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Biomedical subjects

G O Johnson

Publications and source records attributed to G O Johnson.

At least 37 records · Page 2Linked to original sources
PubMed1998

The effect of leg flexion angle on the mechanomyographic responses to isometric muscle actions.

The purpose of this investigation was to examine the effect of leg flexion angle on the relationship between mechanomyographic (MMG) amplitude and isometric torque production. Adult males (n = 9) performed isometric muscle actions of the leg extensors at 25, 50, 75, and 100 percent maximal voluntary contraction (%MVC) on a calibrated CYBEX 6000 dynamometer at 25, 50, and 75 degrees below full extension. A piezoelectric MMG recording device was placed over the mid-portion of the rectus femoris. At 25 degrees of leg flexion, the MMG amplitude increased to 100%MVC. At 50 and 75 degrees of leg flexion, however, MMG amplitude increased to 75%MVC, and then did not change significantly (P > 0.05) between 75 and 100%MVC. These findings indicate that the MMG amplitude-isometric torque relationship is joint angle specific and may be the result of leg flexion angle differences in: (1) muscle stiffness, or (2) motor unit activation strategies.

Adult↗
PubMed1998

Gender comparisons of the mechanomyographic responses to maximal concentric and eccentric isokinetic muscle actions.

PURPOSE: The purpose of this study was to determine whether there is a gender difference in the velocity-related patterns of mechanomyographic (MMG) responses to maximal isokinetic concentric (CON) and eccentric (ECC) muscle actions. METHODS: Adult males (N = 15) and females (N = 16) performed maximal CON and ECC muscle actions of the leg extensors on a calibrated Cybex 6000 dynamometer at velocities of 30, 90, and 150 degrees.s-1. MMG was detected by a piezoelectric crystal contact sensor placed over the vastus lateralis muscle. RESULTS: The results indicated that there were decreases in CON peak torque (PT) across velocities, while ECC PT remained constant with increasing velocity for both genders. MMG amplitude increased significantly (P < 0.05) with velocity in both the males and females for CON and ECC muscle actions. There was a gender difference in the velocity-related patterns of MMG responses to maximal isokinetic CON muscle actions; however, there was no gender difference in the pattern of ECC MMG responses. CONCLUSIONS: The gender difference in CON MMG responses may be attributed to the greater percent decline in CON PT across velocity for the females than the males. In addition, the males displayed greater CON and ECC MMG amplitudes at all muscle action velocities than the females, possibly because of gender differences in muscle mass and/or thickness of the adipose tissue layer.

Acceleration↗
PubMed1997

Mechanomyographic responses to concentric isokinetic muscle contractions.

The purpose of this investigation was to examine the effects of velocity of contraction on the mechanomyographic (MMG) responses to maximal concentric isokinetic leg extension movements. Eight adult males [mean (SD) age, 22.3 (1.3) years] performed maximal leg extensions on a calibrated Cybex 6000 dynamometer at velocities of 60, 120, 180, 240, 300, and 360 degrees x s(-1) x MMG responses were detected by a piezoelectric recording device placed over the vastus lateralis muscle. Intraclass reliability correlations ranged from R = 0.84 to 0.97 and from 0.90 to 0.99 for peak torque and MMG amplitude values, respectively, with no significant differences (P > 0.05) between the mean values for test versus retest at any contraction velocity. There were significant differences (P < 0.05) in peak torque at all velocities except 240 [135 (27) Nm] versus 300 [127 (27) Nm], and 300 versus 360 degrees x s(-1) [115 (37) Nm]. The mean MMG amplitude at 60 degrees x s(-1) [61 (67) mV] was significantly less (P < 0.05) than that at 360 degrees x s(-1) [452 (451) mV]. These results indicate a velocity-related dissociation between MMG amplitude and peak torque. Furthermore, it was hypothesized that the increases in MMG amplitude were due to velocity-related decreases in muscle stiffness which allowed for greater muscle fiber oscillations.

Adult↗
PubMed1997

Effect of an aerobic training program on physical working capacity at heart rate threshold.

The purpose of this investigation was to determine the effect of an aerobic training program on physical working capacity at the heart rate threshold (PWCHRT). A total of 19 subjects volunteered for this investigation. The control group (CG) consisted of three females and four males, while the training group (TG) included four females and eight males. All subjects were pretested and posttested for maximal oxygen consumption rate (VO2max) and PWCHRT. The training consisted of riding a cycle ergometer for 30 min at 85% of maximal heart rate (HR) three times per week for 8 weeks. Two univariate (VO2max and PWCHRT) 2 x 2 mixed factorial analyses of variance [group (CG, TG) x time (pretraining, posttraining)] were used to analyze the data. A significant (P < 0.05) group by time interaction for PWCHRT was followed by paired t-tests to analyze the simple main effects. The PWCHRT for the TG increased significantly (P < 0.05) as a result of the training program, whereas no change occurred for the CG. The VO2max did not change significantly for either the TG or the CG. The results of this investigation demonstrate that PWCHRT was sensitive to the effects of an 8-week aerobic training program.

Adult↗
PubMed1997

Mechanomyography and oxygen consumption during incremental cycle ergometry.

The purpose of this investigation was to describe and compare the relationships for mechanomyography (MMG) and oxygen consumption rate (VO2) versus power output during incremental cycle ergometry. Twenty four adult males [mean (SD) age, 22.1 (2.0) years] volunteered to perform an incremental test to exhaustion on a cycle ergometer. A MMG piezoelectric recording device was placed mid-thigh over the vastus lateralis muscle and VO2 was measured using standard open circuit procedures. The r2 values for the MMG and VO2 versus power output relationships ranged from 0.79 to 0.99 and 0.97 to 0.99, respectively. In 20 of the 24 subjects there was no significant (P > 0.10) difference between the slope values for the normalized MMG and VO2 (expressed as a percentage of maximal values) versus power output relationships. The results of this study indicate that MMG procedures can be used to quantify muscular activity and monitor changes in exercise intensity during cycle ergometry. Furthermore, the present findings demonstrated a close association between the mechanical (MMG) and metabolic (VO2) aspects of muscular contraction during incremental cycle ergometry.

Adult↗
PubMed1997

Validity of near-infrared interactance for estimating relative body fat in female high school gymnasts.

The present investigation examined the validity of near-infrared interactance (NIR) estimates of relative body fat (% fat) from the Futrex-5000 (F5000), Futrex-5000A (F5000 A), and Futrex-1000 (F1000) instruments. Ninety-eight female high school gymnasts (X age+/-SD = 15.7+/-1.2 yr) participated in this investigation. Subsamples were used to cross-validate the F5000 (n = 52), F5000A (n = 46), and F1000 (n = 89) instruments. The NIR % fat estimates were validated against criterion % fat from underwater weighing (UWW) using the adult conversion constants of Brozek et al. (6) (UWWB) and the female age-specific constants of Lohman (23) (UWWL). The cross-validation statistical analysis included examination of the constant error (CE), standard error of estimate (SEE), r, and total error (TE). In addition, full-model multiple regression analyses were used to predict UWWB or UWWL from body weight (BW) and height (HT). BW and HT were correlated with % fat at R = 0.65-0.70, while the validity coefficients for the NIR instruments ranged from r = 0.40-0.78. The F5000 resulted in nonsignificant CE values (-0.3 % fat vs UWWB and 1.5 % fat vs UWWL; p > 0.008) as well as the lowest TE values (TE = 3.1 % fat vs UWWB and 4.0 % fat vs UWWL). All other NIR estimates of % fat resulted in TE values > or = 6.3 % fat. In addition, for all NIR instruments there were negative correlations for the plots of the CE versus the mean of predicted and criterion (UWWB and UWWL) % fat. Therefore, the present findings indicated that the F5000 provided more accurate estimates of % fat than the F5000A or F1000 instruments, but may underestimate the desired minimal body weight for female gymnasts at the low end of the % fat distribution.

Adipose Tissue↗
PubMed1997

Validity of bioelectrical impedence equations for estimating fat-free weight in high school female gymnasts.

The present study examined the validity of bioelectrical impedence (BIA) equations for estimating fat-free weight (FFW) in female gymnasts by comparing the values to those obtained from underwater weighing (UWW). Ninety-seven female Caucasian high school gymnasts (mean age +/- SD = 15.7 +/- 1.1 yr) participated in the study. FFW from UWW was calculated from percent fat using the revised formula of Brozek et al. (mean FFW +/- SD = 43.8 +/- 4.5 kg) and the age-specific constants of Lohman (mean FFW +/- SD = 44.8 +/- 4.6 kg). Cross-validation analyses included examination of the constant error (CE), SEE, r, and total error (TE). The results indicated similar trends between equations when based on either the Brozek or Lohman conversions; however, the CE, SEE, and TE values were consistently lower for the majority of the equations using the revised formula of Brozek et al. Based upon the results of the cross-validation analyses, the equation of Houtkooper et al. and the interlaboratory equations of Van Loan et al. and Lohman, which resulted in identical TE values of 2.4 kg are recommended for use with young high school gymnasts.

Adolescent↗
PubMed1997

Mechanomyographic responses to maximal eccentric isokinetic muscle actions.

The purpose of the present investigation was to examine the mechanomyographic (MMG) responses to maximal eccentric isokinetic muscle actions. Eight adult male volunteers [age 22 +/- 2 (SD) yr] performed maximal eccentric muscle actions of the leg extensors at 60, 90, 120, and 180 degrees /s on a Cybex 6000 isokinetic dynamometer. MMG was detected by a piezoelectric crystal contact sensor placed over the vastus lateralis muscle. Test-retest intraclass correlations ranged from R = 0.88 to 0.97 for peak torque and from R = 0.97 to 0.98 for root mean square MMG amplitude values. There was no significant (P > 0.05) velocity-related change in eccentric peak torque; however, there was a significant (P < 0.05) increase in MMG between 60 [119 +/- 44 (SE) mV] and 180 degrees/s (302 +/- 128 mV). These findings indicated a velocity-related dissociation between MMG and peak torque for maximal eccentric isokinetic muscle actions.

Adult↗
PubMed1996

Effect of workbout duration on the physical working capacity at fatigue threshold (PWCFT) test.

The purpose of this study was twofold: (1) to determine the effect of increasing the duration of the workbout at each power output during the physical working capacity at fatigue threshold (PWCFT) test from 2 to 3 or 4 min, and (2) to examine the time to exhaustion during continuous workbouts at the PWCFT. Twelve adult males (means +/- SD = 22.4 +/- 3.0 years) volunteered to perform three PWCFT tests using workbout durations of 2, 3, and 4 min. Following the determination of the PWCFT values, nine of the subjects performed continuous workbouts at PWCFT2 and PWCFT4 for as long as possible. The mean PWCFT value using 4-min workbouts (PWCFT4 = 168.8 +/- 45.1 W) was significantly less (p < 0.05, 19.1%) than that using 2-min workbouts (PWCFT2 = 208.9 +/- 59.0 W). However, only two subjects were able to complete 60 min at PWCFT4 and none of the subjects were able to complete 60 min at PWCFT2. Therefore, although increasing the duration of the workbout at each power output resulted in a lower PWCFT4, these findings do not support a recommendation for a change in the PWCFT test protocol.

Adult↗
PubMed1996

Validity of bioelectrical impedance equations for estimating percent fat in males.

The present study examined the validity of selected bioelectrical impedance (BIA) equations for estimating percent fat (% fat) in males and compared their validity with that of a commonly used skinfold equation (Sum3). One-hundred twenty-two Caucasian males (X +/- SD = 12.5 +/- 5.8% fat, as determined by underwater weighing) served as subjects. Selection of the BIA equations was dependent upon meeting at least one of three criteria: 1) developed from a previous interlaboratory investigation, 2) derived on a large sample size (> 200), or 3) previously been shown to accurately estimate body composition when cross-validated against a criterion method. Cross-validation analyses included examination of the constant error, standard error of estimate (SEE), r, and total error (TE). The Sum3 equation which resulted in the lowest SEE and TE values (2.6% fat and 3.4% fat, respectively) and the highest validity coefficient (r = 0.90, P < 0.001), most accurately estimated % fat and, therefore, was recommended over BIA equations for estimating body composition in Caucasian males with lean to average body fatness. The fat-specific interlaboratory BIA equation of Segal et al. for males < or = 20% fat (N = 107) which resulted in a TE value of 3.6% fat and the generalized equation of Guo et al. (TE = 4.1% fat) may, however, be considered as acceptable alternatives.

Adipose Tissue↗
PubMed1996

Age, fat-free weight, and isokinetic peak torque in high school female gymnasts.

The purpose of this study was to examine the covariate influence of fat-free weight (FFW) on age-related increases in isokinetic peak torque for leg flexion and extension in high school female gymnasts. Seventy-two gymnasts (X age +/- SD = 15 7 +/- 1.2 yr) volunteered to be measured for isokinetic leg flexion and extension strength using a calibrated Cybex II dynamometer at 30, 180, and 300 degrees*s(-1) as well as for body composition from underwater weighing. The results indicated that there were significant (P < 0.05) zero-order correlations for age versus leg flexion (r = 0.36-0.47) and extension (r = 0.51-0.57) peak torque, as well as FFW versus leg flexion (r = 0.50-0.66) and extension (r = 0.620.73) peak torque. There were also significant (P < 0.05) first-order partial correlations between age and peak torque (covaried for FFW) for leg extension at 30 (r = 0.25), 180 (r = 0.36-0.39), and 300 degrees*s(-1) (r = 0.25-0.28) but not for leg flexion. These findings indicated that for the high school female gymnasts in the present study, there were age-related increases in strength that could not be accounted for by changes in FFW. It is possible that factors such as an increase in muscle mass per unit of FFW and/or neural maturation contribute to strength increases during adolescence in female athletes.

Adolescent↗
PubMed1996

Validity of skinfold estimates of percent fat in high school female gymnasts.

A preseason estimation of body composition may be useful for assigning a safe minimal body weight for female gymnasts. The present investigation examined the validity of 11 skinfold equations for predicting percent body fat (%fat) in high school female gymnasts (X age +/- SD = 15.7 +/- 1.2 yr) by comparing the values with those obtained from underwater weighing. Seventy-three gymnasts (X %fat +/- SD = 18.6 +/- 4.5%fat) volunteered to serve as subjects. The statistical analyses included examination of the constant error (CE), standard error of estimate (SEE), correlation coefficient (r), and total error (TE). The results of this investigation indicated that 7 of the 11 equations resulted in TE values that were < or = 3.9%fat (range, 3.3-3.9%fat). Of these, the quadratic sum-of-three skinfold equation of Thorland et al. (37) satisfies the most cross-validation criteria and, therefore, is recommended for estimating body composition and minimal body weight in high school female gymnasts. The other six equations with TE values of < or = 3.9%fat should be considered acceptable alternatives.

Adipose Tissue↗
PubMed1996

Neuromuscular fatigue thresholds of the vastus lateralis, vastus medialis and rectus femoris muscles.

The purpose of this study was to compare the power outputs associated with the onset of neuromuscular fatigue in the superficial muscles of the quadriceps femoris group by simultaneously monitoring the EMG activity from the vastus lateralis (VL), vastus medialis (VM) and rectus femoris (RF) muscles during a Physical Working Capacity at the Fatigue Threshold (PWCFT) test. Eleven adult males (mean +/- SD = 24 +/- 3y) served as subjects. There were no significant (p > 0.05) differences among the mean PWCFT values for the VL (226 +/- 58 W), VM (223 +/- 58 W) and RF (203 +/- 54 W) muscles. These findings indicates that the superficial muscles of the quadriceps femoris group generally responded as a unit with respect to the onset (threshold) of neuromuscular fatigue as measured by the PWCFT test.

Adult↗
PubMed1995

Electromyographic fatigue thresholds of the superficial muscles of the quadriceps femoris.

The purpose of this investigation was to compare the thresholds of neuromuscular fatigue determined simultaneously from the vastus lateralis (VL), vastus medialis (VM) and rectus femoris (RF) muscles using the electromyographic fatigue threshold (EMGFT) test. Eight adult volunteers [mean (SD) age, 33 (10) years] served as subjects for this investigation. The results of a one-way repeated measured ANOVA indicated that there was a significant (P < 0.05) difference among the mean EMGFT values for the VL [248(31)W], VM [223(43)W] and RF [220(30)W] muscles. Tukey post-hoc comparisons indicated that the EMGFT for the RF was significantly (P < 0.05) lower than that of the VL. These findings suggested that during cycle ergometry there is a dissociation in neuromuscular fatigue characteristics of the superficial muscles of the quadriceps femoris group.

Adult↗
PubMed1995

Effects of unilateral isometric strength training on joint angle specificity and cross-training.

The purpose of this study was to examine the effects of unilateral isometric leg extension strength training on the strength and integrated electromyogram (IEMG) of both the trained and untrained limbs at multiple joint angles. A training (TRN) group [nine women; mean (SD) age, 20(1) years] exercised for 6 weeks with isometric leg extensions at 80% of maximal isometric torque. A control (CTL) group [eight women; 21(1) years] did not exercise. The training was performed three times per week on a Cybex II isokinetic dynamometer at a joint angle where the lever arm was 0.79 rad below the horizontal plane. The subjects were tested pre- and posttraining for maximal unilateral isometric torque in both limbs at joint angles of zero, 0.26, 0.79, 1.31, and 1.57 rad below the horizontal plane. Bipolar surface electrodes were used to record the IEMG of the vastus lateralis (VL) and vastus medialis (VM) during the isometric tests. Three univariate (torque, IEMG-VL, and IEMG-VM) four-way (group x time x limb x angle) mixed factorial ANOVAs were used to analyze the data. The results indicated joint angle specificity for isometric torque in the TRN group only, with significant increases in torque at 0.79 (P = 0.0004) and 1.31 (P = 0.0039) rad. No significant increases in torque were found in the untrained limb of the TRN group or in either limb of the CTL group. Similarly, there were no significant changes in IEMG as a result of the training for the VL or VM.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1995

Electromyographic responses at the neuromuscular fatigue threshold.

The purposes of this investigation were to: (a) examine the electromyographic (EMG) responses from the vastus lateralis during a continuous ride to exhaustion at the neuromuscular fatigue threshold (EMGFT) and (b) determine what percentage of the power output at VO2max the EMGFT represents. Fifteen adult males (mean +/- SD = 22 +/- 2 years) volunteered to perform an EMGFT test and a continuous ride to exhaustion at the EMGFT on separate days. A subsample of ten subjects completed an incremental cycle ergometer test for the determination of VO2max. The mean (+/- SD) EMGFT for the total sample (n = 15) was 286 +/- 40W and the mean time to exhaustion at the EMGFT was 250 +/- 85 sec. For each subject, the slope of the iEMG versus time relationship during the continuous ride to exhaustion at the EMGFT was significantly (p < 0.05) greater than zero slope. For the subsample (n = 10), the mean EMGFT (296 +/- 45W) was not significantly (p > 0.05) different from the power output at VO2max (282 +/- 47W). These findings did not support the contention that the EMGFT can be sustained without electromyographic evidence of neuromuscular fatigue and indicated that the EMGFT was comparable to the power output at VO2max.

Adult↗
PubMed1995

Anthropometric estimation of thigh muscle cross-sectional area.

The purpose of this investigation was to derive and validate circumference and skinfold equations for estimating the anatomical cross-sectional area (CSA) of the quadriceps, hamstrings, and total thigh muscles. Forty-three adult male (mean age +/- SD = 25 +/- 5 yr) volunteers underwent magnetic resonance imaging (MRI) to determine the CSA of the thigh muscles at the midfemur level as well as midthigh circumference and anterior thigh skinfold assessment. Multiple regression analyses were used to derive equations for predicting quadriceps, hamstrings, and total thigh muscle CSA of the dominant limb from the anthropometric dimensions on a random sample of 30 of the subjects. Cross-validation (CV) analyses were performed for each equation on: (a) the nondominant thigh of the derivation group (N = 30); (b) the dominant thigh of the CV group (N = 13); and (c) the nondominant thigh of the CV group (N = 13). The CV total error values for the quadriceps, hamstrings, and total thigh muscle CSA ranged from 5.4 to 14.4, 3.3 to 5.5, and 10.0 to 25.4 cm2, respectively. The anthropometric equations are recommended for one-time estimates of muscle CSA values in healthy, well-nourished young adult males when more sophisticated procedures are not available.

Adult↗