Study design and ethics
A methodological study was conducted at the Healthy Aging Research Laboratory and Isokinetic Dynamometry Laboratory of the Physical Therapy Department of Federal University of São Carlos (UFSCar), São Paulo, Brazil. This study was approved by Research Ethics Committee for Human Beings of the UFSCar (certificate number: 88921118.4.0000.5504). The study was conducted according to the guidelines of the Declaration of Helsinki. Verbal and written informed consent was obtained from all participants. In the groups of participants with Alzheimer’s disease, the caregivers or legally authorized representatives gave informed consent in name of their care recipients and verbal informed consent was obtained from the participants on the day of the evaluations. Moreover, the caregiver or legally authorized representatives of illiterate participants provided informed consent for the study. The data were collected between February and December 2019. The checklist of the Guidelines for Reporting Reliability and Agreement Studies (GRRAS) were followed [23, 24].
Eligible participants were community-dwelling older adults without and with AD, ≥ 65 years of age, both sexes and with no musculoskeletal disorders of the knee or ankle, such as fractures, pain, osteoporosis or previous surgeries. The participants were recruited from the Health School Unit of UFSCar, Open University for Older Adults and Family Health Programs in the city of São Carlos.
The inclusion criteria for the older adults with AD were (1) medical diagnosis of AD based on the Diagnostic and Statistical Manual of Mental Disorders (DSM-V) , (2) being classified in the mild or moderate stages of AD based on the Clinical Dementia Rating (CDR) scale [26, 27], and (3) score below the cutoff point for the detection of dementia on the Mini Mental State Examination (MMSE) adjusted for years of education: 20 (illiterate), 25 (1 to 4 years of education), 26.5 (5 to 8 years of education), 28 (9 to 11 years of education) and 29 (≥ 12 years of education) [28, 29], and, (4) being clinically stable. The exclusion criteria for the older adults with AD were (1) comprehension difficulties (e.g., inability to state one’s own name, hand over or receive an object when requested to do so, etc.), (2) dementia of other etiologies (e.g., Lewy body, vascular, frontotemporal, etc.), (3) other neurodegenerative diseases besides AD (e.g., Parkinson’s disease) or non-neurodegenerative diseases (e.g., stroke), and (4) diagnosis of depression.
The medical diagnosis of AD was performed by geriatricians or neurologists based on the DSM-V . The classification of AD stage was performed by the researchers based on the CDR, who have amply experience proved by previous studies [21, 30, 31]. This scale quantifies the severity of dementia and is composed of six domains: memory, orientation, judgment and problem solving, community affairs, home and hobbies and personal care. The final score is used to classify very mild (CDR = 0.5), mild (CDR = 1), moderate (CDR = 2) and severe (CDR = 3) dementia [26, 27].
The inclusion criteria for the older adults without-AD were (1) preserved cognition, with score above the cutoff point on the MMSE adjusted for years of education [28, 29], (2) not meeting the criteria for Mild Cognitive Impairment or dementia (e.g., AD) [25, 32], and (3) being clinically stable. The exclusion criteria were (1) neurodegenerative and non-neurodegenerative diseases, and (2) diagnosis of depression.
Clinical and demographic measures
Global cognitive function was assessed using the MMSE, which addresses orientation, memory, language and visuospatial skills. The maximum score is 30 points, with higher scores indicating a better performance [28, 29]. Grip strength was measured using the JAMAR® Hydraulic Hand Dynamometer (Model PC-5030J1, Fred Sammons, Inc., Burr Ridge, IL, USA). The participants were instructed to use the greatest possible strength and maintain the contraction for six seconds. Three readings were taken on the dominant hand and the average was calculated, with higher scores indicating a better performance . Depressive symptoms were assessed using the Geriatric Depression Scale, which is composed of 15 affirmative or negative questions (yes = presence of symptom; no = absence of symptom), with a score > 5 indicating the presence of depressive symptoms [34, 35]. Physical activity level was assessed using the Modified Baecke Questionnaire (MBQ), which is composed of 10 items related to activities of daily living as well as the investigation of free time and physical activity, with lower scores indicating a lower physical activity level [36, 37]. For those with AD, the MBQ was administered to the caregiver to obtain information on the participant. A caregiver was considered a family member or guardian who spent at least half of the day with the older person four times a week. The procedures for the application of the tests of the clinical measures followed the orientations recommended in the original version.
The isokinetic evaluations were performed using the Biodex System isokinetic dynamometer (Biodex Multi Joint System PRO, Shirley, New York, USA) with a sampling frequency of 100 Hz.
Description of isokinetic evaluation of knee and ankle muscle strength
The isokinetic evaluations of knee extension and flexion were performed with angular velocities of 60º/s (five maximum voluntary repetitions) and 180°/s (15 maximum voluntary repetitions) in a total range of 70º starting from 90° knee flexion (0° = complete extension). The isokinetic evaluations of ankle plantar flexion and dorsiflexion were performed with an angular velocity of 30º/s (five maximum voluntary repetitions) in a total range of 45º starting from 35° of plantar flexion to 10º of dorsiflexion (0° = neutral position).
As an unprecedented study involving older adults with AD, the parameters of the isokinetic evaluations were based on previous reliability studies conducted with healthy older adults [13, 14]. The capacity of the knee extensors and flexors to generate maximum force seems to occur at low and high velocities (60°/s and 180°/s) in healthy older adults. The production of maximum plantar flexor and dorsiflexor strength in healthy older adults seems to occur at low velocities (30°/s), which justifies the choice of the angular velocities used in the present study [38, 39].
The participants underwent evaluations in the sitting position on the isokinetic dynamometer. The positioning and procedures for the collection of data were performed following the specifications of the manufacturer . The isokinetic evaluations of the knee and ankle muscle strength were performed with the dominant lower limb. To determine dominance, the participants were asked to kick a ball with a much strength as possible. The isokinetic measures used for the analyses were peak torque, average peak torque and total work; higher values of these measures indicate a better performance. A comprehensive evaluation of maximum muscle strength should include these three measures. Peak torque and average peak torque were normalized by individual body mass (isokinetic measure / body mass (kg) × 100).
Adaptation and standardization of isokinetic evaluations
The adaptation and standardization of the isokinetic evaluations were ensured with communication strategies adopted for the participants with AD, such as maintaining eye contact when speaking, speaking slowly and clearly, explaining the actions prior to their execution as well as repeating the explanation of the correct execution and demonstrating the tests. Prior to the isokinetic evaluations on D1 and D2, an explanation was given and the tests were demonstrated on the non-dominant lower limb, followed by familiarization on the dominant lower limb for each test and angular velocity with three submaximal repetitions and one maximum repetition [9,10,11]. After familiarization, the main examiner asked the participants if they understood the execution of the tests. The evaluations were initiated after verbal confirmation from the participants that they understood the instructions of the tests. After three minutes of rest, the isokinetic evaluation was performed. During the evaluations, standardized, vigorous verbal commands were given: Ready, set, go! (the word “go” was repeated throughout the time of the contractions). During the three five-minute rest intervals, the explanation of the correct execution of the tests was repeated and the participants verbally confirmed that they had understood the explanation. These procedures were adopted due to the limitations of older adults with AD (e.g., difficulties in communication and the comprehension of verbal commands). During the isokinetic evaluations, heart rate, blood oxygen saturation and blood pressure were monitored to ensure the safety of the older adults. For the purposes of comparison, the isokinetic evaluations were the same for all three groups (mild-AD, moderate-AD and without-AD).
The evaluations were conducted on two days (D1 = test; D2 = retest) with a minimum interval of three days for muscle recovery and a maximum of seven days [41, 42]. On D1, clinical and demographic data were collected, followed by the isokinetic evaluation of the knee and ankle muscle strength in random order. On D2, the isokinetic evaluation was performed again in the same order as that used on D1. The isokinetic evaluations were performed by a single examiner. The examiner on D2 was blinded to the results of the tests on D1, as the data were not exported from the isokinetic dynamometer. Every effort was made to maintain the factors related to the evaluation sessions consistent (same time of the day [morning or afternoon] and the same members of the team assisting in the evaluations).
The sample size was determined a priori using the method proposed by Walter, Eliasziw and Donner (1998) . Based on this method, a sample of 22 participants per group was needed to achieve an 80% test power, considering a 5% significance level, acceptable intraclass correlation coefficient (ICC) of 0.3 and ICC of 0.70 for the two measures (test and retest).
The normality and homoscedasticity of the data were analyzed using the Shapiro-Wilk and Levene tests, respectively. The clinical and demographic characteristics of the participants allocated to the mild-AD, moderate-AD and without-AD groups were compared using one-way ANOVA and Tukey’ post hoc test (parametric variables), considering p < 0.05; Kruskal-Wallis and Mann-Whitney test with the Bonferroni correction (nonparametric variables), considering p < 0.016; and chi-square test (categorical variables), considering p < 0.016.
Relative reliability was determined using the ICC two-way mixed model of a single-measure (ICC3,1) at the 95% confidence interval. The ICC3,1 was interpreted based on the Munro’s classification (very low: 0.00-0.25, low: 0.26–0.49, moderate: 0.50–0.69, high: 0.70–0.89 and very high: 0.90-1.00) [44, 45]. Absolute reliability was determined by the standard error of measurement (SEM) and minimal detectable change at the 95% confidence interval (MDC95). Standard error of measurement and MDC95 values were interpreted based on absolute and relative values. Absolute values were calculated using the following equations: SEM = SD√1-ICC and MDC95 = SEM×√2 × 1.64 . Relative values were reported in percentage of error (SEM%) and change (MDC95%). A SEM% value ≤ 10% and MDC% value ≤ 30% were considered acceptable . Relative values were calculated using the following formulas: SEM% = SEM / mean of measures x 100% and MDC95% = MDC95 / mean of measures x 100% . The mild-AD, moderate-AD and without-AD groups were considered separately in the reliability analyses. The data were managed and analyzed using SPSS 23 (IBM; Chicago, IL, USA) and Excel 2010 (Microsoft Corporation, Redmond, Washington, USA).