Developmental changes in prehension during childhood

 Recent evidence has shown that the anticipatory control of grip and load force is not innate and develops over several years in childhood. The present study examined the development of grasping behavior by quantifying the relationship between grip force and the vertical acceleration of an object. Children and adults were requested to use a precision grip to lift an instrumented object which varied in size and weight. Grip force, grip force rate and the vertical position and acceleration profiles of the test object were measured or calculated. The results demonstrated the presence of distinct developmental milestones in the maturation of precision grip from 2 to 9 years of age. With 2-year-old children, the peak acceleration was negatively correlated (r=–0.51, n=34, P<0.01) with peak grip force during lifting. By 3 years of age, peak acceleration and peak grip force during lifting became positively correlated (r=0.28, n=104, P<0.01) and the correlation continued to strengthen up to 9 years of age. Variations in the temporal coupling of both peak grip force and peak acceleration also decreased with maturation. Furthermore, starting at 4-years-old, children clearly controlled the acceleration and deceleration of the object in a symmetrical pattern and used a single burst of grip force rate to grasp the object with some regularity, suggesting that the emergence of an anticipatory control strategy had already begun. Received: 23 November 1996 / Accepted: 21 September 1998     

Precision in isometric precision grip force is reduced in middle-aged adults

We investigated age related changes in the control of precision grip in 29 healthy adults spanning early adulthood to middle age (21–67 years). Subjects performed a visually guided, isometric precision grip ramp-and-hold force-tracking task. Target force levels were 3, 6, and 9 N. Precision and performance of force regulation was quantified. Larger errors were made during the ramp than during the hold phase. Age correlated positively with the amount of error at the lowest (3 N) force level in both phases. Force onsets were systematically earlier in middle-aged subjects and the average slope of the force during the ramp decreased with increasing age. The results show that precision during low grip force control decreases already during middle age and those subjects may modify their force generation strategies to compensate for early and subtle degenerative changes in the motor system before decline in grip strength is apparent.     

Aging affects the predictive control of grip force during object manipulation

We examined the effects of aging on the predictive control of grip force during object manipulation under various external force fields. Participants rhythmically moved a hand-held object (m = 0.4 kg) in the horizontal plane under three experimental conditions: (1) with an elastic cord attached to the upper arm (ARM), (2) with the elastic cord attached to the object (OBJECT), and (3) without any elastic cord (NO ELAST). Performance was evaluated in terms of both metric and spectral characteristics of the grip force (GF) profile, in relation to the movement-induced variations in load at the object-finger interface (LFO). The performance of a group of 12 older adults (mean age = 66.3 years) was compared to the performance of a group of 12 young adults (mean age = 25.0 years), whose metric characteristics were reported earlier (Exp. Brain Res. 172:331, 2006). Although elderly participants exerted a larger mean GF, a tight linear coupling between GF and LFO was found for both groups in OBJECT. In ARM and NO ELAST, coefficients of cross-correlations were markedly lower, the more so for the elderly participants. Adjustments in GF occurred slightly in advance of variations in LFO in young adults (+7 ms) and somewhat delayed in the elderly (−26 ms). Spectral analyses revealed that in OBJECT, LFO and GF varied primarily at the frequency of movement. In ARM and NO ELAST, where LFO varied at twice this frequency, GF modulations contained a substantial frequency component at the frequency of movement, with this effect being more pronounced for the elderly participants. We conclude that both young and older adults demonstrate a predictive control of GF, capable of separating external force fields acting on the arm or on object–finger interface. However, in the presence of variations in LFO occurring at twice the frequency of movement, the spectral profile of GF exhibits a non-functional component of variation at the frequency of movement. Aging amplifies this latter effect, thereby affecting the efficiency of the predictive control of grip force.     

Failure to disrupt the ‘sensorimotor’ memory for lifting objects with a precision grip

When repetitively lifting an object with mechanical properties that vary from lift-to-lift, the fingertip forces for gripping and lifting are influenced strongly by the previous lift, revealing a ‘sensorimotor’ memory. Two recent reports indicate that the sensorimotor memory for grip force is easily disrupted by an unrelated task like a strong pinch or vibration, even when the lift was performed with the hand contralateral to the vibration or preceding pinch. These findings indicate that this memory may reflect sensory input or muscle contraction levels, rather than object properties or the specific task of gripping and lifting. Here we report that the predictive scaling of lift force was not disrupted by conditioning tasks that featured exerting a vertical isometric force with the upper extremity. When subjects lifted a 2 N object repetitively the peak lift force rate was 26.4 N/s. The lift force rate increased to 36.1 N/s when the 2 N object was lifted (regardless of hand) after lifting the 8 N object with the right hand, which reveals the expected ‘sensorimotor’ memory. The lift force rate did not increase (24.8 vs. 26.4 N/s for the control condition) when a bout of isometric exertion (9.8 N) in the vertical direction with the distal right forearm preceded lifts of the 2 N object. This finding was confirmed with another isometric task designed to more closely mimic lifting an object with a precision grip. This difference in the sensitivity of grip versus lift force to a preceding isometric contraction indicates that separate sensorimotor memories contribute to the predictive scaling of the commands for gripping and lifting an object.     

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.     

Effects of chronic median nerve compression at the wrist on sensation and manual skills

 The aim of this study was to analyse the functional impairments caused by chronic median nerve compression at the wrist on hand sensation and manual skill. Hand function was assessed in 11 patients (8 women and 3 men) with severe carpal tunnel syndrome (CTS) and compared with that of an age- and sex-matched control group. Apart from CTS, the subjects were healthy and the electrodiagnostic examination was normal. The pressure and vibration detection thresholds of the index finger were partially impaired and statistically different (P<0.05) when compared with controls, suggesting a reduction of tactile acuity in the territory of the median nerve. The thermal thresholds were identical in both groups, suggesting that the small-diameter fibres were not affected. When a small object was lifted and positioned in space, the coordination between the grip force and the vertical lifting force did not seem to be affected in our patients. They were able to modify their grip force according to the friction between the fingertips and the object, i.e. the more slippery the object, the higher the grip force. The unimanual Purdue Pegboard subtest results suggest that digital dexterity was also not significantly perturbed in our sample of CTS patients when compared with controls. Despite the severe abnormalities of median nerve conduction, our results suggest that chronic median nerve compression occurring in CTS induces partial impairment of tactile sensibility with minor impact on grasp force regulation and digital dexterity. Received: 25 September 1998 / Accepted: 10 December 1998     

The stability of precision grip forces during cyclic arm movements with a hand-held load

In this paper we examine the coordination of grip force and load during brisk cyclic arm movements with a hand-held object under a range of conditions. We show that, regardless of the surface texture of the object or movement frequency, grip force is modulated in parallel with load. Thus, the tight coupling between grip force and load observed in short-duration tasks such as lifting or point-to-point movements is also seen in longer-duration cyclic movements. Moreover, the gain of the relation between grip force and load remains essentially constant over time. Across conditions, we find a dissociation between the gain of the relation between grip force and load and the grip force offset. With a more slippery surface texture both the gain and offset increase, whereas increases in frequency lead to an increase in the offset but a decrease in gain. This suggests that these two parameters are under independent high-level control. We also observe that when subjects were instructed to maintain a high-baseline grip force during the movement, grip force was still modulated with load even though an increase in grip was not necessary to prevent slip. This suggests that there is an obligatory coupling between grip force and load. This coupling might be subserved by low-level mechanisms not under high-level control.     

Aging reduces the ability to change grip force and balance control simultaneously

The purpose of this study was to investigate the change in the fingertip forces and balance control of young adults and older adults. The subjects lifted an object of constant weight (i.e., 1500 g) using their right hand, first in a seated position and then in a standing position. We quantified the ability of the participants to adjust their fingertip forces across trials by comparing the percentage of change in the peak grip force, peak load force and the ratio between peak grip force and peak load force. Moreover, we quantified their ability to stabilize their balance following the lifting of the object in the standing condition. The results showed that in both conditions young adults reduced their peak grip force much more than older adults across trials. In the seated condition, young adults increased slightly their peak load force, across trials, while older adults reduced it. In the standing condition, both groups showed similar change in peak load force across trials. Remarkably, older adults improved their balance stability similarly to young adults in the standing condition. This observation suggests that the ability of the older adults to modulate grip force applied to an object while standing is diminished probably to dedicate more attention to the balance control task rather than fine-tuning the grip force. Reducing balance instability following repetitive lifting is certainly more beneficial as the consequences of a fall could be more dramatic than dropping a cup of coffee.     

Basic motor capacity in relation to object manipulation and general manual ability in young children with spastic cerebral palsy

Objective: Limited resources in terms of elementary functions may be a limiting factor for functional activities. The objective of the study was to examine basic hand motor capacities in young children with bilateral spastic cerebral palsy (BSCP) and to compare with deficits in functional activities. Method: Eighty-eight children with BSCP, 3–6 years of age, manipulated a grip object (200 g) equipped with a uniaxial force sensor. Basic motor capacity was assessed based upon (1) maximal grip strength and (2) production of fast repetitive grip force changes (FFC) while holding the object on the table. Subjects’ performance on this task was compared to the grip force amplitude and force rate assessed while the subject was lifting the same object. Results were compared between different degrees of manual ability according to the Manual Ability Classification System (MACS). Results: In children with BSCP, even in high-functioning children with MACS 1, fast grip force changes and grip strength were 2 SDs and more below the mean of controls. Differences increased from MACS 2 to 4 but not between MACS 1 and 2. During lifting children with BSCP used considerable proportions of their maximum grip strength (40–90%) and of their grip force rates during 70% vs. 86%. In some children with low manual abilities (MACS 3/4), grip force rates during lifting were higher than during FFC. Conclusion: In children with BSCP, basic motor capacity may influence manual ability, particularly in children with MACS 3 and 4. In some of these children, the underlying processes during lifting may also differ qualitatively.     

Maximal force,force/time and activation/coactivation characteristics of the neck muscles in extension and flexion in healthy men and women at different ages

This study examined the force production characteristics, activation/coactivation and endurance capacity of the neck extension and flexion muscles in healthy men (n=29) and women (n=28) divided into three age groups (18–26 years, 30–37 years and 45–55 years). Force and electromyography (EMG) measurements were performed during the maximal voluntary isometric extension and flexion actions. This was followed by an endurance test (ET; 60% force level of maximal voluntary contraction sustained until exhaustion), after which the force and EMG recordings were repeated. Men were both stronger and had higher values (P<0.001) for explosive force (rate of force development, RFD) than women in both actions. Younger subjects of both genders exhibited larger (P<0.05 in women) RFD values than older subjects in extension. The coactivation of the antagonist muscles during the maximal extension or flexion did not differ significantly between men and women, but the coactivation of the antagonists was larger (P<0.05) in the older age groups than in the youngest group. Women maintained the 60% force level longer than men in both actions (extension, P<0.001; flexion, not significant). The fatiguing loading led to significant decreases in maximal isometric force (P<0.001) and RFD (P<0.01–0.001), but these relative decreases did not differ between the groups. In conclusion, large gender differences in the voluntary extension and flexion force production characteristics of the neck muscles did exist, as reported earlier for other muscles of the body. No age-related differences were observed in maximal force of the extension and flexion actions within the age ranges of the subject groups studied here, but the older subjects exhibited greater coactivation and produced lower force values in the early portions of the force/time curve of the extension than the youngest group. The data indicate that explosive force production may be sensitive to aging earlier than maximal strength in the case of the neck extensor muscles. Electronic Publication     

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.     

Development of human precision grip

The adaptation of the grip forces to the frictional condition between the digits and an object relies on feedforward sensorimotor mechanisms that use tactile afferent input to intermittently update a sensorimotor memory that controls the force coordination, i.e., the ratio between grip force (normal to the grip surface) and load force (tangential to the grip surface). The present study addressed the development of these mechanisms. Eighty-nine children and 15 adults lifted an instrumented object with exchangeable grip surfaces measuring the grip and load forces. Particularly in trials with high friction (sandpaper), the youngest children used a high grip force to load force ratio. Although this large safety margin against slips indicated an immature capacity to adapt to the frictional condition, higher grip forces were produced for more slippery material (silk versus sandpaper). The safety margin decreased during the first 5 years of age, in parallel with a lower variability in the grip force and a better adaptation to the current frictional condition. The youngest children (18 months) could adapt the grip force to load force ratio to the frictional condition in a series of lifts when the same surface structure was presented in blocks of trials, but failed when the surface structure was unpredictably changed between subsequent lifts. The need for repetitive presentation suggests a poor capacity to form a sensorimotor memory representation of the friction or an immature capacity to control the employed ratio from this representation. The memory effects, reflected by the influences of the frictional condition in the previous trial, gradually increased with age. Older children required a few lifts and adults only one lift to update their force coordination to a new friction. Hence, the present finding suggests that young children use excessive grip force, a strategy to avoid frictional slips, to compensate for an immature tactile control of the precision grip.     

Tactile impairments cannot explain the effect of age on a grasp and lift task

This experiment addressed the often-posed theory that age-related declines in manual dexterity result from diminished tactile function. We measured the time ’young’ subjects (n=33; mean=45 years) and ’old’ subjects (n=33; mean=74 years) needed to grip (thumb and index finger), lift, and transport a small metal sphere when vision was permitted and when blindfolded. Subjects began each trial by reaching for the sphere and were instructed to complete the entire task quickly. In the absence of visual information, placement of the finger and thumb for a secure grip and lift cannot be performed efficiently without tactile information. If age-related tactile changes are functionally significant for this task, then without visual information the ’old’ group should show a disproportionate increase in the duration of the grip and lift phase of the task compared to the ’young’ group. Perceptual thresholds for tactile pressure stimuli (Semmes-Weinstein filaments) confirmed well-known age-related changes. Age and vision effects were manifest mainly during the grip-lift phase (time from object contact to lift-off from its support surface), with the expected finding that the ’old’ group required more time than ’young’ group, regardless of visual condition. The main finding was that the ’grip-lift’ duration in the ’no-vision’ condition was about twice the duration observed in the ’vision’ condition for both age groups (ratios of 2.1 and 2.3 for ’young’ and ’old’, respectively). This similar relative slowing for the two groups fails to support the hypothesis that old adults’ ability to grip and lift the object was limited by changes in the availability or use of tactile information. Received: 30 September 1997 / Accepted: 26 January 1998     

Development of human precision grip

When an object held by a precision grip is subjected to an abrupt vertical load perturbation, somatosensory input from the digits triggers an increase in grip force to restore an adequate safety margin, preventing frictional slips. In adults the response occurs after a latency of 60–80 ms. In the present study, children from 2 years old upward and adults grasped and lifted an object using a precision grip. Sudden, unpredicted increases in load force (tangential to the grip surfaces) were induced by the experimenter by dropping a small disc on to a receptacle attached to the object. The impact elicited a grip force response which in young children had a longer latency and a smaller amplitude than was seen in adults. The grip response latency gradually become shorter and its amplitude increased with increasing age, reaching adult values at 6–10 years. The muscle activity underlying the response could have several bursts. The adults showed one brisk response, appearing 40–50 ms after impact, in extrinsic and intrinsic hand muscles, while younger children also exhibited a short-latency burst, appearing about 20 ms after impact. It is suggested that the short-latency response was mediated via spinal pathways, and that these pathways are disengaged by supraspinal centers during development. In a predictable loading situation, when subjects dropped the disc themselves into the receptacle using the contralateral hand, they changed strategy. Adults induced a well-timed anticipatory grip force increase prior to the impact that was scaled to the weight of the object. The youngest children did not time the force increase properly in relation to the impact. Yet, they could scale their anticipatory grip force increase with respect to the weight of the dropped disc. This suggests a well-developed capacity to use information about the weight of objects held by one hand to parameterize a programmed force output to the other hand.     

Force recovery after eccentric exercise in males and females

In this study we investigated force loss and recovery after eccentric exercise, and further characterized profound losses in muscle function (n=192 subjects – 98 males, 94 females; population A). Maximal voluntary contractile force (MVC) was assessed before, immediately after, and at 36 and 132 h after eccentric exercise. Two groups were then established (A₁ and A₂). Group A₁ demonstrated a >70% reduction in MVC immediately after exercise, but were recovering at 132 h after exercise. These subjects performed a follow-up MVC 26 days later (n=32). Group A₂ demonstrated a >70% reduction in MVC immediately post-exercise, but still exhibited a >65% reduction in force at 132 h post-exercise; these subjects also performed a follow-up MVC every 7 days until full recovery was established (n=9). In population A, there was a 57% reduction in MVC immediately post-exercise and a 67% recovery by 132 h post-exercise (P < 0.01), with no significant gender differences (P > 0.05). In group A₁, although more females (two-thirds) showed large force losses after exercise, these females demonstrated greater %MVC recovery at 132 h post-exercise (59% vs 44%) and at 26 days post-exercise (93% vs 81%) compared to the males. In group A₂, MVC recovery occurred between 33 and 47 days post-exercise. In conclusion, 21% of all subjects showed a delayed recovery in MVC after high-force eccentric exercise. Although there were no significant gender differences in force loss, a disproportionately larger number of females demonstrated force reductions of >70%. However, their recovery of force was more rapid than that observed for the males who also demonstrated a >70% force loss. Accepted: 2 October 2000     

Protein supplementation before and after resistance training in older men

We determined the effects of protein supplementation immediately before (PRO-B) and after (PRO-A) resistance training (RT; 12 weeks) in older men (59–76 years), and whether this reduces deficits in muscle mass and strength compared to younger men (18–40 years). Older men were randomized to PRO-B (0.3 g/kg protein before RT + placebo after RT, n=9), PRO-A (placebo before + protein after RT, n=10), or PLA (placebo before and after RT, n=10). Lean tissue mass, muscle thickness of the elbow, knee, and ankle flexors and extensors, and leg and bench press strength were measured before and after RT and compared to databases of younger subjects (n=22–60). Myofibrillar protein degradation (3-methylhistidine) and bone resorption (cross-linked N-telopeptides) were also measured before and after RT. Lean tissue mass, muscle thickness (except ankle dorsi flexors), and strength increased with training (P<0.05), with little difference between groups. There were no changes in 3-methylhistidine or cross-linked N-telopeptides. Before RT, all measures were lower in the older compared to younger groups (P<0.05), except for elbow extensor muscle thickness. Following training, muscle thickness of the elbow flexors and ankle dorsi flexors and leg press strength were no longer different than the young, and elbow extensor muscle thickness was greater in the old men (P<0.05). Supplementation with protein before or after training has no effect on muscle mass and strength in older men. RT was sufficient to overcome deficits in muscle size of the elbow flexors and ankle dorsi flexors and leg press strength in older compared to younger men.     

Roles of glabrous skin receptors and sensorimotor memory in automatic control of precision grip when lifting rougher or more slippery objects

Summary To be successful, precision manipulation of small objects requires a refined coordination of forces excerted on the object by the tips of the fingers and thumb. The present paper deals quantitatively with the regulation of the coordination between the grip force and the vertical lifting force, denoted as the load force, while small objects were lifted, positioned in space and replaced by human subjects using the pinch grip. It was shown that the grip force changed in parallel with the load force generated by the subject to overcome various forces counteracting the intended manipulation. The balance between the two forces was adapted to the friction between the skin and the object providing a relatively small safety margin to prevent slips, i.e. the more slippery the object the higher the grip force at any given load force. Experiments with local anaesthesia indicated that this adaptation was dependent on cutaneous afferent input. Afferent information related to the frictional condition could influence the force coordination already about 0.1 s after the object was initially gripped, i.e. approximately at the time the grip and load forces began to increase in parallel. Further, secondary, adjustments of the force balance could occur later in response to small short-lasting slips, revealed as vibrations in the object. The new force balance following slips was maintained, indicating that the relationship between the two forces was set on the basis of a memory trace. Its updating was most likely accounted for by tactile afferent information entering intermittently at inappropriate force coordination, e.g. as during slips. The latencies between the onset of such slips and the appearance of the adjustments (0.06–0.08 s) clearly indicated that the underlying neural mechanisms operated highly automatically.     

Manipulation of a fragile object by elderly individuals

We investigated strategies of healthy elderly participants (74–84 years old) during prehension and transport of an object with varying degrees of fragility. Fragility was specified as the maximal normal force that the object could withstand without collapsing. Specifically, kinetic and kinematic variables as well as and force covariation indices were quantified and compared to those shown by young healthy persons (19–28 years old). We tested three hypotheses related to age-related changes in two safety margins (slip safety margin and crush safety margin) and indices of force covariation. Compared to young controls, elderly individuals exhibited a decrease in object acceleration and an increase in movement time, an increase in grip force production, a decrease in the correlation between grip and load forces, an overall decrease in indices of multi-digit synergies, and lower safety margin indices computed with respect to both dropping and crushing the object. Elderly participants preferred to be at a relatively lower risk of crushing the object even if this led to a higher risk of dropping it. Both groups showed an increase in the index of synergy stabilizing total normal force produced by the four fingers with increased fragility of the object. Age-related changes are viewed as a direct result of physiological changes due to aging, not adaptation to object fragility. Such changes in overall characteristics of prehension likely reflect diminished synergic control by the central nervous system of finger forces with aging. The findings corroborate an earlier hypothesis on an age-related shift from synergic to element-based control.     

The mechanical properties of the heel pad in elderly adults

The shock absorbing characteristics of the heel pad in vivo were examined in two groups of active elderly individuals whose ages ranged between 60 and 67 years (n = 10) and between 71 and 86 years (n = 10). For comparative purposes, young adults (n = 10) aged between 17 and 30 years were also examined. A free-fall impact testing device which consisted of an instrumented shaft (mass 5 kg), accelerometer and position detection transducer was used to obtain deceleration and deformation of the heel during impact. The data were obtained from impact velocities of 0.57 m · s^–1 (slow) and 0.94 m · s^–1 (fast). Peak values of the deceleration and deformation, as well as the time to these peaks from onset of impact, and energy absorption were evaluated. At the slow impact velocity, no age effect was found for the parameters examined except for the energy absorption. At the fast impact velocity, there was higher peak deceleration and smaller deformation for the elderly than for the younger adults. The energy absorbed was less for the elderly than for the younger adults. It was concluded that the capacity for shock absorbency of the heel pad declines with age.     

The influence of competing perceptual and motor priors in the context of the size–weight illusion

When lifting objects of identical mass but different sizes, people perceive the smaller objects as weighing more than the larger ones (the ‘size–weight’ illusion, SWI). While individual’s grip and load force rates are rapidly scaled to the objects’ actual mass, the magnitude of the force used to lift these SWI-inducing objects is rarely discussed. Here, we show that participants continue to apply a greater loading force to a large SWI-inducing cube than to a small SWI cube, lift after lift. These differences in load force persisted long after initial errors in grip and load force rates had been corrected. Interestingly, participants who showed the largest illusion made the smallest errors in load force. This unexpected relationship suggests that the motor system is consistently biased toward the expectations of heaviness for a particular stimulus in a Bayesian fashion, and that this loading error is subsequently reduced by SWI perceptual errors in the opposite direction.