INTRODUCTION
Aging is associated with body composition changes that have important implications for functional capacity, nutritional status, and risk for chronic diseases.1 The decline in lean body mass, also called sarcopenia, and a loss of muscle strength have been well-documented in older populations.2–4 Sarcopenia has become an umbrella term that covers both a set of cellular processes (denervation, mitochondrial dysfunction, inflammatory, and hormonal changes) and a set of outcomes (loss in muscle strength, loss in movement and function, increased fatigue, increased risk of metabolic disorders, falls, and skeletal fractures).5 Baumgartner et al6 developed criteria for the diagnosis of sarcopenia on the basis of “relative skeletal muscle mass index” (RSMI), which is appendicular skeletal muscle mass (ASM) (kg) measured by dual-energy x-ray absorptiometry (DXA) expressed relative to height (m2). Men with RSMI values less than 7.26 kg/m2 and women with RSMI values less than 5.45 kg/m2 are diagnosed with sarcopenia.6 Recently, the European Working Group on Sarcopenia in Older People expanded this sarcopenia classification system by including levels of muscle function to the reduced muscle mass criterion.7 According to this group, 3 categories of sarcopenia are as follows: (1) Presarcopenia includes individuals who have only reduced muscle mass; (2) sarcopenia includes those with reduced muscle mass and either low muscle strength or low physical performance; and (3) severe sarcopenia includes those with reduced muscle mass and both low muscle strength and physical performance.
Previous research has shown that muscle strength is an important contributor to balance8,9 and risk for falls in older populations.10 We are very interested in lower extremity strength because of its associations with these functional activities as well as with bone health. Specifically, individuals with lower quadriceps strength were found to have multiple falls,10 increased body sway,10 and lower static and dynamic balance8 compared with their stronger counterparts. Hip abduction strength also has been shown to be a strong predictor of falls in older adults.11 Regarding bone health, Verschueren et al12 reported that older men with presarcopenia and sarcopenia were more likely to be osteoporotic than men with normal muscle mass and muscle function. Interestingly, lower extremity strength (quadriceps) accounted for a larger proportion of the variance in bone mineral density than upper extremity strength (hand grip) in this study. These findings highlight the importance of knee extensor and hip abduction strength in preventing falls and fall-related fractures in older populations.
In a recent systematic review, Mijnarends et al13 summarized the validity and reliability of tests for measuring the components of sarcopenia in older adults. On the basis of the literature, they concluded that the leg press (LP) and handheld dynamometry provide reliable measures of muscle strength, although a lack of standardized protocols for handheld dynamometry existed. Gait speed and the Short Physical Performance Battery (gait speed, standing balance, and chair rises) were found to be reliable physical performance tests. One physical performance measurement tool not addressed by Mijnarends et al13 is jumping mechanography, a procedure that uses maximal countermovement jumps to assess muscular power. Jumping mechanography has been shown to be reproducible in a wide age range (24-88 years) of individuals.14 Runge et al15 reported significant negative correlations between jump power (JPow) and age in men and women, which were stronger than the correlations between chair standing power and age. Buehring et al16 found that this procedure did not cause injury or an increase in self-reported pain levels; thus, it was well-tolerated in men and women, 60 years and older. They also reported that JPow was positively correlated with proximal femur lean mass. In addition, JPow has been shown to be positively correlated with other physical performance tests14,15 and tongue strength.17 To date, JPow differences based on sarcopenia status in older individuals have not been determined.
Purpose
The primary purpose of this study was to compare jump performance and lower extremity muscle strength based on sarcopenia status in men and women, 55 to 75 years of age. We also examined the relationships between the 2 muscle function tests (jump test and muscle strength). A secondary purpose was to assess gender differences in the jump test and muscle strength variables. We hypothesized that JPow and muscle strength would be significantly lower in the sarcopenia group and that jump test variables would be positively correlated with muscle strength. We expected that women would have lower JPow and muscle strength than men in this age range.
METHODS
Participants
The participants for this study included 60 individuals (27 men and 33 women), 55 to 75 years of age, who were recruited from the general community in the Oklahoma City metro area. Before participating, volunteers were required to obtain medical clearance from their personal physicians to ensure that they were medically stable and capable of undergoing the strength and jump testing safely. Individuals were excluded from the study if they had any condition that precluded them from fully performing the physical tasks or if they had a metabolic/endocrine disorder that is known to affect musculoskeletal function. Specific exclusion criteria were (1) hypo/hyperthyroidism/parathyroidism; (2) uncontrolled hypertension; (3) metal screws/plates/rods in the body; (4) back surgery/myocardial infarction/congestive heart failure/cataract surgery/stroke within the previous 6 months; (5) known prior vertebral fracture; (6) known fragility fracture within the last year; (7) tobacco use within the previous 10 years; (8) body weight greater than the weight limit of the DXA (136 kg); and (9) current use of medications that affect muscle/bone, such as hormone replacement therapy or corticosteroids. This study was approved by the University of Oklahoma institutional review board.
Study Design and Procedures
Our study utilized a cross-sectional research design where male and female participants were classified into 2 groups, normal or sarcopenia, on the basis of their RSMI values. Potential subjects were prescreened using a screening checklist questionnaire via phone or e-mail. Our study required 3 visits to the laboratory. During the first visit, participants reviewed and signed the written informed consent form and a health status questionnaire to determine whether they met any of the exclusion criteria. Those who were enrolled in the study then were given a medical clearance form to bring to their personal physician for approval. After obtaining medical clearance, the second visit to the laboratory was scheduled to complete the physical activity questionnaire, menstrual history questionnaire (women only), and DXA measurements for body composition. In addition, familiarization for strength testing (two LP and right and left hip abduction [RHAb and LHAb]) and the jump test were carried out in the Neuromuscular Laboratory after completion of the total body scan. On the third visit (2-6 days after visit 2), participants underwent the jump test and strength testing, in that order. These muscle function tests were conducted by trained research assistants who were blinded to the group classification. All the testing took place at Huston Huffman Center at University of Oklahoma, Norman campus.
Questionnaires
Participants filled out a health status questionnaire for medical history and a physical activity questionnaire, the International Physical Activity Questionnaire, which was used to estimate the physical activity levels (mets/week).18 A menstrual history questionnaire was completed by women to confirm that they were not taking any hormone replacement therapy.
Anthropometric Measurements
Height was measured in centimeters with participants standing with their backs against a wall stadiometer (Novel Products Inc, Rockton, Illinois) without shoes, heels against the wall, head facing forward and level, arms at the side, and breath held. Weight was measured in minimal clothing, without shoes, and empty pockets by a digital body weight scale (Tanita Corporation of America, Arlington Heights, Illinois) in kilograms.
Body Composition
Dual-energy x-ray absorptiometry (GE Lunar Prodigy, enCORE 2010 Software, Version 13.31.016, GE Medical Systems, Madison, Wisconsin) was used to measure body composition by a single technician. A daily calibration of the DXA was performed using the quality assurance feature of the machine. A total body scan was used to determine percent body fat, fat mass, bone free lean body mass (BFLBM), and fat free mass (FFM). The subject's torso thickness at the umbilicus was used to determine scan speeds for the total body scan. The short term in vivo precision coefficients of variation (CV%) are 1.24% for percent body fat, 0.64% for BFLBM, 1.16% for fat mass, and 0.83% for FFM in our laboratory. Appendicular skeletal muscle mass is the sum of the lean soft tissue mass of the arms and legs. Relative skeletal muscle mass index was calculated to determine the diagnosis of sarcopenia using the formula: RSMI (kg/m2) = ASM ÷ height2.6 An RSMI value less than 7.26 in men and less than 5.45 in women was used to classify an individual as having sarcopenia.6
Muscular Strength Testing
As mentioned, participants were familiarized with resistance exercises and the correct weight lifting techniques on a day prior to the strength testing session. All the participants performed 1-repetition maximum (1RM) tests for 2 LP, and RHAb and LHAb, respectively, isotonic resistance exercises using Cybex weight machines (Cybex International, Medway, Massachusetts). A successful lift was defined as completion of the lift over the full range of motion for the specific muscle group. All participants had a warm-up for 5 minutes at a comfortable pace on Monark 828E stationary bicycle ergometer, after which they performed 1 set of 10 submaximal repetitions for each exercise at about 50% of their perceived maximal effort. After 1-minute rest, the weight was increased progressively in increments of 9.1 to 18.2 kg for LP and 2.8 to 5.6 kg for hip abduction until maximum effort to failure was reached. A 90-second rest interval was provided between successive attempts and all 1RM measurements were determined within 5 attempts.
Jump Test Measurement
A Tendo FiTRODYNE power and speed analyzer (Tendo Sports Machines, Trencin, Slovak Republic) was used to assess JPow and jump velocity. The Tendo unit has 2 components: (1) a velocity sensor unit and (2) a microcomputer.19 A Velcro-strap-enabled cable attached the velocity sensor unit to a standard barbell that was positioned on the floor near to the jump mat while another Velcro strap was attached around the waist near the iliac crest. Prior to jump test, body weight was measured by a digital weighing scale (Tanita Corporation of America, Arlington Heights, Illinois) and the value was entered into the microcomputer while the participant stood still on the jump mat. According to the manufacturer, the Tendo equipment measures average velocity of the weight lifted in the vertical plane, and the microcomputer multiplies the mass of the lifter in kilograms by the acceleration from gravity to estimate average force in newtons (N). Average power is then estimated by multiplying average force and average velocity.19
Jump height and airtime were measured using “Just Jump” handheld equipment (Probotics Inc, Huntsville, Alabama). It has 2 components: (1) a just jump contact mat which measures 27″ × 27″ and (2) a just jump handheld computer.20 After placing the just jump mat on a level surface, the cord from the mat was connected to the hand-held computer device. The computer was turned on while the participant was off the jump mat. A “step on mat” instruction was displayed in the computer when it was ready to record jump trials. The participant was asked to step on the mat and stand with both feet shoulder width apart; at this time point, the Velcro strap from the velocity-sensor unit of the Tendo was attached to the participant's waist.
The participant was then instructed to do a countermovement jump with nonrestricted arm motion as high and as fast as possible without tucking the legs and to land with both feet on the mat. When the participant landed on the mat, AT and JHt were displayed by the just jump handheld computer and JPow and JVel were displayed by the Tendo microcomputer. If a participant tucked the legs or bent the knees while in air, it was considered an unsuccessful jump. A total of 3 successful jumps were recorded for each participant and the average of the 3 trials was used for data analyses. A rest interval of a minimum of 60 seconds to as long as needed was provided between consecutive jump trials, although none of our participants required more than 60 seconds of rest. Trained spotters stood on either side of the subject to help with balance, if needed. In addition, a transfer belt was fastened around the waist of the subject that was held by spotters to stabilize the participants in case any balance issues arose. No balance issues arose and no testing-related injuries were reported by any of our participants. The same tester recorded each trial for the participants and the same 2 spotters were responsible for the safety aspects of this test and for monitoring jump technique.
Jumping mechanography is a reliable measurement technique in adults, 24 to 88 years of age, with a 3.6% short-term precision error.14 In addition, Farias et al21 found that the use of a contact mat, instead of a force platform, provided reliable vertical JPow results in women, 60+ years of age (intraclass correlation coefficient of 0.91). In our laboratory, short-term precisions CV% for the jump test procedures in young adults are 4.0%, 3.9%, and 3.3% for JPow, JVel, and JHt, respectively.
Statistical Analysis
All descriptive statistics are reported as mean (standard error [SE]). Data analysis was conducted using SPSS 19.0 software (SPSS Inc, Chicago, Illinois). Independent t tests were used to determine significant group differences in dependent variables based on sarcopenia status. Gender comparisons were performed using independent t tests as a secondary analysis. Pearson correlation coefficients were used to determine relationships between muscle mass and strength variables (ASM, BFLBM, LP, RHAb, and LHAb) with jump test variables (JHt, JPow, and JVel). To minimize the inflation of the type I error, a Bonferroni correction was used to adjust the level of significance for the 4 jump test variables (.05 ÷ 4 = .0125) and the 3 muscle strength variables (.05 ÷ 3 = .017). The level of significance for the correlation coefficient analyses was set at P ≤ .01 (.05 ÷ 5). The level of significance was .05 for all other analyses.
RESULTS
Participant Characteristics Based on Gender
Table 1 shows physical characteristics of the participants based on gender. Significant differences were found in height, weight, and FFM (all P ≤ .0001); men showed higher values than women. Significant differences were also found in percent body fat (P < .0001) between men and women; men showed lower values than women. Table 2 shows muscle mass, muscle strength, and jump performance characteristics based on gender. Men had significantly higher values for surrogate measures of muscle mass (all P ≤ .0001) and muscle strength (all P ≤ .0001) than women. Men also had significantly better performance than women for JPow (P ≤ .0001), JHt (P ≤ .0001), and AT (P ≤ .0001).
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Table 1: Physical Characteristics Based on Gendera
Table 2: Muscle Mass, Muscular Strength, and Jump Test Variables Based on Gendera
Sarcopenia Status and Jump Performance
Based on the RSMI criteria, sarcopenia was found in 20% of the entire sample: 15% of men and 24% of women. The sarcopenia group had significantly lower body weight (P = .012), BMI (P = .02), and BFLBM (P = .012) than the normal group (Table 3). Muscular strength for LP (P = .317), RHAb (P = .110), and LHAb (P = .197) was similar for the 2 groups, even when expressed relative to body weight. Physical activity level (mets/week) was not significantly different (P = .816) between the sarcopenia and normal groups, 4992.29 (2302.81) versus 4621.15 (558.08), respectively.
Table 3: Physical Characteristics and Muscular Strength Based on Sarcopenia Statusa
Table 4 shows jump test characteristics based on sarcopenia status. The sarcopenia group had significantly lower JPow (P = .001) than the normal group, although only a trend was found for a group difference when JPow was expressed relative to body weight (P = .031). A trend was also observed for a lower JVel (P = .016) in the sarcopenia group, but AT (P = .113) and JHt (P = .125) were not significantly different between groups. The jump test variables, JPow, JVel, and JHt, were significantly positively correlated with each other (r = 0.56-0.59; all P < .0001). Muscle strength measures were positively related to the jump test variables, although the correlations were stronger with JPow and JHt (Table 5). In addition, JPow and JHt were found to be significantly (P ≤ .0001) positively correlated with ASM (r = 0.75, r = 0.54, respectively) and BFLBM (r = 0.77, r = 0.51, respectively) (Table 5). Figure 1 depicts the relationships between JPow and strength and lean tissue mass variables.
Figure 1: Scatter plots for jump power vs leg press strength (A), right hip abduction strength (B), left hip abduction strength (C), appendicular skeletal muscle mass (D), and bone free lean body mass (E) in individuals, 55 to 75 years of age (n = 60).
Table 4: Jump Test Characteristics Based on Sarcopenia Statusa
Table 5: Pearson Zero Order Correlations (r) Between Jump Test, Muscle Strength, and Lean Tissue Mass Variables (n = 60)
DISCUSSION
The primary finding of our study was that JPow was significantly lower in individuals classified as sarcopenic compared with their non-sarcopenic counterparts. In contrast, group differences in jump height and airtime did not reach statistical significance, and only a trend was observed for group effect in jump velocity. Although age-related decreases in JPow have been documented previously,15,16 jump performance based on sarcopenia status is unique as it has not been specifically examined in the literature. Recently, Buehring et al16 reported age group differences in JPow and jump height, which were lower in the older group (mean age = 77.8 years) than in the young group (mean age = 26.9 years). They found that proximal femur lean mass was moderately positively correlated with JPow in the older group; however, they did not report sarcopenia status of their sample. In another study, Buehring et al17 examined the association of isometric tongue strength, handgrip strength, muscle function tests (jump performance, gait speed, chair rise time, and balance) in elderly adults (70-95 years). Sarcopenia was diagnosed in 23.7% (n = 13) of their sample based on the RSMI criteria. They found that tongue strength was significantly related to grip strength and to JPow but not to the other muscle function tests or RSMI. They also reported gender differences in JPow and muscle strength, similar to our findings of significantly lower jump performance and muscle strength in women compared with men. Previous studies have expressed JPow relative to body weight to adjust for the effects of body size.14–17 Our study found only a trend for a group difference in relative JPow, possibly because the participants had already been divided into groups on the basis of RSMI, which takes into account lean tissue mass (ASM) and body size (height).
Another unique aspect of this study was the assessment of lower extremity muscular strength. We found significant moderate positive correlations between the strength variables and JPow and jump height. Leg press and hip abduction strengths were not significantly different between the sarcopenia and normal groups, both in the raw units and when normalized to body weight. This finding did not support our hypothesis as we expected that lower lean tissue mass in the sarcopenia group would result in lower force production. The lack of strength differences in conjunction with reduced JPow seems to be incongruent findings; however, discordant relationships have been previously reported between age, muscle size, muscle strength, and jump performance. Runge et al15 found that calf muscle cross-sectional area was not correlated with age and only weakly correlated with JPow whereas JPow showed significant strong negative correlations with age. Buehring et al17 reported that tongue strength was not associated with RSMI, but it was correlated with JPow. These findings suggest that other physiological factors such as fiber-type distribution, fiber pennation angle, and velocity of contraction may play a greater role than muscle size in muscle force and power changes with aging.15
Our findings suggest that JPow may be a better indicator of muscle performance in the older individuals compared to the traditional isotonic muscle strength testing techniques that involve measurement of a specific muscle action by an externally applied resistance. This method effectively tests how well a muscle contracts against a load; however, it does not control the velocity of contraction. In contrast, jumping mechanography tests an individual's JPow, which is a surrogate measure of muscle force and its contracting velocity.15 Frail older individuals have been shown also to have a slow gait; thus, both decreased muscle force production and contraction velocity may be important for fall risk in these individuals.22 Since fast twitch (type II) muscle fibers are important for the performance of explosive activities, the decrease in type II muscle fiber number and area with age2 may be a possible explanation for the inability to maintain JPow with age.15
Study Limitations
When interpreting the results of this study, the constraints of our cross-sectional research design must be considered. Another limitation of this study was the small number of participants who met the criterion for sarcopenia. Although the overall sarcopenia prevalence of 20% was similar to previous studies,12,17 we had only 12 individuals in total who met the criteria for sarcopenia. Since this was a preliminary study, we limited participant recruitment to healthy men and women, 55 to 75 years of age, because of potential safety concerns with the jump protocol. This protocol was safely conducted in our laboratory with no injuries or balance issues occurring for any of the participants. Longitudinal assessments of jump performance in a larger age range of participants are needed to determine the effectiveness of using jump test results for the prediction of sarcopenia, frailty, and osteoporosis. Although growing evidence suggests that jumping mechanography is a useful way to assess muscle performance in older individuals, much work is yet to be done in this area.
CONCLUSION
Community-dwelling individuals classified as sarcopenic had significantly lower JPow than their counterparts with normal amounts of muscle mass. Jump test variables, especially JPow and jump height, were found to be positively correlated with lean tissue and lower extremity strength in adults, 55 to 75 years of age. On the basis of our findings, JPow may be useful for sarcopenia screening in the middle-aged and older adults; however, more research is needed to determine the utility of this method in clinical populations.
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