashed line, n = 22) adults’ Achilles tendon (AT) force and length changes during passive dorsiflexion (left), AT stiffness (kAT) (middle), and peak plantarflexor torque obtained from maximal-voluntary plantarflexion contractions (right). Statistical significance is denoted by asterisk(s) and statistical outliers are denoted by plus signs. (p = 0.05).
Fig 3.
Plantarflexor strength and Achilles tendon stiffness relations to margins of stability during perturbed walking.
Individual average peak plantarflexor torque, Achilles tendon stiffness (kAT), mediolateral (MoSLat) and anteroposterior (MoSAntt) direction margins of stability for older (solid line and solid dot, n = 21) and younger (dashed line and open dot, n = 22) adults during the recovery step following treadmill-induced slip perturbations. Lines of best fit for both groups are shown and significant correlations are depicted per group by their corresponding r-value accompanied by an asterisk. (p = 0.05).
Fig 4.
Plantarflexor strength and Achilles tendon stiffness relations to whole-body angular momentum during perturbed walking.
Individual average peak plantarflexor torque, Achilles tendon stiffness (kAT), and whole-body angular momentum in the sagittal (WBAMSagRange), frontal (WBAMFrontRange), and transverse planes (WBAMTransRange) for older (solid line and solid dot, n = 21) and younger (dashed line and open dot, n = 22) adults during during the recovery step following treadmill-induced slip perturbations. Lines of best fit for both groups are shown and significant correlations are depicted per group by their corresponding r-value accompanied by an asterisk. (p = 0.05).
Fig 5.
Plantarflexor strength and Achilles tendon stiffness relations to relative margins of stability during perturbed walking.
Individual average peak plantarflexor torque, Achilles tendon stiffness (kAT), relative mediolateral (ΔMoSLat) and anteroposterior (ΔMoSAnt) direction margins of stability for older (solid line and solid dot, n = 21) and younger (dashed line and open dot, n = 22) adults during the recovery step following treadmill-induced slip perturbations. Lines of best fit for both groups are shown and significant correlations are depicted per group by their corresponding r-value accompanied by an asterisk. (p = 0.05).
Fig 6.
Plantarflexor strength and Achilles tendon stiffness relations to relative whole-body angular momentum during perturbed walking.
Individual average peak plantarflexor torque, Achilles tendon stiffness (kAT), and relative whole-body angular momentum in the sagittal (ΔWBAMSagRange), frontal (ΔWBAMFrontRange), and transverse planes (ΔWBAMTransRange) for older (solid line and solid dot, n = 21) and younger (dashed line and open dot, n = 22) adults during the recovery step following treadmill-induced slip perturbations. Lines of best fit for both groups are shown and significant correlations are depicted per group by their corresponding r-value accompanied by an asterisk. (p = 0.05).
Table 1.
Relations between Achilles tendon stiffness and plantarflexor torque and balance outcomes during perturbed walking.