Publications

The interrelationship between muscle strength, motor unit (MU) number, and age is poorly understood, and in this study we sought to determine whether age-related differences in muscle strength are moderated by estimates of functioning MU number and size. Eighteen older adults (OA; 67 +/- 1.20 years) and 24 young adults (YA; 22 +/- 0.74 years) participated in this study. Maximum voluntary pinch-grip strength of the nondominant hand was determined and estimates of MU number were obtained from the abductor pollicis brevis muscle using the noninvasive motor unit number index (MUNIX) technique. The MUNIX technique was also utilized to derive a motor unit size index (MUSIX). An analysis of covariance (Age Group x MUNIX or MUSIX) was used to test heterogeneity of regression slopes, with body mass and gender serving as covariates. We observed that the slope of pinch-grip strength on the estimated number of MUs between YA and OA differed, indicated by an Age Group x MUNIX interaction (p = 0.04). Specifically, after controlling for the effect of body mass and gender, the slope in OA was significantly positive (0.14 +/- 0.06 N/MUs, p = 0.03), whereas no such relationship was found in YA (-0.08 +/- 0.09 N/MUs, p = 0.35). A significant Age Group x MUSIX interaction was also observed for strength (p 0.01). In contrast to MUNIX, the slope in younger adults was significantly positive (0.48 +/- 0.11 N/muV, p 0.01), whereas no such relationship was found in older adults (-0.30 +/- 0.22 N/muV, p = 0.18). These findings indicate that there is an interrelationship between muscle strength, MU numbers, and aging, which suggests that a portion of muscle weakness in seniors may be attributable to the loss of functioning motor units.

Mal de debarquement syndrome (MdDS) is a poorly characterized and understood disorder of perceived motion. We sought to characterize postural control and the psychological impact of MdDS. Additionally, we explored whether patients with MdDS exhibit altered corticospinal and intracortical excitability. In a case-control study we compared patients with MdDS to age- and sex-matched controls (n=8/group). Postural stability (sigmar) was quantified from plane phase plots based on center or pressure, and psychological indices of depression, fatigue and kinesiophobia were obtained. Transcranial magnetic stimulation (TMS) was used to assess corticospinal excitability by quantifying the motor evoked potential (MEP) amplitude of the flexor carpi radialis, and intracortical excitability was assessed by quantifying indices of intracortical facilitation (ICF), and short-interval and long-interval intracortical inhibition using a paired-pulse TMS paradigm. The patients with MdDS exhibited greater mean (+/-standard error of the mean) sigmar during semi-tandem stance (10.9 +/- 1.5 compared to 7.1 +/- 0.7, p=0.04), higher levels of kinesiophobia (41.6 +/- 2.8 compared to 27.3 +/- 2.2), and higher levels of fatigue (27.0 +/- 4.1 compared to 48.4 +/- 1.0). Patients with MdDS exhibited a higher mean motor threshold (MT) (58.1 +/- 2.5 compared to 47.4 +/- 2.7% of stimulator output), and larger MEP (13.1 +/- 3.1 compared to 5.1 +/- 1.2% of maximal compound muscle action potential) but there was no difference in measures of intracortical excitability. These findings suggest that patients with MdDS exhibit impaired postural stability, and high levels of kinesiophobia and fatigue. Additionally, we observed that patients with MdDS exhibit higher MT and large MEP amplitudes, but do not exhibit differences in measures of intracortical excitability, compared to controls. These findings help characterize MdDS, and provide insight into the physiology of MdDS.

BACKGROUND AND AIMS: The purpose of this study was to evaluate the prevalence of symptomatic fatigue and its relation to physical function and self-perceived health in a sample of older, rural community-dwelling adults with commonly-used clinical fatigue scales. METHODS: This is an exploratory, descriptive study of 30 subjects from 4 rural counties. All subjects were 70+ years of age and had no recent history of hospitalization. Subjects were assessed in their homes and completed a standard test of physical function, twelve functional assessment instruments, and two commonly-used clinical fatigue scales: the Fatigue Severity Scale and the Functional Assessment of Chronic Illness Therapy-Fatigue Scale. RESULTS: Depending on the fatigue instrument and criteria used, 23-47% of subjects exhibited symptomatic fatigue. Regardless of the scale, fatigue was associated with several negative consequences: decreased physical function performance, lower morale, and reduced physical composite scores on the Short Form-36 quality of life questionnaire. Of note, these differences remained significant even after accounting for depression scale scores. In addition, fatigue was associated with a greater incidence of risk for malnutrition. CONCLUSIONS: Together, these findings suggest that symptomatic fatigue may be quite prevalent in older individuals in rural settings, and warrants further consideration when presented in the clinical setting, as it may be associated with several negative health outcomes.

The aim of this study was to evaluate the changes in electromyographic (EMG) activity of the lower limb muscles, and hip, knee and ankle kinematics during gait while wearing low- (4-cm) and high-heeled (10-cm) shoes in 31 young and 15 middle-aged adult women. We observed an increase in knee flexion and decrease in ankle eversion associated with elevated heel heights suggesting that compensatory mechanisms attenuating ground reaction forces may be compromised during gait with higher-heeled shoes. Additionally, we observed increased muscle activity during high-heeled gait that may exacerbate muscle fatigue. Collectively, these findings suggest that permanent wearing of heeled footwear could contribute to muscle overuse and repetitive strain injuries.

OBJECTIVE: Wearing high-heeled shoes may produce deleterious effects on the musculoskeletal system. The purpose of this study was to evaluate the changes in electromyographic (EMG) activity of the erector spinae muscles and pelvis kinematics during gait while wearing low- and high-heeled shoes in both young and middle-aged adult women. DESIGN: In 31 young women (20-25 yrs) and 15 middle-aged women (45-55 yrs) without back pain, the EMG activity of the erector spinae muscle and pelvis kinematics in the sagittal, frontal, and transverse planes were assessed during gait on flat surface at natural speeds in three conditions: without shoes and in low- (4 cm) and high- (10 cm) heeled shoes. RESULTS: In younger women, significant differences in lumbar erector spinae EMG activity were observed during gait at initial ground contact as well as in toe off between the three conditions, with an increasing amount of EMG activity being observed in association with increased heel height. In middle-aged women, significantly higher lumbar erector spinae EMG activity was noted during gait with high-heeled shoes compared with gait without shoes. Interestingly, younger women exhibited an increase in pelvic range of motion in the sagittal plane during high-heeled gait compared with low-heeled gait and walking without shows; however, this compensatory response was not observed in middle-aged women. CONCLUSIONS: From a clinical perspective, increased lumbar erector spinae muscle activity associated with wearing high-heeled shoes could exacerbate muscle overuse and lead to low back problems. The lower pelvic range of motion associated with wearing high heels in middle-aged women may indicate that tissues in the lumbopelvic region become more rigid with age and that the harmful effect of high-heeled shoes on posture and spinal tissues may be more pronounced with advancing age.

OBJECTIVE: Resistance exercise (RE) stimulates growth hormone (GH) secretion in a load-dependent manner, with heavier loads producing larger GH responses. However, new research demonstrates that low-load RE performed with blood flow restriction (BFR) produces potent GH responses that are similar to or exceed those produced following high-load RE. We hypothesized that low-load RE with vascular restriction would attenuate the known age-related reduction in GH response to RE. DESIGN: In a randomized crossover design, ten young (28 +/- 7.8 years) and ten older (67.4 +/- 4.6 years) men performed bilateral knee extension RE with low-load [20% of one-repetition maximum (1RM)] with BFR and high-load (80% 1RM) without BFR. GH and lactate were measured every 10 minutes throughout a 150-minute testing session (30 minutes prior to and 120 minutes following completion of the exercise); IGF-I was measured at baseline and 60 minutes post-exercise. RESULTS: Area under the GH curve indicated that both age groups responded similarly to each exercise condition. However, young men had a significantly greater maximal GH response to low-load RE with BFR than the high-load condition without BFR. Additionally, younger men had greater maximal GH concentrations to low-load RE with BFR than older men (p=0.02). The GH responses were marginally correlated to lactate concentration (r=0.13, p=0.002) and IGF-I levels were unchanged with RE. CONCLUSIONS: GH responses to low-load RE with vascular restriction are slightly higher than high-load RE without vascular restriction in young men. However, low-load RE with vascular restriction did not attenuate the known age-related reduction in GH response with exercise. These data suggest that while low-load RE with vascular restriction is as effective for inducing a GH response than traditionally-based high-load RE, there is a more potent response in young men.

In 2008, we published an article arguing that the age-related loss of muscle strength is only partially explained by the reduction in muscle mass and that other physiologic factors explain muscle weakness in older adults (Clark BC, Manini TM. Sarcopenia =/= dynapenia. J Gerontol A Biol Sci Med Sci. 2008;63:829-834). Accordingly, we proposed that these events (strength and mass loss) be defined independently, leaving the term "sarcopenia" to be used in its original context to describe the age-related loss of muscle mass. We subsequently coined the term "dynapenia" to describe the age-related loss of muscle strength and power. This article will give an update on both the biological and clinical literature on dynapenia-serving to best synthesize this translational topic. Additionally, we propose a working decision algorithm for defining dynapenia. This algorithm is specific to screening for and defining dynapenia using age, presence or absence of risk factors, a grip strength screening, and if warranted a test for knee extension strength. A definition for a single risk factor such as dynapenia will provide information in building a risk profile for the complex etiology of physical disability. As such, this approach mimics the development of risk profiles for cardiovascular disease that include such factors as hypercholesterolemia, hypertension, hyperglycemia, etc. Because of a lack of data, the working decision algorithm remains to be fully developed and evaluated. However, these efforts are expected to provide a specific understanding of the role that dynapenia plays in the loss of physical function and increased risk for disability among older adults.

PURPOSE: Sprint training is associated with several beneficial adaptations in skeletal muscle, including an enhancement of sarcoplasmic reticulum (SR) Ca(2+) release. Unfortunately, several patient populations (e.g., the elderly, those with cardiac dysfunction) that might derive great benefit from sprint exercise are unlikely to tolerate it. The purpose of this report was to describe the development of a tolerable neuromuscular electrical stimulation (NMES) protocol that induces skeletal muscle adaptations similar to those observed with sprint training. METHODS: Our NMES protocol was modeled after a published sprint exercise protocol and used a novel electrode configuration and stimulation sequence to provide adequate training stimulus while maintaining subject tolerance. Nine young, healthy subjects (four men) began and completed the training protocol of the knee extensor muscles. RESULTS: All subjects completed the protocol, with ratings of discomfort far less than those reported in studies of traditional NMES. Training induced significant increases in SR Ca(2+) release and citrate synthase activity ( 16% and 32%, respectively), but SR Ca(2+) uptake did not change. The percentage of myosin heavy chain IIx isoform was decreased significantly after training. At the whole muscle level, neither central activation nor maximum voluntary isometric contraction force were significantly altered, although isometric force did exhibit a trend toward an increase ( 3%, P = 0.055). Surprisingly, the NMES training produced a significant increase in muscle cross-sectional area ( 3%, P = 0.04). CONCLUSIONS: It seems that an appropriately designed NMES protocol can mimic many of the benefits of sprint exercise training, with a low overall time commitment and training volume. These findings suggest that NMES has the potential to bring the benefits of sprint exercise to individuals who are unable to tolerate traditional sprint training.