Modulation of the tibial nerve H-reflex by mechanical stimulation of afferents in the lumbar spine.

The purpose of this study was to describe the convergence of afferent discharges from the ligament-muscular system of the lumbar spine to the segmentally-related gastrocnemius muscle. The subjects were 32 healthy, young volunteers recruited from a college student population. Afferent discharges from the ligament-muscular system of the lumbar spine were evoked by manually moving the trunk into either flexion (n = 16) or left lateral bending (n = 16) using a multi-directional adjustable treatment table (Zenith Cox Flexion Table). Using linear potentiometers affixed to the treatment table and interfaced with a computer data acquisition system, manual movements of the table were visually guided to generate passive trunk movements at velocities of 10 degrees, 20 degrees, and 40 degrees per second. Tibial nerve H-reflex responses were recorded from the right gastrocnemius muscle as the trunk approached its end range of motion. Regardless of velocity for the flexion movement, the H/M(Max) ratio significantly decreased from 28.0% to 20.9% (p < .05). During lateral bending, the H/M(Max) ratio significantly decreased from 27.4% to 24.0% at velocities of 10 degrees and 20 degrees per second (p < .05) with a subsequent decrease to 20.5% at a velocity of 40 degrees per second (p < .05). The nature of these decreases in the H/M(Max) ratios across the different velocities during lateral bending significantly departed from linearity (p < .05). These data provide sufficient evidence to suggest that heteronymous conditioning effects from the ligament-muscular system of the lumbar spine during passive trunk movements attenuate alpha motoneuronal activity of the segmentally-related gastrocnemius muscle.

The short golf backswing: effects on performance and spinal health implications.

BACKGROUND: Full recoil golf swings have been implicated in back pain and injury in golfers. Evidence suggests that a restricted backswing may reduce the potential for injury without compromising performance. OBJECTIVE: To examine both golf swing performance and selected muscular actions of the trunk and shoulder during a full recoil swing as compared with a modified short backswing. METHODS: Electromyographic (EMG) recordings were taken bilaterally from the lumbar, external oblique, latissimus dorsi, and right pectoral muscles in 7 golfers during a full recoil swing and a modified short backswing. High-speed videotape was used to measure back swing angle reduction. Clubhead velocity (CHV) and ball-contact accuracy were quantified by using a swing speed indicator and clubface contact tape, respectively. RESULTS: Shortening of the backswing by 46.5 degrees +/- 24.7 degrees had no effect on stroke accuracy as measured by mean deviation from the target spot on the club (19.0 +/- 7.8 mm vs 19.3 +/- 9.2 mm). CHV was not significantly reduced (33.9 +/- 2.5 m/s vs 31.2 +/- 2.2 m/s). However, EMG root-mean-square was decreased 19% in the right oblique muscle from 750 to 250 ms before impact (P < .05). During the acceleration phase, activation of left lumbar muscle decreased by 12%, whereas activation of right latissimus muscle increased by 21%. Although left lumbar muscle activity during the follow-through increased 14%, there was a substantial (17%) but nonsignificant decrease of activation of trunk muscles (P = .11). There was a general trend toward an increased activation of the shoulder musculature from 250 ms before impact to 500 ms after impact. CONCLUSION: These data support the idea that short backswings in golf may reduce trunk muscle activation and possibly reduce back injury and pain without negatively impacting swing accuracy or CHV. However, the short swing increases shoulder muscle activation and may, in turn, promote risk for shoulder injury.

Comparison of effects of spinal manipulation and massage on motoneuron excitability.

The purpose of this study was to compare the magnitude and duration of motoneuron inhibition occurring as a sequel to spinal manipulation or paraspinal and limb massage. The physiologic mechanisms involved in spinal manipulative therapy and massage therapy are largely unknown. One possible hypothesis is based upon the theory that these two distinct and different modalities may attenuate the activity of alpha motoneurons. Both modalities have been reported to produce short-term inhibition of motoneurons. Asymptomatic volunteers were randomly assigned to either a spinal manipulation, massage, or control group. Baseline tibial nerve H-reflex amplitudes were obtained prior to the application of either lumboscaral spinal manipulation or paralumbar and limb massage. Post-interventional H-reflex recordings were recorded immediately following the application of either modality. Spinal manipulation significantly (p < 0.05) attenuated alpha motoneuronal activity immediately post-therapy, as measured by the amplitude of the tibial nerve H-reflex. Massage subjects exhibited no significant reduction in motoneuronal activity immediately following administration. Spinal manipulation produced a transient attenuation of alpha motoneuronal excitability. Paraspinal and limb massage did not inhibit the motoneuron pool as measured immediately post-therapy. These findings support the supposition that spinal manipulation procedures lead to short-term inhibitory effects on motoneuron excitability to a greater magnitude than massage.

Spinal reflex attenuation associated with spinal manipulation.

STUDY DESIGN: This study evaluated the effect of lumbosacral spinal manipulation with thrust and spinal mobilization without thrust on the excitability of the alpha motoneuronal pool in human subjects without low back pain. OBJECTIVES: To investigate the effect of high velocity, low amplitude thrust, or mobilization without thrust on the excitability of the alpha motoneuron pool, and to elucidate potential mechanisms in which manual procedures may affect back muscle activity. SUMMARY OF BACKGROUND DATA: The physiologic mechanisms of spinal manipulation are largely unknown. It has been proposed that spinal manipulation may reduce back muscle electromyographic activity in patients with low back pain. Although positive outcomes of spinal manipulation intervention for low back pain have been reported in clinical trials, the mechanisms involved in the amelioration of symptoms are unknown. METHODS: In this study, 17 nonpatient human subjects were used to investigate the effect of spinal manipulation and mobilization on the amplitude of the tibial nerve Hoffmann reflex recorded from the gastrocnemius muscle. Reflexes were recorded before and after manual spinal procedures. RESULTS: Both spinal manipulation with thrust and mobilization without thrust significantly attenuated alpha motoneuronal activity, as measured by the amplitude of the gastrocnemius Hoffmann reflex. This suppression of motoneuronal activity was significant (P < 0.05) but transient, with a return to baseline values exhibited 30 seconds after intervention. CONCLUSIONS: Both spinal manipulation with thrust and mobilization without thrust procedures produce a profound but transient attenuation of alpha motoneuronal excitability. These findings substantiate the theory that manual spinal therapy procedures may lead to short-term inhibitory effects on the human motor system.

The effect of activity history and current activity on static and dynamic postural balance in older adults.

The purpose of this study was to investigate the effects of former athleticism and current activity status on static and dynamic postural balance in older adults. Fifty-six subjects participated in four study groups including former athletes, currently active (AA; n = 15; 69.1+/-4.4 years.; 77.8+/-9.8 kg), former athletes, currently inactive (AI; n = 12; 66.7 years.; 87.2+/-15.1 kg), controls currently active (CA; n = 14; 68.6 +/- 4.5 years.; 73.9+/-15 kg), and controls currently inactive (CI; n = 15; 72.8+/-4.8 years; 81.1+/-14.8). All subjects were tested for height, weight, flexibility, thigh circumference, and static (sharpened Romberg/unipedal stance), and dynamic (step length and width) balance tests. The sharpened Romberg (eyes open) test showed that AA (60.0+/-0 s) and CA (59.4+/- 0.5 s) balanced significantly longer than AI (41.5+/-7.2 s), and CI (41.8+/-6.1 s) (p<0.05). The unipedal (eyes open) test balance scores for AA, CA, AI, and CI were respectively 40.0+/-4.5, 55.1+/- 3.4, 33.0+/-7.1, and 27.5+/-6.1 s, with CA significantly better than CI (p<0.05). In dynamic balance AA and CA (746.1+/-28.0 and 724.6+/-24.3 mm) showed significantly longer step lengths (p<0.05) than CI (643.7+/-26.5 mm). The eyes closed test results for relative group comparisons were similar. Overall, two-way analysis of variance showed a significant activity main effect for all dependent variables measured (p<0.05). The results indicated that current activity status plays a key role on balance performance in older adults. Furthermore, former athletic activity history provides no protection for the age related onset of postural imbalance.

Hemodynamic responses to the stroop and cold pressor tests after submaximal cycling exercise in normotensive males.

The effect of 30 min of cycling exercise at 60% VO2max on hemodynamic responses to the Stroop and cold pressor tests in 12 normotensive males was examined. Subjects were randomly assigned in a counterbalanced design to perform the stressors pre- and postexercise and served as their own controls. Cardiac output (CO), heart rate (HR), and stroke volume were measured continuously by impedance cardiography. Blood pressure was measured beat to beat using a photoplethysmographic volume transducer. Total peripheral resistance (TPR) was calculated. The systolic blood pressure response (elevation) to the Stroop test was significantly attenuated postexercise (4.2 +/- 1.4%, p < 0.05) as compared to preexercise and control (10.1 +/- 1.8%). Postexercise HR reactivity was also significantly attenuated to the Stroop test postexercise (0.3 +/- 1.7%) compared to preexercise and control (6.5 +/- 2.3%, p < 0.05). Hemodynamic variables among treatment groups, with the possible exception of HR, appeared to be unaffected during the cold pressor postexercise. Neither central (i.e., CO) nor peripheral (i.e., TPR) responses appeared to be solely responsible for the attenuated blood pressure response to the Stroop stressor.

Blood pressure responses to acute exercise in type A and B females and males.

Cardiovascular responses to moderate steady-state exercise were studied in Type A and Type B males and females. Eighteen men and 17 women, ages 20 to 40 years, were chosen for Type A (males = 10, females = 10) and Type B (males = 8, females = 7) groups by means of the Jenkins Activity Survey (JAS). Subjects exercised on a electronic cycle ergometer at 60% O2peak for 20 min. Repeated measures analysis of variance (ANOVA) showed a significant Time (F(8,17) = 16.1, p < 0.001) and Group x Sex interaction (F(1,24) = 7.84, p < 0.01), with no differences evident between the male systolic blood pressures (SBPs), but an elevation in the SBP of the Type A female subject. SBP between all males and females were also significantly different (F(1,24) = 68.8, p < 0.0001). Males showed a greater SBP throughout the resting, exercise, and recovery time intervals when compared to females as a group, diastolic blood pressures (DBP) showed a significant Time effect (F(8,17) = 11.8, p < 0.0001) with a significant Time x Group x Sex interaction (F(8,17) = 3.1, p < 0.02). Significant Sex effects (F(1,24) = 12.4, p < 0.002) and a marginal Group x Sex interaction were also observed (F(1,24) = 3.67, p < 0.067), with Type B females having the greatest reduction in DBP at 15 min (59.0 +/- 5.6 vs. 71.7 +/- 3.2 mmHg) and 20 min (50.0 +/- 3.3 vs. 67.3 +/- 3.9 mmHg) or exercise when compared to Type As. There were significant DBP differences between males and females throughout rest, at 5 and 20 min of exercise, and in both recovery time periods. The data only reveal significant differences in DBP between Type A and B males at 20 min of exercise [64.2 +/- 3.3 (A) and 72.6 +/- 1.2 (B) mmHg]. No significant Group main effect was evident. These data suggest that hemodynamic and other physiological responses to exercise in females, particularly Type A, vary from males according to behavioral types associated with differing physiology.

Comparison of anaerobic components of the Wingate and Critical Power tests in males and females.

The purpose of the study was to reexamine the relationship between the Wingate and Critical Power tests of anaerobic capacity (AC) and anaerobic reserve (AR), respectively. A second purpose was to observe gender differences. Both tests were administered to 16 female and 13 male subjects (N = 29) on a Monark cycle ergometer with six subjects repeating AR measurement. The results show that AC (240.2 +/- 30.5 J-kg-1, calculated from total work for 30 s) and AR (184.0 +/- 1.2 J.kg-1) were not well-correlated (r = 0.07, P > 0.72). When expressed as total energy independent of body mass, the relationship was significant but low (r = 0.41, P > 0.02). Since AR was 23% lower than AC, which is believed to underestimate true anaerobic capacity, the data suggest that the Critical Power and Wingate tests do not assess the same anaerobic compartments. AR from the Critical Power test may not include the energy component of anaerobic glycolysis. Therefore, intrinsic methodological and theoretical differences between the tests make the absolute comparison of AC and AR problematic.

The effect of sleep deprivation and exercise load on isokinetic leg strength and endurance.

Isokinetic leg strength and fatigue were measured in 24 male U.S. Marine Corps volunteers in a simulated sleep loss and unusually heavy work scenario. Knee extension and flexion peak torque (PT) were measured at three isokinetic speeds (1.57, 2.62 and 3.66 rad.s-1) followed by 45 consecutive maximal reciprocal contractions at 3.14 rad.s-1 to measure fatigue index (FI). All subjects were retested 2 days later following 30-h sleep deprivation (SD). The exercise group (n = 12) spent 25 1-h sessions performing computer tasks, filling out questionnaires and walked 1.61 km with a 50% gross body mass pack load, during each of the 25 sessions. The control group (n = 12) did likewise but did not exercise. Repeated measures ANOVA indicated that flexion PT at 1.57 rad.s-1 decreases (P < 0.013) after SD. Exercise did not affect FI but did decrease PT. It was concluded that carrying a 50% load produces decrements in PT for both extension and flexion but more so for flexion. SD affected PT but had no effect on FI.

The effects of strength training and centrifuge exposure on +Gz tolerance.

Weight training (WT) as a countermeasure to +Gz intolerance seems to be an accepted modality, but lacks convincing support in the research literature. This study examines factors associated with WT in an operational environment as a countermeasure to +Gz intolerance. Fourteen naval aviators were assigned to a WT group (n = 7) and a non-weight-trained control (NWT) group (n = 7). All subjects were tested before and after a 10-week training period for aerobic capacity (VO2max), dynamic muscular strength (Cybex II), body fat, gradual onset rate (GOR) for +Gz at a rate of +1 G.15 s-1, and finally a simulated air combat maneuver (SACM) consisting of 15 s at +4.5 Gz and +7 Gz alternating challenges to exhaustion. The results indicate the following: 1) no significant differences in any of the centrifuge variables measured between WT and NWT groups while both groups showed a time or learning effect in SACM from 175 +/- 109 to 239 +/- 110 s (grouped data df = 16, t = 2.97, p < 0.09); 2) no improvement in SACM time for subjects achieving a 300-s SACM pretest value (n = 4); and 3) for SACM times below 300 s, the WT subjects showed a greater improvement than NWT subjects (n = 11, F = 9.12, p < 0.02). The data suggest that although some subjects showed considerable strength gains, WT in a naval aviation operational setting may not provide a universal desired effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Isokinetic strength and endurance during 30-day 6 degrees head-down bed rest with isotonic and isokinetic exercise training.

The purpose of our study was to determine if an intensive, intermittent, isokinetic, lower extremity exercise training program would attenuate or eliminate the decrease of muscular strength and endurance during prolonged bed rest (BR). The 19 male subjects (36 +/- 1 yr, 178 +/- 2 cm, 76.5 +/- 1.7 kg) were allocated into a no exercise (NOE) training group (N = 5), an isotonic (lower extremity cycle ergometer) exercise (ITE) training group (N = 7), and an isokinetic (isokinetic knee flexion-extension) exercise (IKE) training group (N = 7). Peak knee (flexion and extension) and shoulder (abduction-adduction) functions were measured weekly in all groups with one 5-repetition set. After BR, average knee extension total work decreased by 16% with NOE, increased by 27% with IKE, and was unchanged with ITE. Average knee flexion total work and peak torque (strength) responses were unchanged in all groups. Force production increased by 20% with IKE and was unchanged with NOE and ITE. Shoulder total work was unchanged in all groups, while gross average peak torque increased by 27% with ITE and by 22% with IKE, and was unchanged with NOE. Thus, while ITE training can maintain some isokinetic functions during BR, maximal intermittent IKE training can increase other functions above pre-BR control levels.

Blood pressure responses to acute exercise in type A females.

Cardiovascular responses to moderate steady state exercise were studied in type A females. Twenty-one women, ages 18 to 43 years (mean 29.7 +/- 2.0), were divided into type A (n = 11) and type B (n = 10) groups by means of the Jenkins Activity Survey (JAS). Baseline measurement included EMG, BP, and psychometric data. Subjects exercised on a treadmill at 60% VO2 peak for 20 min. Postexercise systolic and diastolic BP (SBP, DBP) reductions were similar in the type A and type B groups [111/72 to 104/66 and 108/70 to 98/62 mmHg, respectively, (F(1, 38) = 4.03, p < 0.05]. However, DBP responses during exercise for type B, but not the type A, were lower [71 +/- 2 to 60 +/- 2 mmHg, preexercise to steady state exercise, (F(1, 38) = 7.14, p < 0.01]. Also, steady state exercise SBP was lower in type B compared to type A [159 +/- 3 vs. 150 +/- 4 mmHg, (F(1, 38) = 3.97, p < 0.05]. These data with psychometric data, suggest that hemodynamic and other physiological responses to exercise in females vary according to behavioral types associated with differing physiology.

Aerobic fitness and opioidergic inhibition of cardiovascular stress reactivity.

The role of endogenous opioids in aerobic fitness-induced decrements in cardiovascular stress reactivity was examined by comparing the effects of opioid antagonism with naltrexone on responses to stress in young adults with high versus low levels of aerobic fitness. Two hundred forty subjects were given an activity questionnaire and males with the highest (Fit) and lowest (Nonfit) aerobic activity profiles were recruited for maximal oxygen consumption (VO2max) treadmill testing and psychological stress testing (final sample N = 28). Heart rate and blood pressures were measured during performance on a computer-controlled arithmetic task after pretreatment with either naltrexone (Trexan, DuPont) or a placebo. During placebo challenges, Fit subjects, compared with Nonfit, showed lower heart rate reactivity during stress and lower mean arterial blood pressures immediately before and during recovery from stress. Naltrexone eliminated these reactivity differences by increasing heart rate reactivity and raising mean arterial blood pressure in Fit subjects. These data suggest that aerobic fitness is associated with enhanced opioidergic inhibition of circulatory stress reactivity. Opioidergic modulatory effects on stress reactivity may comprise an important mechanism in fitness-associated risk reduction for cardiovascular disease.

Effect of downhill running on motoneuron pool excitability.

The purpose of this study was to compare alterations in motoneuron pool excitability after eccentric-biased (ECC-B) downhill running exercise with non-biased (NO-B) level running exercise. Six male subjects (25-34 yr) participated in the study, which included ECC-B exercise (-10% grade) and NO-B exercise (0% grade) at 50% of maximal O2 uptake for 20 min. The control trial consisted of 20 min of quiet rest with all subjects participating in all conditions (repeated measures). Motoneuron pool excitability was measured by the Hoffman reflex (H-wave), which was expressed as a ratio (H/M ratio) of the maximal electrically stimulated muscle action potential (M-wave). NO-B exercise resulted in a 9.3 +/- 2.7% (SE) reduction in the H/M ratio. ECC-B exercise resulted in a 24.6 +/- 5.7% reduction in the ratio (P less than 0.05 for both). The two exercise treatment conditions were also significantly different from one another (P less than 0.05). Twenty-four-hour postexercise H/M ratios were similar to baseline (P greater than 0.05). Postexercise subjective muscle soreness assessment (DOMS) produced significant increases in DOMS of 36 and 166% immediately and 24 h after exercise, respectively, for the ECC-B trial only (P less than 0.001). The data show that ECC-B exercise results in greater postexercise H/M ratio reductions than NO-B exercise and that H/M ratio changes post-ECC-B exercise are not solely associated with DOMS.

Exercise-training protocols for astronauts in microgravity.

The question of the composition of exercise protocols for use by astronauts in microgravity is unresolved. Based on our knowledge of physical working requirements for astronauts during intra- and extravehicular activity and on the findings from bed-rest studies that utilized exercise training as a countermeasure for the reduction of aerobic power, deterioration of muscular strength and endurance, decrements in mood and cognitive performance, and possibly for bone loss, two exercise protocols are proposed. One assumes that, during microgravity, astronaut exercise physiological functions should be maintained at 100% of ground-based levels; the other assumes that maximal aerobic power in flight can be reduced by 10% of the ground-based level. A recommended prescription for in-flight prevention or partial suppression of calcium (bone) loss cannot be written until further research findings are obtained that elucidate the site, the magnitude, and the mechanism of the changes. Hopefully these proposed exercise prescriptions will stimulate further research and discussion resulting in even more efficient protocols that will help ensure the optimal health and well-being of our astronauts.

Body composition in paraplegic male athletes.

The body composition and anthropometric characteristics of male paraplegic athletes (PARA, N = 22) were contrasted to an able-bodied ectomorphic (N = 22) and mesomorphic (N = 31) comparison group of moderately and highly trained male subjects. The validity of 12 body composition [density (Db)] prediction equations reported in the literature, 4 generalized, were determined (tested) on this special group of athletes (PARA). On the whole, the prediction equations over-predicted Db in PARA by 0.0039 to 0.0166 g X cm-3 (under-predicted relative fat by 1.8 to 7.4%). Five diameter, 11 circumference, and 7 skinfold measures were used in a SAS-STEPWISE multiple regression procedure with hydrostatically determined Db to develop several suitable Db prediction equations for the paraplegic athlete. Diameters were poor predictors (r = 0.60, SEE = 0.0164), while skinfolds, circumferences, or a combination of measures were acceptable, with the combined equation being best (r = 0.95, SEE = 0.0064). The findings of this study suggest that even generalized equations do not adequately predict Db in PARA and that paraplegic specific equations are presently best suited for predicting Db in paraplegic athletes. The results further indicate that although these equations meet many of the criteria of Lohman, the SEE and total error values are unusually high and make prediction of body composition using anthropometry in a heterogeneous group of PARA athletes slightly unreliable.

Motor neuron excitability: the Hoffmann reflex following exercise of low and high intensity.

The current exercise literature lacks physiological evidence for a stress reduction induced by high intensity aerobic work. The purpose of this study was to investigate the effectiveness of high intensity aerobic work on stress reduction and to evaluate the currently tenable hypotheses in view of the data on the response to high intensity work. The subjects participated in a control trial and two 20-min treadmill exercise trials at low and high (40% and 75% VO2max) intensity. The H- and M-wave responses to artificial stimulation of the tibial nerve (Angel and Hofmann, 1963) were used to assess spinal cord activation level which constituted the major dependent variable in the study. Low intensity exercise showed a 12.8% reduction in the H max/M max ratio (P less than 0.05) while high intensity exercise reduced the H max/M max ratio by 21.5% (P less than 0.0002). The two exercise treatment levels were also significantly different from each other (P less than 0.02). These data provide objective evidence for the efficacy of high intensity exercise in stress reduction as measured by the Hoffmann reflex and provide indirect evidence for a possible thermal hypothesis for exercise-induced stress reduction.

Physical training and +Gz tolerance reevaluated.

The effect of physical training on +Gz tolerance is of vital interest in the aerospace community. The data on the effect of physical training on orthostatic tolerance or simulated air combat maneuvers is equivocal. The effects of aerobic and strength training programs is briefly reviewed. The data suggest a need for careful reinterpretation of research results in light of conflicting reports and methodological shortcomings. Aerobic training cannot be assumed to always be detrimental nor can strength training be assumed to be universally effective in improving +Gz tolerance. In selecting appropriate screening criteria and training regimens for aircraft personnel, it seems prudent to reinvestigate strength and endurance training effects on +Gz tolerance using multivariate research paradigms. Special attention should be directed to commonly accepted physiological principles which may vary under conditions of altered gravitation.