How quickly can healthy adults move their hands to intercept an approaching object? Age and gender effects

A method for simulating forward falls and controlling impact velocity

Assessment of protective arm reactions associated with forward falls are typically performed by dropping research participants from a height onto a landing surface. The impact velocity is generally modulated by controlling the total height of the fall. This contrasts with an actual fall where the fall velocity is dependent on several factors in addition to fall height and not likely predictable at the onset of the fall. A counterweight and pulley system can be used to modulate the fall velocity in simulated forward falls in a manner that is not predictable to study participants, enhancing experimental validity. However, predicting the fall velocity based on participant height and weight and counterweight mass is not straightforward. In this article, the design of the FALL simulator For Injury prevention Training and assessment (FALL FIT) system is described. A dynamic model of the FALL FIT and counterweight system is developed and model parameters are fit using nonlinear optimization and experimental data. The fitted model enables prediction of fall velocity as a function of participant height and weight and counterweight load. The method can be used to provide controllable perturbations thereby elucidating the control strategy used when protecting the body from injury in a forward fall, how the control strategy changes because of aging or dysfunction or as a method for progressive protective arm reaction training.•Construction of device to simulate forward falls with controllable impact velocity using material that are commercially available is described•A dynamic model of the FALL FIT is developed to estimate the impact velocity of a simulated forward fall using participant height and counterweight load•The dynamic model is validated using data from 3 previous studies.

Age-related changes in protective arm reaction kinematics, kinetics, and neuromuscular activation during evoked forward falls

Fall related injuries in older adults are a major healthcare concern. During a fall, the hands and arms play an important role in minimizing trauma from ground impact. Although older adults are able to orient the hands and arms into a protective orientation after falling and prior to ground impact, an inability to avoid increased body impact occurs with age. Previous investigations have generally studied rapid arm movements in the pre-impact phase or absorbing energy in the post-impact phase. There are no known studies that have directly examined both the pre-impact and post-impact phase in sequence in a forward fall. The aim of this study was to identify age-related biomechanical and neuromuscular changes in evoked arm reactions in response to forward falls that may increase fall injury risk. Fourteen younger and 15 older adults participated. Falls were simulated while standing with torso and legs restrained via a moving pendulum system from 4 different initial lean angles. While there was not a significant age-related difference in the amount of energy absorbed post-impact (p = 0.68), older adults exhibited an 11% smaller maximum vertical ground reaction force when normalized to body weight (p = 0.031), and 8 degrees less elbow extension at impact (p = 0.045). A significant interaction between age and initial lean angle (p = 0.024), indicated that older adults required 54%, 54%, 41%, and 57% greater elbow angular displacement after impact at the low, medium, medium-high, and high initial lean angles compared to younger adults. These results suggested older adults may be at greater risk of increased body impact due to increased elbow flexion angular displacement after impact when the hands and arms are able to contact the ground first. Both groups exhibited robust modulation to the initial lean angle with no observed age-related differences in the initial onset timing or amplitude of muscle activation levels. There were no significant age-related differences in the EMG timing, amplitude or co-activation of muscle activation preceding impact or following impact indicating comparable neuromotor response patterns between older and younger adults. These results suggest that aging changes in muscular elements may be more implicated in the observed differences than changes in neuromuscular capacity. Future work is needed to test the efficacy of different modalities (e.g. instruction, strength, power, perturbation training, fall landing techniques) aimed at reducing fall injury risk.

Age-related deficits in rapid visuomotor decision-making

Many goal-directed actions that require rapid visuomotor planning and perceptual decision-making are affected in older adults, causing difficulties in execution of many functional activities of daily living. Visuomotor planning and perceptual identification are mediated by the dorsal and ventral visual streams, respectively, but it is unclear how age-induced changes in sensory processing in these streams contribute to declines in visuomotor decision-making performance. Previously, we showed that in young adults, task demands influenced movement strategies during visuomotor decision-making, reflecting differential integration of sensory information between the two streams. Here, we asked the question if older adults would exhibit deficits in interactions between the two streams during demanding motor tasks. Older adults (n = 15) and young controls (n = 26) performed reaching or interception movements toward virtual objects. In some blocks of trials, participants also had to select an appropriate movement goal based on the shape of the object. Our results showed that older adults corrected fewer initial decision errors during both reaching and interception movements. During the interception decision task, older adults made more decision- and execution-related errors than young adults, which were related to early initiation of their movements. Together, these results suggest that older adults have a reduced ability to integrate new perceptual information to guide online action, which may reflect impaired ventral-dorsal stream interactions.NEW & NOTEWORTHY Older adults show declines in vision, decision-making, and motor control, which can lead to functional limitations. We used a rapid visuomotor decision task to examine how these deficits may interact to affect task performance. Compared with healthy young adults, older adults made more errors in both decision-making and motor execution, especially when the task required intercepting moving targets. This suggests that age-related declines in integrating perceptual and motor information may contribute to functional deficits.

Does functional performance and upper body strength predict upper extremity reaction and movement time in older women?

BACKGROUND

Reaction time to initiate upper limb movement and movement time to place hands on the landing surface may be important factors in forward fall landing and impact, contributing to injury reduction. The aim was to investigate the relationship of physical function and upper body strength to upper limb reaction and movement time in older female participants.

METHODS

75 female participants (72 ± 8 yrs) performed 5 arm response trials. Reaction time (signal to initiation of movement), and movement time (initial movement to contact), were collected using 3D motion capture. Additional variables were: handgrip; sit-to-stand; shoulder flexion and elbow extension strength measured by hand-held dynamometry; one-legged balance; fall risk; and physical activity scores. Prediction variables for reaction and movement time were determined in separate backward selection multiple regression analyses. Significance was set at P < 0.05.

FINDINGS

Significant regression equations for RT (r² = 0.08, P = 0.013) found a relationship between stronger handgrip (Beta = -0.002) and faster reaction time, accounting for 8% variance. For movement time (r² = 0.06, P = 0.036) greater shoulder flexion strength (Beta = -0.04) was related to faster movement time, explaining 6% variance. Stronger SF strength was related to a decrease in MT by 4%.

DISCUSSION

A relationship between arm strength measures and faster upper body reaction and movement time was shown, with 10-20% higher strength associated with a 5% faster response time. Even though this was a relatively weak relationship, given that strength is a modifiable component this provides a potential avenue for future intervention efforts. This in turn could have an impact on forward fall landing and potential reduction of injury risk.

Protective arm movements are modulated with fall height

Protective arm reactions were evoked in 14 younger adults to determine the effect of fall height on protective arm reaction biomechanics. Participants were supported in a forward-leaning position on top of an inverted pendulum that isolated arm reaction by preventing any fall arresting contribution that may come from the ankle, knees, or hip. At an unpredictable time, the pendulum was released requiring participants to rapidly orient their arms to protect the head and body. Vertical ground reaction force (vGRF), arm kinematics, and electromyographic (EMG) measures of the biceps and triceps were compared at four initial lean angles. The time following perturbation onset and prior to impact consisted of two phases: rapid extension of the elbows and co-activation of the biceps and triceps in preparation for impact. The rapid orientation phase was modulated with fall height while the co-activation of the biceps and triceps in preparation for landing was minimally affected. Larger lean angles resulted in increased vGRF, increased elbow extension at impact, decreased elbow angular extension velocity at impact, and increased neck velocity at impact while hand velocity at impact was not significantly affected. The neuromuscular control strategy appears to optimize elbow extension angle/angular velocity prior to co-activation of the biceps and triceps that occurs about 100 ms prior to impact. Future work should investigate how the neuromuscular control strategy handles delayed deployment of protective arm reactions.

Analysis of the visual spatiotemporal properties of American Sign Language

Careful measurements of the temporal dynamics of speech have provided important insights into phonetic properties of spoken languages, which are important for understanding auditory perception. By contrast, analytic quantification of the visual properties of signed languages is still largely uncharted. Exposure to sign language is a unique experience that could shape and modify low-level visual processing for those who use it regularly (i.e., what we refer to as the Enhanced Exposure Hypothesis). The purpose of the current study was to characterize the visual spatiotemporal properties of American Sign Language (ASL) so that future studies can test the enhanced exposure hypothesis in signers, with the prediction that altered vision should be observed within, more so than outside, the range of properties found in ASL. Using an ultrasonic motion tracking system, we recorded the hand position in 3-dimensional space over time during sign language production of signs, sentences, and narratives. From these data, we calculated several metrics: hand position and eccentricity in space and hand motion speed. For individual signs, we also measured total distance travelled by the dominant hand and total duration of each sign. These metrics were found to fall within a selective range, suggesting that exposure to signs is a specific and unique visual experience, which might alter visual perceptual abilities in signers for visual information within the experienced range, even for non-language stimuli.

Biomechanical and physiological age differences in a simulated forward fall on outstretched hands in women

BACKGROUND

Falling on the outstretched hands, a protective mechanism to arrest the body and avoid injury, requires upper limb and trunk motor control for effective body descent. Older women are particularly susceptible to injury from a forward fall, but the biomechanical and physiological (e.g., muscle strength) factors related to this increased risk are poorly understood. Determining age differences in the modifiable neuromuscular factors related to a forward fall landing and descent could help to inform injury prevention strategies. The purpose was to investigate age related differences in upper extremity strength and fall arrest strategy differences during a simulated fall and to evaluate the relationships between muscle strength and biomechanical variables.

METHODS

Nineteen younger (mean age 23.0 yrs., SD 3.8) and 16 older (mean age 68.2 yrs., SD 5.3) women performed five trials of simulated falls. Biomechanical measures and electromyographic muscle activity were recorded during the descents. Concentric, isometric and eccentric strength of the non-dominant upper limb was measured via a dynamometer using a customized protocol.

FINDINGS

Older women demonstrated lower concentric elbow extension strength compared to younger women (p = 0.002). Landing strategies differed where younger women had significantly greater elbow joint angle (p = 0.006) and velocity (p = 0.02) at impact. Older women demonstrated diminished capacity to absorb energy and control descent on outstretched hands compared to younger women (p = 0.001).

INTERPRETATION

The landing strategy used by older women along with decreased energy absorption may increase risk of fall-related injury and increase the likelihood of trunk or head impact with the ground.

Motor cognitive processing speed estimation among the primary schoolchildren by deriving prediction formula: A cross-sectional study

OBJECTIVES

Motor cognitive processing speed (MCPS) is often reported in terms of reaction time. In spite of being a significant indicator of function, behavior, and performance, MCPS is rarely used in clinics and schools to identify kids with slowed motor cognitive processing. The reason behind this is the lack of availability of convenient formula to estimate MCPS. Thereby, the aim of this study is to estimate the MCPS in the primary schoolchildren.

MATERIALS AND METHODS

Two hundred and four primary schoolchildren, aged 6-12 years, were recruited by the cluster sampling method for this cross-sectional study. MCPS was estimated by the ruler drop method (RDM). By this method, a metallic stainless steel ruler was suspended vertically such that 5 cm graduation of the lower was aligned between the web space of the child's hand, and the child was asked to catch the moving ruler as quickly as possible, once released from the examiner's hand. Distance the ruler traveled was recorded and converted into time, which is the MCPS. Multiple regression analysis of variables was performed to determine the influence of independent variables on MCPS.

RESULTS

Mean MCPS of the entire sample of 204 primary schoolchildren is 230.01 ms ± 26.5 standard deviation (95% confidence interval; 226.4-233.7 ms) that ranged from 162.9 to 321.6 ms. By stepwise regression analysis, we derived the regression equation, MCPS (ms) = 279.625-5.495 × age, with 41.3% (R = 0.413) predictability and 17.1% (R² = 0.171 and adjusted R² = 0.166) variability.

CONCLUSION

MCPS prediction formula through RDM in the primary schoolchildren has been established.

Factors influencing ball-player impact probability in youth baseball

Background:

Altering the weight of baseballs for youth play has been studied out of concern for player safety. Research has shown that decreasing the weight of baseballs may limit the severity of both chronic arm and collision injuries. Unfortunately, reducing the weight of the ball also increases its exit velocity, leaving pitchers and nonpitchers with less time to defend themselves. The purpose of this study was to examine impact probability for pitchers and nonpitchers.

Hypothesis:

Reducing the available time to respond by 10% (expected from reducing ball weight from 142 g to 113 g) would increase impact probability for pitchers and nonpitchers, and players’ mean simple response time would be a primary predictor of impact probability for all participants.

Study Design:

Nineteen subjects between the ages of 9 and 13 years performed 3 experiments in a controlled laboratory setting: a simple response time test, an avoidance response time test, and a pitching response time test.

Methods:

Each subject performed these tests in order. The simple reaction time test tested the subjects’ mean simple response time, the avoidance reaction time test tested the subjects’ ability to avoid a simulated batted ball as a fielder, and the pitching reaction time test tested the subjects’ ability to avoid a simulated batted ball as a pitcher.

Results:

Reducing the weight of a standard baseball from 142 g to 113 g led to a less than 5% increase in impact probability for nonpitchers. However, the results indicate that the impact probability for pitchers could increase by more than 25%.

Conclusion:

Pitching may greatly increase the amount of time needed to react and defend oneself from a batted ball.

Clinical Relevance:

Impact injuries to youth baseball players may increase if a 113-g ball is used.

How gender and task difficulty affect a sport-protective response in young adults

We tested the hypotheses that gender and task difficulty affect the reaction, movement, and total response times associated with performing a head protective response. Twenty-four healthy young adults (13 females) performed a protective response by raising their hands from waist level to block a foam ball fired at their head from an air cannon. Participants initially stood 8.25 m away from the cannon ('low difficulty'), and were moved successively closer in 60 cm increments until they failed to block at least five of eight balls ('high difficulty'). Limb motion was quantified using optoelectronic markers on the participants' left wrist. Males had significantly faster total response times (P = 0.042), a trend towards faster movement times (P = 0.054), and faster peak wrist velocity (P < 0.001) and acceleration (P = 0.032) than females. Reaction time, movement time, and total response time were significantly faster under high difficulty conditions for both genders (P < 0.001). This study suggests that baseball and softball pitchers and fielders should have sufficient time to protect their head from a batted ball under optimal conditions if they are adequately prepared for the task.

Reducing hip fracture risk during sideways falls: Evidence in young adults of the protective effects of impact to the hands and stepping

Hip fracture is rare in young adults, despite evidence that the energy available in a fall is sufficient to fracture the young proximal femur. This might be explained by protective responses that allow young individuals to avoid hip impact during sideways falls. To test this hypothesis, we conducted experiments with 44 individuals (31 women and 13 men) aged 19-26 years, who were instructed to try to maintain balance after a sudden unpredictable sideways translation was applied to the platform they stood upon. While the surface adjacent to the platform was formed of gymnasium mats, we provided no information on surface compliance, or the direction and speed of the perturbation. Ninety percent of participants fell and impacted the pelvis, and 98% of those cases involved direct impact to the hip region. Impact occurred to the hand in 98% of falls, and preceded impact to the pelvis by 50 ms on average (SD=40, range=-12-175 ms). The impact velocity of the pelvis decreased 3.6% for every 10 ms increase in the interval between hand and pelvis impact, and was reduced by 22% on average by stepping prior to impact. Our results suggest that the lack of hip fractures in young adults cannot be explained by avoidance of hip impact during sideways falls. Rather, it probably relates to use of the hands and stepping, and by simply possessing sufficient bone strength to withstand the direct blow to the greater trochanter that tends to accompany sideways falls.

Time Requirement for Young and Elderly Women to Move Into a Position for Breaking a Fall With Outstretched Hands

BACKGROUND

Risk for hip fracture during a fall is reduced by contacting the ground first with the outstretched hands. However, it is unclear whether the time required for young and elderly individuals to move the hands into a protective position exceeds that available during a typical fall.

METHODS

We tested whether young (n = 30; aged 18-35 years) and elderly women (n = 30; aged 70-88 years) differed in the time required to move their hands into a protective position for breaking a fall. Participants stood either facing or sideways to shoulder-height targets (simulating forward and sideways falls, respectively), which they were instructed to contact as quickly as possible after hearing an aural go cue. Total contact time was partitioned into reaction time and movement time.

RESULTS

Young women contacted the targets faster than elderly women in both forward (530 +/- 60 vs 615 +/- 88 ms; p <.001) and sideways trials (658 +/- 80 vs 799 +/- 145 ms; p <.001). This difference was due to faster movement times for young participants. There was no difference between groups in reaction time.

CONCLUSIONS

Previous studies have shown that during actual falls from standing, wrist and pelvis contact occur at 680 +/- 116 and 715 +/- 160 ms, respectively. Comparing these values to our results suggests that the typical elderly woman should be able to move her hands quickly enough to break a forward fall, but not a sideways fall.

On reducing hand impact force in forward falls: results of a brief intervention in young males

OBJECTIVE

To test the working hypotheses that after a brief (10 min) intervention, (a) young adults can volitionally reduce fall-related wrist impact forces, and (b) no difference in impact force would exist between intervention and control groups at 3-weeks or 3-months follow-up.

BACKGROUND

The wrist is the most commonly fractured site in the body at any age, most often as a result of impact with the ground while arresting a forward fall.Methods. Twenty-nine healthy young male volunteers participated. A 3-month intervention group (n=10) performed five standardized forward falls before and after a 10-min instructional intervention aimed at reducing wrist impact forces during the baseline visit. They, along with a 3-month control group (n=11) who did not receive the intervention, were remeasured in five trials at 3-weeks and 3-months follow-up, without intervening practice. A baseline control group (n=8) performed the five trials, then repeated them at the baseline visit without receiving the intervention. Unilateral body segment kinematics and bilateral hand-ground impact forces were measured and the hypotheses were tested using repeated measures analysis of variance.

RESULTS

At the baseline visit, a significant group-by-trial-block interaction was found (P=0.02): the 3-month intervention group reduced their average maximum impact forces by 18% from initial values (P=0.002); the baseline control group did not do so (0.5% increase, P=0.91). The 3-month intervention (20 falls) and control (15 falls) groups did not differ at the 3-month follow-up (P=0.62); however, when the groups were combined their maximum impact force had decreased significantly (8.9%, P=0.04) over that time.

CONCLUSIONS

Healthy young males learned in 10 min to significantly reduce wrist impact forces in forward falls, but retention was poor at 3-weeks follow-up. Irrespective of group, however, after the 5 falls at 3-weeks subjects had taught themselves to reduce their impact forces at the 3-months follow-up.

RELEVANCE

A brief educational intervention can significantly reduce forward fall-related impact forces in the short term. However, with or without the brief intervention, the experience of performing between 5-10 forward falls 3 weeks apart apparently resulted in decreased impact forces over the next 2 months, thereby reducing the risk of injury in these forward falls.

Fall-related upper body injuries in the older adult: a review of the biomechanical issues

Although the epidemiology of fall-related injuries is well established for the elderly population over 65 years of age, the biomechanics of how, when and why injuries do and do not occur when arresting a fall have received relatively little attention. This paper reviews the epidemiological literature in the MEDLINE data base pertinent to the biomechanics of fall-related injuries, including data on fall rates, fall-related injury rates, fall directions and types of injuries available. It also covers primary sources not listed on MEDLINE, along with the pertinent biomechanics literature. Many falls in older adults are in a forward direction, and as a result the upper extremities are one of the most commonly injured structures, presumably in protecting the head and torso. In this review emphasis is placed on what is, and what is not, known of the biomechanical factors that determine the impact forces and injury risk associated with upper extremity injuries in forward falls. While decreased bone mineral density may be contributory, it is not a reliable predictor of fracture risk. Evidence is presented that fall-related impact forces can be reduced by appropriate volitional arrest strategies. Further theoretical and experimental research is needed to identify appropriate fall-arrest strategies for the elderly, as well as the physical capacities and skills required to do so. Inexpensive interventions might then be developed to teach safe fall-arrest techniques to older individuals.

Biomechanical simulations of forward fall arrests: effects of upper extremity arrest strategy, gender and aging-related declines in muscle strength

Computer simulation was used to predict the extent to which age-related muscle atrophy may adversely affect the safe arrest of a forward fall onto the arms. The biomechanical factors affecting the separate risks for wrist fracture or head impact were examined using a two-dimensional, 5-link, forward dynamic model. The hypothesis was tested in older females that age-related loss in muscular strength renders the use of the arms ineffective in arresting a forward fall without either a torso impact exceeding 0.5m/s or distal forearm loads sufficient to fracture the wrist. The results demonstrate that typical age-related decline in arm muscle strength substantially reduces the ability to arrest a forward fall without the elbows buckling and, therefore, a risk of torso and/or head impact. The model predicted that older women with below-average bone strength risk a Colles fracture when arresting typical falls, particularly with an extended arm.

Fall arrest strategy affects peak hand impact force in a forward fall

We measured the peak hand impact force involved in bimanually arresting a forward fall to the ground from a 1-m shoulder height in five healthy young males. The effects of three different subject instruction sets: "arrest the fall naturally"; "keep the head as far from the ground as possible"; and "minimize the peak hand forces" were studied by measuring body segment kinematics, ground reaction forces, and upper-extremity myoelectric activity. The hypotheses were tested that the (a) arrest strategy did not influence peak impact force, (b) arm configuration, impact velocity and upper-extremity electromyography (EMG) levels correlate to the peak impact force (c) and impacting the ground with one hand leading the other does not increase the impact force over that obtained with simultaneous hand use. The results show that these subjects were able to volitionally decrease the peak impact force at the wrist by an average of 27% compared with a "natural landing" (p=0.014) and 40% compared with a "stiff-arm landing" (p<0.0005). The magnitude of the peak unilateral wrist force varied from 0.65 to 1.7 body weight for these moderate falls onto a padded surface. Peak force correlated with the elbow angle at impact, wrist velocity at impact and with pre-EMG triceps activity. The force was not significantly higher for non-simultaneous hand impacts. We conclude that fall arrest strategy can substantially alter the peak impact forces applied to the distal forearm during a fall arrest. Therefore, the fall arrest strategy likely influences wrist injury risk independent of bone strength.