The hallucinogen 1-[2,5-dimethoxy-4-iodophenyl]-2-aminopropane (DOI) increases cortical extracellular glutamate levels in rats

We examined how young and older adults adapt their posture to static balance tasks of increasing difficulty. Participants stood barefoot on a force platform in normal quiet, Romberg-sharpened and one-legged stance. Center of pressure (CoP) variations, electromyographic (EMG) activity of ankle and hip muscles and kinematic data were recorded. Both groups increased postural sway as a result of narrowing the base of support. Greater CoP excursions, EMG activity and joint displacements were noted in old compared to younger adults. Older adults displayed increased hip movement accompanied by higher hip EMG activity, whereas no similar increase was noted in the younger group. It is concluded that older adults rely more on their hip muscles when responding to self induced perturbations introduced by increased task constraints during quiet standing.     

Adaptations to long-term strength training of ankle joint muscles in old age

The aim of this study was to enquire whether older adults, who continue plantar-flexion (PF) strength training for an additional 6-month period, would achieve further improvements in neuromuscular performance, in the ankle PFs, and in the antagonist dorsi-flexors (DFs). Twenty-three healthy older volunteers (mean age 77.4 ± 3.7 years) took part in this investigation and 12 of them followed a 1-year strength-training program. Both neural and muscular factors were examined during isometric maximal voluntary contraction (MVC) torques in ankle PF and DF pre-training, post 6 and post 12 months. The main finding was that 6 months of additional strength training of the PFs, beyond 6 months, allowed further improvements in neuromuscular performance at the ankle joint in older adults. Indeed, during the first 6 months of progressive resistance training, there was an increase in the PF MVC torque of 11.1 ± 19.9 N m, and then of 11.1 ± 17.9 N m in the last 6-month period. However, it was only after 1 year that there was an improvement in the evoked contraction at rest in PF (+ 8%). The strength training of the agonist PF muscles appeared to have an impact on the maximal resultant torque in DF. However, it appeared that this gain was first due to modifications occurring in the trained PFs muscles, then, it seemed that the motor drive of the DFs per se was altered. In conclusion, long-term strength training of the PFs resulted in continued improvements in neuromuscular performance at the ankle joint in older adults, beyond the initial 6 months.     

Effects of a 5-h hilly running on ankle plantar and dorsal flexor force and fatigability

This study aimed to examine the effects of a 5-h hilly run on ankle plantar (PF) and dorsal flexor (DF) force and fatigability. It was hypothesised that DF fatigue/fatigability would be greater than PF fatigue/fatigability. Eight male trail long distance runners (42.5 ± 5.9 years) were tested for ankle PF and DF maximal voluntary isokinetic contraction strength and fatigue resistance tests (percent decrement score), maximal voluntary and electrically evoked isometric contraction strength before and after the run. Maximal EMG root mean square (RMS(max)) and mean power frequency (MPF) values of the tibialis anterior (TA), gastrocnemius lateralis (GL) and soleus (SOL) EMG activity were calculated. The peak torque of the potentiated high- and low-frequency doublets and the ratio of paired stimulation peak torques at 10 Hz over 100 Hz (Db10:100) were analysed for PF. Maximal voluntary isometric contraction strength of PF decreased from pre- to post-run (-17.0 ± 6.2%; P < 0.05), but no significant decrease was evident for DF (-7.9 ± 6.2%). Maximal voluntary isokinetic contraction strength and fatigue resistance remained unchanged for both PF and DF. RMS(max) SOL during maximal voluntary isometric contraction and RMS(max) TA during maximal voluntary isokinetic contraction were decreased (P < 0.05) after the run. For MPF, a significant decrease for TA (P < 0.05) was found and the ratio Db10:100 decreased for PF (-6.5 ± 6.0%; P < 0.05). In conclusion, significant isometric strength loss was only detected for PF after a 5-h hilly run and was partly due to low-frequency fatigue. This study contradicted the hypothesis that neuromuscular alterations due to prolonged hilly running are predominant for DF.     

Standing strength training of the ankle plantar and dorsiflexors in older women, using concentric and eccentric contractions

Many studies have reported strength gains in older adults following high-intensity resistance training. However, the muscle contraction types examined have been primarily isometric (static) or concentric (CONC; shortening). Less is known about how eccentric (ECC) strength in older adults responds to training or about the efficacy of ECC contractions as training stimuli in these subjects, even though muscle contractions of this type are performed in most training regimens and daily physical activities. In this study, 15 physically active, healthy older women [68 (5) years; mean (SD)] completed an 8-week resistance training program of two sessions per week. Training consisted of three sets of eight repetitions of CONC ankle plantar flexion (PF) and ECC dorsiflexion (DF), at greater than 80% of the initial peak torque, in a standing position only. Subjects were tested in standing and supine positions for: (1) strength over a range of 10° DF to 20° PF for both CONC and ECC; DF and PF (2) passive resistive torque of the plantar flexors at 6°/s; and (3) DF and PF rate of torque development. All strength testing and training was done at 30°/s. Significant increases (P < 0.01) were found for both CONC DF (↑30%) and ECC DF (↑17%) peak torque in the standing position. No significant changes occurred for DF strength as measured with the subjects in the supine position, PF strength in either position, passive resistive torque, or rate of torque development. In summary, strength gains occurred only in the dorsiflexors, which were trained using ECC contractions. Improvements in DF strength were specific to the position of training, which has implications for the transferability of strength gains to functional tasks such as maintaining gait.     

Age-related fatigability of the ankle dorsiflexor muscles during concentric and eccentric contractions

This study compares the fatigability of the ankle dorsiflexors during five sets of 30 maximal concentric and eccentric contractions in young and elderly adults. The torque produced by the ankle dorsiflexors and the average surface electromyogram (aEMG) of the tibialis anterior were continuously recorded. The contribution of central and peripheral mechanisms to muscle fatigue was tested before, after each set of contractions, and during a 30 min recovery period by the superimposed electrical stimulation method. The compound muscle action potential (M-wave), the mechanical response to single (twitch) and paired (doublet) stimulation, and the postactivation potentiation were also recorded. Compared with young subjects, elderly adults exhibited a greater loss of torque for concentric (50.2 vs. 40.9%; P<0.05) and eccentric (42.1 vs. 27.1%; P < 0.01) contractions. Although young subjects showed a lesser decrease in torque during the eccentric compared with concentric contractions, elderly adults experienced similar fatigability for the two types of contractions despite a comparable depression in the EMG activity of both groups and contraction types (10–20%). As tested by the interpolated-twitch method and aEMG/M-wave ratio, voluntary activation was not altered during either type of contraction or for either age group. During the two fatigue tasks, only elderly adults experienced a decrease in M-wave area (26.4–35.4%; P < 0.05). All together, our results suggest that the fatigue exhibited by both young and elderly adults during maximal concentric and eccentric contractions mainly involved peripheral alterations and that elderly adults may also have experienced a decline in neuromuscular propagation.     

Force control is impaired in the ankle plantarflexors of elderly adults

This study determined the amplitude of force fluctuations for the ankle dorsiflexor (DF) and plantarflexor (PF) muscles of young and elderly adults. Maximal voluntary contraction (MVC) force and isometric DF and PF steadiness (2.5–80% MVC) was assessed in 11 young (23 ± 3 years, 5 women, 6 men) and 10 elderly (73 ± 6 years, 5 women, 5 men) adults. The coefficient of variation (CV) and power spectrum of the force was measured from the steadiness trials. MVC force was lower for elderly adults for PF (38% lower, P = 0.002) but not DF (20% lower, P = 0.14). For PF, the CV of force was greater for elderly than young adults at 2.5% (2.64 vs. 1.71%) and 5% MVC (1.78 vs. 1.24%), similar at 10, 50, and 80% MVC, and greater for young than elderly at the 30% MVC target force. For DF, the CV of force was similar for young and elderly at all target forces (P > 0.05). The CV of force was 49% lower for the PF compared with DF muscles across all target forces (P < 0.0001). This difference was significantly greater at the 2.5 (58%), 5 (58%), and 10% MVC (44%) target forces compared with higher target forces. The power spectra of the force fluctuations for both muscles were consistently dominated by frequencies below 2 Hz. For elderly adults, the neuromuscular factors that underlie both muscle strength and force fluctuations during low-force contractions are impaired in the ankle plantarflexors but not the dorsiflexors.     

Static balance improvement in elderly after dorsiflexors electrostimulation training

The purpose of the present study was to determine the effect of dorsiflexors ElectroStimulation (ES) training, on postural tasks of increasing difficulty in the elderly. Twenty-one elderly adults were randomly assigned into one of two groups: a Training (TG) and a Control Group (CG). The TG (n=10) performed (4 weeks, 4 s/week, 40 min/session) superimposed (electrically evoked and voluntary activation) isometric dorsiflexions (ankle 100°) while seated. Biphasic, rectangular symmetrical pulses (300 ms, 70 Hz, 20–60 mA) were used to provoke maximal muscle activation. Participants performed three static balance tasks (Normal Quiet Stance, Sharpened Romberg, and One-Legged Stance) during which postural sway was quantified using maximum range and standard deviation of Centre of Pressure displacement (Kistler 9281C, 1,000 Hz). Bipolar surface electrodes were used to record the Electromyographic activity (EMG) of Tibialis Anterior, Medial Gastrocnemius, Rectus Femoris and Semi-Tendineous. Two-dimensional kinematic data were collected (60 Hz) and analyzed using the APAS Motion Analysis software. The body was modeled as a five-segment rigid link system. Isometric dorsiflexion moment/angular position relationship was also established using a Cybex dynamometer. ES training resulted in decreased postural sway (P<0.05), greater ankle muscles EMG activity (P<0.001), greater stability of the ankle joint (P<0.05) and significant changes in mean position of all three joints of the lower limb. In addition, dorsiflexion moment significantly (P<0.001) increased as a result of ES training. It is concluded that dorsiflexors ES training, could reduce postural sway and the use of ankle muscles, more characteristic of young adults, might appear in the elderly as well.     

Age-related differences in postural control: effects of the complexity of visual manipulation and sensorimotor contribution to postural performance

Patterns of adaptive changes to the exposure to a sinusoidal visual stimulus can be influenced by stimulus characteristics as well as the integrity of the sensory and motor systems involved in the task. Sensorimotor deficits due to aging might alter postural responses to visual manipulation, especially in more demanding tasks. The purpose of this study was to compare postural control between young and older adults at different levels of complexity and to examine whether possible sensory and/or motor changes account for postural performance differences in older adults. Older and young adults were submitted to the following tests: postural control assessments, i.e., body sway during upright stance and induced by movement of a visual scene (moving room paradigm); sensory assessments, i.e., visual (acuity and contrast sensitivity) and somatosensory (tactile foot sensitivity and detection of passive ankle motion); and motor assessments, i.e., isometric ankle torque and muscular activity latency after stance perturbation. Older adults had worse sensory and motor performance, larger body sway amplitude during stance and stronger coupling between body sway and moving room motion than younger adults. Multiple linear regression analyses indicated that the threshold for the detection of passive ankle motion contributed the most to variances in body sway and this contribution was more striking when visual information was manipulated in a more unpredictable way. The present study suggests that less accurate information about body position is more detrimental to controlling body position, mainly for older adults in more demanding tasks.     

Tension regulation during lengthening and shortening actions of the human soleus muscle

In the present study we investigated tension regulation in the human soleus (SOL) muscle during controlled lengthening and shortening actions. Eleven subjects performed plantar flexor efforts on an ankle torque motor through 30° of ankle displacement (75°–105° internal ankle angle) at lengthening and shortening velocities of 5, 15 and 30° · s⁻¹. To isolate the SOL from the remainder of the triceps surae, the subject's knee was flexed to 60° during all trials. Voluntary plantar flexor efforts were performed under two test conditions: (1) maximal voluntary activation (MVA) of the SOL, and (2) constant submaximal voluntary activation (SVA) of the SOL. SVA trials were performed with direct visual feedback of the SOL electromyogram (EMG) at a level resulting in a torque output of 30% of isometric maximum. Angle-specific (90° ankle angle) torque and EMG of the SOL, medial gastrocnemius (MG) and tibialis anterior (TA) were recorded. In seven subjects from the initial group, the test protocol was repeated under submaximal percutaneous electrical activation (SEA) of SOL (to 30% isometric maximal effort). Lengthening torques were significantly greater than shortening torques in all test conditions. Lengthening torques in MVA and SVA were independent of velocity and remained at the isometric level, whereas SEA torques were greater than isometric torques and increased at higher lengthening velocities. Shortening torques were lower than the isometric level for all conditions. However, whereas SVA and SEA torques decreased at higher velocities of shortening, MVA torques were independent of velocity. These results indicate velocity- and activation-type-specific tension regulation in the human SOL muscle. Accepted: 11 October 1999     

Age-related differences in postural control in humans in response to a sudden deceleration generated by postural disturbance

Age-related differences in postural control in response to a relatively large deceleration resulting from postural disturbance were investigated in eight normal elderly men (age range 67–72 years) and eight young men as controls (age range 19–22 years) using a moving platform. Data were obtained for the hip, knee and ankle angles, position of the centre of foot pressure (CFP), head acceleration, and muscle activity of the leg muscles. The elderly subjects had slower and larger ankle and hip joint movements, and CFP displacement in response to the disturbance compared to the young controls. The elderly subjects also had a delayed occurrence, and greater magnitude of peak acceleration of head rotation than did the young subjects. For the elderly subjects, the CFP was closely related to angular changes in the hip joint movement, but not to those of the ankle and knee joint movements. For the young subjects, on the other hand, the CFP was significantly correlated with angular change in the ankle joint. Co-contraction of the tibialis anterior and gastrocnemius muscles was observed in the elderly subjects. The results indicated that a movement pattern for postural correction in the elderly adults was different from that of the young adults. The elderly relied more on hip movements while the young controls relied on ankle movements to control postural stability. Electronic Publication     

The development of balance control in children: comparisons of EMG and kinetic variables and chronological and developmental groupings

This cross-sectional study examined electromyographic (EMG) and kinetic variables during reactive balance responses in children grouped according to developmental level as compared with chronological age. Purposes were to explore relationships between the two types of variables and the effectiveness of the two grouping methods. Forty-four children between 9 months and 10 years old were tested for reactive balance control on a moveable platform. Surface electrodes measured EMG activity in the gastrocnemius (GA), hamstrings (HA), paraspinals (PS), tibialis anterior (TA), quadriceps (QA), and abdominal (AB) muscles. Timing and distance of center-of-pressure (COP) movements and peak muscle torques at the ankle, knee, and hip were also examined. Significant relationships and group differences were found between postural muscle activity and both the torque generated in the lower limbs and the timing and distance of COP adjustments employed to restabilize balance. As postural muscle activity increased and became more coordinated in timing, peak torque at the ankle and hip also increased, while the distance of and time to complete COP readjustments decreased. Children in younger/developmentally lower groups had smaller-magnitude and less-synergic muscle activity, lower peak torques, longer times to restabilize the COP, and greater COP paths than older/higher developmental groupings. Grouping by developmental level produced more statistical differences than did grouping by age. The correspondence of GA, HA, and PS muscle activity with COP measures and joint peak torques confirms that these muscles are key contributors to the balance synergy correcting for induced forward sway. Additionally, developmental level appears to be a much better predictor of balance improvement than chronological age.     

The organization of torque and EMG activity during bilateral handle pulls by standing humans

Summary This study examined whether the torques and EMG activity that precede and accompany bilateral arm pulls made by standing humans demonstrate a pulse height form of organization. Nine adults made abrupt bilateral pulls in the sagittal plane against a handle, to force targets equal to 5, 10, 20, 40, 60, 80 and 95% of their maximal pulling force (%MPF). The force applied at the handle, ground reaction forces, the center of pressure (CP), and EMG activity in gastrocnemius (GS), biceps femoris (BF), tibialis anterior (TA) and quadriceps (QD) muscles were recorded. Our analysis divided the action into a pre-pull phase (events prior to the increase of handle force) and a pulling phase (while handle force was greater than zero). We evaluated the effects of %MPF on the durations and peak amplitudes of the pre-pull and pulling angular impulses about the ankle joint and on pre-pull EMG patterns. The results showed that the angular impulse associated with the pulling torque (due to the reactive force on the body during the pull) had a pulse height organization: peak torque increased linearly with %MPF, and the durations of the pulling torque were relatively constant. In contrast, a pulse height organization did not characterize the pre-pull period for either the angular impulse associated with ankle torque (due to net ground reaction force) or EMG activity in the leg muscles. Rather, peak ankle torque typically increased up to some submaximal %MPF and then plateaued, perhaps due to a constraining effect of foot length on CP. The durations of pre-pull ankle torques increased over the whole range of %MPF, thereby compensating for the limit on ankle torque. Depending on the subject, the muscles were recruited in two different orders: GS-BF-TA-QD, or GS-TA-BF-QD. As the %MPF increased, the EMG onset times of all four muscles occurred earlier, and there was a greater likelihood that the BF, TA and QD muscles would be recruited on a given trial. The changes in the ankle torque and EMG patterns were gradual, suggesting that the pre-pull phase could have one underlying form of organization, with parameters that are tuned to task goals and anatomical constraints.     

Reduced plantarflexor specific torque in the elderly is associated with a lower activation capacity

Previous studies have reported a decrease in muscle torque per cross-sectional area in old age. This investigation aimed at determining the influence of agonists muscle activation and antagonists co-activation on the specific torque of the plantarflexors (PF) in recreationally active elderly males (EM) and, for comparison, in young men (YM). Twenty-one EM, aged 70–82 years, and 14 YM, aged 19–35 years, performed isometric maximum voluntary contractions (MVC). Activation was assessed by comparing the amplitude of interpolated supramaximal twitch doublets at MVC, with post-tetanic doublet peak torque. Co-activation of the tibialis anterior (TA) was evaluated as the ratio of TA-integrated EMG (IEMG) activity during PF MVC compared to TA IEMG during maximal voluntary dorsiflexion. Triceps surae muscle volume (VOL) was assessed using magnetic resonance imaging (MRI), and PF peak torque was normalised to VOL (PT/VOL) since the later approximates physiological cross-sectional area (CSA) more closely than anatomical CSA. Also, physical activity level, assessed by accelerometry, was significantly lower (21%) in the elderly males. In comparison to the YM group, a greater difference in PT (39%) than VOL (19%) was found in the EM group. PT/VOL and activation capacity were respectively lower by 25% and 21% in EM compared to YM, whereas co-activation was not significantly different. In EM PT/VOL correlated with activation (R²=0.31, P<0.01). In conclusion, a reduction in activation capacity may contribute significantly to the decline in specific torque in the plantar flexors of elderly males. The hypothesis is put forward that reduced physical activity is partialy responsible for the reduced activation capacity in the elderly.     

The influence of age and surface compliance on changes in postural control and attention due to ankle neuromuscular fatigue

The reduction in the quality and integration of sensory information with aging could increase the alterations in postural control associated with muscle fatigue observed in younger adults. This study aimed to compare changes in postural control and attentional demands due to ankle muscle fatigue, with intact and reduced proprioceptive information at the ankle, between young and older adults. Eleven young (24 ± 4 years) and 13 older (65 ± 4 years) men stood quietly on a force platform (blindfolded) under four experimental conditions (combinations of firm (FS)/compliant (CS) surfaces and single/dual tasks), before and immediately after a fatiguing exercise. The fatiguing exercise, performed on a dynamometer, consisted of maintaining an isometric contraction of the plantarflexors at 50 % of maximum until exhaustion. Both COP sway area and COP sway velocity were greater on the CS compared to FS and increased with fatigue for both groups in all conditions. COP sway area showed a greater increase with fatigue in older adults when standing on the CS. Reaction time (secondary task) increased significantly after fatigue, but only for older adults when standing on the CS. The effects of fatigue on postural control are more important when proprioceptive information at the ankle is altered. In particular, older adults had more difficulty and may have needed more attention to stand quietly, compared with young adults.     

Development of a sliding mode control model for quiet upright stance

Human upright stance appears maintained or controlled intermittently, through some combination of passive and active ankle torques, respectively representing intrinsic and contractile contributions of the ankle musculature. Several intermittent postural control models have been proposed, though it has been challenging to accurately represent actual kinematics and kinetics and to separately estimate passive and active ankle torque components. Here, a simplified single-segment, 2D (sagittal plane) sliding mode control model was developed for application to track kinematics and kinetics during upright stance. The model was implemented and evaluated using previous experimental data consisting of whole body angular kinematics and ankle torques. Tracking errors for the whole-body center-of-mass (COM) angle and angular velocity, as well as ankle torque, were all within ∼10% of experimental values, though tracking performance for COM angular acceleration was substantially poorer. The model also enabled separate estimates of the contributions of passive and active ankle torques, with overall contributions estimated here to be 96% and 4% of the total ankle torque, respectively. Such a model may have future utility in understanding human postural control, though additional work is needed, such as expanding the model to multiple segments and to three dimensions.     

A stretching program increases the dynamic passive length and passive resistive properties of the calf muscle-tendon unit of unconditioned younger women

This study examined the effects of a 6-week stretching program on the dynamic passive elastic properties of the calf muscle-tendon unit (MTU) of unconditioned younger women. After random assignment of 12 women (age 18-31 years) to a stretching group (SG) or to a control group (CG), six subjects in the SG and four subjects in the CG completed the study. For the initial tests, a Kin-Com dynamometer moved the ankle from plantarflexion to maximal dorsiflexion (DF) with negligible surface EMG activity in the soleus, gastrocnemius and tibialis anterior muscles. Angular displacement, passive resistive torque, area under the curve (passive elastic energy) and stiffness variables were reduced from the passive DF torque curves. The SG then completed ten static wall stretches held 15 s each, five times a week for 6 weeks, the CG did not. The tests were repeated and the changes between the tests and retests were examined for group differences (Mann-Whitney U). The SG had significant increases in the maximal passive DF angle (7 degrees +/- 4 degrees ), maximal passive DF torque (11.2 +/- 8.3 N m), full stretch range of motion (23 degrees +/- 24 degrees ), full stretch mean torque (3.4 +/- 2.1 N m), and area under the full stretch curve (22.7 +/- 23.5 degrees N m) compared to the CG (P < or = 0.019). The passive stiffness did not change significantly. The results showed that a stretching program for unconditioned calf MTUs increased the maximal DF angle and length extensibility, as well as the passive resistive properties throughout the full stretch range of motion. The adaptations within the calf MTU provide evidence that stretching enhances the dynamic passive length and passive resistive properties in unconditioned younger women.     

Subjective, physiological and biomechanical responses to prolonged manual work performed standing on hard and soft surfaces

The aim of this laboratory study was to examine the subjective, physiological and biomechanical responses to prolonged light repetitive manual work during standing on soft (polyurethane standard mat) and hard (aluminum casting) surfaces. The subjects stood on the hard (10 subjects) and on the soft surfaces (11 subjects) for 2 h. Intensity of unpleasantness, shank circumference, electromyograph (EMG) activities from the right soleus and tibialis anterior muscles, mean amplitude and total angular displacement around the left and right ankle in the saggital plane, centre of pressure (CoP) displacement in the frontal and saggital planes, calf surface temperature, and pain intensity in experimentally induced muscle pain were recorded. Maximal voluntary contraction and fatigue tests were performed before and after the 2 h experiment. Standing on a soft surface caused a lower intensity of unpleasantness. During standing on a hard surface compared to a soft one the results showed an enhanced swelling of the shank, an increased EMG activity (right soleus muscle) of the lower leg, a greater amplitude and total angular displacement, and a larger CoP displacement in the frontal plane. Indications of more pronounced muscle fatigue while standing on the hard surface were also noticed. After 105 min, experimental muscle pain was elicited by injecting hypertonic saline. The intensity of the induced pain was lower when standing on the soft surface. Amplitude, angular distance and CoP displacement showed a tendency to be greater after injection of the hypertonic saline. It was found that the experimentally induced pain influenced postural activity, underlining central interactions between proprioceptors and nociceptors. The results highlighted a higher feeling of comfort when standing on the soft surface. In addition, postural activity was lower when standing on the soft surface, but the activity was sufficient to prevent swelling of the lower legs. Accepted: 27 June 1997     

Post-effect of forward and backward locomotion on body orientation in space during quiet stance

Neural circuits responsible for stance control serve other motor tasks as well. We investigated the effect of prior locomotor tasks on stance, hypothesizing that postural post-effects of walking are dependent on walking direction. Subjects walked forward (WF) and backward (WB) on a treadmill. Prior to and after walking they maintained quiet stance. Ground reaction forces and centre of foot pressure (CoP), ankle and hip angles, and trunk inclination were measured during locomotion and stance. In WF compared to WB, joint angle changes were reversed, trunk was more flexed, and movement of CoP along the foot sole during the support phase of walking was opposite. During subsequent standing tasks, WB induced ankle extension, hip flexion, trunk backward leaning; WF induced ankle flexion and hip extension. The body CoP was displaced backward post-WB and forward post-WF. The post-effects are walking-direction dependent, and possibly related to foot-sole stimulation pattern and trunk inclination during walking.     

Normalized lengthening peak torque is associated with temporal twitch characteristics in elderly women but not young women

Aim: To determine if greater normalized torque during maximal effort lengthening actions in elderly women compared with young women is related to age‐associated adjustments in neural activation and/or contractile function. Methods: The right knee extensors of 14 young women (21–30 years) and 12 elderly women (65–78 years) were assessed for isometric, shortening and lengthening peak torque, electromyography (EMG) activity, and isometric twitch contractile properties. Knee extensor contractile tissue volume was determined using magnetic resonance imaging. Normalized torque was determined as peak torque per unit of knee extensor contractile tissue volume. Results: Normalized torque during the isometric and shortening actions was similar between age groups (P > 0.05); however, lengthening normalized torque was significantly higher for the elderly women (P < 0.05). In the young women, a significant relationship existed between normalized torque and EMG for all muscle actions (P < 0.05), while no association was found between normalized torque and temporal twitch characteristics for any muscle action (P > 0.05). In the elderly women, a significant relationship existed between normalized torque and EMG for the isometric and shortening muscle actions (P < 0.05), but not for lengthening normalized torque and EMG (P > 0.05). Furthermore, no association existed between isometric and shortening normalized torque, and temporal twitch characteristics in the elderly women (P > 0.05); however, a significant relationship existed between lengthening normalized torque, and the rate of relaxation and contraction duration (P < 0.05). Conclusions: The greater capacity to develop lengthening peak torque relative to contractile tissue volume in the elderly women appeared to be associated with age‐related adjustments in the temporal twitch characteristics rather than neural activation.