Participants
A total of 152 healthy participants older than 65 years (age range, 65–88 years; male/female ratio, 59:93) were recruited from local senior citizen clubs (called rojin clubs in Japanese). None of the participants had any known musculoskeletal or neuromuscular conditions that would limit their mobility or their ability to perform the STS movement. Research protocol of this study followed the Declaration of Helsinki. The ethical committee of our university approved the study protocol (reference number: 2014–231), and all participants provided written informed consent.
LS Risk Test to Classify LS Grade
The participants were categorized into three groups according to the severity of LS, determined by the LS risk test [3, 19]. The LS risk test consisted of three parts: (1) the 25-question GLFS, (2) the two-step test, and (3) the stand-up test.
(1) The 25-Question GLFS
This scale is a self-administered, comprehensive measure, consisting of 25 items that include 4 questions on pain, 16 questions on ADLs, 3 questions on social functions, and 2 questions on mental health status. Each question is graded with a 5-point scale, from no impairment (0 points) to severe impairment (4 points). Participants are scored for a total by adding the points for each question (minimum = 0; maximum = 100 points). Thus, a higher score is associated with decreased mobility performance.
(2) Two-Step Test
This test measures the stride length to assess walking ability, including muscle strength, balance, and flexibility of the lower limbs. The procedure is as follows. Participants were instructed to take two long steps, as long as possible, and then to align both feet. The score is calculated by normalizing the maximal length of the two steps taken by the participant based on the participant’s height.
(3) Stand-Up Test
This test assesses leg strength. Participants stand up on one leg or both legs from decreasing a chair height from 40 to 10 cm, with a 10 cm stepdown. Lowering the chair height leads to a greater biomechanical demand while standing up from a seated position. Participants were instructed to stand up on one leg or both legs from each chair height without leaning back and then maintain the standing posture for 3 seconds. They were scored based on their height level using one leg and both legs. Scores ranged from 0 to 8 depending on their difficulty in standing up [4].
After these tests, participants were diagnosed by their severity of LS, either non-LS or stage Ⅰ or Ⅱ, based on the JOA protocol [20]. Stage Ⅰ LS represents the beginning of the decline in mobility. An individual who cannot perform a one-leg stand-up movement from a seat 40 cm high, or whose 25-question GLFS score is ≥ 7, or whose two-step test score is < 1.3 is diagnosed with stage Ⅰ LS. Stage Ⅱ LS represents progression to a decline in mobility. An individual who cannot perform a both-leg stand-up movement from a seat 20 cm high, or whose 25-question GLFS score is ≥ 16, or whose two-step test score is < 1.1 is diagnosed with stage Ⅱ.
STS Score
Details of the STS testing procedure were described in our previous study [6]. Participants were instructed to stand up from a chair as quickly as possible, immediately recover their balance, and stand as still as possible in an upright posture for 5 seconds. To perform this test, participants were seated on an armless, backless chair, with the seat height adjusted to 100% of the participant’s knee height (Fig. 1). The participants crossed their arms over their chest during testing. Both feet were placed shoulder-width apart on the WBB that was used as a force plate. The WBB is designed to serve as a video game controller that is increasingly used to assess postural control in rehabilitation [21–26]. The WBB, which consists of a rigid platform with four strain gauge-based vertical-load transducers located in the feet at each corner, was used to calculate the VGRF and COP. During each STS movement, WBB data were streamed to a laptop computer at approximately 100 Hz. Before data were recorded, each participant was allowed the opportunity to practice the procedure. Each participant performed two trials, separated by a 1minute interval. We did not observe any accidents, such as falling, during this test.
The STS score, which represents motor performance, was defined as a combination of the speed and balance scores. These parameters were calculated using the VGRF (in N), COP position in X and Y directions (Cx, Cy; in cm), and COP trajectory distance (in cm) during STS movement (Fig. 2). The VGRF was normalized to the participant’s body weight. The speed score (s− 1) was defined as a linear slope of the VGRF–time curve. The balance score (m− 1) was defined as the inverse of COP trajectory distance when Cy was minimized to + 3 seconds. The STS score ((ms)−1) was then defined as the product of the speed and balance scores to quantify STS movement performance. Higher STS scores indicated better motor performance. These two indices have a tradeoff relationship because typically if the movement speed (speed score) increases, it becomes difficult to control balance and remain as still as possible (balance score).
Indices were calculated from each set of trial data using a custom MATLAB program (MathWorks, Natick, RI, USA). For each participant, a practice trial was followed by two timed trials, and the average value was used as a result for further analysis.
Peak Power during STS
We all calculated the peak power during the STS movement using the WBB data. All ADLs require the body’s center of mass to move from one place to another by producing adequate force from the muscles. Explosive power is proven to be a good indicator to identify the functional ability of aging people [27]. The rising phase during STS was detected based on the VGRF [28]. Power during this period was calculated by integrating the acceleration derived by the VGRF (velocity) followed by taking the scalar product of the force and velocity [29].
TUG Time
The TUG test, a well-known clinical test, was developed to improve evaluations of functional performance and mobility [30]. This test measured the time needed for a participant to rise from a chair, walk 3 m, turn around, walk back, turn around, and sit down again. A shorter TUG time indicates better performance. Participants were instructed to walk as quickly and safely as possible. For each participant, a practice trial was followed by two timed trials, and the fastest trial was selected for further analysis.
Handgrip Strength
Handgrip strength is a good index to positively correlate with motor function and ADL performance [31]. The strength was measured bilaterally in a standing position using a handgrip dynamometer. Both hands were tested two times, and the maximum value was used to characterize the participant’s handgrip strength.
Statistical Analyses
Statistical analyses were performed using SPSS 22.0 (IBM SPSS, Chicago, IL, USA). One-way analysis of variance and the Dunnett T3 multiple comparison test were used to analyze the differences among the three groups, as defined by the severity of LS (non-LS, stage Ⅰ LS, stage Ⅱ LS). A p value of < 0.05 was considered statistically significant.