Biomedical subjects

E J Metter

Publications and source records attributed to E J Metter.

At least 37 records · Page 2Linked to original sources

PubMed2000

Longitudinal influence of age, menopause, hormone replacement therapy, and other medications on parotid flow rates in healthy women.

BACKGROUND: Recent investigations have demonstrated that parotid salivary dysfunction is not a normal process of aging, but may be the consequence of systemic conditions and their treatment, including medications and menopause. The purpose of this study was to assess longitudinally the influence of age, menopausal status, hormone replacement therapy, and other medications on stimulated parotid flow rates (SPFRs) in healthy women. METHODS: Medical diagnoses, menopausal status, medication utilization, and 2% citric acid stimulated parotid salivas were collected from 396 women, aged 21 to 96 years, from the Baltimore Longitudinal Study of Aging (National Institute on Aging, National Institutes of Health) over a 17-year span by three investigators. RESULTS: There was no overall longitudinal effect of time on SPFR. Age at first visit was a significant predictor of a decrease in SPFR when adjusted for time and xerostomic medications. However, the deleterious effect of taking one xerostomic medication was equivalent to approximately 14 years of aging. Menopausal status and hormone replacement therapy were not consistently associated with diminished SPFR. CONCLUSIONS: These results suggest that menopause and hormone replacement therapy are not associated with parotid salivary dysfunction. Aging may have a statistically significant yet small deleterious influence on SPFR; however, the adverse influence of xerostomic medications is much larger.

Adult↗

PubMed2000

Effects of strength training and detraining on muscle quality: age and gender comparisons.

Maximal force production per unit of muscle mass (muscle quality, or MQ) has been used to describe the relative contribution of non-muscle-mass components to the changes in strength with age and strength training (ST). To compare the influence of age and gender on MQ response to ST and detraining, 11 young men (20-30 years), nine young women (20-30 years), 11 older men (65-75 years), and 11 older women (65-75 years), were assessed for quadriceps MQ at baseline, after 9 weeks of ST, and after 31 weeks of detraining. MQ was calculated by dividing quadriceps one repetition maximum (IRM) strength by quadriceps muscle volume determined by magnetic resonance imaging. All groups demonstrated significant increases in IRM strength and muscle volume after training (all p < .05). All groups also increased their MQ with training (all p < .01), but the gain in MQ was significantly greater in young women than in the other three groups (p < .05). After 31 weeks of detraining, MQ values remained significantly elevated above baseline levels in all groups (p < .05), except the older women. These results indicate that factors other than muscle mass contribute to strength gains with ST in young and older men and women, but those other factors may account for a higher portion of the strength gains in young women. These factors continue to maintain strength levels above baseline for up to 31 weeks after cessation of training in young men and women, and in older men.

Adult↗

PubMed2000

Leisure-time physical activities and their relationship to cardiorespiratory fitness in healthy men and women 18-95 years old.

PURPOSE: We examined leisure-time physical activities (LTPA) and their contribution to peak oxygen consumption (VO2) in healthy men (N = 619) and women (N = 497) aged 18-95 yr (mean 51 +/- 17) who were participants of the Baltimore Longitudinal study of Aging. METHODS: Calculations of LTPA were based on the average self-reported time spent performing 97 activities and converted into MET-min x 24 h(-1). The activities were divided into three levels of LTPA based on absolute intensity. Peak VO2 was determined from a maximal treadmill exercise test. RESULTS: Total LTPA was inversely related to age in both sexes (r = -0.26, P < 0.0001 in men and r = -0.23, P < 0.0001 in women), mediated primarily by less high-intensity activities in older subjects, with only minor differences in moderate- and low-intensity activities across age. Peak VO2 correlated positively with LTPA; the correlations were strongest for high-intensity LTPA (r = 0.33 in men and 0.27 in women, each P < 0.0001), intermediate for moderate-intensity activity (r = 0.12, P < 0.004 in men and r = 0.17, P < 0.0001 in women) and minimal for low-intensity activity (r = 0.08, P = 0.05 in men and r = 0.06, P = 0.20 in women). On univariate analysis, total LTPA accounted for 12.9% of peak VO2 variance for men and 10.6% for women. By multivariate analysis, LTPA independently accounted for 1.6% of the peak VO2 variance in men and 1.8% in women after controlling for age and body mass index. CONCLUSIONS: In healthy adults across a broad age range, LTPA is a relatively minor independent contributor to aerobic capacity.

Adolescent↗

PubMed2000

Age and gender responses to strength training and detraining.

PURPOSE: The purpose of this study was to examine the effects of age and gender on the strength response to strength training (ST) and detraining. METHODS: Eighteen young (20-30 yr) and 23 older (65-75 yr) men and women had their one-repetition maximum (1 RM) and isokinetic strength measured before and after 9 wk of unilateral knee extension ST (3 d x wk(-1)) and 31 wk of detraining. RESULTS: The young subjects demonstrated a significantly greater (P < 0.05) increase in 1 RM strength (34+/-3%; 73+/-5 vs 97+/-6 kg; P < 0.01) than the older subjects (28+/-3%; 60+/-4 vs 76+/-5 kg, P < 0.01). There were no significant differences in strength gains between men and women in either age group with 9 wk of ST or in strength losses with 31 wk of detraining. Young men and women experienced an 8+/-2% decline in 1 RM strength after 31 wk of detraining (97+/-6 vs 89+/-6 kg, P < 0.05). This decline was significantly less than the 14+/-2% decline in the older men and women (76+/-5 vs 65+/-4 kg, P < 0.05). This strength loss occurred primarily between 12 and 31 wk of detraining with a 6+/-2% and 13+/-2% decrease in the young and older subjects, respectively, during this period. DISCUSSION: These results demonstrate that changes in 1 RM strength in response to both ST and detraining are affected by age. However, ST-induced increases in muscular strength appear to be maintained equally well in young and older men and women during 12 wk of detraining and are maintained above baseline levels even after 31 wk of detraining in young men, young women, and older men.

Adult↗

PubMed2000

High-volume, heavy-resistance strength training and muscle damage in young and older women.

To determine possible age differences in muscle damage response to strength training, ultrastructural muscle damage was assessed in seven 20- to 30-yr-old and six 65- to 75-yr-old previously sedentary women after heavy-resistance strength training (HRST). Subjects performed unilateral knee-extension exercise 3 days/wk for 9 wk. Bilateral muscle biopsies from the vastus lateralis were assessed for muscle damage via electron microscopy. HRST resulted in a 38 and 25% increase in strength in the young and older women, respectively (P < 0.05), but there were no between-group differences. In the young women, 2-4% of muscle fibers exhibited damage before and after training in both the trained and untrained legs (P = not significant). In contrast, muscle damage increased significantly after HRST, from 5 to 17% of fibers damaged (P < 0.01), in the older women in the trained leg compared with only 2 and 5% of fibers damaged in the untrained leg before and after training, respectively. The present results indicate that older women exhibit higher levels of muscle damage after chronic HRST than do young women.

Adult↗

PubMed2000

Serum levels of insulin-like growth factor I (IGF-I), IGF-II, IGF-binding protein-3, and prostate-specific antigen as predictors of clinical prostate cancer.

Insulin-like growth factors (IGFs) may play a role in prostate growth, hyperplasia, and malignancy. High plasma IGF-I has been associated with increased prostate cancer risk. In a prospective, cohort, case-control study in the Baltimore Longitudinal Study on Aging population, we examined prostate volume by magnetic resonance imaging, and prostate-specific antigen (PSA), IGF-I, IGF-II, and IGF-binding protein-3 (IGFBP-3) in sera obtained approximately 9 yr before diagnosis of prostate cancer in cases (n = 72) or age-matched controls (n = 127) and in 76 additional Baltimore Longitudinal Study on Aging men (normal subjects) with measured prostate volumes and no prostate cancer. We calculated adjusted odds ratios (OR) by logistic regression, relative risks for significant ORs, and receiver operator curves for prostate cancer, using serum measures alone and in combination. Adjusted ORs for the high vs. low tertile were: for IGF-I, 3.1 [confidence interval (CI), 1.1-8.7]; for IGF-II, 0.2 (CI, 0.07-0.6); for IGFBP-3, 0.71 (CI, 0.3-1.7); and for PSA, 12.5 (CI, 3.8-40.9). For significant ORs, relative risk estimates remained significant at 2.0 for IGF-I, 0.3 for IGF-II, and 5.5 for PSA. Receiver operator curves showed PSA to be the most powerful predictor of prostate cancer. Adding IGF-II to PSA improved prediction. IGF-II was significantly and inversely related (r = -0.219; P < 0.01) and PSA was directly and significantly related (r = 0.461; P < 0.0001) to prostate volume, whereas IGF-I and IBFBP-3 were not. High IGF-I and low IGF-II are independently associated with increased risk of prostate cancer, but PSA level is a much stronger predictor of prostate cancer in the ensuing 10 yr than either IGF-I or IGF-II. The absence of a relationship of IGF-I to prostate size is inconsistent with increased ascertainment in men with large prostates as the source of greater prostate cancer risk associated with IGF-I. Our data suggest that IGF-II may inhibit both prostate growth and development of prostate cancer.

Baltimore↗

PubMed1999

Prostate-specific antigen testing of older men.

BACKGROUND: Elevated serum prostate-specific antigen (PSA) levels are predictive of a future diagnosis of prostate cancer. To test the hypothesis that older men with low PSA levels may require less intensive PSA testing because of a reduced prostate cancer detection rate, we evaluated the association between age, baseline PSA level, and prostate cancer detection. METHODS: We conducted a prospective cohort study among participants in a study of aging who had serial PSA measurements taken from age 60 or 65 years until they either were diagnosed with prostate cancer (cancer case subjects) or reached the age of 75 years (subjects without prostate cancer). The time of cancer detection among cancer case subjects was defined as the measurement date on which a PSA level above 4.0 ng/mL was detected (i.e., PSA conversion). Cancer case subjects and subjects without prostate cancer were analyzed according to baseline PSA level and age. RESULTS: All cancer case subjects in the 60-year-old cohort had baseline PSA levels above 0.5 ng/mL, and 14 of 15 cancer cases that would have been detected by a PSA conversion among the 65-year-old cohort were associated with baseline PSA levels of 1.1 ng/mL or more. If PSA testing were discontinued in men aged 65 years with PSA levels of 0.5 ng/mL or less, 100% (95% confidence interval [CI] = 78%-100%) of the cancers would still be detected by age 75 years; if PSA testing were discontinued in men aged 65 years who had PSA levels of 1.0 ng/mL or less, 94% (95% CI = 70%-100%) of the cancers would still be detected by age 75 years. CONCLUSIONS: These data suggest that a decrease in the intensity of screening among older men with low PSA values may not lead to an increase in undetected prostate cancer.

Age Factors↗

PubMed1999

Influence of age and postmenopausal estrogen replacement therapy on carotid arterial stiffness in women.

OBJECTIVE: This study examines the influence of age and current estrogen replacement therapy (ERT) on common carotid arterial (CCA) stiffness in women. METHODS: The subjects comprised 172 women (age 55.6 +/- 16.4 years) from the Baltimore Longitudinal Study of Aging, including 37 current postmenopausal ERT users. The ERT users included 18 women taking estrogen alone and 19 women taking estrogen and progesterone. Bilateral CCA were examined by B-mode carotid ultrasonography, and the stiffness index was defined as the logarithm of the ratio of systolic to diastolic blood pressure (BP) divided by the fractional diameter increase during the cardiac cycle. RESULTS: The stiffness index increased linearly with age (r = 0.69, p < 0.001), and was lower in ERT users than in postmenopausal nonusers (8.0 +/- 2.0 vs 9.7 +/- 3.1, p < 0.01). Furthermore, the effects of age (beta = 0.67, p < 0.0001) and ERT (beta = -0.23, p < 0.001) on the stiffness index persisted after adjustments for systolic BP (beta = 0.23, p < 0.01), diastolic BP (beta = -0.26, p < 0.001) and other cardiovascular risk factors (model r2 = 0.59, p < 0.0001). The stiffness index was similar in both ERT users with and without progesterone and lower than in postmenopausal nonusers (p < 0.05) after considering the age effects. CONCLUSION: Age-associated increases in CCA stiffness are reduced by postmenopausal ERT.

Aged↗

PubMed1999

Carotid arterial stiffness as a surrogate for aortic stiffness: relationship between carotid artery pressure-strain elastic modulus and aortic pulse wave velocity.

Common carotid arterial (CCA) stiffness can be assessed during carotid ultrasonography, but its association with aortic stiffness, a well-defined cardiovascular risk factor, has not been clarified. This study examines the relationship between CCA and aortic stiffness. CCA pressure-strain elastic modulus (Ep) and aortic pulse wave velocity (APWV) were evaluated in 110 healthy volunteers (age 56.2 +/- 14.6 y) by B-mode and Doppler ultrasonography. CCA Ep increased linearly with age and was higher in men than in women (model r2 = 0.50, p < 0.001). APWV increased quadratically with age (model r2 = 0.54, p < 0.001), similarly for women and men. Both CCA Ep and APWV were linearly associated with systolic blood pressure (BP) (r = 0.53 and 0.46, respectively) but not with diastolic BP. A linear relationship was found between CCA Ep and APWV (APWV = 194.7 + 5.67 x Ep [model r2 = 0.42, p < 0.001]). CCA Ep was associated with APWV (p < 0.001) independent of age, gender, and BP (model r2 = 0.62, p < 0.001), and the most parsimonious model to explain APWV included CCA Ep and age (APWV = 601.73 - 15.64 x age + 0.223 x age2 + 2.69 x Ep [model r2 = 0.60, p < 0.001]). Thus, CCA Ep is moderately associated with APWV. CCA stiffness as assessed by B-mode may be useful as a surrogate for aortic stiffness.

Aortic Diseases↗

PubMed1999

Decay Index: a new carotid Doppler waveform measure associated with the Windkessel function of elastic arteries.

The Windkessel function of elastic arteries determines the postpeak contour of the internal carotid artery (ICA) Doppler waveform. To introduce an indicator for the function, the postpeak contour was reproduced by an exponential function, and the exponential coefficient was termed "Decay Index" (DI). DI obtained from 108 stenosis-free ICAs (55.2 +/- 15.8 y) was compared with surrogate measures for the Windkessel function (aortic pulse wave velocity, pulse pressure) and ICA peripheral resistance (resistive index, pulsatility index, systolic-to-diastolic ratio). DI was moderately correlated with the Windkessel and the resistance measures (r ranged 0.44-0.57). To clarify these intercorrelations, principal component analysis revealed two components. The first was loaded by the resistance measures (component load > or = 0.93) although the second was loaded by the Windkessel measures and DI (> or = 0.70), supporting an association between DI and the Windkessel function. Thus, DI is likely to represent an indicator for the Windkessel function.

Aorta↗

PubMed1999

The relationship of motor unit size, firing rate and force.

OBJECTIVE: Using a clinical electromyographic (EMG) protocol, motor units were sampled from the quadriceps femoris during isometric contractions at fixed force levels to examine how average motor unit size and firing rate relate to force generation. METHODS: Mean firing rates (mFRs) and sizes (mean surface-detected motor unit action potential (mS-MUAP) area) of samples of active motor units were assessed at various force levels in 79 subjects. RESULTS: MS-MUAP size increased linearly with increased force generation, while mFR remained relatively constant up to 30% of a maximal force and increased appreciably only at higher force levels. A relationship was found between muscle force and mS-MUAP area (r2 = 0.67), mFR (r2 = 0.38), and the product of mS-MUAP area and mFR (mS-MUAP x mFR) (r2 = 0.70). CONCLUSIONS: The results support the hypothesis that motor units are recruited in an orderly manner during forceful contractions, and that in large muscles only at higher levels of contraction ( > 30% MVC) do mFRs increase appreciably. MS-MUAP and mFR can be assessed using clinical EMG techniques and they may provide a physiological basis for analyzing the role of motor units during muscle force generation.

Action Potentials↗

PubMed1999

Muscle quality and age: cross-sectional and longitudinal comparisons.

We addressed whether muscle quality (force per unit muscle mass) changes with age in cross-sectional and longitudinal analyses from three groups from the Baltimore Longitudinal Study of Aging: (1) Isometric arm strength studied cross-sectionally in 617 subjects with muscle mass estimated by cross-sectional area (CSA) from arm circumference and by 24-hour urinary creatinine excretion (CREAT); (2) longitudinal study for 10 to 25 years in 412 men using the same measures as the first group; and (3) isometric knee extensor strength studied cross-sectionally in 675 subjects; muscle mass estimated by CREAT, CSA from thigh circumference, and leg nonosseous fat free mass (FFM) from dual energy x-ray absorptiometry. Muscle quality declined in both arm and leg with age in cross-sectional analyses using CSA and FFM, but not CREAT. No age-associated arm muscle quality declines were observed longitudinally using CREAT or CSA. The relationship between muscle quality and age is dependent on how muscle mass is estimated and on whether subjects are studied cross-sectionally or longitudinally. In addition, CREAT may measure a muscle property not accounted for by CSA or FFM.

Absorptiometry, Photon↗

PubMed1999

Muscle quality. I. Age-associated differences between arm and leg muscle groups.

To determine the differences between arm and leg muscle quality (MQ) across the adult life span in men and women, concentric (Con) and eccentric (Ecc) peak torque (PT) were measured in 703 subjects (364 men and 339 women, age range 19-93 yr) and appendicular skeletal muscle mass (MM) was determined in the arm and leg in a subgroup of 502 of these subjects (224 men and 278 women). Regression analysis showed that MQ, defined as PT per unit of MM, was significantly higher in the arm ( approximately 30%) than in the leg across age in both genders (P < 0.01). Arm and leg MQ declined at a similar rate with age in men, whereas leg MQ declined approximately 20% more than arm MQ with increasing age in women (P </= 0.01 and P < 0.05 for Con and Ecc PT, respectively). Moreover, the age-associated decrease in arm MQ was steeper in men than in women whether Con or Ecc PT was used (both P < 0.05). Arm MQ as determined by Con PT showed a linear age-related decline in men and women (28 and 20%, respectively, P < 0.001), whereas arm MQ as determined by Ecc PT showed a linear age-related decline in men (25%, P < 0.001) but not in women (not significant). In contrast, both genders exhibited an age-related quadratic decline in leg MQ as determined by Con PT ( approximately 40%) and Ecc PT ( approximately 25%; both P < 0.001), and the rate of decline was similar for men and women. Thus MQ is affected by age and gender, but the magnitude of this effect depends on the muscle group studied and the type of muscle action (Con vs. Ecc) used to assess strength.

Adult↗

PubMed1999

Muscle quality. II. Effects Of strength training in 65- to 75-yr-old men and women.

To determine the effects of strength training (ST) on muscle quality (MQ, strength/muscle volume of the trained muscle group), 12 healthy older men (69 +/- 3 yr, range 65-75 yr) and 11 healthy older women (68 +/- 3 yr, range 65-73 yr) were studied before and after a unilateral leg ST program. After a warm-up set, four sets of heavy-resistance knee extensor ST exercise were performed 3 days/wk for 9 wk on the Keiser K-300 leg extension machine. The men exhibited greater absolute increases in the knee extension one-repetition maximum (1-RM) strength test (75 +/- 2 and 94 +/- 3 kg before and after training, respectively) and in quadriceps muscle volume measured by magnetic resonance imaging (1,753 +/- 44 and 1, 955 +/- 43 cm3) than the women (42 +/- 2 and 55 +/- 3 kg for the 1-RM test and 1,125 +/- 53 vs. 1,261 +/- 65 cm3 for quadriceps muscle volume before and after training, respectively, in women; both P < 0.05). However, percent increases were similar for men and women in the 1-RM test (27 and 29% for men and women, respectively), muscle volume (12% for both), and MQ (14 and 16% for men and women, respectively). Significant increases in MQ were observed in both groups in the trained leg (both P < 0.05) and in the 1-RM test for the untrained leg (both P < 0.05), but no significant differences were observed between groups, suggesting neuromuscular adaptations in both gender groups. Thus, although older men appear to have a greater capacity for absolute strength and muscle mass gains than older women in response to ST, the relative contribution of neuromuscular and hypertrophic factors to the increase in strength appears to be similar between genders.

Aged↗

PubMed1999

Ultrastructural muscle damage in young vs. older men after high-volume, heavy-resistance strength training.

This study assessed ultrastructural muscle damage in young (20-30 yr old) vs. older (65-75 yr old) men after heavy-resistance strength training (HRST). Seven young and eight older subjects completed 9 wk of unilateral leg extension HRST. Five sets of 5-20 repetitions were performed 3 days/wk with variable resistance designed to subject the muscle to near-maximal loads during every repetition. Biopsies were taken from the vastus lateralis of both legs, and muscle damage was quantified via electron microscopy. Training resulted in a 27% strength increase in both groups (P < 0.05). In biopsies before training in the trained leg and in all biopsies from untrained leg, 0-3% of muscle fibers exhibited muscle damage in both groups (P = not significant). After HRST, 7 and 6% of fibers in the trained leg exhibited damage in the young and older men, respectively (P < 0.05, no significant group differences). Myofibrillar damage was primarily focal, confined to one to two sarcomeres. Young and older men appear to exhibit similar levels of muscle damage at baseline and after chronic HRST.

Actin Cytoskeleton↗

PubMed1999

Increased carotid artery intimal-medial thickness: risk factor for exercise-induced myocardial ischemia in asymptomatic older individuals.

Asymptomatic coronary artery disease (CAD) is prevalent in the general population and has been associated with an increased risk for symptomatic CAD. Although the diagnosis of asymptomatic CAD is currently dependent on exercise testing and coronary angiography, other vascular diagnostic techniques could potentially be of aid in the assessment. Increased intimal-medial thickness (IMT) of the common carotid artery as assessed by B-mode ultrasonography is a purported index of atherosclerosis, and is associated with symptomatic CAD. Based on a recent report, this article will focus on the relationship between IMT and asymptomatic CAD as evidenced by exercise ECG, and in combination with exercise thallium scintigraphy. It was found that exercise-induced ST segment depression was associated with increased IMT independent of age, coronary risk factors and manifest CAD. After adjustment for age, IMT progressively increased from healthy subjects to asymptomatic subjects with positive exercise ECG alone, to those with concordant positive ECG and thallium scintigraphic findings who had IMT virtually identical to that in subjects with manifest CAD. Each 0.1 mm increase in IMT was associated with a 1.91-fold (95% CI 1.46-2.50) increased risk for concordant positive exercise tests or manifest CAD, independent of other coronary risk factors. These findings and the review of the literature suggest the potential utility of carotid ultrasonography in identifying asymptomatic individuals at higher risk for CAD.

Aged↗

PubMed1998

Increased carotid artery intimal-medial thickness in asymptomatic older subjects with exercise-induced myocardial ischemia.

BACKGROUND: Previous studies have shown an association between symptomatic coronary artery disease (CAD) and increased intimal-medial thickness of the common carotid artery (CCA IMT), a purported index of atherosclerosis. This study determines whether CCA IMT is increased in asymptomatic older subjects with an ischemic ST-segment response to treadmill exercise. METHODS AND RESULTS: CCA IMT was measured by B-mode ultrasound in community-dwelling volunteers from the Baltimore Longitudinal Study of Aging, including 397 healthy subjects (age, 58.5+/-15.8 years) with normal ECG responses to maximum treadmill exercise, 72 asymptomatic subjects (age, 66.1+/-13.4 years) with exercise-induced horizontal or downsloping ST-segment depression >/=1 mm, and 38 subjects (age, 77. 4+/-7.8 years) with clinically manifest CAD as diagnosed by medical history and resting ECG. Forty-three subjects with abnormal exercise ECGs also underwent exercise thallium scintigraphy. Exercise-induced ST-segment depression was associated with increased IMT (P<0.0001) independent of age and manifest CAD. After adjustment for age, IMT values progressively increased from healthy subjects to asymptomatic subjects with positive exercise ECG alone to those with concordant positive ECG and thallium scintigraphic findings who had virtually identical IMT to subjects with manifest CAD. Each 0.1-mm increase in IMT was associated with a 1.91-fold (95% CI, 1.46 to 2.50; P<0.0001) increased risk for concordant positive exercise tests or manifest CAD, independent of other significant predictors of CAD. CONCLUSIONS: CCA IMT is increased in older subjects with asymptomatic myocardial ischemia as evidenced by exercise ECG alone or in combination with thallium scan. Carotid ultrasound may help to identify asymptomatic individuals with CAD.

Adult↗

PubMed1998

Firing rate analysis using decompostion-enhanced spike triggered averaging in the quadriceps femoris.

Electromyographic signals detected from the quadriceps femoris during various constant force contractions were decomposed to identify individual motor unit discharges and mean firing rates (FRs). Subject and group mean FRs were calculated for each force level. Mean FR values and FR variability increased with force. Individual, subject, and group mean FRs showed slight increases until 30% of maximum voluntary contraction and larger increases thereafter. Findings are discussed in relation to motor unit recruitment, frequency modulation, and fatigue.

Action Potentials↗