Annie A. Butler

Age and gender differences in seven tests of functional mobility

BackgroundThe objective of this study was to examine age and gender differences in seven tests of functional mobility.MethodsThe study included 50 young participants aged 20 to 39 years, and 684 older participants aged 75 to 98 years. Functional mobility measures included the coordinated stability test, the near tandem balance test, the six metre walk test, the sit to stand test with five repetitions, the alternate step test and the stair ascent and descent tests.ResultsOlder participants performed significantly worse than the younger participants in all of the functional mobility tests (p < 0.001), with the older women performing worse than the older men in all of the tests (p < 0.05). Significant correlations were found within the older group among all the functional mobility tests scores (r = 0.24–0.87, p < 0.001), and between functional mobility performance and age (r = 0.14–0.35, p < 0.001). People with arthritis and stroke performed worse than people without these conditions in these tests.ConclusionThis study provides a normative database for performance of young and older community-dwelling people in a battery of validated and reliable functional mobility tests. The results confirm age-related differences in functional mobility between young and older adults.

Muscle weakness impairs the proprioceptive control of human standing

The leg muscles have two distinct roles in human standing. They are a principle source of the sensory input used to detect body sway and they also produce the contractile force that corrects body sway. In this population study, we provide evidence for a link between these contractile and sensory functions of muscle. In subjects classified as having weak or strong leg muscles, we compared body sway with and without vision. Subjects (17) with weakness through prior-polio were compared with age-matched controls (34) and from 174 subjects aged 60-69, those classified weak (<15 Nm ankle dorsiflexion) were compared with the strong (>or=15 Nm). The weaker and stronger groups from these populations had equivalent visual acuity and lower-limb sensory function. However, the weaker swayed disproportionately more than the stronger on closing the eyes. Strength alone could not cause this increased sway of the weaker subjects because they were as stable as the strong subjects when the eyes were open. This effect of strength was not apparent in an older group (>or=70 years, n = 276), where eye closure increased sway by similar amounts in the weak and strong. This appears to be related to visual and somatosensory impairments and increased morbidity in the weak of this group, an association not present in the younger groups. We conclude that there is a relative failure of proprioceptive postural control associated with muscle weakness. This indicates a functional link between contractile and sensory muscular processes and shows that multiple sensory inputs are more important for people with muscle weakness.

Ability Versus Hazard: Risk-Taking and Falls in Older People

BACKGROUND Among older people, undue risk taking could lead to falls, irrespective of physical ability. We investigated the interaction between risk-taking behavior and physical ability and its contribution to falls. METHODS Participants (N = 294, age ≥ 70) were asked to walk as quickly as possible to a visible destination by choosing one of six paths. Each contained a raised plank that had to be walked along without falling. The shortest path had the narrowest and tallest plank and the longest had the widest and lowest. Behavioral risk was defined as the probability of falling off the chosen plank. This was estimated from a ground path walking task because, for safety, participants were stopped before crossing the plank. Self-reported everyday risk-taking behavior, fear of falling, physical functioning, and 1-year prospective fall rates were measured. RESULTS Older participants and those with poor physical ability chose easier planks to cross. Participants with good physical ability consistently took a slight behavioral risk, whereas those with poor physical ability took either very-high behavioral risks or chose the overly safe path with no risk. Unexpectedly, participants reporting cautious behavior on the everyday risk-taking behavior scale took greater behavioral risks. Independent of physical performance, behavioral risk was significantly associated with falls during the subsequent year. CONCLUSIONS Assessing behavioral choice in relation to physical ability can identify risk-taking but neither the difficulty of a chosen action nor self-reports of risk-taking behavior are sufficient. Risk-taking behavior is an independent risk factor for falls and management of undue risk-taking might complement existing fall prevention strategies.

Is this my finger? Proprioceptive illusions of body ownership and representation

•  The brain keeps a representation of which things are part of our body. This sense of ownership is easily manipulated using brushing of the skin or movement of a limb to create an illusion of ownership over an inanimate object, such as a rubber hand. •  We induced a sense of ownership of an artificial finger using movement of the index finger without vision of the hands. As cutaneous receptors had been anaesthetised, this illusion depended on proprioceptive signals from muscle receptors. •  In addition, we found a new grasp illusion in which perceived distance between the index fingers decreases when subjects hold an artificial finger. •  These results increase understanding of how the brain generates our body representation and may help in understanding diseases in which the sense of ownership is disrupted.

Body ownership and a new proprioceptive role for muscle spindles

Knowledge of which body parts belong to us is referred to as the sense of body ownership. There is increasing evidence that this important aspect of human proprioception is highly malleable. Research into ownership of individual body parts was stimulated by Botvinick and Cohens rubber‐hand illusion (Nature 391,1998, 756), which demonstrated that an artificial body part can be incorporated in ones body representation and can cause real body parts to be sensed erroneously. Here, we review key studies that have advanced our understanding of the sense of body ownership, including the important role played by multisensory integration and spatiotemporal congruence of sensory signals. We also discuss our recent discovery that body ownership can be induced in response to movement stimuli by signals from a single class of sensory receptor, namely muscle spindles.

Activation of human inspiratory muscles in an upside-down posture.

During quiet breathing, activation of obligatory inspiratory muscles differs in timing and magnitude. To test the hypothesis that this coordinated activation can be modified, we determined the effect of the upside-down posture compared with standing and lying supine. Subjects (n=14) breathed through a pneumotachometer with calibrated inductance bands around the chest wall and abdomen. Surface electromyographic activity (EMG) was recorded from the scalene muscles. Crural diaphragmatic EMG and oesophageal and gastric pressures were measured in a subset of six subjects. Quiet breathing and standard lung function manoeuvres were performed. The upside-down posture reduced end-expiratory lung volume. During quiet breathing, for the same inspiratory airflow and tidal volume, ribcage contribution decreased, abdominal contribution increased and transdiaphragmatic pressure swing doubled in the upside-down posture compared to standing (p<0.05). Despite this, crural diaphragm EMG was unchanged, whereas scalene muscle EMG was reduced by ∼half (p<0.05). Thus, the mechanical effect of an upside-down posture differentially affects inspiratory muscle activation.

Time, touch and temperature affect perceived finger position and ownership in the grasp illusion

The brains internal model of the body and the sense of body ownership are fundamental to interaction with the world. It is thought that temporally congruent, repetitive multisensory stimuli are required to elicit a sense of body ownership. Here we investigate the ability of static cutaneous stimuli – passively grasping an artificial finger – to induce body ownership and alter perceived body position; we also investigate how physical characteristics of grasped objects alter these senses. We show that static cutaneous stimuli can alter perceived body position and induce an illusion of ownership and also that signals of temperature, texture and shape of grasped finger‐sized objects influence body ownership. Thus, these aspects of human proprioception can be altered by a single sustained sensory stimulus and by the physical characteristics of held objects.

Perceptions of Speed and Risk: Experimental Studies of Road Crossing by Older People

Crossing a road safely is a complex task requiring good sensorimotor function and integration of information about traffic speed, distances and one’s own speed. Poor judgement through age-related sensorimotor or cognitive impairment or a predisposition to take risks could lead to errors with serious consequences. On a simulated road, 85 participants (age ≥70 years) were asked to cross in front of an approaching car with a clearance as small as considered safe in two conditions; (1) with nothing else to attend to (free crossing) and (2) with an additional ball-gathering task while waiting to cross (task crossing). Participants were categorised according to their crossing outcome (failed to cross, ‘hit’, exact, safe, cautious). Participants also performed two sub-studies; (1) the perception of the time-to-arrival of moving objects and (2) the perception of own gait speed. Physical and cognitive function and everyday risk-taking behaviour were also assessed. In free crossing, clearances varied but no participants were “hit” by the car. In task crossing, participants allowed smaller clearances and 10% of participants would have been hit while 13% missed the opportunity to cross altogether. Across a wide range of physical and cognitive measures, including perceived and actual gait speed, a consistent pattern was observed in the task crossing condition. The exact group performed best, the ‘hit’, safe and cautious groups performed less well while those who missed the opportunity (fail) performed worst. The exact group reported taking the greatest risks in everyday life whereas the remaining groups reported being cautious. In conclusion, we found older people with poorer perceptual, physical and cognitive function made inappropriate and risky decisions in a divided attention road-crossing task despite self-reports of cautious behaviour in everyday life.

Time course of human motoneuron recovery after sustained low-level voluntary activity.

Motoneurons often fire repetitively and for long periods. In sustained voluntary contractions the excitability of motoneurons declines. We provide the first detailed description of the time course of human motoneuron recovery after sustained activity at a constant discharge rate. We recorded the discharge of single motor units (MUs, n = 30) with intramuscular wire electrodes inserted in triceps brachii during weak isometric contractions. Subjects (n = 15) discharged single MUs at a constant frequency (∼10 Hz) with visual feedback for prolonged durations (3-7 min) until rectified surface electromyogram (sEMG) of triceps brachii increased by ∼100%. After a rest of 1-2, 15, 30, 60, 120, or 240 s, subjects briefly resumed the contraction with the target MU at the same discharge rate. Each MU was tested with three to four rest periods. The magnitude of sEMG was increased when contractions were resumed, and the target motoneuron discharged at the test frequency following rest intervals of 2-60 s (P = 0.001-0.038). The increased sEMG indicates that greater excitatory drive was needed to discharge the motoneuron at the test rate. The increase in EMG recovered exponentially with a time constant of 28 s but did not return to baseline even after a rest period of ∼240 s. Thus the decline in motoneuron excitability from a weak contraction takes several minutes to recover fully.

Human motoneurone excitability is depressed by activation of serotonin 1A receptors with buspirone

In the adult turtle spinal cord, action potential generation in motoneurones is inhibited by spillover of serotonin to extrasynaptic serotonin 1A (5‐HT1A) receptors at the axon initial segment. We explored whether ingestion of the 5‐HT1A receptor partial agonist, buspirone, decreases motoneurone excitability in humans. Following ingestion of buspirone, two tests of motoneurone excitability showed decreases. F‐wave areas and persistence in an intrinsic muscle of the hand were reduced, as was the area of cervicomedullary motor evoked potentials in biceps brachii. Our findings suggest that activation of 5‐HT1A receptors depresses human motoneurone excitability. Such a depression could contribute to decreased motoneurone output during fatiguing exercise if there is high serotonergic drive to the motoneurones.