Aging of the somatosensory system: a translational perspective

Challenging stability limits in old and young individuals with a functional reaching task

OBJECTIVES

The aim of this study was to evaluate the extent to which adding a challenge to a reach test would increase the limit of stability (LOS) in young and old individuals.

DESIGN

While standing on a force plate, with infrared markers on bony landmarks for motion analysis, 15 old (mean [SD], 70 [7] yrs old) and 15 young (mean [SD], 24 [2] yrs old) participants completed the modified functional reach test, in which they were asked to touch the farthest target possible in a series. They were then challenged to touch additional targets (functional reach challenge test) until they lost balance.

RESULTS

The young participants reached farther than the old participants on both the modified functional reach and functional reach challenge tests (P = 0.005 and P = 0.003), but no group differences were found in absolute distance gains seen with the addition of the challenge. The participants in both groups displaced the center of pressure farther and used 10% more of their anatomic stability allowance in the functional reach challenge test than in the modified functional reach test. The young participants increased the LOS from 80.5% to 90.9% of their anatomic allowance, whereas the old participants increased theirs from 72.1% to 82.8%.

CONCLUSIONS

The challenge improved reaching and LOS similarly in the young and old participants, but the old participants used smaller absolute percentages of LOS. Knowing the flexibility of LOS is useful for rehabilitation practitioners in assessing balance and designing therapeutic exercises that challenge stability during performance of functional arm movements and train individuals to use their LOS safely.

Discrepancy in the involution of the different neural loops with age

The purpose of the study was to compare the effects of sensory manipulations on postural control for subjects of different ages. A young group of subjects (n = 17; 20.0 ± 1.3 years) and an old group of subjects (n = 17; 74.7 ± 6.3 years) were compared in 14 postural conditions [2 reference conditions and 12 sensory manipulation conditions: eyes closed, cervical collar, tendon vibration, electromyostimulation, galvanic vestibular stimulation (2 designs), foam surface] on a force platform. Spatio-temporal parameters of the center of foot pressure displacement were analyzed. When vestibular or proprioceptive afferences were manipulated, the old group was more disturbed than the young group. In addition, when myo-articular proprioceptive afferences were the only non-manipulated information source, the old group was also more disturbed than the young group. Hence, the inability to correctly interpret proprioceptive information and/or the impairment of myo-articular information would appear to be the major factor causing postural control deterioration. Moreover, concerning the vestibular system, it may be that aging alters the central integration of vestibular afferences. These results suggest that aging differently affects the functional ability of the different neural loops in postural control.

When standing on a moving support, cutaneous inputs provide sufficient information to plan the anticipatory postural adjustments for gait initiation

Gait initiation is preceded by initial postural adjustments whose goal is to set up the condition required for the execution of the focal stepping movement. For instance, the step is preceded by a shift of the body’s center of mass towards the stance foot unloading the stepping leg. This displacement is produced by exerting forces on the ground (i.e., thrust) while the body is still motionless. The purpose of this study was to identify whether the mere cutaneous inputs from the feet soles evoked by a lateral translation of the support could be used to scale the initial postural adjustments. Participants stood with their eyes closed on a force platform that could be moved laterally with a low acceleration (between 0.14 m/s² and 0.30 m/s²) to reach a constant velocity of 0.02 m/s. This translation resulted in a change in the somatosensory cues from the feet soles without modifying vestibular inputs. Participants were instructed to produce a step with the right foot as soon as they felt the platform start to move (on either side) or heard an auditory cue. In the latter case, the platform stayed stationary. We found that the thrust duration was lengthened when the platform moved towards the supporting foot. In this condition, the cutaneous stimulation provided information related to a body shift towards the stepping leg. This increased thrust duration likely helped overcoming the non-functional body shift perceived towards the stepping leg. This result highlights the accuracy with which the actual standing position can be determined from foot sole cutaneous cues in the absence of visual and vestibular or proprioceptive inputs.

Age-related changes in the joint position sense of the human hand

Age-related changes in lower limb joint position sense and their contributions to postural stability are well documented. In contrast, only a few studies have investigated the effect of age on proprioceptive hand function. Here, we introduce a novel test for measuring joint position sense in the fingers of the human hand. In a concurrent matching task, subjects had to detect volume differences between polystyrene balls grasped with their dominant (seven test stimuli: 126-505 cm(3)) and their nondominant hand (three reference stimuli: 210, 294, and 505 cm(3)). A total of 21 comparisons were performed to assess the number of errors, the weight of errors (ie, the volume difference between test and reference stimuli), and the direction of errors (ie, over- or underestimation of test stimulus). The test was applied to 45 healthy subjects aged 21 to 79 years. Our results revealed that all variables changed significantly with age, with the number of errors showing the strongest increase. We also assessed tactile acuity (two-point discrimination thresholds) and sensorimotor performance (pegboard performance) in a subset of subjects, but these scores did not correlate with joint position sense performance, indicating that the test reveals specific information about joint position sense that is not captured with pure sensory or motor tests. The average test-retest reliability assessed on 3 consecutive days was 0.8 (Cronbach's alpha). Our results demonstrate that this novel test reveals age-related decline in joint position sense acuity that is independent from sensorimotor performance.

Effects of core instability strength training on trunk muscle strength, spinal mobility, dynamic balance and functional mobility in older adults

BACKGROUND

Age-related postural misalignment, balance deficits and strength/power losses are associated with impaired functional mobility and an increased risk of falling in seniors. Core instability strength training (CIT) involves exercises that are challenging for both trunk muscles and postural control and may thus have the potential to induce benefits in trunk muscle strength, spinal mobility and balance performance.

OBJECTIVE

The objective was to investigate the effects of CIT on measures of trunk muscle strength, spinal mobility, dynamic balance and functional mobility in seniors.

METHODS

Thirty-two older adults were randomly assigned to an intervention group (INT; n = 16, aged 70.8 ± 4.1 years) that conducted a 9-week progressive CIT or to a control group (n = 16, aged 70.2 ± 4.5 years). Maximal isometric strength of the trunk flexors/extensors/lateral flexors (right, left)/rotators (right, left) as well as of spinal mobility in the sagittal and the coronal plane was measured before and after the intervention program. Dynamic balance (i.e. walking 10 m on an optoelectric walkway, the Functional Reach test) and functional mobility (Timed Up and Go test) were additionally tested.

RESULTS

Program compliance was excellent with participants of the INT group completing 92% of the training sessions. Significant group × test interactions were found for the maximal isometric strength of the trunk flexors (34%, p < 0.001), extensors (21%, p < 0.001), lateral flexors (right: 48%, p < 0.001; left: 53%, p < 0.001) and left rotators (42%, p < 0.001) in favor of the INT group. Further, training-related improvements were found for spinal mobility in the sagittal (11%, p < 0.001) and coronal plane (11%, p = 0.06) directions, for stride velocity (9%, p < 0.05), the coefficient of variation in stride velocity (31%, p < 0.05), the Functional Reach test (20%, p < 0.05) and the Timed Up and Go test (4%, p < 0.05) in favor of the INT group.

CONCLUSION

CIT proved to be a feasible exercise program for seniors with a high adherence rate. Age-related deficits in measures of trunk muscle strength, spinal mobility, dynamic balance and functional mobility can be mitigated by CIT. This training regimen could be used as an adjunct or even alternative to traditional balance and/or resistance training.

Database of movement control in the cervical spine. Reference normal of 182 asymptomatic persons

In this study, the first normative database of movement control in the cervical spine has been established. For this purpose the Fly Test was used, which is a reliable and valid clinical test capable of detecting deficient movement control of the cervical spine in patients with neck pain and its associated disorders. One hundred and eighty-two asymptomatic persons, eighty-three men and ninety-nine women, aged 16-74 years, divided into six age groups, were recruited. The Fly Test, using a 3-space Fastrak device, recorded the accuracy of cervical spine movements when tracking three incrementally difficult movement patterns. Amplitude accuracy (AA), directional accuracy (DA), and jerk index (JI) were compared across patterns and age groups. A multivariate analysis of variance revealed a significant effect for age (p < 0.001) but not gender (p > 0.05). Lower accuracy for AA and DA in all three movement patterns was observed in the groups of subjects aged 55-64 and 65-74 years, and also for JI in the easy and medium patterns. Knowledge of normative values for the Fly Test is important and useful in identifying impaired movement control and monitoring the effectiveness of treatment interventions in patients with neck pain of traumatic and non-traumatic origin.

[Falls and fall risk factors. Are they relevant in ENT outpatient medical care?]

BACKGROUND AND OBJECTIVE

The risk of falling increases with age. The problem of falls in medical care constantly rises in priority due to demographic changes in Germany. Therefore, the risk of falling should be assessed in all patients. The present study was intended to evaluate the frequency of falls in daily ENT outpatient medical care and the importance of risk factors.

PATIENTS/METHODS

A multicentre study was performed in six outpatient facilities based on a survey of falls and fall risk factors in patients living at home (n = 673) aged 60 years and older.

RESULTS

Falls are a frequent problem in the elderly. A total of 23% had one and 13.7% more than one fall within the previous year. Of these, 15% suffered injury, while only 13% used some form of fall prevention. Joint pain and problems of the hip and lower extremities (42.6%), abnormal gait (41.3%), dizziness and balance disorders (38%) as well as impaired activities of daily living (35%) are the most common risk factors for falls and multiple falls in all age groups. Impaired hearing is a fall risk factor in elderly patients.

CONCLUSIONS

Falls are a frequent problem in patients in ENT outpatient medical care. Fall prevention is advisable in the context of an interdisciplinary approach.

Plantar vibrotactile detection deficits in adults with chronic ankle instability

PURPOSE

The purpose of this study was to investigate the vibrotactile detection thresholds of the plantar cutaneous afferents in subjects with chronic ankle instability compared with healthy control subjects.

METHODS

Eight adults with chronic ankle instability and eight adults with no ankle sprain history participated. Vibrotactile detection thresholds were assessed using a mechanical stimulus generator system, mounted onto an articulated microscope arm, which delivered sinusoidal vibrotactile inputs to the foot sole at three different sites: head of the first metatarsal, base of the fifth metatarsal, and the heel. Vibrotactile stimulation was delivered at a range of test frequencies that corresponded to the known responsiveness of cutaneous mechanoreceptors in the glabrous skin of the foot sole (10, 25, and 50 Hz). Probe displacement measures (dB) from the last eight displacement trials that contained 50% positive detection responses were averaged to obtain a single threshold estimate for each test frequency and site combination.

RESULTS

The results of this study indicate that no significant group-by-site interactions were found for any test frequencies (P > 0.29). However, group main effects were present at the 10-Hz (P < 0.0001), 25-Hz (P = 0.03), and 50-Hz (P = 0.04) test frequencies, indicating that subjects with chronic ankle instability had significantly higher detection thresholds or less sensitivity when stimulation sites were pooled.

CONCLUSIONS

The results of this study indicate that subjects with chronic ankle instability may demonstrate decreased sensitivity on the plantar surface of the foot. These alterations in plantar cutaneous somatosensation may help explain the underlying mechanisms associated with the prolonged sensorimotor system impairments in postural control and gait commonly exhibited by people with chronic ankle instability.

Age-related changes in motor cortical properties and voluntary activation of skeletal muscle

Aging is associated with dramatic reductions in muscle strength and motor control, and many of these agerelated changes in muscle function result from adaptations in the central nervous system. Aging is associated with widespread qualitative and quantitative changes of the motor cortex. For example, advancing age has been suggested to result in cortical atrophy, reduced cortical excitability, reduced cortical plasticity, as well as neurochemical abnormalities.The associated functional effects of these changes likely influence numerous aspects of muscle performance such as muscle strength and motor control. For example, there is evidence to suggest that the muscle weakness associated with aging is partially due to impairments in the nervous system's ability to fully activate motor neurons- particularly in the larger proximal muscle groups. In this review article we discuss age-related changes in the motor cortex, as well as the abilityor lack thereof- of older adults to voluntarily activate skeletal muscle. We also provide perspectives on scientific and clinical questions that need to be addressed in the near future.

Correlação entre postura da cabeça, intensidade da dor e índice de incapacidade cervical em mulheres com queixa de dor cervical

A dor cervical é o sintoma mais comum das disfunções cervicais, frequentemente relacionado à manutenção de posturas inadequadas. As alterações posturais da cabeça estão associadas à ocorrência de dor cervical, sendo a anteriorização a alteração mais comum. O objetivo foi investigar a correlação entre postura da cabeça, intensidade da dor e índice de incapacidade cervical - neck disability index (NDI). O grupo estudo (GE) foi composto por mulheres na faixa entre 20 e 50 anos com queixas de dor cervical por mais de três meses, e o grupo controle (GC), por mulheres assintomáticas. A intensidade da dor foi avaliada pela escala visual analógica (EVA), a incapacidade pelo NDI e a postura da cabeça pelo ângulo craniovertebral (CV). A normalidade dos dados foi verificada pelo teste de Lilliefors, e a comparação entre os grupos pelo teste t de Student para amostras independentes e a associação entre as variáveis pela correlação de Spearman. O nível de significância foi de 5%. O GE apresentou médias menores para o ângulo CV (p=0,02). O ângulo CV demonstrou correlação negativa com a EVA (r=-0,48) e o NDI (r=-0,15), sugerindo que quanto menor o ângulo, maior a intensidade da dor e a incapacidade cervical. O NDI e a EVA apresentaram correlação positiva (r=0,59). O ângulo CV em indivíduos com dor cervical foi significativamente menor que em assintomáticos, apresentando correlação moderada com o NDI e a dor.

Age-related changes in the behavior of the muscle-tendon unit of the gastrocnemius medialis during upright stance

Mechanical properties of the muscle-tendon unit change with aging, but it is not known how these modifications influence the control of lower leg muscles during upright stance. In this study, young and elderly adults stood upright on a force platform with and without vision while muscle architecture and myotendinous junction movements (expressed relative to the change in the moment on the x-axis of the force platform) were recorded by ultrasonography and muscle activity by electromyography. The results show that the maximal amplitude of the sway in the antero-posterior direction was greater in elderly adults (age effect, P < 0.05) and was accompanied by an increase in lower leg muscle activity compared with young adults. Moreover, the data highlight that fascicles shorten during forward sway and lengthen during backward sways but more so for young (−4 ± 3 and −4 ± 3 mm/Nm, respectively) than elderly adults (−0.7 ± 3 and 0.8 ± 3 mm/Nm, respectively; age × sway, P < 0.001). Concurrently, the pennation angle increased and decreased during forward and backward sways, respectively, with greater changes in young than elderly adults (age × sway, P < 0.001). In contrast, no significant differences were observed between age groups for tendon lengthening and shortening during sways. The results indicate that, compared with young, elderly adults increase the stiffness of the muscular portion of the muscle-tendon unit during upright stance that may compensate for the age-related decrease in tendon stiffness. These observations suggest a shift in the control strategy used to maintain balance.

Adult norms of the perceptual threshold of touch (PTT) in the hands and feet in relation to age, gender, and right and left side using transcutaneous electrical nerve stimulation

There is a lack of standardized and quantifiable measures of touch function, for clinical work. Furthermore, it is not possible to make accurate diagnostic judgments of touch function before normative values are estimated. The objectives of this study were to establish adult norms of the perceptual threshold of touch (PTT) for the hands and feet according to age and gender and to determine the effect of right/left side, handedness, height, weight, and body mass index (BMI) on the PTT. The PTT was assessed by using a high-frequency transcutaneous electrical nerve stimulator (Hf/TENS) with self-adhesive skin electrodes in 346 adults. The PTT was identified as the level registered in mA at which the participants perceived a tingling sensation. The PTT for all participants was a median of 3.75 mA (range 2.50-7.25) in the hands and a median of 10.00 (range 5.00-30.00) in the feet. With increasing age an increase of the PTT was found. Men reported higher PTT than women. The right hand had higher PTT than the left. Handedness, height, weight, and BMI did not affect the PTT. Adult norms of the PTT in the hands for age, gender, and right/left side are presented for four age groups. The present study's estimate of the PTT in the hands could be used as adult norms. Adult norms for the feet could not be estimated because the PTT values in the feet showed a great variance.

Effects of early-stage aging on locomotor dynamics and hindlimb muscle force production in the rat

Attenuation of locomotor function is common in many species of animals as they age. Dysfunctions may emerge from a constellation of age-related impairments, including increased joint stiffness, reduced ability to repair muscle tissue, and decreasing fine motor control capabilities. Any or all of these factors may contribute to gait abnormalities and substantially limit an animal's speed and mobility. In this study we examined the effects of aging on whole-animal locomotor performance and hindlimb muscle mechanics in young adult rats aged 6–8 months and ‘early aged’ 24-month-old rats (Rattus norvegicus, Fischer 344 × Brown Norway crosses). Analyses of gaits and kinematics demonstrated that aged rats moved significantly more slowly, sustained longer hindlimb support durations, moved with a greater proportion of asymmetrical gaits, were more plantigrade, and moved with a more kyphotic spinal posture than the young rats. Additionally, the external mechanical energy profiles of the aged animals were variable across trials, whereas the younger rats moved predominantly with bouncing mechanics. In situ analyses of the ankle extensor/plantar flexor muscle group (soleus, plantaris, and medial and lateral gastrocnemii) revealed reduced maximum force generation with aging, despite minimal changes in muscle mass. The weakened muscles were implicated in the degradation of hindfoot posture, as well as variability in center-of-mass mechanics. These results demonstrate that the early stages of aging have consequences for whole-body performance, even before age-related loss of muscle mass begins.

Compromised encoding of proprioceptively determined joint angles in older adults: the role of working memory and attentional load

Perceiving the positions and movements of one's body segments (i.e., proprioception) is critical for movement control. However, this ability declines with older age as has been demonstrated by joint angle matching paradigms in the absence of vision. The aim of the present study was to explore the extent to which reduced working memory and attentional load influence older adult proprioceptive matching performance. Older adults with relatively HIGH versus LOW working memory ability as determined by backward digit span and healthy younger adults, performed memory-based elbow position matching with and without attentional load (i.e., counting by 3 s) during target position encoding. Even without attentional load, older adults with LOW digit spans (i.e., 4 digits or less) had larger matching errors than younger adults. Further, LOW older adults made significantly greater errors when attentional loads were present during proprioceptive target encoding as compared to both younger and older adults with HIGH digit span scores (i.e., 5 digits or greater). These results extend previous position matching results that suggested greater errors in older adults were due to degraded input signals from peripheral mechanoreceptors. Specifically, the present work highlights the role cognitive factors play in the assessment of older adult proprioceptive acuity using memory-based matching paradigms. Older adults with LOW working memory appear prone to compromised proprioceptive encoding, especially when secondary cognitive tasks must be concurrently executed. This may ultimately result in poorer performance on various activities of daily living.

Measures of sensation in neurological conditions: a systematic review

Objective: To systematically review the psychometric properties and clinical utility of measures of sensation in neurological conditions to inform future research studies and clinical practice.

Data sources: Electronic databases (MEDLINE, CINAHL, EMBASE and AMED) were searched from their inception to December 2010.

Review methods: Search terms were used to identify articles that investigated any sensory measures in neurological conditions. Data about their psychometric properties and clinical utility were extracted and analyzed independently. The strength of the psychometric properties and clinical utility were assessed following recommendations. 1

Results: Sixteen sensory measures were identified. Inter-rater reliability and redundancy of testing protocols are particular issues for this area of assessment. Eleven were rejected because they were not available for a researcher or clinician to use. Of the remaining five measures, the Erasmus MC modifications of the Nottingham Sensory Assessment and the Sensory section of the Fugl–Meyer Assessment showed the best balance of clinical utility and psychometric properties.

Conclusion: Many measures of sensory impairment have been used in research but few have been fully developed to produce robust data and be easy to use. At present, the sensory section of the Fugl–Meyer Assessment and the Erasmus MC modifications of the Nottingham Sensory Assessment show the most effective balance of usability and robustness, when delivered according to the operating instructions.

Multisensory integration across the senses in young and old adults

Stimuli are processed concurrently and across multiple sensory inputs. Here we directly compared the effect of multisensory integration (MSI) on reaction time across three paired sensory inputs in eighteen young (M=19.17 years) and eighteen old (M=76.44 years) individuals. Participants were determined to be non-demented and without any medical or psychiatric conditions that would affect their performance. Participants responded to randomly presented unisensory (auditory, visual, somatosensory) stimuli and three paired sensory inputs consisting of auditory-somatosensory (AS) auditory-visual (AV) and visual-somatosensory (VS) stimuli. Results revealed that reaction time (RT) to all multisensory pairings was significantly faster than those elicited to the constituent unisensory conditions across age groups; findings that could not be accounted for by simple probability summation. Both young and old participants responded the fastest to multisensory pairings containing somatosensory input. Compared to younger adults, older adults demonstrated a significantly greater RT benefit when processing concurrent VS information. In terms of co-activation, older adults demonstrated a significant increase in the magnitude of visual-somatosensory co-activation (i.e., multisensory integration), while younger adults demonstrated a significant increase in the magnitude of auditory-visual and auditory-somatosensory co-activation. This study provides first evidence in support of the facilitative effect of pairing somatosensory with visual stimuli in older adults.

Role of sensory stimulation in amelioration of obstructive sleep apnea

Obstructive sleep apnea (OSA), characterized by recurrent upper airway (UA) collapse during sleep, is associated with significant morbidity and disorders. Polysomnogram is employed in the evaluation of OSA and apnea-hypopnea number per hour reflects severity. For normal breathing, it is essential that the collapsible UA is patent. However, obstruction of the UA is quite common in adults and infants. Normally, important reflex mechanisms defend against the UA collapse. The muscle activity of UA dilators, including the genioglossus, tensor palatini (TP), and pharyngeal constrictors, is due to the integrated mechanism of afferent sensory input → to motor function. Snoring is harsh breathing to prevent UA obstruction. Unfortunately, snoring vibrations, pharyngeal suction collapse, negative pressure, and hypoxia cause pathological perturbations including dysfunctional UA afferent sensory activity. The current paper posits that peripheral sensory stimulation paradigm, which has been shown to be efficacious in improving several neurological conditions, could be an important therapeutic strategy in OSA also.

Influence of simulated neuromuscular noise on the dynamic stability and fall risk of a 3D dynamic walking model

Measures that can predict risk of falling are essential for enrollment of older adults into fall prevention programs. Local and orbital stability directly quantify responses to very small perturbations and are therefore putative candidates for predicting fall risk. However, research to date is not conclusive on whether and how these measures relate to fall risk. Testing this empirically would be time consuming or may require high risk tripping experiments. Simulation studies therefore provide an important tool to initially explore potential measures to predict fall risk. This study performed simulations with a 3D dynamic walking model to explore if and how dynamic stability measures predict fall risk. The model incorporated a lateral step controller to maintain lateral stability. Neuronal noise of increasing amplitude was added to this controller to manipulate fall risk. Short-term (λ(S)(*)) local instability did predict fall risk, but long-term (λ(L)(*)) local instability and orbital stability (maxFM) did not. Additionally, λ(S)(*) was an early predictor for fall risk as it started increasing before fall risk increased. Therefore, λ(S)(*) could be a very useful tool to identify older adults whose fall risk is about to increase, so they can be enrolled in fall prevention programs before they actually fall.

Aging effects on the control of grip force magnitude: an fMRI study

Functional neuroimaging techniques have allowed for investigations into the mechanisms of age-related deterioration in motor control. This study used functional Magnetic Resonance Imaging (fMRI) to investigate age related differences in the control of grip force magnitude. Using an event-related design, fMRI scans were completed on 13 older adults, and 13 gender matched younger adults, while using their dominant hand to squeeze a rubber bulb for 4s at 10%, 40% or 70% of their maximum voluntary contraction. Both groups were able to match the relative force targets, however the older adults produced significantly lower levels of absolute force. fMRI analysis consisted of a 1) region of interest (ROI) approach to detect differences in selected motor areas within brain and 2) a voxel-wise whole brain comparison to find areas of differential activation that were not defined a priori between the older and younger group. The ROI analysis revealed that despite producing lower levels of absolute force, the older adults showed higher levels of activity predominantly in subcortical structures (putamen, thalamus and cerebellum) when compared to the younger group. The older adults also showed higher levels of activity in the ipsilateral ventral premotor cortex. A total of 19 of the 22 ROIs analyzed showed a significant main effect of the required force-level. In the majority of the ROIs that showed a significant force effect there were no significant differences in the magnitude of the blood-oxygen-level-dependent (BOLD) signal between the 10% and 40% conditions but a significantly higher BOLD signal in the 70% condition, suggesting that the modulation of brain activation with grip force may not be controlled in a linear fashion. It was also found that the older adult group demonstrate higher levels of activation in 7 areas during a force production task at higher force levels using a voxel-wise analysis. The 7 clusters that showed significant differences tended to be areas that are involved in visual-spatial and executive processing. The results of this study revealed that older adults require significantly higher activation of several areas to perform the same motor task as younger adults. Higher magnitudes of the BOLD signal in older adults may represent a compensatory pattern to counter age related deterioration in motor control systems.

Soleus H-reflex and its relation to static postural control

The Hoffmann reflex (H-reflex) test has been extensively used to investigate the responsiveness of Ia afferent spinal loop in animal and human studies. The H-reflex response is influenced by multiple neural pathways and the assessment of H-reflex variation is a useful tool in understanding the neural mechanisms in control of movement. Recently, several studies have examined the relationship between the H-reflex modulation and postural stability. For example, it has been reported that the amplitude of soleus (SOL) H-reflex is depressed in relation to increased body sway during upright standing on a soft surface compared to that on a solid surface. It has been suggested that the SOL H-reflex modulation under such condition is predominately affected by the presynaptic inhibitory mechanisms for avoiding oversaturation of the spinal motoneurons. It has also been reported that after balance training, the SOL H-reflex amplitude is down-modulated in parallel with improvement in balance control, suggesting a functional adaptation at the supraspinal levels. The aim of this review is to examine the current literature on the relationship between H-reflex modulation and postural control for a better understanding of the physiological mechanisms involved in control of posture in humans.

Age-related difference on weight transfer during unconstrained standing

The ability to transfer weight from one lower limb to the other is essential for the execution of daily life activities and little is known about how weight transfer during unconstrained natural standing is affected by age. This study examined the weight transfer ability of elderly individuals during unconstrained standing (for 30 min) in comparison to young adults. The subjects (19 healthy elderly adults, range 65-80 years, and 19 healthy young adults, range 18-30 years) stood with each foot on a separate force plate and were allowed to change their posture freely at any time. The limits of stability and base of support width during standing, measures of mobility (using the timed up and go and the preferred walking speed tests), and fear of falling were also measured. In comparison to the young adults, during unconstrained standing the elderly adults produced four times fewer weight transfers of large amplitude (greater than half of their body weight). The limits of stability and base of support width were significantly smaller for the elderly adults but there were no significant differences in the measures of mobility and in the fear of falling score compared to young adults. The observed significant age-related decrease in the use of weight transfer during unconstrained standing, despite any difference in the measured mobility of the subjects, suggests that this task reveals unnoticed and subtle differences in postural control, which may help to better understand age related impairments in balance that the elderly population experiences.

Biofeedback for training balance and mobility tasks in older populations: a systematic review

CONTEXT

An effective application of biofeedback for interventions in older adults with balance and mobility disorders may be compromised due to co-morbidity.

OBJECTIVE

To evaluate the feasibility and the effectiveness of biofeedback-based training of balance and/or mobility in older adults.

DATA SOURCES

PubMed (1950-2009), EMBASE (1988-2009), Web of Science (1945-2009), the Cochrane Controlled Trials Register (1960-2009), CINAHL (1982-2009) and PsycINFO (1840-2009). The search strategy was composed of terms referring to biofeedback, balance or mobility, and older adults. Additional studies were identified by scanning reference lists.

STUDY SELECTION

For evaluating effectiveness, 2 reviewers independently screened papers and included controlled studies in older adults (i.e. mean age equal to or greater than 60 years) if they applied biofeedback during repeated practice sessions, and if they used at least one objective outcome measure of a balance or mobility task.

DATA EXTRACTION

Rating of study quality, with use of the Physiotherapy Evidence Database rating scale (PEDro scale), was performed independently by the 2 reviewers. Indications for (non)effectiveness were identified if 2 or more similar studies reported a (non)significant effect for the same type of outcome. Effect sizes were calculated.

RESULTS AND CONCLUSIONS

Although most available studies did not systematically evaluate feasibility aspects, reports of high participation rates, low drop-out rates, absence of adverse events and positive training experiences suggest that biofeedback methods can be applied in older adults. Effectiveness was evaluated based on 21 studies, mostly of moderate quality. An indication for effectiveness of visual feedback-based training of balance in (frail) older adults was identified for postural sway, weight-shifting and reaction time in standing, and for the Berg Balance Scale. Indications for added effectiveness of applying biofeedback during training of balance, gait, or sit-to-stand transfers in older patients post-stroke were identified for training-specific aspects. The same applies for auditory feedback-based training of gait in older patients with lower-limb surgery.

IMPLICATIONS

Further appropriate studies are needed in different populations of older adults to be able to make definitive statements regarding the (long-term) added effectiveness, particularly on measures of functioning.

Effects of muscle fatigue on gait characteristics under single and dual-task conditions in young and older adults

BACKGROUND

Muscle fatigue and dual-task walking (e.g., concurrent performance of a cognitive interference (CI) while walking) represent major fall risk factors in young and older adults. Thus, the objectives of this study were to examine the effects of muscle fatigue on gait characteristics under single and dual-task conditions in young and older adults and to determine the impact of muscle fatigue on dual-task costs while walking.

METHODS

Thirty-two young (24.3 ± 1.4 yrs, n = 16) and old (71.9 ± 5.5 yrs, n = 16) healthy active adults participated in this study. Fatigue of the knee extensors/flexors was induced by isokinetic contractions. Subjects were tested pre and post fatigue, as well as after a 5 min rest. Tests included the assessment of gait velocity, stride length, and stride length variability during single (walking), and dual (CI+walking) task walking on an instrumented walkway. Dual-task costs while walking were additionally computed.

RESULTS

Fatigue resulted in significant decreases in single-task gait velocity and stride length in young adults, and in significant increases in dual-task gait velocity and stride length in older adults. Further, muscle fatigue did not affect dual-task costs during walking in young and older adults. Performance in the CI-task was improved in both age groups post-fatigue.

CONCLUSIONS

Strategic and/or physiologic rationale may account for the observed differences in young and older adults. In terms of strategic rationale, older adults may walk faster with longer strides in order to overcome the feeling of fatigue-induced physical discomfort as quickly as possible. Alternatively, older adults may have learned how to compensate for age-related and/or fatigue-induced muscle deficits during walking by increasing muscle power of synergistic muscle groups (e.g., hip flexors). Further, a practice and/or learning effect may have occurred from pre to post testing. Physiologic rationale may comprise motor unit remodeling in old age resulting in larger proportions of type I fibres and thus higher fatigue-resistance and/or increased muscle spindle sensitivity following fatigue leading to improved forward propulsion of the body. These findings are preliminary and have to be confirmed by future studies.

Sensory system function and postural stability in men aged 30-80 years

OBJECTIVE

To examine age-related changes in postural stability and sensory system functioning in men aged 30-80 years.

DESIGN

Observational, cross-sectional study.

METHODS

One hundred six healthy men aged 30-80 years participated. Personal characteristics were recorded and outcome measures included: velocity of sway during bilateral stance on a firm and foam surface (eyes open (EO) and eyes closed (EC)), balancing on one leg (EO and EC), lower limb somatosensation (tactile acuity, vibration threshold and joint position error), high-contrast visual acuity (HCVA) and low-contrast visual acuity (LCVA), edge contrast sensitivity and vestibular-ocular reflex (VOR) control.

RESULTS

Men in their 60s and 70s were found to be less stable than the younger age decades when standing on a firm or foam surface. Reduced stability was evident from the 40s to 50s for one-leg-stance (EC). Lower limb somatosensation and HCVA and LCVA were significantly reduced by the 60s but edge contrast sensitivity reduced by the 50s. Age-related changes in VOR control did not emerge until the 70s in this study cohort.

CONCLUSIONS

The results show reduction in postural stability and sensory system functioning in men by the 60s supporting pre-emptive assessment of workers in industries where falls are frequent.

Visuomotor adaptation and proprioceptive recalibration in older adults

Previous studies have shown that both young and older subjects adapt their reaches in response to a visuomotor distortion. It has been suggested that one's continued ability to adapt to a visuomotor distortion with advancing age is due to the preservation of implicit learning mechanisms, where implicit learning mechanisms include processes that realign sensory inputs (i.e. shift one's felt hand position to match the visual representation). The present study examined this proposal by determining if changes in sense of felt hand position (i.e. proprioceptive recalibration) follow visuomotor adaptation in older subjects. As well, we examined the influence of age on proprioceptive recalibration by comparing young and older subjects' estimates of the position at which they felt their hand was aligned with a visual reference marker before and after aiming with a misaligned cursor that was gradually rotated 30 degrees clockwise of the actual hand location. On estimation trials, subjects moved their hand along a robot-generated constrained pathway. At the end of the movement, a reference marker appeared and subjects indicated if their hand was left or right of the marker. Results indicated that all subjects adapted their reaches at a similar rate and to the same extent across the reaching trials. More importantly, we found that both young and older subjects recalibrated proprioception, such that they felt their hand was aligned with a reference marker when it was approximately 6 degrees more left (or counterclockwise) of the marker following reaches with a rotated cursor. The leftward shift in both young and older subjects' estimates was in the same direction and a third of the extent of adapted movement. Given that the changes in the estimate of felt hand position were only a fraction of the changes observed in the reaching movements, it is unlikely that sensory recalibration was the only source driving changes in reaches. Thus, we propose that proprioceptive recalibration combines with adapted sensorimotor mappings to produce changes in reaching movements. From the results of the present study, it is clear that changes in both sensory and motor systems are possible in older adults and could contribute to the preserved visuomotor adaptation.

Influence of simulated neuromuscular noise on movement variability and fall risk in a 3D dynamic walking model

People at risk of falling exhibit increased gait variability, which may predict future falls. However, the causal mechanisms underlying these correlations are not well known. Increased neuronal noise associated with aging likely leads to increased gait variability, which could in turn lead to increased fall risk. This paper presents a model of how changes in neuromuscular noise independently affect gait variability and probability of falling, and aims to determine the extent to which changes in gait variability directly predict fall risk. We used a dynamic walking model that incorporates a lateral step controller to maintain lateral stability. Noise was applied to this controller to approximate neuromuscular noise in humans. Noise amplitude was varied between low amplitudes that did not induce falls and high amplitudes for which the model always fell. With increases in noise amplitude, the model fell more often and after fewer steps. Gait variability increased with noise amplitude and predicted increased probability of falling. Importantly, these relationships were not linear. At either low gait variability or very high gait variability, small increases in noise and variability affected probability of falling very little. Conversely, at intermediate noise and/or variability levels, the same small increases resulted in large increases in probability of falling. Our results validate the idea that age-related increases in neuromuscular noise likely play a direct contributing role in increasing fall risk. However, neuromuscular noise remains only one of many important factors that need to be considered. These findings have important implications for fall prevention research and practice.

Functional mobility and balance in community-dwelling elderly submitted to multisensory versus strength exercises

It is well documented that aging impairs balance and functional mobility. The objective of this study was to compare the efficacy of multisensory versus strength exercises on these parameters. We performed a simple blinded randomized controlled trial with 46 community-dwelling elderly allocated to strength ([GST], N = 23, 70.2-years-old ± 4.8 years) or multisensory ([GMS], N = 23, 68.8-years-old ± 5.9 years) exercises twice a week for 12 weeks. Subjects were evaluated by blinded raters using the timed ‘up and go’ test (TUG), the Guralnik test battery, and a force platform. By the end of the treatment, the GMS group showed a significant improvement in TUG (9.1 ± 1.9 seconds (s) to 8.0 ± 1.0 s, P = 0.002); Guralnik test battery (10.6 ± 1.2 to 11.3 ± 0.8 P = 0.009); lateromedial (6.1 ± 11.7 cm to 3.1 ± 1.6 cm, P = 0.02) and anteroposterior displacement (4.7 ± 4.2 cm to 3.4 ± 1.0 cm, P = 0.03), which were not observed in the GST group. These results reproduce previous findings in the literature and mean that the stimulus to sensibility results in better achievements for the control of balance and dynamic activities. Multisensory exercises were shown to be more efficacious than strength exercises to improve functional mobility.

Do mechanical tests of glove stiffness provide relevant information relative to their effects on the musculoskeletal system? A comparison with surface electromyography and psychophysical methods

The main purpose of the present study was to test the construct validity of two mechanical tests of glove stiffness using a surface electromyography (SEMG) methodology that would allow estimating the effect of glove stiffness on forearm muscle activation during a standardized grip contraction. The mechanical tests [free-deforming multidirectional test (FDMT) and Kawabata Evaluation System for Fabrics (KESF)] were applied on 27 gloves covering a wide range of stiffness. In 30 human subjects, a psychophysical assessment of these gloves was also carried on in addition to the SEMG test. The results showed that the sensitivity of the different tests to glove stiffness differences was slightly better for the FDMT (75% sensitivity) than for the psychophysical assessment (72%), while the SEMG test showed much lower sensitivity (13-31%, depending on the muscle). The SEMG test was highly correlated to the psychophysical assessment (0.88-0.95, depending on the muscle tested), and the FDMT (0.88-0.94) and KESF (0.77-0.86) mechanical tests, showing the construct validity of mechanical tests, particularly for the FDMT. It was concluded that mechanical tests provide relevant information relative to the effect of glove stiffness on the musculoskeletal system of the forearm.

Aging and the force-velocity relationship of muscles

Aging in humans is associated with a loss in neuromuscular function and performance. This is related, in part, to the reduction in muscular strength and power caused by a loss of skeletal muscle mass (sarcopenia) and changes in muscle architecture. Due to these changes, the force-velocity (f-v) relationship of human muscles alters with age. This change has functional implications such as slower walking speeds. Different methods to reverse these changes have been investigated, including traditional resistance training, power training and eccentric (or eccentrically-biased) resistance training. This review will summarise the changes of the f-v relationship with age, the functional implications of these changes and the various methods to reverse or at least partly ameliorate these changes.