Age-related differences in lower-limb force–time relation during the push-off in rapid voluntary stepping
Introduction
Muscle strength is an important determinant of functional performances such as getting up from a chair, climbing stairs, and walking (Bassey and Short, 1990, Ikezoe et al., 1997). It is well documented that lower-limb muscle strength peaks around the age of 30 and decreases at a rate of 1–2% per year after the fifth decade of life (Borges, 1989, Freedson et al., 1993). Muscle quality or specific tension (i.e., maximal voluntary strength per unit of cross-sectional area) is an assumed indicator of both muscle strength and contractile velocity (De Vito et al., 1998, Krivickas et al., 2001), which indeed also decrease with age (Doherty, 2004). Because the marked increase in the incidence of falls in older adults (Tinetti et al., 1988) coincides with the progressive reduction in lower-limb muscle mass, strength has been considered a primary factor associated with falling. Yet, the age-related decline in muscle power is greater (3–4% per year) than the decline in muscle strength (Bassey and Short, 1990, Bean et al., 2003, Skelton et al., 1994). Thus, muscle power has also been considered a major factor related to increase in the risk of falling in older adults (Skelton et al., 2002). However, some studies have shown minimal or no differences in lower-limb muscle strength between fallers and non-fallers (Wolfson et al., 1993, Judge et al., 1994). That is, the evidence for the suggested relation between muscle strength or muscle power to falling in older adults is controversial.
There could be several reasons for such a controversy. For example, muscle strength in older adults has been assessed using dynamometric techniques (Judge et al., 1994, Melzer et al., 2000), while neglecting important functional aspects of muscle capability in real-life situations (e.g., the ability to step rapidly). Another controversy relates to differences in the definition of power under different perspectives. Mechanical and metabolic definitions of power (or energy; i.e., the integral of power) are often assumed to be synonymous, although the relation between the two is ambiguous. For example, different types of fast walking or running (in humans or animals) are meant to maintain the vertical speed of the center of mass near zero and the speed in the direction of locomotion nearly constant (i.e., near zero vertical and horizontal accelerations). Indeed, this should optimize mechanical energy. However, it is known that mechanical parameters associated with power or energy do not strongly correlate with metabolic cost under different speeds of locomotion (e.g., see review by McMahon, 1984). In this regard, older people may show decreased ability to produce metabolic power or energy in a continuous gait, but not necessarily decrease in mechanical power as expressed in a single discrete stepping action that may prevent a fall. Power in the last case strongly depends on the ability to release energy fast enough with the right timing.
In the current study, we adopt a mechanical definition where forward stepping power results from the propulsive tangential force and velocity. We argue that in older adults it is not the magnitude of the exerted power per se but the time evolution of the force generation that may be associated with the ability to react quickly and avoid falling. It should be noted that such a suggestion is compatible with previous findings using similar stepping paradigms, which showed slow anticipation and stepping reaction times in older adults (Melzer et al., 2007a).
Thus, the general aim of the current study was to verify whether propulsive force and power differ in young and older adults during the push-off in a rapid step, and in what aspects they differ. In particular, we were interested in the relationship between the generation of propulsive force and the time of force application because this relationship may reflect the ability to generate muscle power (Liebermann and Katz, 2003). We assumed that delays in generating forces during the push-off stage are critical for fast forward stepping actions that may prevent falls in older adults. This assumption is partially supported by Pijnappels et al. (2005), who showed delays in the onset of knee-movement generation in the support limb in older fallers during the recovery after tripping, accompanied by a lower peak ankle-moment during push-off reactions.
Specifically, we examined age-related differences in push-off ability from force–time data obtained in rapid voluntary step executions based on the assumption that the ability to perform a rapid step may be important for preventing a fall during locomotion (Madigan and Lloyd, 2005). Previous work showed longer step execution times in older compared to young adults (Bean et al., 2003, Melzer and Oddsson, 2004, Melzer et al., 2007a, Melzer et al., 2007b). In this study, we hypothesized that older adults would indeed achieve lower force and power magnitudes compared with young adults during the push-off phase of a forward step execution. Such differences may be attributed to the decreased ability to reach peak force values in short time intervals. Thus, we further hypothesized that older people would achieve peak force and power in significantly longer times compared to young people, during the preparatory and swing phases of the push-off. Lastly, we hypothesized that such differences would be observed in the preparatory phase of the push-off rather than in the swing phase.
Section snippets
Participants and procedures
Fifteen healthy older adult (9 males) and 15 young (8 males) volunteers were recruited for the study. Older participants were included based on the following criteria: no previous neurological or orthopedic disorders, a score greater than 45 on the Berg Balance Scale (Berg et al., 1992), a Mini-Mental Score greater than 24 indicating the absence of moderate to severe dementia (Folstein et al., 1975), and the ability to ambulate independently. The use of a cane was allowed but not a walker.
Results
Descriptive data for the two groups and the two step phases are presented in Table 2.
The mean results shown in Table 2 suggest that the ability to exert peak power and force in the direction of locomotion differs in young and older adults, as expected. This was observed in the AP direction (averaged over both phases, young = 958 Nm/s exerted more propulsive power than older adults = 634 Nm/s, averaged over both phases) but not necessarily in the vertical direction (young = 48 [Nm/s]/BW compared to
Discussion
This study showed age-related differences in lower-limb push-off force and power in the AP direction, but only a difference in force (and not power) in the vertical direction. One would normally expect that if power measures in older and young adults are not different, while forces are lower in former group, then there should be a compensatory effect for such low peak forces. One way to compensate could be by adjusting the timing of force application. Nonetheless, this cannot be the case in the
Conclusions
The present study shows evidence suggesting that the time evolution of a propulsive force and, to a lesser degree, the force or power magnitudes, differentiate between young and older adults. Time to peak force appears to be a valid measure related to muscle power. It may capture the requirement of performing work within short time intervals in order to prevent a fall. This parameter seems to be sensitive enough to differentiate between people of different age groups performing rapid voluntary
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