Measures of motor segmentation from rapid isometric force pulses are reliable and differentiate Parkinson's disease from age-related slowing.

Some people with Parkinson's disease (PD) have disruptions in motor output during rapid isometric muscle contractions. Measures of such disruptions (motor segmentation) may help clarify disease subtype, progression, or effects of therapeutic interventions. We investigated the potential utility of segmentation measures by testing two hypotheses that are fundamental to measurement and evaluation. First, measures of motor segmentation are reliable from day to day (intraclass correlation coefficient > 0.8). Second, that measures of motor segmentation have the sensitivity to differentiate between people with PD and older adults. 10 subjects with PD had a mean age of 70.1 years, Hoehn-Yahr stage < 3, and median levodopa equivalent daily dose of 350 mg. Older adult (mean age 81.9 years) reference data are from a previously published study. Each subject provided approximately 87 rapid isometric index finger abduction force pulses up to 65% of their maximal isometric force for calculation of force pulse measures. Measures were computed for the excitation, transition, and relaxation phases of each force pulse. Measures of motor segmentation had high reliability and presented large (Cohen's D > 0.8) and significant (p < 0.05) group differences. In bivariate plots of selected measures, motor segmentation marked a departure of PD from age-related slowing. Across all subjects, greater segmentation was associated with greater impairments in rate control and a longer time to reach peak force (all Spearman's ρ > 0.8). These results support the potential utility of the motor segmentation measures by satisfying requirements for reliability and the sensitivity to indicate deviations from age-related slowing in motor output.

Single-Task and Dual-Task Tandem Gait Performance Across Clinical Concussion Milestones in Collegiate Student-Athletes.

OBJECTIVE: To determine the length of time after concussion that impaired tandem gait performance is observed. DESIGN: Clinical measurement, prospective longitudinal. SETTING: NCAA collegiate athletic facility. PARTICIPANTS: Eighty-eight concussed NCAA Division I student-athletes and 30 healthy controls. INDEPENDENT VARIABLES: Group (concussion/control) and time (Baseline, Acute, Asymptomatic, and RTP). MAIN OUTCOME MEASURES: Participants completed 4 single-task and dual-task tandem gait trials. The concussion group completed tests at the following time points: preseason (Baseline), within 48 hours after concussion (Acute), on the day symptoms were no longer reported (Asymptomatic), and when cleared to return to sports (RTP). Controls completed the same protocol at similar intervals. The dual-task trials involved minimental style cognitive questions answered simultaneously during tandem gait. We analyzed the best time of the 4 trials, comparing groups with a linear mixed model. RESULTS: Acutely after concussion, the concussion group performed single-task tandem gait slower (worse) than controls (concussion: 11.36 ± 2.43 seconds, controls: 9.07 ± 1.78 seconds, P < 0.001). The concussion group remained significantly slower than controls (9.95 ± 2.21 vs 8.89 ± 1.65 seconds, P = 0.03) at Asymptomatic day but not RTP. There were significant group (P < 0.001) and time (P < 0.001) effects for dual-task tandem gait. The groups were not significantly different at baseline for single-task (P = 0.95) or dual-task (P = 0.22) tandem gait. CONCLUSIONS: Our results indicate that tandem gait performance is significantly impaired acutely after concussion, compared with both preseason measures and controls. Postural control impairments were not present when the student-athletes were cleared for RTP. This information can assist clinicians when assessing postural control and determining recovery after a concussive injury.

Site-Specific Transmission of a Floor-Based, High-Frequency, Low-Magnitude Vibration Stimulus in Children With Spastic Cerebral Palsy.

OBJECTIVE: To determine the degree to which a high-frequency, low-magnitude vibration signal emitted by a floor-based platform transmits to the distal tibia and distal femur of children with spastic cerebral palsy (CP) during standing. DESIGN: Cross-sectional study. SETTING: University research laboratory. PARTICIPANTS: Children with spastic CP who could stand independently (n=18) and typically developing children (n=10) (age range, 4-12y) participated in the study (N=28). INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: The vibration signal at the high-frequency, low-magnitude vibration platform (approximately 33Hz and 0.3g), distal tibia, and distal femur was measured using accelerometers. The degree of plantar flexor spasticity was assessed using the Modified Ashworth Scale. RESULTS: The high-frequency, low-magnitude vibration signal was greater (P<.001) at the distal tibia than at the platform in children with CP (.36±.06g vs .29±.05g) and controls (.40±.09g vs .24±.07g). Although the vibration signal was also higher at the distal femur (.35±.09g, P<.001) than at the platform in controls, it was lower in children with CP (.20±.07g, P<.001). The degree of spasticity was negatively related to the vibration signal transmitted to the distal tibia (Spearman ρ=-.547) and distal femur (Spearman ρ=-.566) in children with CP (both P<.05). CONCLUSIONS: A high-frequency, low-magnitude vibration signal from a floor-based platform was amplified at the distal tibia, attenuated at the distal femur, and inversely related to the degree of muscle spasticity in children with spastic CP. Whether this transmission pattern affects the adaptation of the bones of children with CP to high-frequency, low-magnitude vibration requires further investigation.

Impaired performance of rapid grip in people with Parkinson's disease and motor segmentation.

Bradykinesia, or slow movement, is a defining symptom of Parkinson's disease (PD), but the underlying neuromechanical deficits that lead to this slowness remain unclear. People with PD often have impaired rates of motor output accompanied by disruptions in neuromuscular excitation, causing abnormal, segmented, force-time curves. Previous investigations using single-joint models indicate that agonist electromyogram (EMG) silent periods cause motor segmentation. It is unknown whether motor segmentation is evident in more anatomically complex and ecologically important tasks, such as handgrip tasks. Aim 1 was to determine how handgrip rates of force change compare between people with PD and healthy young and older adults. Aim 2 was to determine whether motor segmentation is present in handgrip force and EMG measures in people with PD. Subjects performed rapid isometric handgrip pulses to 20-60% of their maximal voluntary contraction force while EMG was collected from the grip flexors and extensors. Dependent variables included the time to 90% peak force, the peak rate of force development, the duration above 90% of peak force, the number of segments in the force-time curve, the number of EMG bursts, time to relaxation from 90% of peak force, and the peak rate of force relaxation. People with PD had longer durations and lower rates of force change than young and older adults. Six of 22 people with PD had motor segmentation. People with PD had more EMG bursts compared to healthy adults and the number of EMG bursts covaried with the number of segments. Thus, control of rapid movement in Parkinson's disease can be studied using isometric handgrip. People with PD have impaired rate control compared to healthy adults and motor segmentation can be studied in handgrip.

Motor unit rate coding is severely impaired during forceful and fast muscular contractions in individuals post stroke.

Information regarding how motor units are controlled to produce forces in individuals with stroke and the mechanisms behind muscle weakness and movement slowness can potentially inform rehabilitation strategies. The purpose of this study was to describe the rate coding mechanism in individuals poststroke during both constant (n = 8) and rapid (n = 4) force production tasks. Isometric ankle dorsiflexion force, motor unit action potentials, and surface electromyography were recorded from the paretic and nonparetic tibialis anterior. In the paretic limb, strength was 38% less and the rate of force development was 63% slower. Linear regression was used to describe and compare the relationships between motor unit and electromyogram (EMG) measures and force. During constant force contractions up to 80% maximal voluntary contraction (MVC), rate coding was compressed and discharge rates were lower in the paretic limb. During rapid muscle contractions up to 90% MVC, the first interspike interval was prolonged and the rate of EMG rise was less in the paretic limb. Future rehabilitation strategies for individuals with stroke could focus on regaining these specific aspects of motor unit rate coding and neuromuscular activation.

Evaluation of novel tests of neuromuscular function based on brief muscle actions.

Although widely used, the standard strength test (SST) is known to provide moderate correlations with functional measures, while being based on sustained maximum forces and a relatively large number of trials. The aim of this study was to compare the concurrent (with respect to SST) and external validity (with respect to the standard balance and maximum power output tests) of 2 alternate tests of neuromuscular function based on brief isometric actions. The first test provides a slope between the rates of torque development (RTD) and peak torques (T) measured from a number of consecutive rapid actions performed across a wide range of T levels (brief force pulses, BFP). The second test (alternating consecutive maximum contractions, ACMC) provides T and RTD from multiple cycles of rapid alternating maximum actions of 2 antagonistic muscle groups. The results obtained from 29 young and healthy subjects revealed moderate-to-high concurrent validity of ACMC (median r = 0.56, p < 0.05) and its similar, if not higher external validity than SST. Conversely, both the concurrent and external validity of BFP seemed to be relatively low (r = 0.23, p > 0.05). Because ACMC could also have advantage over SST by being based on somewhat lower and transitional muscle forces exerted and fewer trials are needed for testing 2 antagonistic muscles, the authors conclude that ACMC could be considered as either an alternative or complementary test to SST for testing the ability for rapid exertion of maximum forces. Conversely, BFP may offer a measure of the neuromuscular system "as a whole" that is complementary to SST by providing outcomes that are relatively independent of muscle size and function.

Imaging the lower limb network in Parkinson's disease.

BACKGROUND: Despite the significant impact of lower limb symptoms on everyday life activities in Parkinson's disease (PD), knowledge of the neural correlates of lower limb deficits is limited. OBJECTIVE: We ran an fMRI study to investigate the neural correlates of lower limb movements in individuals with and without PD. METHODS: Participants included 24 PD and 21 older adults who were scanned while performing a precisely controlled isometric force generation task by dorsiflexing their ankle. A novel MRI-compatible ankle dorsiflexion device that limits head motion during motor tasks was used. The PD were tested on their more affected side, whereas the side in controls was randomized. Importantly, PD were tested in the off-state, following overnight withdrawal from antiparkinsonian medication. RESULTS: The foot task revealed extensive functional brain changes in PD compared to controls, with reduced fMRI signal during ankle dorsiflexion within the contralateral putamen and M1 foot area, and ipsilateral cerebellum. The activity of M1 foot area was negatively correlated with the severity of foot symptoms based on the Movement Disorder Society-Sponsored Revision of the Unified Parkinson's Disease Rating Scale (MDS-UPDRS-III). CONCLUSION: Overall, current findings provide new evidence of brain changes underlying motor symptoms in PD. Our results suggest that pathophysiology of lower limb symptoms in PD appears to involve both the cortico-basal ganglia and cortico-cerebellar motor circuits.

Rate control deficits during pinch grip and ankle dorsiflexion in early-stage Parkinson's disease.

BACKGROUND: Much of our understanding of the deficits in force control in Parkinson's disease (PD) relies on findings in the upper extremity. Currently, there is a paucity of data pertaining to the effect of PD on lower limb force control. OBJECTIVE: The purpose of this study was to concurrently evaluate upper- and lower-limb force control in early-stage PD and a group of age- and gender-matched healthy controls. METHODS: Twenty individuals with PD and twenty-one healthy older adults participated in this study. Participants performed two visually guided, submaximal (15% of maximum voluntary contractions) isometric force tasks: a pinch grip task and an ankle dorsiflexion task. PD were tested on their more affected side and after overnight withdrawal from antiparkinsonian medication. The tested side in controls was randomized. Differences in force control capacity were assessed by manipulating speed-based and variability-based task parameters. RESULTS: Compared with controls, PD demonstrated slower rates of force development and force relaxation during the foot task, and a slower rate of relaxation during the hand task. Force variability was similar across groups but greater in the foot than in the hand in both PD and controls. Lower limb rate control deficits were greater in PD with more severe symptoms based on the Hoehn and Yahr stage. CONCLUSIONS: Together, these results provide quantitative evidence of an impaired capacity in PD to produce submaximal and rapid force across multiple effectors. Moreover, results suggest that force control deficits in the lower limb may become more severe with disease progression.

Association Between Collision Sport Career Duration and Gait Performance in Male Collegiate Student-Athletes.

BACKGROUND: Investigations of estimated age of first exposure to repetitive head impacts from collision and contact sports have shown no associations with neurocognitive or neurobehavioral function at the collegiate level, but the effect of career duration may be a more comprehensive factor. Understanding whether longer career duration influences gait performance would provide insights into potential neurological impairment. PURPOSE: To examine the relationship between career duration of collision sports and single/dual-task gait performance in collegiate student-athletes. STUDY DESIGN: Cross-sectional study; Level of evidence, 3. METHODS: We recruited 168 male student-athletes from collision sports: football, lacrosse, ice hockey, and wrestling (mean ± SD age, 19.2 ± 1.3 years; height, 184.5 ± 7.2 cm; mass, 94.3 ± 15.9 kg; estimated age of first exposure, 8.6 ± 3.1 years; career duration, 10.6 ± 3.0 years). All participants completed a baseline single- and dual-task gait assessment before the start of their athletic season. Inertial measurement units were used to measure gait speed and stride length. During the dual task, participants were asked to perform working memory cognitive tasks while walking. The dependent variables were single/dual-task gait speed and stride length, cognitive accuracy, and dual-task cost. The relationship between career duration, analyzed as a continuous variable, and the dependent variables was analyzed using a linear regression. RESULTS: There were no significant associations between career duration and single-task gait speed (1.16 ± 0.16 m/s; ß = -0.004; P = .35; 95% CI = -0.012 to 0.004; η2 = 0.005) or dual-task gait speed (1.02 ± 0.17 m/s; ß = -0.003; P = .57; 95% CI = -0.011 to 0.006; η2 = 0.002). There were also no significant associations between career duration and single/dual-task stride length, cognitive accuracy, or dual-task cost. CONCLUSION: Career duration among collegiate collision sport athletes was not associated with single- or dual-task gait performance, suggesting that a greater exposure to repetitive head impacts is not detrimental to dynamic postural control at the college level. However, the effects of diminished gait performance over the lifetime remain to be elucidated.

The rate of force development scaling factor (RFD-SF): protocol, reliability, and muscle comparisons.

Performing a set of isometric muscular contractions to varied amplitudes with instructions to generate force most rapidly reveals a strong linear relationship between peak forces (PF) achieved and corresponding peak rates of force development (RFD). The slope of this relationship, termed the RFD scaling factor (RFD-SF), quantifies the extent to which RFD scales with contraction amplitude. Such scaling allows relative invariance in the time required to reach PF regardless of contraction size. Considering the increasing use of this relationship to study quickness and consequences of slowness in older adults and movement disorders, our purpose was to further develop the protocol to measure RFD-SF. Fifteen adults (19-28 years) performed 125 rapid isometric contractions to a variety of force levels in elbow extensors, index finger abductors, and knee extensors, on 2 days. Data were used to determine (1) how the number of pulses affects computation of the RFD-SF, (2) day-to-day reliability of the RFD-SF, and (3) the nature of RFD-SF differences between diverse muscle groups. While sensitive to the number of pulses used in its computation (P<.05), RFD-SF was reliable when computed with >50 pulses (ICC>.7) and more so with 100-125 pulses (ICC=.8-.92). Despite differences in size and function across muscles, RFD-SF was generally similar (i.e., only 8.5% greater in elbow extensors than in index finger abductors and knee extensors; P=.049). Results support this protocol as a reliable means to assess how RFD scales with PF in rapid isometric contractions as well as a simple, non-invasive probe into neuromuscular health.

Rates of neuromuscular excitation during cycling in Parkinson's disease compared to healthy young and older adults.

BACKGROUND: Bradykinesia affects mobility in some people with Parkinson's. Fall risk makes the neural control of maximal speed ambulatory movements difficult to study in Parkinson's. Stationary recumbent bicycling favors the use of electromyography at high movement speeds, and may better reveal neuromuscular rate limiters. METHODS: Subjects were 18 adults with Parkinson's, 14 older adults and 14 young adults. Electromyograms were recorded from two muscles during stationary recumbent bicycling at 60, 80, 100, 120 RPM and peak voluntary cadence. Rate of electromyogram rise was calculated. Subjects performed the timed up and go and four square step test. Parkinson's sub-groups were formed based on whether they could pedal ≥120 RPM. Mixed models were used to compare groups and spearman's correlations quantified relationships. FINDINGS: Eight people with Parkinson's and four older adults could not complete the 120 RPM condition. Faster people with Parkinson's (n = 10) had greater maximum cadence (F = 42.85, P < 0.05), higher rates of electromyogram rise in both muscles (F > 16.9, P < 0.05), and faster mobility test times (F > 6.5, P < 0.05) than slower people with Parkinson's (n = 8). In Parkinson's, correlations between vastus lateralis rate of electromyogram rise and four square step test (ρ = -0.62), timed up and go (ρ = -0.53), and peak cadence (ρ = 0.76) were significant (all P < 0.05). INTERPRETATION: People with Parkinson's with slower peak pedaling cadence had slower mobility performance and lower vastus lateralis excitation rates at higher cadences. Vastus lateralis excitation rates had moderate to strong relationships with peak cadence and mobility. Exercise interventions may seek to improve peak cadence or excitation rates in people with Parkinson's.

Motor unit firing rates during constant isometric contraction: establishing and comparing an age-related pattern among muscles.

Motor unit (MU) firing rates (FRs) are lower in aged adults, compared with young, at relative voluntary contraction intensities. However, from a variety of independent studies of disparate muscles, the age-related degree of difference in FR among muscles is unclear. Using a standardized statistical approach with data derived from primary studies, we quantified differences in FRs across several muscles between younger and older adults. The data set included 12 different muscles in young (18-35 yr) and older adults (62-93 yr) from 18 published and one unpublished study. Experiments recorded single MU activity from intramuscular electromyography during constant isometric contraction at different (step-like) voluntary intensities. For each muscle, FR ranges and FR variance explained by voluntary contraction intensity were determined using bootstrapping. Dissimilarity of FR variance among muscles was calculated by Euclidean distances. There were threefold differences in the absolute frequency of FR ranges across muscles in the young (soleus 8-16 and superior trapezius 20-49 Hz), but in the old, FR ranges were more similar and lower for nine out of 12 muscles. In contrast, the explained FR variance from voluntary contraction intensity in the older group had 1.6-fold greater dissimilarity among muscles than the young (P < 0.001), with FR variance differences being muscle dependent. Therefore, differences between muscle FR ranges were not explained by how FRs scale to changes in voluntary contraction intensity within each muscle. Instead, FRs were muscle dependent but were more dissimilar among muscles in the older group in their responsiveness to voluntary contraction intensity.NEW & NOTEWORTHY The mean frequency of motor unit firing rates were compared systematically among several muscles and between young and older adults from new and published data sets. Firing rates among muscles were lower and more similar during voluntary isometric contraction in older than younger adults. Firing rate responses from voluntary contraction intensity were muscle dependent and more dissimilar among muscles in the older than young adults.

Gait Performance Is Associated with Subsequent Lower Extremity Injury following Concussion.

PURPOSE: The purpose was to examine gait characteristics between collegiate athletes who did and did not sustain a lower-extremity musculoskeletal (LEMSK) injury in the year after concussion. METHODS: Thirty-four NCAA collegiate athletes with diagnosed concussions were divided into two groups based on if they did (n = 16) or did not (n = 18) sustain a LEMSK in the year after concussion. Participants completed baseline testing before the start of the season and again at return to play postconcussion. Injuries were tracked using an electronic medical database. Participants were instrumented with three APDM Opal triaxial accelerometers and performed five single-task (ST) and five dual-task (DT) gait trials. Participants traversed a 10-meter walkway, turned around a specified endpoint, and returned to the original line. During DT, participants simultaneously walked and answered mini-mental style questions. A linear mixed-effects model assessed interactions and/or main effects between groups for gait speed, double support time, cadence, stride length, and cognitive accuracy. RESULTS: The LEMSK group walked slower (ST, 1.15 ± 0.10 m·s; DT, 1.01 ± 0.10 m·s) than the uninjured group (ST, 1.23 ± 0.11 m·s; DT, 1.10 ± 0.11 m·s) during both ST (P = 0.04) and DT (P = 0.03). The injury group spent longer in double support (ST, 20.19% ± 2.34%; DT, 21.92% ± 2.13%) than the uninjured group (ST, 18.16% ± 2.60%; DT, 20.00% ± 2.32%) during both ST (P = 0.02) and DT (P = 0.02). The injury group had a significantly lower cognitive accuracy (89.56% ± 6.48%) than the uninjured group (95.40% ± 7.08%) across time points (P = 0.02). CONCLUSIONS: There were significant differences in gait characteristics and cognitive accuracy between those who did and did not sustain a LEMSK injury after concussion. The LEMSK group demonstrated a conservative gait strategy both before and after their concussive injury.

Relationships between motor unit size and recruitment threshold in older adults: implications for size principle.

As a part of the aging process, motor unit reorganization occurs in which small motoneurons reinnervate predominantly fast-twitch muscle fibers that have lost their innervation. We examined the relationship between motor unit size and the threshold force for recruitment in two muscles to determine whether older individuals might develop an alternative pattern of motor unit activation. Young and older adults performed isometric contractions ranging from 0 to 50% of maximal voluntary contraction in both the first dorsal interosseous (FDI) and tibialis anterior (TA) muscles. Muscle fiber action potentials were recorded with an intramuscular needle electrode and motor unit size was computed using spike-triggered averaging of the global EMG signal (macro EMG), which was also obtained from the intramuscular needle electrode. As expected, older individuals exhibited larger motor units than young subjects in both the FDI and the TA. However, moderately strong correlations were obtained for the macro EMG amplitude versus recruitment threshold relationship in both the young and older adults within both muscles, suggesting that the size principle of motor unit recruitment seems to be preserved in older adults.

Comparison of neural excitation measures from the surface electromyogram during rate-dependent muscle contractions.

Peak power and peak rate of isometric force development (RFD) predict performance and functional mobility. Surface electromyography (EMG) is used to quantify the amplitude and rate of neuromuscular excitation. To inform the selection of EMG measures in research on rate-dependent muscle contractions, this methodological study compared amplitude-, area- and rate-based measures based on their correlations with RFD. Considering populations in whom a quiet EMG baseline is challenging, we included measures that do not require the determination of EMG onset. Twenty-one young adults performed isometric dorsiflexion contractions to 40% of their maximal force at increasing RFD. EMG was recorded from tibialis anterior. Relationships between EMG measures and RFD were quantified with Spearman's rho. RMS amplitude of the initial 75 ms of EMG had the strongest correlation with peak RFD (ρ = 0.80) among measures computed from EMG onset. Peak rate of EMG rise (RER) had the strongest relationship with peak RFD (ρ = 0.69) among measures that did not require determination of EMG onset. The strength of the relationship between RER and RFD and the strong correlation between RER and RMS75 during rapid contractions (ρ = 0.86) supports the use of RER in experiments where neural excitation might not be initiated from a quiet baseline.

Evidence of bilinearity in the relationship between rate of neuromuscular excitation and rate of force development.

The purpose was to examine the relationship between the rate of neural excitation (rate of rise in the electromyogram, EMG) and the rate of isometric force development (RFD) to determine whether surface EMG measures can detect nonlinearity that is expected due to underlying motor unit discharge behavior and the summation of progressively larger motor unit potentials throughout recruitment. Due to interest in obtaining a change point, a bilinear model was hypothesized to provide the best fit of the EMG-RFD relationship compared to a linear model, exponential model and log-transformed data. 21 young adult participants performed isometric dorsiflexion contractions to 40% of their maximal voluntary contraction (MVC) force. Contractions were performed in RFD conditions ranging from slow (20 %MVC/s) to fast (peak volitional rate). The Akaike Information Criterion supported nonlinear best fit models in 16 of the 21 participants with the greatest overall support for the bilinear model (n = 13). The bilinear models indicated a mean change point at 204%MVC/s. The present data do not identify the specific motor unit control mechanisms at play and the influence of amplitude cancellation on the electromyogram must be carefully considered.

Neural training for quick strength gains in the elderly: strength as a learned skill.

A sample of 20 older adults (76 +/- 6 years) participated in a two-session training intervention with the goal of eliciting rapid and functionally meaningful strength gains in the ankle plantarflexors. Tests were conducted on a Monday-Wednesday-Friday schedule with two training sessions after tests one and two. During each test, subjects performed five maximal explosive force production contractions (MVCs) from which maximal voluntary torque (MVT) and peak rates of torque development (RTDs) were obtained. To augment the quick strength gains typically observed in response to serial strength measurements, these MVCs were supplemented with exercises consisting of high-velocity, low-force movements at the ankle joint. These exercises were chosen to elicit high rates of neural stimulation without high resistance. Maximal voluntary torque increased by 15% from 53.9 +/- 36 to 62.2 +/- 36.2 N x m (p = 0.02). There was a parallel trend toward increases in RTD based on RTD measures computed over various timescales (0.11 < p < 0.21). The nonsignificant increase in RTD was from 223.9 +/- 153.6 to 248.4 +/- 147.8 N x m x s(-1). This preliminary study has determined that rapid strength gains of functional magnitude are possible in the plantarflexors of the elderly. Subsequent work is necessary to test the translation of such gains to function in the frail elderly and to determine the specific contributions of the selected low-resistance exercises to overall gains.

Modulation of motor unit firing rates during a complex sinusoidal force task in young and older adults.

This study compared motor unit rate coding and muscular force control in the first dorsal interosseous muscle of older (n = 11, mean 72.3 yr) and young (n = 12, mean 18.7 yr) adults. Rate coding during a sinusoidal isometric force-matching task was evaluated using spectral analysis of the time-varying changes in firing rate. The task required force modulations to match a trajectory comprising the sum of 0.15- and 0.45-Hz sine waves. Based on the amplitude of spectral peaks at 0.15 and 0.45 Hz, the amplitude of force modulation was similar in young and older adults at both frequencies (F = 1.9, P = 0.17). Force modulation gain (FMG) was computed as the ratio of the amplitude of force modulation to the amplitude of firing rate modulation. To account for rate coding differences related to the properties of the motoneuron, recruitment threshold force was used as a covariate in age-group comparisons. At both task frequencies, firing rate was modulated with less amplitude (F = 0 14, P < 0.001) and FMG was greater (F = 0 27, P < 0.001) in the older adults. In its transformation of neural input to mechanical output, muscle is known to act as a low-pass filter. Compared with modulation at 0.15 Hz, less change in force per change in firing rate at 0.45 Hz (lower FMG; F = 0 67, P < 0.001), independent of age group, is consistent with this filtering effect. Our conclusion is that there is a reduced amplitude of firing rate modulation in older adults.

Explosive force and fractionated reaction time in elderly low- and high-active women.

PURPOSE: The purpose of this study was to determine whether muscle power, activation time, and neuromuscular stimulation are related to physical activity patterns in older women. METHODS: Forty women (65-84 yr) were assigned to high-active (HA) and low-active (LA) groups on the basis of a historical activity questionnaire, and then they performed a series of maximal, voluntary, isometric knee extensions in a visually cued RT task. Fractionated RT measures were taken using three landmarks in the data: the presentation of the visual stimulus, the beginning of the EMG burst, and the initiation of force development. The rate and magnitude of neural stimulation were taken from surface EMG. RESULTS: Peak torque was 15% greater, rate of torque development was 26% greater, motor time was 17% shorter, rate of EMG rise was 25% faster, and onset EMG magnitude was 15% greater in HA than in LA subjects (P<0.05). CONCLUSION: These results indicate that older women with a history of vigorous activity can generate greater force, power, and motor output in comparison with their less active peers. The lower-body mass index of the HA subjects and 310% greater volume of physical activity are likely to account for the enhanced neuromuscular function seen. It is plausible that in addition to aging, physical inactivity is responsible for a large portion of the loss of neuromuscular function seen in older adults.