Estimated age of first exposure to American football and outcome from concussion

OBJECTIVE

To examine the association between estimated age at first exposure (eAFE) to American football and clinical measures throughout recovery following concussion.

METHODS

Participants were recruited across 30 colleges and universities as part of the National Collegiate Athletic Association (NCAA)-Department of Defense Concussion Assessment, Research and Education Consortium. There were 294 NCAA American football players (age 19 ± 1 years) evaluated 24-48 hours following concussion with valid baseline data and 327 (age 19 ± 1 years) evaluated at the time they were asymptomatic with valid baseline data. Participants sustained a medically diagnosed concussion between baseline testing and postconcussion assessments. Outcome measures included the number of days until asymptomatic, Immediate Post-Concussion Assessment and Cognitive Testing (ImPACT) composite scores, Balance Error Scoring System (BESS) total score, and Brief Symptom Inventory 18 (BSI-18) subscores. The eAFE was defined as participant's age at the time of assessment minus self-reported number of years playing football.

RESULTS

In unadjusted regression models, younger eAFE was associated with lower (worse) ImPACT Visual Motor Speed (R ² = 0.031, p = 0.012) at 24-48 hours following injury and lower (better) BSI-18 Somatization subscores (R ² = 0.014, p = 0.038) when the athletes were asymptomatic. The effect sizes were very small. The eAFE was not associated with the number of days until asymptomatic, other ImPACT composite scores, BESS total score, or other BSI-18 subscores.

CONCLUSION

Earlier eAFE to American football was not associated with longer symptom recovery, worse balance, worse cognitive performance, or greater psychological distress following concussion. In these NCAA football players, longer duration of exposure to football during childhood and adolescence appears to be unrelated to clinical recovery following concussion.

King-Devick Test Reliability in National Collegiate Athletic Association Athletes: A National Collegiate Athletic Association-Department of Defense Concussion Assessment, Research and Education Report

CONTEXT

The King-Devick (KD) test has received considerable attention in the literature as an emerging concussion assessment. However, important test psychometric properties remain to be addressed in large-scale independent studies.

OBJECTIVE

To assess (1) test-retest reliability between trials, (2) test-retest reliability between years 1 and 2, and (3) reliability of the 2 administration modes.

DESIGN

Cross-sectional study.

SETTING

Collegiate athletic training facilities.

PATIENTS OR OTHER PARTICIPANTS

A total of 3248 intercollegiate student-athletes participated in year 1 (male = 55.3%, age = 20.2 ± 2.3 years, height = 1.78 ± 0.11 m, weight = 80.7 ± 21.0 kg) and 833 participated in both years.

MAIN OUTCOME MEASURE(S)

Time, in seconds, to complete the KD error free. The KD test reliability was assessed between trials and between annual tests over 2 years and stratified by test modality (spiral-bound cards [n = 566] and tablet [n = 264]).

RESULTS

The KD test was reliable between trials (trial 1 = 43.2 ± 8.3 seconds, trial 2 = 40.8 ± 7.8 seconds; intraclass correlation coefficient [ICC] (2,1) = 0.888, P < .001), between years (year 1 = 40.8 ± 7.4 seconds, year 2 = 38.7 ± 7.7 seconds; ICC [2,1] = 0.827, P < .001), and for both spiral-bound cards (ICC [2,1] = 0.834, P < .001) and tablets (ICC [2,1] = 0.827, P < .001). The mean change between trials for a single test was -2.4 ± 3.8 seconds. Although most athletes improved from year 1 to year 2, 27.1% (226 of 883) of participants demonstrated worse (slower) KD times (3.2 ± 3.9 seconds) in year 2.

CONCLUSIONS

The KD test was reliable between trials and years and when stratified by modality. A small improvement of 2 seconds was identified with annual retesting, likely due to a practice effect; however, 27% of athletes displayed slowed performance from year 1 to year 2. These results suggest that the KD assessment was a reliable test with modest learning effects over time and that the assessment modality did not adversely affect baseline reliability.

Relationship Between the King-Devick Test and Commonly Used Concussion Tests at Baseline

CONTEXT

Comprehensive assessments are recommended to evaluate sport-related concussion (SRC). The degree to which the King-Devick (KD) test adds novel information to an SRC evaluation is unknown.

OBJECTIVE

To describe relationships at baseline among the KD and other SRC assessments and explore whether the KD provides unique information to a multimodal baseline concussion assessment.

DESIGN

Cross-sectional study.

SETTING

Five National Collegiate Athletic Association institutions participating in the Concussion Assessment, Research and Education (CARE) Consortium.

PATIENTS OR OTHER PARTICIPANTS

National Collegiate Athletic Association student-athletes (N = 2258, age = 20 ± 1.5 years, 53.0% male, 68.9% white) in 11 men's and 13 women's sports.

MAIN OUTCOME MEASURE(S)

Participants completed baseline assessments on the KD and (1) the Symptom Inventory of the Sport Concussion Assessment Tool-3rd edition, (2) the Brief Symptom Inventory-18, (3) the Balance Error Scoring System, (4) the Standardized Assessment of Concussion (SAC), (5) the Immediate Post-Concussion Assessment and Cognitive Testing (ImPACT) test battery, and (6) the Vestibular/Ocular Motor Screening tool during their first year in CARE. Correlation coefficients between the KD and the 6 other concussion assessments in isolation were determined. Assessments with ρ magnitude >0.1 were included in a multivariate linear regression analysis to evaluate their relative association with the KD.

RESULTS

Scores for SAC concentration, ImPACT visual motor speed, and ImPACT reaction time were correlated with the KD (ρ = -0.216, -0.276, and 0.164, respectively) and were thus included in the regression model, which explained 16.8% of the variance in baseline KD time (P < .001, Cohen f² = 0.20). Better SAC concentration score (β = -.174, P < .001), ImPACT visual motor speed (β = -.205, P < .001), and ImPACT reaction time (β = .056, P = .020) were associated with faster baseline KD performance, but the effect sizes were small.

CONCLUSIONS

Better performance on cognitive measures involving concentration, visual motor speed, and reaction time was weakly associated with better baseline KD performance. Symptoms, psychological distress, balance, and vestibular-oculomotor provocation were unrelated to KD performance at baseline. The findings indicate limited overlap at baseline among the CARE SRC assessments and the KD.

Neither total muscle activation nor co-activation explains the youthful walking economy of older runners

BACKGROUND

Older adults (≥65 years) exhibit greater metabolic rates during walking (worse walking economy) compared to young adults. Yet, previous research suggests that habitual running, but not habitual walking, exercise mitigates the age-related deterioration of walking economy.

RESEARCH QUESTION

Does total leg muscle activation and/or agonist-antagonist co-activation explain the superior walking economy of older runners versus older walkers?

METHODS

We quantified metabolic power, leg muscle activation, and co-activation in older walkers and older runners during walking at 0.75, 1.25, and 1.75 m/s.

RESULTS

While accounting for multiple comparisons, neither total stride (each speed p ≥ 0.024), stance- (each speed p ≥ 0.217), nor swing- (each speed p ≥ 0.170) phase EMG amplitude differed between older walkers and older runners at 0.75, 1.25, or 1.75 m/s. Stride averaged medial gastrocnemius and biceps femoris activation was lower in older runners than older walkers at 1.25 and 1.75 m/s (all p ≤ 0.025). We also calculated shank, thigh, and overall (shank and thigh) agonist-antagonist leg muscle co-activation over each stride, and the only difference between groups was a greater shank co-activation in older runners at 0.75 m/s (p = 0.024). Across groups, stride, stance-, and swing-phase total muscle activation positively correlated with gross metabolic power (R2 = 0.58-0.66; all p < 0.001). Paradoxically, across groups, stride, stance-, and swing-phase muscle co-activation indices were negatively correlated with gross metabolic power (R2 = 0.08-0.29; all p ≤ 0.007).

SIGNIFICANCE

Neither total leg muscle activation nor co-activation explains the superior walking economy of older runners versus older walkers.

Influences of Mental Illness, Current Psychological State, and Concussion History on Baseline Concussion Assessment Performance

BACKGROUND

A student-athlete's mental state, including history of trait anxiety and depression, or current psychological state may affect baseline concussion assessment performance.

PURPOSE

(1) To determine if mental illness (anxiety, depression, anxiety with depression) influences baseline scores, (2) to determine if psychological state correlates with baseline performance, and (3) to determine if history of concussion affects Brief Symptom Inventory-18 (BSI-18) subscores of state anxiety, depression, and somatization.

STUDY DESIGN

Cross-sectional study; Level of evidence, 3.

METHODS

A sample of 8652 collegiate student-athletes (54.5% males, 45.5% females) participated in the Concussion Assessment, Research and Education (CARE) Consortium. Baseline assessments included a demographic form, a symptom evaluation, Standardized Assessment of Concussion, Balance Error Scoring System, a psychological state assessment (BSI-18), and Immediate Post-concussion Assessment and Cognitive Test. Baseline scores were compared between individuals with a history of anxiety (n = 59), depression (n = 283), and anxiety with depression (n = 68) and individuals without a history of those conditions (n = 8242). Spearman's rho correlations were conducted to assess the relationship between baseline and psychological state subscores (anxiety, depression, somatization) (α = .05). Psychological state subscores were compared between individuals with a self-reported history of concussions (0, 1, 2, 3, 4+) using Kruskal-Wallis tests (α = .05).

RESULTS

Student-athletes with anxiety, depression, and anxiety with depression demonstrated higher scores in number of symptoms reported (anxiety, 4.3 ± 4.2; depression, 5.2 ± 4.8; anxiety with depression, 5.4 ± 3.9; no anxiety/depression, 2.5 ± 3.4), symptom severity (anxiety, 8.1 ± 9.8; depression, 10.4 ± 12.4; anxiety with depression, 12.4 ± 10.7; no anxiety/depression, 4.1 ± 6.9), and psychological distress in state anxiety (anxiety, 3.7 ± 4.7; depression, 2.5 ± 3.6; anxiety with depression, 3.8 ± 4.2; no anxiety/depression, 0.8 ± 1.8), depression (anxiety, 2.4 ± 4.0; depression, 3.2 ± 4.5; anxiety with depression, 3.8 ± 4.8; no anxiety/depression, 0.8 ± 1.8), and somatization (anxiety, 2.3 ± 2.9; depression, 1.8 ± 2.8; anxiety with depression, 2.2 ± 2.4; no anxiety/depression, 0.9 ± 1.7). A moderate positive relationship existed between all BSI-18 subscores and total symptom number (n = 8377; anxiety: r_s = 0.43, P < .001; depression: r_s = 0.42, P < .001; somatization: r_s = 0.45, P < .001), as well as total symptom severity (anxiety: r_s = 0.43, P < .001; depression: r_s = 0.41, P < .001; somatization: r_s = 0.45, P < .001). Anxiety, depression, and somatization subscores were greater among student-athletes that self-reported more concussions.

CONCLUSION

Clinicians should be cognizant that student-athletes with a history of trait anxiety, depression, and anxiety with depression may report higher symptom score and severity at baseline. Individuals with extensive concussion history may experience greater state anxiety, depression, and somatization.

Short-Term Immunosuppression Enhances Long-Term Survival of Bovine Chromaffin Cell Xenografts in Rat Cns

Xenogeneic donors, a largely untapped resource, would solve many of the problems associated with the limited availability of human donor tissue for neural transplantation. Previous work in our laboratory has revealed that xenografts of isolated bovine chromaffin cells survive transplantation into the periaqueductal gray (PAG) of immunosuppressed adult rats. Electron microscopic analysis reveals that graft sites contain healthy chromaffin cells, but do not contain host immune cells typical of graft rejection.

The aim of the current study was to assess the necessary conditions for long-term survival of bovine chromaffin cell xenografts in the central nervous system (CNS). In particular, the need for short-course vs. permanent immunosuppressive therapy with cyclosporine A (CsA) for the long-term survival of grafted bovine chromaffin cells was addressed. Grafts from animals receiving continuous CsA treatment for either 3, 6, or 12 wk contained large clumps of dopamines-β-hydroxylase (DBH) positive cells in contrast to the few surviving cells observed in nonimmunosuppressed animals. In addition, grafts from animals that had CsA treatment terminated at 3 or 6 wk contained similarly large clumps of DBH-positive cells. Furthermore, short-term immunosuppression (3 wk) appeared to enhance the long-term survival of grafted cells, since clumps of DBH staining cells could still be positively identified in the host PAG at least 1 yr after transplantation.

Complete rejection of graft tissue depends on several factors, such as blood–brain barrier integrity, the presence of major histocompatability complex (MHC) antigens in either the host or graft, and the status of the host immune system. By using a suspension of isolated bovine chromaffin cells, potential MHC antigen presenting cells, such as endothelial cells, are eliminated. In addition, CsA treatment may negate the immunologic consequences of increased blood–brain barrier permeability following surgical trauma by attenuating the host cell mediated response. In summary, long-term survival of isolated chromaffin cell xenografts in the rat CNS may be attained by a short-term course of CsA.

Older Runners Retain Youthful Running Economy despite Biomechanical Differences

PURPOSE

Sixty-five years of age typically marks the onset of impaired walking economy. However, running economy has not been assessed beyond the age of 65 yr. Furthermore, a critical determinant of running economy is the spring-like storage and return of elastic energy from the leg during stance, which is related to leg stiffness. Therefore, we investigated whether runners older than 65 yr retain youthful running economy and/or leg stiffness across running speeds.

METHODS

Fifteen young and 15 older runners ran on a force-instrumented treadmill at 2.01, 2.46, and 2.91 m·s(-1). We measured their rates of metabolic energy consumption (i.e., metabolic power), ground reaction forces, and stride kinematics.

RESULTS

There were only small differences in running economy between young and older runners across the range of speeds. Statistically, the older runners consumed 2% to 9% less metabolic energy than the young runners across speeds (P = 0.012). Also, the leg stiffness of older runners was 10% to 20% lower than that of young runners across the range of speeds (P = 0.002), and in contrast to the younger runners, the leg stiffness of older runners decreased with speed (P < 0.001).

CONCLUSIONS

Runners beyond 65 yr of age maintain youthful running economy despite biomechanical differences. It may be that vigorous exercise, such as running, prevents the age related deterioration of muscular efficiency and, therefore, may make everyday activities easier.

Running for exercise mitigates age-related deterioration of walking economy

INTRODUCTION

Impaired walking performance is a key predictor of morbidity among older adults. A distinctive characteristic of impaired walking performance among older adults is a greater metabolic cost (worse economy) compared to young adults. However, older adults who consistently run have been shown to retain a similar running economy as young runners. Unfortunately, those running studies did not measure the metabolic cost of walking. Thus, it is unclear if running exercise can prevent the deterioration of walking economy.

PURPOSE

To determine if and how regular walking vs. running exercise affects the economy of locomotion in older adults.

METHODS

15 older adults (69 ± 3 years) who walk ≥ 30 min, 3x/week for exercise, "walkers" and 15 older adults (69 ± 5 years) who run ≥ 30 min, 3x/week, "runners" walked on a force-instrumented treadmill at three speeds (0.75, 1.25, and 1.75 m/s). We determined walking economy using expired gas analysis and walking mechanics via ground reaction forces during the last 2 minutes of each 5 minute trial. We compared walking economy between the two groups and to non-aerobically trained young and older adults from a prior study.

RESULTS

Older runners had a 7-10% better walking economy than older walkers over the range of speeds tested (p = .016) and had walking economy similar to young sedentary adults over a similar range of speeds (p =  .237). We found no substantial biomechanical differences between older walkers and runners. In contrast to older runners, older walkers had similar walking economy as older sedentary adults (p =  .461) and ∼ 26% worse walking economy than young adults (p<.0001).

CONCLUSION

Running mitigates the age-related deterioration of walking economy whereas walking for exercise appears to have minimal effect on the age-related deterioration in walking economy.

Effects of aging on mechanical efficiency and muscle activation during level and uphill walking

PURPOSE

The metabolic cost of walking is greater in old compared to young adults. This study examines the relation between metabolic cost, muscular efficiency, and leg muscle co-activation during level and uphill walking in young and older adults.

PROCEDURES

Metabolic cost and leg muscle activation were measured in young (22.3 ± 3.6 years) and older adults (74.5 ± 2.9 years) walking on a treadmill at six different slopes (0.0-7.5% grade) and a speed of 1.3 ms(-1). Across the range of slopes, 'delta mechanical efficiency' of the muscular system and antagonist muscle co-activation were quantified.

MAIN FINDINGS

Across all slopes, older adults walked with a 13-17% greater metabolic cost, 12% lower efficiency, and 25% more leg muscle co-activation than young adults. Among older adults, co-activation was weakly correlated to metabolic cost (r=.233) and not correlated to the lower delta efficiency.

CONCLUSION

Lower muscular efficiency and increased leg muscle co-activation contribute to the greater metabolic cost of uphill slope walking among older adults but are unrelated to one another.

High efficiency in human muscle: an anomaly and an opportunity?

Can human muscle be highly efficient in vivo? Animal muscles typically show contraction-coupling efficiencies <50% in vitro but a recent study reports that the human first dorsal interosseous (FDI) muscle of the hand has an efficiency value in vivo of 68%. We examine two key factors that could account for this apparently high efficiency value: (1) transfer of cross-bridge work into mechanical work and (2) the use of elastic energy to do external work. Our analysis supports a high contractile efficiency reflective of nearly complete transfer of muscular to mechanical work with no contribution by recycling of elastic energy to mechanical work. Our survey of reported contraction-coupling efficiency values puts the FDI value higher than typical values found in small animals in vitro but within the range of values for human muscle in vivo. These high efficiency values support recent studies that suggest lower Ca(2+) cycling costs in working contractions and a decline in cost during repeated contractions. In the end, our analysis indicates that the FDI muscle may be exceptional in having an efficiency value on the higher end of that reported for human muscle. Thus, the FDI muscle may be an exception both in contraction-coupling efficiency and in Ca(2+) cycling costs, which makes it an ideal muscle model system offering prime conditions for studying the energetics of muscle contraction in vivo.

Influence of zolpidem and sleep inertia on balance and cognition during nighttime awakening: a randomized placebo-controlled trial

OBJECTIVES

To determine whether sleep inertia (grogginess upon awakening from sleep) with or without zolpidem impairs walking stability and cognition during awakenings from sleep.

DESIGN

Three within-subject conditions hypnotic medication (zolpidem), placebo (sleep inertia), and wakefulness control randomized using balanced Latin square design.

SETTING

Sleep laboratory.

PARTICIPANTS

Twelve older and 13 younger healthy adults.

INTERVENTION

Five milligrams of zolpidem or placebo 10 minutes before scheduled sleep (double-blind: zolpidem or sleep inertia); placebo before sitting in bed awake for 2 hours after their habitual bedtime (single-blind: wakefulness control).

MEASUREMENTS

Tandem walk on a beam and cognition, measured using computerized performance tasks, approximately 120 minutes after treatment.

RESULTS

No participants stepped off the beam on 10 practice trials. Seven of 12 older adults stepped off the beam after taking zolpidem, compared with none after sleep inertia and three after wakefulness control. Fewer young adults stepped off the beam: three after taking zolpidem, one after sleep inertia, and none after wakefulness control. Number needed to harm analyses showed one tandem walk failure for every 1.7 (95% confidence interval (CI)=1.4-2.0) older and 5.5 (95% CI=5.2-5.8) younger adults treated with zolpidem. Cognition was significantly more impaired after zolpidem exposure than with wakefulness control in older and younger participants (working memory: older, -4.3 calculations, 95% CI=-7.0 to -1.7; younger, -12.4 calculations, 95% CI=-18.2 to -6.7; Stroop: older, 76-ms increase (95% CI=13.5-138.4 ms); younger, 126-ms increase, 95% CI=34.7-217.5 ms), whereas sleep inertia significantly impaired cognition in younger but not older participants.

CONCLUSION

Zolpidem produced clinically significant balance and cognitive impairments upon awakening from sleep. Because impaired tandem walk predicts falls and hip fractures and because impaired cognition has important safety implications, use of nonbenzodiazepine hypnotic medications may have greater consequences for health and safety than previously recognized.

Effects of aging and arm swing on the metabolic cost of stability in human walking

To gain insight into the mechanical determinants of walking energetics, we investigated the effects of aging and arm swing on the metabolic cost of stabilization. We tested two hypotheses: (1) elderly adults consume more metabolic energy during walking than young adults because they consume more metabolic energy for lateral stabilization, and (2) arm swing reduces the metabolic cost of stabilization during walking in young and elderly adults. To test these hypotheses, we provided external lateral stabilization by applying bilateral forces (10% body weight) to a waist belt via elastic cords while young and elderly subjects walked at 1.3m/s on a motorized treadmill with arm swing and with no arm swing. We found that the external stabilizer reduced the net rate of metabolic energy consumption to a similar extent in elderly and young subjects. This reduction was greater (6-7%) when subjects walked with no arm swing than when they walked normally (3-4%). When young or elderly subjects eliminated arm swing while walking with no external stabilization, net metabolic power increased by 5-6%. We conclude that the greater metabolic cost of walking in elderly adults is not caused by a greater cost of lateral stabilization. Moreover, arm swing reduces the metabolic cost of walking in both young and elderly adults likely by contributing to stability.

Individual limb work does not explain the greater metabolic cost of walking in elderly adults

Elderly adults consume more metabolic energy during walking than young adults. Our study tested the hypothesis that elderly adults consume more metabolic energy during walking than young adults because they perform more individual limb work on the center of mass. Thus we compared how much individual limb work young and elderly adults performed on the center of mass during walking. We measured metabolic rate and ground reaction force while 10 elderly and 10 young subjects walked at 5 speeds between 0.7 and 1.8 m/s. Compared with young subjects, elderly subjects consumed an average of 20% more metabolic energy ( P = 0.010), whereas they performed an average of 10% less individual limb work during walking over the range of speeds ( P = 0.028). During the single-support phase, elderly and young subjects both conserved ∼80% of the center of mass mechanical energy by inverted pendulum energy exchange and performed a similar amount of individual limb work ( P = 0.473). However, during double support, elderly subjects performed an average of 17% less individual limb work than young subjects ( P = 0.007) because their forward speed fluctuated less ( P = 0.006). We conclude that the greater metabolic cost of walking in elderly adults cannot be explained by a difference in individual limb work. Future studies should examine whether a greater metabolic cost of stabilization, reduced muscle efficiency, greater antagonist cocontraction, and/or a greater cost of generating muscle force cause the elevated metabolic cost of walking in elderly adults.

The amplitude of force variability is correlated in the knee extensor and elbow flexor muscles

The purpose of this study was to determine motor output variability for different muscles in the same individuals. Ten young (21.7 +/- 3.4 years) and ten elderly (72.1 +/- 3.9 years) men underwent assessment of maximal isometric (MVC) and dynamic (1-RM) strength, and performed constant-force (2-50% MVC), constant-load (5-50% 1-RM load), and unloaded postural contractions as steadily as possible with the first dorsal interosseus (FDI), elbow flexors (EF), and knee extensors (KE). The coefficient of variation (CV) of force for isometric contractions and the standard deviation (SD) of acceleration for concentric, eccentric, and postural contractions were calculated. The 1-RM load, the CV of force for four of five isometric target forces, and the SD of acceleration during postural contractions were correlated between the EF and KE muscles. MVC force, 1-RM load, and SD of acceleration during postural contractions were not correlated between the FDI/EF or FDI/KE. The CV of force was correlated between the FDI/EF and FDI/KE for two of five isometric target forces. The SD of acceleration during concentric and eccentric contractions was not correlated between muscles. The normalized fluctuations during isometric contractions were greater for the FDI compared with the EF and KE. Elderly adults displayed greater fluctuations only for the FDI during low-force isometric and postural contractions. The dominant frequency of fluctuations was similar for the EF and KE muscles. The correlated fluctuations for the EF and KE muscles, within subjects, suggests that the two motor neuron pools transform the various neural inputs similarly.

Minimizing center of mass vertical movement increases metabolic cost in walking

A human walker vaults up and over each stance limb like an inverted pendulum. This similarity suggests that the vertical motion of a walker's center of mass reduces metabolic cost by providing a mechanism for pendulum-like mechanical energy exchange. Alternatively, some researchers have hypothesized that minimizing vertical movements of the center of mass during walking minimizes the metabolic cost, and this view remains prevalent in clinical gait analysis. We examined the relationship between vertical movement and metabolic cost by having human subjects walk normally and with minimal center of mass vertical movement ("flat-trajectory walking"). In flat-trajectory walking, subjects reduced center of mass vertical displacement by an average of 69% (P = 0.0001) but consumed approximately twice as much metabolic energy over a range of speeds (0.7-1.8 m/s) (P = 0.0001). In flat-trajectory walking, passive pendulum-like mechanical energy exchange provided only a small portion of the energy required to accelerate the center of mass because gravitational potential energy fluctuated minimally. Thus, despite the smaller vertical movements in flat-trajectory walking, the net external mechanical work needed to move the center of mass was similar in both types of walking (P = 0.73). Subjects walked with more flexed stance limbs in flat-trajectory walking (P < 0.001), and the resultant increase in stance limb force generation likely helped cause the doubling in metabolic cost compared with normal walking. Regardless of the cause, these findings clearly demonstrate that human walkers consume substantially more metabolic energy when they minimize vertical motion.

Task differences with the same load torque alter the endurance time of submaximal fatiguing contractions in humans

Endurance time, muscle activation, and mean arterial pressure were measured during two types of submaximal fatiguing contractions that required each subject to exert the same net muscle torque in the two tasks. Sixteen men and women performed isometric contractions at 15% of the maximum voluntary contraction (MVC) force with the elbow flexor muscles, either by maintaining a constant force while pushing against a force transducer (force task) or by supporting an equivalent inertial load while maintaining a constant elbow angle (position task). The endurance time for the force task (1402 +/- 728 s) was twice as long as that for the position task (702 +/- 582 s, P < 0.05), despite a similar reduction in the load torque at exhaustion for each contraction. The rate of increase in average electromyographic activity (EMG, % peak MVC value) for the elbow flexor muscles was similar for the two tasks. However, the average EMG was greater at exhaustion for the force task (22.4 +/- 1.2%) compared with the position task (14.9 +/- 1.0%, P < 0.05). In contrast, the rates of increase in the mean arterial pressure, the rating of perceived exertion, anterior deltoid EMG, and fluctuations in motor output (force or acceleration) were greater for the position task compared with the force task (P < 0.05). Furthermore, the rate of bursts in EMG activity, which corresponded to the transient recruitment of motor units, was greater for the brachialis muscle during the position task. These results indicate that the briefer endurance time for the position task was associated with greater levels of excitatory and inhibitory input to the motor neurons compared with the force task.

Influence of the CNS environment on chromaffin cell survival and catecholamine secretion patterns

Chromaffin cells implanted into the CNS have been used as a potential source of sustained catecholamine delivery, although their survival and continued catecholamine secretion are controversial. In addition, chromaffin cells exhibit a high degree of neurochemical plasticity in response to environmental factors. The present aims were to determine whether the CNS provides a supportive environment for sustained catecholamine production in transplanted chromaffin cells and to assess whether this novel environment alters patterns of catecholamine secretion. Catecholamine release from bovine chromaffin cells implanted into the rat midbrain was determined in brain slices. In addition, alterations in catecholamine secretion patterns, particularly adrenaline/noradrenaline ratios, were compared in vitro versus in transplants. Results indicated that brain slices containing chromaffin cell implants released high basal and nicotine-stimulated levels of adrenaline and noradrenaline. It is surprising that although adrenaline/noradrenaline ratios steadily declined in culture, this did not occur when cells were transplanted to the CNS in the early postharvesting phases. However, if cells were transplanted following longer periods in culture, adrenaline/noradrenaline ratios remained low. Together, these results suggest that the CNS can provide a supportive environment for chromaffin cell survival and that the pattern of catecholamine secretion can be optimized by prior in vitro manipulation.

Origin of afferent projections into bovine chromaffin cell implants in the rat periaqueductal gray determined by retrograde and anterograde tracing

We have previously described long-term survival of isolated bovine chromaffin cell suspension grafts in the periaqueductal gray of adult rats. Electron microscopic analysis of the graft sites revealed synapses on the transplanted chromaffin cells. The origin of these synapses is not known, but they are probably derived from the host since the initial grafts were suspensions of chromaffin cells that were essentially free of other cell types. In order to determine the origin of the observed synapses, retrograde and anterograde tracer analyses were performed on grafted rats at 4 and 8 weeks after transplantation. Following injection of the retrograde tracer (Fluoro-Gold) into graft sites, four major host sites were labeled: hindbrain reticular formation, substantia nigra, lateral hypothalamus, and cingulate cortex. Injection of anterograde tracer (rhodamine-conjugated dextranamine) into the substantia nigra, lateral hypothalamus, and cingulate cortex produced labeled fibers and terminals in and around 4 and 8 weeks old chromaffin cell graft sites. An increase in both the number of retrogradely labeled cells, as well as in the density of anterogradely labeled fibers and terminals within the graft site, was observed from 4 to 8 weeks. This study shows that graft innervation from the host is primarily from areas that normally project afferent fibers to the periaqueductal gray. The increase in labeled fibers and terminals over 8 weeks suggests that de novo synapse formation on grafted bovine chromaffin cells is a continuous process that is dependent on the regenerative capacity and plasticity of the host neuronal network and the grafted bovine chromaffin cells.

Robust survival of isolated bovine adrenal chromaffin cells following intrastriatal transplantation: a novel hypothesis of adrenal graft viability

Previous investigations have demonstrated that adrenal chromaffin cells survive poorly when grafted into the striatum of rodents, nonhuman primates, and patients with Parkinson's disease. This poor survival has been attributed to the low levels of endogenous NGF within the striatum. However, chromaffin cells isolated from the nonchromaffin constituents of the adrenal medulla (fibroblasts and endothelial cells) have recently been demonstrated to survive grafting into a number of CNS sites. The present study determined whether nonchromaffin constituents of the adrenal medulla may be responsible for poor graft survival. We compared the survival of intrastriatally grafted isolated bovine chromaffin cells with that observed following implantation of either perfused adrenal medullary suspensions containing all adrenal medullary cell types or isolated chromaffin cells that were then reseeded with autologous fibroblasts and endothelial cells. Implants of perfused adrenal medullary cells survived poorly and most graft sites were infiltrated with macrophages. The chromaffin cells in this group that did survive appeared to be in the process of degeneration. In contrast, large numbers of isolated chromaffin cells survived for up to 2 months following transplantation. These cells maintained their endocrine phenotype and stained for all enzymatic markers of catecholamine synthesis as well as chromogranin A. Morphologically, these cells resembled chromaffin cells seen in situ and the perigraft region was essentially devoid of macrophages. When isolated chromaffin cells were reseeded with autologous fibroblasts and endothelial cells, the implants degenerated and few, if any, surviving chromaffin cells were observed. Interestingly, these latter grafts induced a host-derived sprouting response of tyrosine hydroxylase-immunoreactive fibers. These data demonstrate that large numbers of adrenal chromaffin cells can survive intrastriatal implantation in the absence of exposure to exogenous NGF. Rather, the nonchromaffin cells of the adrenal medulla (fibroblasts and endothelial cells) appear to compromise the viability of grafted chromaffin cells. Once they are eliminated from the graft, robust survival of chromaffin cells occurs. If clinical trials employing adrenal medullary grafts are still to be considered for the treatment of Parkinson's disease, isolation of the chromaffin cells should be considered to enhance graft viability.

Subarachnoid adrenal medullary transplants for terminal cancer pain. A report of preliminary studies

BACKGROUND

The prolonged use of opioids to treat intractable pain with currently available therapeutic modalities is often unsatisfactory, usually because of tolerance or complications. Extensive studies carried out in the authors' laboratories have indicated that the transplantation of adrenal medullary tissue into the spinal subarachnoid space can significantly reduce pain in animal pain models, most likely via release of opioid peptides and catecholamines. The current study was undertaken to assess the feasibility and efficacy of subarachnoid adrenal medullary transplantation in alleviating terminal cancer pain in humans.

METHODS

Two milliliters of human adrenal medullary tissue were prepared in the laboratory and then transplanted via lumbar puncture into the subarachnoid space in five patients suffering from terminal cancer pain. Pain scores (VAS), functional activity, and opioid intake were assessed and recorded before and after the transplantation procedure. In addition, CSF samples were collected before and (when possible) at fixed intervals after transplantation for biochemical and cytologic analysis.

RESULTS

Four of the five patients demonstrated progressive decreases in pain scores after the transplant procedure, with concomitant reductions in opioid intake. Three of these four patients remained pain free, two for over 10 months, while the other had a recurrence of her pain after surgery for spinal cord compression secondary to metastases 10 weeks after transplant. The fifth patient had no pain reduction by 1 month after the procedure, and refused further followup. After the transplants, spinal CSF samples revealed increased concentrations of met-enkephalin in three of the five patients, and increased concentrations of catecholamines in the four patients in whom they were determined.

CONCLUSIONS

The results obtained in this study indicate that subarachnoid adrenal medullary transplantation may provide a unique and effective approach to the management of intractable chronic pain in humans.

Pharmacologic characterization of opioid peptide release from chromaffin cell transplants using a brain slice superfusion method

A simple chamber and an inexpensive superfusion system for studying mammalian brain slices containing neural transplants is described. With this method, rat brain slices containing bovine chromaffin cell transplants can be maintained for several hours, allowing for the determination of neurochemical characteristics and pharmacologic responsiveness of the grafted cells. Using this technique, basal and nicotine-stimulated release of metenkephalin from rat periaqueductal gray slices containing bovine chromaffin cell transplants were measured. Results showed that met-enkephalin release can be increased by nicotinic stimulation in slices containing chromaffin cell, but not control implants, for at least 8 weeks postimplantation. Furthermore, this response was dose-related. These results are in good agreement with previous behavioral studies and provide corroborative evidence for the mechanism of pain reduction by the release of opioid peptides from chromaffin cell transplants in the periaqueductal gray. This study demonstrates that neurochemical and pharmacologic analyses of neural transplants using a superfused brain slice method can be a complementary approach in determining the underlying mechanisms of neural transplants in the central nervous system.

Survival and integration of bovine chromaffin cells transplanted into rat central nervous system without exogenous trophic factors

Our previous studies have demonstrated that suspension grafts of isolated bovine chromaffin cells survive in the periaqueductal gray (PAG) of rat midbrain for up to 1 year after transplantation. The current study aimed to determine whether this type of graft could survive transplantation into sites other than the PAG that can benefit from chromaffin cell secretory products. In this study, electron microscope analysis showed that chromaffin cells implanted into the frontal neocortex, striatum, PAG, or the subarachnoid space overlying the spinal cord survived for at least 8 weeks without evidence of degeneration. Intraparenchymally placed grafts appeared relatively avascular and well integrated within the host parenchyma. When blood vessels were found, they were primarily at the host-graft border and were of the nonfenestrated central nervous system (CNS) type. Numerous synapses were present between the grafted cells and host neuronal processes. In addition, extensive intercommunication, via gap junction-like structures, was common in the grafts. Morphologic evidence of granular secretion was most commonly seen in striatal grafts. In contrast, subarachnoid grafts displayed minimal interaction with the host spinal tissue and were heavily vascularized with fenestrated capillaries. Despite morphologic differences between intra- and extraparenchymal grafts, this study demonstrates that isolated suspensions of bovine chromaffin cells survive transplantation into CNS sites without exogenous trophic factors and suggests that these cells are potential candidates for neural transplantation into these regions.

Host-graft relationships of isolated bovine chromaffin cells in rat periaqueductal grey

The transplantation of chromaffin cells from the adrenal medulla into pain modulatory regions of the CNS has previously been shown to reduce pain sensitivity, most likely via local release of neuroactive substances from the transplanted cells. The ready availability of bovine adrenal glands, as well as the high levels of opioid peptides produced by their chromaffin cells, make these glands a potentially valuable donor source for antinociception studies. However, the success of these xenografts depends on their ability to survive and integrate within the host CNS. The aim of the present study was to assess host-graft relationships of bovine chromaffin cells transplanted to the rat CNS. We have found that isolated bovine chromaffin cells survive for at least three months in the rat periaqueductal grey, with no evidence of immunological response following a short-term course of immunosuppressant treatment. In the early stages following transplantation, only minor pathology is found at the injection site, which apparently recovers completely at later stages. The host-graft borders are not well demarcated, in contrast to solid tissue grafts. Neuronal processes of host origin, forming numerous synapses with the transplanted bovine chromaffin cells, are apparent by three weeks following transplantation. Migration also occurs from the graft into the host parenchyma, as evidenced by individual chromaffin cells found near host parenchymal blood vessels. The clusters of chromaffin cells found in the graft itself are generally not very vascular, in contrast to solid tissue grafts. The chromaffin cell clusters are surrounded by blood vessels of the non-fenestrated CNS type at the host-graft border. It is likely that the small size of the graft does not require extensive angiogenesis. The lack of fenestrated peripheral-type endothelial capillaries, normally seen in adrenal medullary tissue grafts, may contribute to the survival of these xenografts in the rat brain.