Review of motor control mechanisms underlying impact absorption from falls

Evidence of invariant lower-limb kinematics in anticipation of ground contact during drop-landing and drop-jumping

Gravity is a ubiquitous external force that must be considered when producing coordinated movements. Drop-landing is a popular task to study how humans cope with gravity, because anticipatory muscle activations can be released before the estimated ground contact. But the consequences of these anticipatory muscle activations have only been interpreted in terms of stiffening the lower-limbs in preparation for ground contact, without considering potential anticipatory kinematic consequences. The objective of this study is to quantify the kinematic consequences of anticipatory muscle activations in two different landing tasks, to clarify whether anticipatory muscle activations are adapted to cope with gravity, to the dynamic constraints of the movement to perform, or both. Twenty young athletes performed drop-landing and drop-jumping from a 35 cm elevated platform. Sagittal angles and angular velocities of the hip, knee, and ankle joints, and acceleration of the foot were computed, as well as the onset of joint flexions and onset of foot vertical acceleration change. We found the same pattern of anticipatory hip and knee flexion, both starting before ground contact in all participants and in both tasks. We found no anticipatory kinematics for the ankle joint. Consecutive to the hip and knee flexion, the foot accelerated upwards before ground contact. Our results show that anticipatory muscle activations used by humans have systematic and invariant kinematic consequences during the air-time phase to cope with gravity: they initiate the hip and knee joints flexion before ground contact. This strategy likely limits the amount of ground reaction forces developed to oppose the gravity external force, and completes the stiffening role already described in the literature. These two complementary consequences -rotation and stiffening- seem to serve the same purpose of protecting the skeletal system. Since gravity is ubiquitous, these automated movements must be considered in other movements involving landing phases, such as heel strikes during gait.

Interactions during falls with environmental objects: evidence from real-life falls in long-term care captured on video

BACKGROUND

Falls are the leading cause of injuries in older adults. Environmental objects (such as furniture, walls, and handrails) may act as hazards or facilitators to balance maintenance and safe landing. There is lack of objective evidence on how older adults interact with objects during falls. We addressed this gap by characterizing body part contacts with objects other than the floor during real-life falls in long-term care.

METHODS

We analyzed videos of 1759 falls experienced by 584 residents to characterize the prevalence of contacts with objects before, during, and after fall initiation. Using generalized estimating equations, we compared the prevalence of falls with versus without contact to objects after fall initiation. Using linear mixed models, we tested for differences across body parts in the probability of contacting objects after fall initiation.

RESULTS

In nearly one-third of falls, interactions with objects (e.g., trips over objects, loss of support with objects) or with other people (e.g., being pushed by another person) had a primary role in causing imbalance and initiating the fall. After fall initiation, participants contacted objects in 60% of falls, with intentional hand contacts to objects via reach-to-grasp or bracing being the most common type of interaction (Probability ± SE = 0.32 ± 0.01), followed by unintentional impacts to the torso (0.21 ± 0.01) and head (0.16 ± 0.01). Intentional hand contact to an object was more common during forward than backward falls (p < 0.001), while head and torso contacts to objects were more common during backward and sideways falls than forward falls (multiple p values ≤ 0.003). The hand most often contacted chairs, wheelchairs or couches, followed by tables or counters, walls, other people, walkers, and handrails. The head, torso, and shoulder most often contacted a wall.

CONCLUSIONS

Most falls in long-term care involved contacts with objects other than the ground, indicating that complex environments often accompany falls in long-term care. Higher probabilities of intentional hand contacts in forward falls, versus unintentional head and torso impacts in backward and sideways falls may reflect the influence of being able to visualize and adjust one's falling patterns to nearby objects.

Anodal tDCS improves the effect of neuromuscular training on the feedforward activity of lower extremity muscles in female taekwondo athletes with dynamic knee valgus

Transcranial direct current stimulation (tDCS) can increase cortical excitability of a targeted brain area. This study aimed to investigate the effect of adding anodal-tDCS (a-tDCS) to neuromuscular training (NMT) on the dynamic knee valgus (DKV) and feedforward activity (FFA) of knee muscles. Thirty-four Taekwondo athletes with DKV, were randomly assigned to either NMT + a-tDCS (N = 17) or NMT + sham tDCS (N = 17). DKV and the knee muscles' FFA at the moment of single and double-leg landing and lateral hopping tasks were evaluated before and after the interventions. DKV and FFA of the knee muscles was improved in all tasks (P < 0.05), however, between-group differences were not significant (P > 0.05). The FFA of the semitendinosus, vastus medialis, gluteus medius, and gastrocnemius muscles in the single-leg landing (P < 0.05), the gluteus medius, gluteus maximus, semitendinosus, biceps femoris, and gastrocnemius muscles in the double-leg landing (P < 0.05), and the gluteus medius, gluteus maximus, and gastrocnemius muscles in the lateral hopping (P < 0.05) tasks were significantly different between the groups. A-tDCS achieved significantly larger improvements in the feedforward activity of lower extremity muscles compared with sham-tDCS. However, between-group comparisons did not show a significant difference in DKV.

Effects of Stroboscopic Vision on Depth Jump Motor Control: A Biomechanical Analysis

Researchers commonly use the 'free-fall' paradigm to investigate motor control during landing impacts, particularly in drop landings and depth jumps (DJ). While recent studies have focused on the impact of vision on landing motor control, previous research fully removed continuous visual input, limiting ecological validity. The aim of this investigation was to evaluate the effects of stroboscopic vision on depth jump (DJ) motor control. Ground reaction forces (GRF) and lower-extremity surface electromyography (EMG) were collected for 20 young adults (11 male; 9 female) performing six depth jumps (0.51 m drop height) in each of two visual conditions (full vision vs. 3 Hz stroboscopic vision). Muscle activation magnitude was estimated from EMG signals using root-mean-square amplitudes (RMS) over specific time intervals (150 ms pre-impact; 30-60 ms, 60-85 ms, and 85-120 ms post-impact). The main effects of and interactions between vision and trial number were assessed using two-way within-subjects repeated measures analyses of variance. Peak GRF was 6.4% greater, on average, for DJs performed with stroboscopic vision compared to full vision (p = 0.042). Tibialis anterior RMS EMG during the 60-85 ms post-impact time interval was 14.1% lower for DJs performed with stroboscopic vision (p = 0.020). Vastus lateralis RMS EMG during the 85-120 ms post-impact time interval was 11.8% lower for DJs performed with stroboscopic vision (p = 0.017). Stroboscopic vision altered DJ landing mechanics and lower-extremity muscle activation. The observed increase in peak GRF and reduction in RMS EMG of the tibialis anterior and vastus lateralis post-landing may signify a higher magnitude of lower-extremity musculotendinous stiffness developed pre-landing. The results indicate measurable sensorimotor disruption for DJs performed with stroboscopic vision, warranting further research and supporting the potential use of stroboscopic vision as a sensorimotor training aid in exercise and rehabilitation. Stroboscopic vision could induce beneficial adaptations in multisensory integration, applicable to restoring sensorimotor function after injury and preventing injuries in populations experiencing landing impacts at night (e.g., military personnel).

Prepared for landing: A simple activation strategy scales muscle force to landing height

Before landing from a jump or fall, animals preactivate muscles to stiffen their limb joints but it is unclear how muscles tune limb stiffness and how collision forcefulness is anticipated. We measured electromyography and force from the lateral gastrocnemius muscle during landings in turkeys, an animal model that allows for direct measurements of muscle force. Many studies of landings in humans and other animals have found the duration of muscle preactivation to be constant, starting approximately 100 ms before impact, irrespective of fall duration. Therefore, we hypothesized a lack of relationship between fall duration (as dictated by drop height), muscle activity onset-time, and force at toe-down. Contrary to our expectations, both muscle activity and force rose from briefly after fall initiation until toe-down. Preactivation duration was proportional to fall height, while the rate of force rise was consistent across drop heights, resulting in force at landing and leg stiffness being proportional to fall height. Onset of muscle activity lagged 22 ± 7 ms (mean ± S.E.M.) from fall initiation, consistent with a reflex response initiation of the force ramp-up. Together, our results suggest that a constant (clock-like) rate of motor unit recruitment, initiated at fall initiation provides a preactivation that is proportional to drop height. The result is a tuning of pre-landing muscle force, providing a limb stiffening that is proportional to impact intensity, possibly without using information about fall distance.

Visual feedback and age affect upper limb reaching accuracy and kinematics in immersive virtual reality among healthy adults

This cross-sectional study aimed to evaluate the effect of visual feedback, age and movement repetition on the upper limb (UL) accuracy and kinematics during a reaching task in immersive virtual reality (VR). Fifty-one healthy participants were asked to perform 25 trials of a reaching task in immersive VR with and without visual feedback of their hand. They were instructed to place, as accurately and as fast as possible, a controller held in their non-dominant hand in the centre of a virtual red cube of 3 cm side length. For each trial, the end-point error (distance between the tip of the controller and the centre of the cube), a coefficient of linearity (CL), the movement time (MT), and the spectral arc length of the velocity signal (SPARC), which is a movement smoothness index, were calculated. Multivariate analyses of variance were conducted to assess the influence of visual feedback, age and trial repetition on the average end-point error, SPARC, CL and MT, and their time course throughout the 25 trials. Providing visual feedback of the hand reduced average end-point error ( P  < 0.001) and MT ( P  = 0.044), improved SPARC ( P  < 0.001) but did not affect CL ( P  = 0.07). Younger participants obtained a lower mean end-point error ( P  = 0.037), a higher SPARC ( P  = 0.021) and CL ( P  = 0.013). MT was not affected by age ( P  = 0.671). Trial repetition increased SPARC ( P  < 0.001) and CL ( P  < 0.001), and reduced MT ( P  = 0.001) but did not affect end-point error ( P  = 0.608). In conclusion, the results of this study demonstrated that providing visual feedback of the hand and being younger improves UL accuracy and movement smoothness in immersive VR. UL kinematics but not accuracy can be improved with more trial repetitions. These findings could guide the future development of protocols in clinical rehabilitation and research.

Bioinspired preactivation reflex increases robustness of walking on rough terrain

Walking on unknown and rough terrain is challenging for (bipedal) robots, while humans naturally cope with perturbations. Therefore, human strategies serve as an excellent inspiration to improve the robustness of robotic systems. Neuromusculoskeletal (NMS) models provide the necessary interface for the validation and transfer of human control strategies. Reflexes play a crucial part during normal locomotion and especially in the face of perturbations, and provide a simple, transferable, and bio-inspired control scheme. Current reflex-based NMS models are not robust to unexpected perturbations. Therefore, in this work, we propose a bio-inspired improvement of a widely used NMS walking model. In humans, different muscles show an increase in activation in anticipation of the landing at the end of the swing phase. This preactivation is not integrated in the used reflex-based walking model. We integrate this activation by adding an additional feedback loop and show that the landing is adapted and the robustness to unexpected step-down perturbations is markedly improved (from 3 to 10 cm). Scrutinizing the effect, we find that the stabilizing effect is caused by changed knee kinematics. Preactivation, therefore, acts as an accommodation strategy to cope with unexpected step-down perturbations, not requiring any detection of the perturbation. Our results indicate that such preactivation can potentially enable a bipedal system to react adequately to upcoming unexpected perturbations and is hence an effective adaptation of reflexes to cope with rough terrain. Preactivation can be ported to robots by leveraging the reflex-control scheme and improves the robustness to step-down perturbation without the need to detect the perturbation. Alternatively, the stabilizing mechanism can also be added in an anticipatory fashion by applying an additional knee torque to the contralateral knee.

The timing and amplitude of the muscular activity of the arms preceding impact in a forward fall is modulated with fall velocity

Protective arm reactions have been shown to be an important injury avoidance mechanism in unavoidable falls. Protective arm reactions have been shown to be modulated with fall height, however it is not clear if they are modulated with impact velocity. The aim of this study was to determine if protective arm reactions are modulated in response to a forward fall with an initially unpredictable impact velocity. Forward falls were evoked via sudden release of a standing pendulum support frame with adjustable counterweight to control fall acceleration and impact velocity. Thirteen younger adults (1 female) participated in this study. Counterweight load explained more than 89% of the variation of impact velocity. Angular velocity at impact decreased (p < 0.001), drop duration increased from 601 ms to 816 ms (p < 0.001), and the maximum vertical ground reaction force decreased from 64%BW to 46%BW (p < 0.001) between the small and large counterweight. Elbow angle at impact (129 degrees extension), triceps (119 ms) and biceps (98 ms) pre-impact time, and co-activation (57%) were not significantly affected by counterweight load (p-values > 0.08). Average triceps and biceps EMG amplitude decreased from 0.26 V/V to 0.19 V/V (p = 0.004) and 0.24 V/V to 0.11 V/V (p = 0.002) with increasing counterweight respectively. Protective arm reactions were modulated with fall velocity by reducing EMG amplitude with decreasing impact velocity. This demonstrates a neuromotor control strategy for managing evolving fall conditions. Future work is needed to further understand how the CNS deals with additional unpredictability (e.g., fall direction, perturbation magnitude, etc.) when deploying protective arm reactions.

Children's strategies in drop-landing

Introduction

Landing is a critical motor skill included in many activities performed in the natural environment by young children. Yet, landing is critically relevance to ensure proper stability and reduce injury. Furthermore, landing is an integral part of many fundamental motor skills which have been linked to greater physical activity, sport participation, and perceived competence in children. Our aim was to examine the drop-landing strategies of young children focusing on the lower extremity with a multi-variant approach.

Methods

Forty-four children divided into four age groups (G1:3–4.5 y, G2:4.5–6 y, G3:6–7.5 y, G4:7.5–9 y) performed 20 drop-land trials in four different conditions: predictable stationary landing, running to the left, to the right, and stay in place. Fifteen reflective markers, two force plates, and ten surface electromyography (sEMG) sensors were used to collect data. MANOVAs (Group x Condition) were conducted separately for the kinematic, kinetic, and sEMG variables.

Results

Only significant group effects were found (kinematic MANOVA p = 0.039, kinetic MANOVA p = 0.007, and sEMG MANOVA p = 0.012), suggesting that younger groups (G1, G2) differed to the older groups (G3, G4). G1 showed less knee flexion and slower ankle dorsi-flexion during the braking phase compared to G3, while G2 presented smaller ankle dorsi-flexion at the braking phase and smaller ankle range of motion than G3. Overall kinetic variables analysis showed a group difference but no group differences for any single kinetic variable alone was found. Regarding sEMG, G1 during the flight phase exhibited longer tibialis anterior and hamstrings activity than G3 and G3 &amp; G4, respectively; and an earlier start of the hamstrings' impact burst than G4. In addition, distal to proximal control was primarily used by all groups to coordinate muscle activity (in response to impact) and joint motion (after impact).

Discussion

Perhaps a developmental critical point in landing performance exists at 4–5 years of age since G1 presented the largest differences among the groups. This suggests that to improve landing strategies could start around this age. Future studies should examine if playground environments that include equipment conducive to landing and practitioners in the kindergarten schools are adequate vehicles to empower this type of intervention.

Motor control of landing in an unsteady environment

BACKGROUND

When landing from a jump or a drop, muscles contract before touchdown to anticipate imminent collision with the ground, soften ground contact and allow to return to a stable standing position without stepping or rebounding.

RESEARCH QUESTION

This study assesses the effect of the unsteadiness of the environment on the motor control of landing. The 'unsteady environment' was induced by asking participants to perform drop landings inside an aircraft that underwent trajectories parallel to Earth's surface. The participants also performed the same task in a 'steady environment' in our laboratory.

METHODS

Ground reaction forces, lower limb joints' movements and the activity of lower limb muscles were recorded. The stability of the landing was assessed by the vertical and anterior-posterior stability indexes, center of pressure measures and by the coefficient of variation of kinetic and kinematic parameters.

RESULTS

On one hand, participants slowdown their joint movements and reduce the knee joint excursion during landing, probably to avoid excessive movements that may induce imbalance. On the other hand, the stability of the landing is reduced while the variability of the movement is increased, illustrating a less stable and less consistent landing. In addition, whatever the environment, landing parameters associated with increased stiffness (i.e., increased impact forces and decreased joint range of motion) are correlated with decreased landing stability.

SIGNIFICANCE

Overall, landings in the'unsteady environment' appear to be more cautious but less stable and less finely tuned. Since the stability of the landing is not directly influenced by the steadiness of the environment, this more cautious behavior could be, at least in part, related to the fear/apprehension induced by sudden acceleration variations of the frame of the aircraft.

Association between landing biomechanics, knee pain, and kinesiophobia in athletes following anterior cruciate ligament reconstruction: A cross-sectional study

INTRODUCTION

Kinesiophobia is a fear of physical movement and activity and is known to impact return to sports and second injury after anterior cruciate ligament reconstruction.

OBJECTIVE

To determine the association among landing biomechanics, knee pain, and kinesiophobia in athletes following anterior cruciate ligament reconstruction.

DESIGN

A cross-sectional study SETTING: Clinical center of sports medicine PARTICIPANTS: This study included 31 athletes who participated in sports after primary, unilateral anterior cruciate ligament reconstruction (age, median [interquartile range]: 20.0 [7.0] years; body mass index: 22.2 [2.6] kg/m² ; time from anterior cruciate ligament reconstruction: 24.0 [23.0] months).

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Kinesiophobia was measured using the Tampa Scale for Kinesiophobia-11. Knee function included joint laxity, strength, and single-leg hop distance. Knee pain intensity during sports activities was measured using a numerical rating scale. Vertical ground reaction force was measured, and electromyography data for the vastus medialis and semitendinosus muscles during single-leg jump landing were collected. Univariate correlation analysis and multiple regression analysis were performed to identify the associations between the Tampa Scale for Kinesiophobia-11 score and outcome measures.

RESULTS

There was no correlation between peak vertical ground reaction force and Tampa Scale for Kinesiophobia-11 score (Spearman's r = -0.172, P = 0.355). On multiple regression analyses, lower preparatory vastus medialis activity during landing (beta coefficient = -0.513, P < 0.001) and knee pain intensity (beta coefficient= 0.569, P < 0.001) were significantly associated with greater Tampa Scale for Kinesiophobia-11 scores.

CONCLUSIONS

In athletes who have participated in sports following anterior cruciate ligament reconstruction, lower preparatory vastus medialis activity during landing and knee pain intensity are associated with greater kinesiophobia. These findings indicate that interventions to address the landing muscle activity pattern and pain control should be performed to decrease kinesiophobia after anterior cruciate ligament reconstruction. This article is protected by copyright. All rights reserved.

Soleus H-reflex modulation during a double-legged drop landing task

Muscle spindle afferent feedback is modulated during different phases of locomotor tasks in a way that facilitates task goals. However, only a few studies have studied H-reflex modulation during landing. This study aimed to characterize soleus (SOL) H-reflex modulation during the flight and early landing period of drop landings. Since landing presumably involves a massive increase in spindle afferent firing due to rapid SOL muscle stretching, we hypothesized H-reflex size would decrease near landing reflecting neural modulation to prevent excessive motoneuron excitation. The soleus H-reflex was recorded during drop landings from a 30 cm height in nine healthy adults. Electromyography (SOL, tibialis anterior (TA), medial gastrocnemius, and vastus lateralis), ankle and knee joint motion and ground reaction force were recorded during landings. Tibial nerve stimulation was timed to elicit H-reflexes during the flight and early ground contact period (five 30 ms Bins from 90 ms before to 60 ms after landing). The H-reflexes recorded after landing (0-30 and 30-60 ms) were significantly smaller (21-36% less) than that recorded during the flight periods (90-0 ms before ground contact; P ≤ 0.004). The decrease in H-reflex size not occurring until after ground contact indicates a time-critical modulation of reflex gain during the last 30 ms of flight (i.e., time of tibial nerve stimulation). H-reflex size reduction after ground contact supports a probable neural strategy to prevent excessive reflex-mediated muscle activation and thereby facilitates appropriate musculotendon and joint stiffness.

What Makes Parkour Unique? A Narrative Review Across Miscellaneous Academic Fields

Parkour is a growing sport that mostly involves jumping, vaulting over obstacles, and climbing in a non-dedicated setting. The authors gathered all known relevant literature across miscellaneous academic fields in order to define parkour with regard to other sports disciplines. Parkour is a lifestyle sport, and as such provides an alternative to mainstream sports, away from strict rules, standardized settings, and necessary competitions. Traceurs (parkour adepts) consider the city as a playground and as an outlet for their creativity, but they also have a strong taste for hard and individualized challenges. They usually train on non-specific structures, at ground level. Although their social background is not clear, they are mostly young and male. Traceurs are stronger than recreational athletes, especially in eccentric exercises. However, their endurance skills may be below average. One of the core specificities of parkour is its precision constraint at landing, which turns a standing long jump into a precision jump, regulated in flight so as to prepare for landing. The running precision jump follows the same landing pattern, and its flight phase contrasts with long jump techniques. Injuries, which are not more frequent than in other sports, often occur at landing and to lower limb extremities. This risk is mitigated by targeting the landing area with the forefoot instead of letting the heel hit the ground like in gymnastics, or with rolling in order to dissipate the impact. Overall, parkour focuses on adaptability to new environments, which leads to specific techniques that have not yet been extensively addressed by the literature.

Influence of Screw-Hole Defect Size on the Biomechanical Properties of Feline Femora in an Ex Vivo Model

OBJECTIVE

 The study aims to evaluate the biomechanical properties of feline femora with craniocaudal screw-hole defects of increasing diameter, subjected to three-point bending and torsion to failure at two different loading rates.

STUDY DESIGN

 Eighty femoral pairs were harvested from adult cat cadavers. For each bending and torsional experiment, there were five groups (n = 8 pairs) of increasing craniocaudal screw-hole defects (intact, 1.5 mm, 2.0 mm, 2.4 mm, 2.7mm). Mid-diaphyseal bicortical defects were created with an appropriate pilot drill-hole and tapped accordingly. Left and right femora of each pair were randomly assigned to a destructive loading protocol at low (10 mm/min; 0.5 degrees/s) or high rates (3,000 mm/min; 90 degrees/s) respectively. Stiffness, load/torque-to-failure, energy-to-failure and fracture morphology were recorded.

RESULTS

 Defect size to bone diameter ratio was significantly different between defect groups within bending and torsional experiments respectively (intact [0%; 0%], 1.5 mm [17.8%; 17.1%], 2.0 mm [22.8%; 23.5%], 2.4 mm [27.8%; 27.6%], 2.7 mm [31.1%; 32.4%]) (p < 0.001). No significant differences in stiffness and load/torque-to-failure were noted with increasing deficit sizes in all loading conditions. Screw-hole (2.7 mm) defects up to 33% bone diameter had a maximum of 20% reduction in bending and torsional strength compared with intact bone at both loading rates. Stiffness and load/torque-to-failure in both bending and torsion were increased in bones subjected to higher loading rates (p < 0.001).

CONCLUSION

 Screw-hole defects up to 2.7 mm did not significantly reduce feline bone failure properties in this ex vivo femoral study. These findings support current screw-size selection guidelines of up to 33% bone diameter as appropriate for use in feline fracture osteosynthesis.

Modulation of spatial and temporal modules in lower limb muscle activations during walking with simulated reduced gravity

Gravity plays a crucial role in shaping patterned locomotor output to maintain dynamic stability during locomotion. The present study aimed to clarify the gravity-dependent regulation of modules that organize multiple muscle activities during walking in humans. Participants walked on a treadmill at seven speeds (1-6 km h-1 and a subject- and gravity-specific speed determined by the Froude number (Fr) corresponding to 0.25) while their body weight was partially supported by a lift to simulate walking with five levels of gravity conditions from 0.07 to 1 g. Modules, i.e., muscle-weighting vectors (spatial modules) and phase-dependent activation coefficients (temporal modules), were extracted from 12 lower-limb electromyographic (EMG) activities in each gravity (Fr ~ 0.25) using nonnegative matrix factorization. Additionally, a tensor decomposition model was fit to the EMG data to quantify variables depending on the gravity conditions and walking speed with prescribed spatial and temporal modules. The results demonstrated that muscle activity could be explained by four modules from 1 to 0.16 g and three modules at 0.07 g, and the modules were shared for both spatial and temporal components among the gravity conditions. The task-dependent variables of the modules acting on the supporting phase linearly decreased with decreasing gravity, whereas that of the module contributing to activation prior to foot contact showed nonlinear U-shaped modulation. Moreover, the profiles of the gravity-dependent modulation changed as a function of walking speed. In conclusion, reduced gravity walking was achieved by regulating the contribution of prescribed spatial and temporal coordination in muscle activities.

Association and Agreement between Reactive Strength Index and Reactive Strength Index-Modified Scores

Since the reactive strength index (RSI) and reactive strength index-modified (RSI-mod) share similar nomenclature, they are commonly referred as interchangeable measures of agility in the sports research literature. The RSI and RSI-mod are most commonly derived from the performance of depth jumping (DJ) and countermovement jumping (CMJ), respectively. Given that DJ and CMJ are plyometric movements that differ materially from biomechanical and neuromotor perspectives, it is likely that the RSI and RSI-mod measure distinct aspects of neuromuscular function. The purpose of this investigation was to evaluate the association and agreement between RSI and RSI-mod scores. A mixed-sex sample of NCAA division I basketball athletes (n = 21) and active young adults (n = 26) performed three trials of DJ from drop heights of 0.51, 0.66, and 0.81 m and three trials of countermovement jumping. Using 2-dimensional videography and force platform dynamometry, RSI and RSI-mod scores were estimated from DJ and CMJ trials, respectively. Linear regression revealed moderate associations between RSI and RSI-mod scores (F = 11.0–38.1; R² = 0.20–0.47; p < 0.001–0.001). Bland–Altman plots revealed significant measurement bias (0.50–0.57) between RSI and RSI-mod scores. Bland–Altman limit of agreement intervals (1.27–1.51) were greater than the mean values for RSI (0.97–1.05) and RSI-mod (0.42) scores, suggesting poor agreement. Moreover, there were significant performance-dependent effects on measurement bias, wherein the difference between and the mean of RSI and RSI-mod scores were positively associated (F = 77.2–108.4; R² = 0.63–0.71; p < 0.001). The results are evidence that the RSI and RSI-mod cannot be regarded as interchangeable measures of reactive strength.

On the impact force analysis of two-leg landing with a flexed knee

This article looks into the effects of the initial knee flexion angle at the contact time on the peak of the impulsive lower limb forces during landing, and how these effects are related to muscular activities. The impact dynamics of drop landing is studied via a musculoskeletal model with eight Hill-type lower-limb muscles. A method is proposed for the representation of two landing strategies: landing with high and low joint stiffness. Then, in each landing strategy, the effect of the initial knee flexion angle on the peak ground reaction force (GRF), the peak knee ligaments force and the peak tibiofemoral contact force is investigated by considering different initial contact postures. It is observed that while landing with a flexed knee decreases the peak GRF in both landing strategies, it decreases the peak tibiofemoral and knee ligaments forces only in landing with low joint stiffness. Specifically, by increasing the initial knee flexion from 0° to 70°, the peak tibiofemoral and knee ligaments forces decrease monotonically by 54% and 82%, in landing with low joint stiffness. For high joint stiffness, however, as the initial knee flexion increases from 10° to 70°, the peak tibiofemoral force is seen to increase monotonically by 42% and the peak knee ligaments force is seen to have a non-monotonic behavior, first decreasing by 42%, and then, increasing by 250%. These results can be considered in training landing strategies to reduce the risk of knee injury.

Patterns of whole-body muscle activations following vertical perturbations during standing and walking

BACKGROUND

Falls commonly occur due to losses of balance associated with vertical body movements (e.g. reacting to uneven ground, street curbs). Research, however, has focused on horizontal perturbations, such as forward and backward translations of the standing surface. This study describes and compares muscle activation patterns following vertical and horizontal perturbations during standing and walking, and investigates the role of vision during standing postural responses.

METHODS

Fourteen healthy participants (ten males; 27±4 years-old) responded to downward, upward, forward, and backward perturbations while standing and walking in a virtual reality (VR) facility containing a moveable platform with an embedded treadmill; participants were also exposed to visual perturbations in which only the virtual scenery moved. We collected bilateral surface electromyography (EMG) signals from 8 muscles (tibialis anterior, rectus femoris, rectus abdominis, external oblique, gastrocnemius, biceps femoris, paraspinals, deltoids). Parameters included onset latency, duration of activation, and activation magnitude. Standing perturbations comprised dynamic-camera (congruent), static-camera (incongruent) and eyes-closed sensory conditions. ANOVAs were used to compare the effects of perturbation direction and sensory condition across muscles.

RESULTS

Vertical perturbations induced longer onset latencies and shorter durations of activation with lower activation magnitudes in comparison to horizontal perturbations (p<0.0001). Downward perturbations while standing generated earlier activation of anterior muscles to facilitate flexion (for example, p=0.0005 and p=0.0021 when comparing the early activators, rectus femoris and tibialis anterior, to a late activator, the paraspinals), whereas upward perturbations generated earlier activation of posterior muscles to facilitate extension (for example, p<0.0001 and p=0.0004, when comparing the early activators, biceps femoris and gastrocnemius, to a late activator, the rectus abdominis). Static-camera conditions induced longer onset latencies (p=0.0085 and p<0.0001 compared to eyes-closed and dynamic-camera conditions, respectively), whereas eyes-closed conditions induced longer durations of activation (p=0.0001 and p=0.0008 compared to static-camera and dynamic-camera, respectively) and larger activation magnitudes. During walking, downward perturbations promptly activated contralateral trunk and deltoid muscles (e.g., p=0.0036 for contralateral deltoid versus a late activator, the ipsilateral tibialis anterior), and upward perturbations triggered early activation of trunk flexors (e.g., p=0.0308 for contralateral rectus abdominis versus a late activator, the ipsilateral gastrocnemius). Visual perturbations elicited muscle activation in 67.7% of trials.

CONCLUSION

Our results demonstrate that vertical (vs. horizontal) perturbations generate unique balance-correcting muscle activations, which were consistent with counteracting vertical body extension induced by downward perturbations and vertical body flexion induced by upward perturbations. Availability of visual input appears to affect response efficiency, and incongruent visual input can adversely affect response triggering. Our findings have clinical implications for the design of robotic exoskeletons (to ensure user safety in dynamic balance environments) and for perturbation-based balance and gait rehabilitation.

Rate of loading, but not lower limb kinematics or muscle activity, is moderated by limb and aerial variation when surfers land aerials

We aimed to determine whether there were any differences in how surfers used their lead and trail limbs when landing two variations of a simulated aerial manoeuvre, and whether technique affected the forces generated at landing. Fifteen competitive surfers (age 20.3 ± 5.6 years, height 178.2 ± 9.16 cm, mass 71.0 ± 10.5 kg) performed a Frontside Air (FA) and Frontside Air Reverse (FAR), while we collected the impact forces, ankle and knee muscle activity, and kinematic data. A principal component analysis (PCA) was used to reduce 41 dependent variables into 10 components. A two-way MANOVA revealed that although there were no limb x aerial variation interactions, surfers generated significantly higher relative loading rates at landing for the trail limb compared to the lead limb (+28.8 BW/s; F_(1,303) = 20.660, p < 0.0001, η² = 0.064). This was likely due to the surfers "slapping" the trail limb down when landing, rather than controlling placement of the limb. Similarly, higher relative loading rates were generated when landing the FA compared to the FAR (+23.6 BW/s; F_(1,303) = 31.655, p < 0.0001, η² = 0.095), due to less time over which the forces could be dissipated. No relationships between aerial variation or limb were found for any of the kinematic or muscle activity data. Practitioners should consider the higher relative loading rates generated by a surfer's trail limb and when surfers perform a FA when designing dry-land training to improve the aerial performance of surfing athletes.

The Association of Visual Impairments of Elite Soccer Players with Concussion and Sports Injuries: A Prospective Cohort Study

Background: Visual skills play a pivotal role in athletic performance. However, in a professional setting, visual assessment is limited to a brief examination of visual acuity by the Snellen chart. This is while visual skills in sport comprise several other components. Objectives: This study aimed to evaluate the potential relationship between visual skills and sports injuries in professional soccer players. Methods: Through a prospective cohort study between September 2017 and October 2018, professional soccer league players were recruited for a complete eye examination including visual acuity, field of vision, and color discrimination as pre-competition examination. Any possible relationship between an abnormal eye finding and sports injury during the upcoming season was investigated. Results: A total of 386 male soccer players in 4 different playing positions were recruited from 16 league teams. Myopia, visual field defects, and green/blue/red color blindness were the most common visual impairments. Overall, there was no significant relationship between abnormal visual skills and the incidence of low back and upper extremity injuries among soccer players. However, a logistic regression model showed that the odds of quadriceps injury is 1.92 times higher (P-value: 0.005) for one diopter increase in both eyes' sum of refractive error. There is also an increased risk of concussion in players who have visual field defects (P-value < 0.005). Conclusions: Visual field defects can put soccer players at a higher risk for concussion. Moreover, uncorrected refractive eye errors will increase the incidence of lower limb injuries, mostly quadriceps injuries.

Acute and Delayed Effects of Fatigue on Ground Reaction Force, Lower Limb Stiffness and Coordination Asymmetries During a Landing Task

Landing is a critical phase of movement for injury occurrence, in which lower limbs should be used equally to better absorb the shock. However, it has been suggested that fatigue can lead to the appearance of asymmetries. The aim of this study was to verify the acute and delayed effects of fatigue on the lower limb asymmetry indexes of peak ground reaction force, leg stiffness and intra-limb coordination during a landing task. Fifteen physically active men performed a fatigue protocol composed of 14 sets of 10 continuous vertical jumps, with a one-minute rest interval between the sets. A step-off landing task was performed before, immediately after, 24 h and 48 h after the fatigue protocol. Two force plates and a video analysis system were used. The symmetry index equation provided the asymmetry indexes. For statistical analysis, ANOVA and effect size analysis were utilized. Inferential statistics did not show the effect of fatigue in the asymmetry indexes for any variable or condition (p > .05). Moderate effect sizes were observed for peak ground reaction force (0.61) and leg stiffness (0.61) immediately after the application of the protocol. In conclusion, fatigue does not seem to significantly change the asymmetries of lower limbs, especially regarding intra-limb coordination. The moderate effects observed for peak ground reaction force and leg stiffness asymmetries suggest that these variables may be acutely affected by fatigue.

Comparison of Modular Control during Side Cutting before and after Fatigue

The purpose of this study was to clarify the coordination between the trunk and lower limb muscles during sidestep and to compare this coordination before and after fatigue intervention. The intervention was lateral jump until exhaustion. Nonnegative matrix factorization (NMF) was used to extract muscle synergies from electromyography. Subsequently, to compare the muscle synergies, a scalar product that evaluates the coincidence of synergies was calculated. Three muscle synergies were extracted before and after the intervention from the NMF analysis. In accordance with the evaluation of the scalar product, these synergies were the same before and after the intervention. One of these synergies that engaged the internal oblique/transversus abdominis, rectus femoris, and adductor muscle was activated from before landing to midstance during sidestep motion; therefore, this synergy is thought to suppress excessive hip abduction. However, the activation timing of this synergy was delayed after the intervention (P = 0.028, effect size: 0.54, Wilcoxon test). This delay is considered to decrease hip stability. Thus, this change may induce a reduction in hip control function.

Neuromuscular Strategies in Stretch-Shortening Exercises with Increasing Drop Heights: The Role of Muscle Coactivation in Leg Stiffness and Power Propulsion

When applying drop jump exercises, knowing the magnitude of the stimulus is fundamental to stabilize the leg joints and to generate movements with the highest power. The effects of different drop heights on leg muscles coactivation, leg stiffness and power propulsion were investigated in fifteen sport science students. Drop jumps from heights of 20, 30, 40, 50, and 60 cm in a random order were performed on a force platform. During each drop jump, the ground reaction force, knee angle displacement, and synchronized surface-electromyography root-mean-square (sEMG_RMS) activity (vastus lateralis, VL; vastus medialis, VM; rectus femoris, RF; biceps femoris, BF; tibialis anterior, TA and lateral gastrocnemius, LG) were recorded. The coactivation in the pre-contact phase, between VL and BF, VM and BF as well as RF and BF, was dependent on the drop height (p < 0.01; effect size (ES) ranged from 0.45 to 0.90). Leg stiffness was dependent on the drop height (p < 0.001; ES = 0.27-0.28) and was modulated by the coactivation of VM-BF (p = 0.034) and RF-BF (p = 0.046) during the braking phase. Power propulsion was also dependent on the drop height (p < 0.001; ES = 0.34); however, it was primarily modulated by the coactivation of LG-TA during the braking phase (p = 0.002). The coactivation of thigh muscles explains leg stiffness adjustments at different drop heights. On the contrary, the coactivation of shank muscles is mostly responsible for the power propulsion.

Neuromuscular control of the ankle during pre-landing in athletes with chronic ankle instability: Insights from statistical parametric mapping and muscle co-contraction analysis

OBJECTIVE

The present study aimed to compare the neuromuscular control of the muscles around the ankle between athletes with CAI and without history of any ankle sprain (Non-CAI) by using statistic parametric mapping (SPM) and co-contraction analyses.

DESIGN

Cross-sectional study; Setting: Laboratory; Participants: 40 athletes (20 CAI, 20 Non-CAI) were pair-matched for age and gender.

MAIN OUTCOME MEASURES

Neuromuscular control was examined using surface electromyography (EMG) amplitude and muscle co-contraction 200 ms before foot-contact with the ground during a jump-landing task.

RESULTS

The EMG amplitude of tibialis anterior, peroneus longus, and gastrocnemius medialis were analyzed using statistic parametric mapping. The CAI group exhibited decreased EMG amplitude of peroneus longus during preparation for foot-contact. There were no significant co-contraction differences between groups.

CONCLUSIONS

Our findings demonstrate that SPM combined with the co-contraction provides a comprehensive EMG analysis to detect the differences of neuromuscular control between athletes with and without chronic ankle instability. Additionally, this finding indicates that CAI contributed to altered neuromuscular control during the pre-landing phase, which may contribute to re-injury mechanisms.

Timing of Muscle Activation Is Altered During Single-Leg Landing Tasks After Anterior Cruciate Ligament Reconstruction at the Time of Return to Sport

OBJECTIVES

It is well known that alterations in landing mechanics persist for years after anterior cruciate ligament reconstruction (ACL-R). Nevertheless, existing literature is controversial in reporting successful or unsuccessful recovery of prelanding muscle activation timing after ACL-R. The study aimed at comparing myoelectric and kinematic patterns during landing tasks between ACL-R and healthy subjects.

DESIGN

Cross-sectional study.

SETTING

Institutional research laboratory.

PATIENTS AND INTERVENTION

Fifteen male athletes after ACL-R using patellar tendon and 11 using hamstrings autograft at the time of return to sport were recruited. Fifteen healthy athletes served as control group. Participants performed 4 different single-leg landing tasks arriving onto a force plate.

MAIN OUTCOME MEASURES

Electromyographic (EMG) activity of knee extensors and flexors, normalized vertical ground reaction force (vGRF), and knee angular displacement were recorded.

RESULTS

In all the tasks, preimpact EMG duration was longer in ACL-R (112 ± 28 ms in the knee extensors; 200 ± 34 ms in the knee flexors) compared with healthy participants (74 ± 19 ms in the knee extensors; 153 ± 29 ms in the knee flexors; P < 0.05). Initial contact (IC) and maximum postimpact knee angle were lower in ACL-R (9 ± 7 degrees at IC; 39 ± 12 degrees at maximum flexion) compared with healthy participants (17 ± 9 degrees at IC; 52 ± 15 degrees at maximum flexion; P < 0.05). Normalized vGRF was higher in ACL-R compared with healthy participants (3.4 ± 0.5 and 2.7 ± 0.6; P < 0.05).

CONCLUSIONS

At the time of return to sport, ACL-R subjects showed altered motor control strategies of single-leg landings. These alterations may lead to uncoordinated movement, hence increasing the risk of reinjury.

Stretch reflex changes in ACL-deficient individuals and healthy controls during normal and surprise landings

No studies in ACL-D individuals have examined neuromuscular adaptations during landing from a jump where an unexpected mechanical event changes the pre-programmed course of movement. The purpose of this study was to compare pre- and post-landing muscle activation in ACL-D individuals and uninjured controls during normal and surprise landings. Nineteen ACL-D and 17 uninjured volunteered. Participants performed repeated single leg landings from 30 and 15 cm heights. During 15 cm landings, a single surprise landing was performed where participants unexpectedly fell through a false surface at 15 cm to the solid floor a further 15 cm below. Electromyography (EMG) amplitude from vastus lateralis (VL), lateral hamstrings (LH), and soleus (Sol) was recorded. Pre-landing (-60 to 0 ms), post-landing short latency (31-60 ms), and post-landing medium latency (61-90 ms) periods were examined. Comparisons in EMG amplitudes were made across limbs (ACL-D, ACL intact, and control) in 30 cm landings. Additionally, the ratio of EMG amplitude in surprise:30 cm normal landings was analyzed. Post-landing LH EMG was reduced in the ACL-D compared to control limbs at short latencies (P < 0.05). Post-landing VL EMG was reduced in the ACL-D and ACL intact compared to the control limb at both latencies (P < 0.05). Surprise landings notably increased post-landing EMG in all muscles, across all limbs (P < 0.001). However, the gain in VL EMG was significantly greater in ACL-D and ACL intact limbs (P < 0.05). These changes in neuromuscular control of ACL-D individuals during expected and surprise landings may have important implications for rehabilitation, instability, and the risk of secondary injury.

Effects of 4-week impairment-based rehabilitation on jump-landing biomechanics in chronic ankle instability patients

OBJECTIVE

To determine effects of 4-weeks of impairment-based rehabilitation on lower extremity neuromechanics during jump-landing.

DESIGN

Descriptive laboratory study.

PARTICIPANTS

Twenty-six CAI subjects (age = 21.4 ± 3.1 sex=(M = 7,F = 19), height = 169.0 ± 8.8 cm, weight = 71.0 ± 13.8 kg) completed 15 jump-landing trials prior to and following 12 supervised rehabilitation sessions.

MAIN OUTCOME MEASURES

Frontal and sagittal lower extremity kinematics and kinetics and sEMG amplitudes (anterior tibialis, peroneus brevis, peroneus longus, and medial gastrocnemius). Means and 90% confidence intervals (CIs) were calculated for 100 ms prior to and 200 ms following ground contact. Areas where pre- and post-rehabilitation CIs did not overlap were considered significantly different. Kinematic and kinetic peaks and kinematic excursion were compared with paired t-test (P ≤ 0.05).

RESULTS

Following rehabilitation, CAI subjects exhibited less ankle (2.1° (0.8, 3.4), P < 0.01) and hip (2.0° (0.5, 3.7), P = 0.01) frontal plane excursion and lower peak hip abduction (2.5° (0.0, 5.0), P = 0.05). There was less ankle (5.0° (1.7, 8.3), P < 0.01) and knee (3.4° (0.8, 6.0), P = 0.01) sagittal plane excursion following rehabilitation. There was decreased peroneus longus activity from 9 ms to 135 ms post ground contact and decreased peak plantar flexion moment (0.08 N∗m/kg (0.01, 0.13), P = 0.02) following rehabilitation.

CONCLUSION

Progressive impairment-based rehabilitation resulted in reductions in kinematic excursion and peroneus longus muscle activity, suggesting a more efficient landing strategy.

Effect of stroboscopic vision on depth jump performance in female NCAA Division I volleyball athletes

Anticipation of ground reaction force (GRF) in depth jumping requires multisensory integration of exteroceptive, vestibular, and proprioceptive inputs. Vision contributes to the anticipation of GRF in drop landings and may influence depth jump performance when disrupted. The purpose of this investigation was to evaluate the effects of stroboscopic vision on depth jump performance. Thirteen female NCAA Division I volleyball athletes completed a testing protocol consisting of 0.38 m depth jumps under condition of full vision and stroboscopic vision at strobe frequencies of 4 and 1.75 Hz. Depth jump performance was assessed via the Reactive Strength Index (RSI) and time-series vertical GRF (vGRF) data. Main effects of stroboscopic vision were evaluated for statistical significance via Repeated Measures Multivariate Analysis of Variance with post hoc multiple paired t-tests (α = 0.05). RSI (p < 0.001) and rebound jump height (p = 0.006) were lower in the 1.75 Hz stroboscopic condition versus full vision, while ground contact time (p = 0.008), and rate of vertical ground reaction force development (p = 0.016) were greater in the 1.75 Hz stroboscopic condition versus full vision. Stroboscopic vision could be used to modify the intensity of depth jumping and considered for inclusion into plyometric training.

The integration of sensory feedback in the modulation of anuran landing preparation

Controlled landing requires preparation. Mammals and bipedal birds vary how they prepare for landing by predicting the timing and magnitude of impact from the integration of visual and non-visual information. Here, we explore how the cane toad Rhinella marina - an animal that moves primarily through hopping - integrates sensory information to modulate landing preparation. Earlier work suggests that toads may modulate landing preparation using predictions of impact timing and/or magnitude based on non-visual sensory feedback during takeoff rather than visual cues about the landing itself. We disentangled takeoff and landing conditions by hopping toads off platforms of different heights while measuring electromyographic (EMG) activity of an elbow extensor (m. anconeus) and capturing high-speed images to quantify whole body and forelimb kinematics. This enabled us to test how toads integrate visual and non-visual information in landing preparation. We asked two questions: (1) when they conflict, do toads correlate landing preparation with takeoff or landing conditions? And (2) for hops with the same takeoff conditions, does visual information alter the timing of landing preparation? We found that takeoff conditions are a better predictor of the onset of landing preparation than landing conditions, but that visual information is not ignored. When hopping off higher platforms, toads start to prepare for landing later when takeoff conditions are invariant. This suggests that, unlike mammals, toads prioritize non-visual sensory feedback about takeoff conditions to coordinate landing, but that they do integrate visual information to fine-tune landing preparation.

The Within-Subjects Effects of Practice on Performance of Drop Landing in Healthy, Young Adults

Most studies of high-speed lower body movements include practice repetitions for facilitating consistency between the trials. We investigated whether 20 repetitions of drop landing (from a 30.5-cm platform onto a force plate) could improve consistency in maximum ground reaction force, linear lower body stiffness, depth of landing, and jump height in 20 healthy, young adults. Coefficient of variation was the construct for variability used to compare the first to the last five repetitions for each variable. We found that the practice had the greatest effect on maximum ground reaction force (p = .017), and had smaller and similar effects on lower body stiffness and depth of landing (p values = .074 and .044, respectively), and no measurable effect on jump height. These findings suggest that the effect of practice on drop landing differs depending upon the variable measure and that 20 repetitions significantly improve consistency in ground reaction force.

Eccentric Resistance Training in Youth: Perspectives for Long-Term Athletic Development

The purpose of this narrative review is to discuss the role of eccentric resistance training in youth and how this training modality can be utilized within long-term physical development. Current literature on responses to eccentric exercise in youth has demonstrated that potential concerns, such as fatigue and muscle damage, compared to adults are not supported. Considering the importance of resistance training for youth athletes and the benefits of eccentric training in enhancing strength, power, speed, and resistance to injury, its inclusion throughout youth may be warranted. In this review we provide a brief overview of the physiological responses to exercise in youth with specific reference to the different responses to eccentric resistance training between children, adolescents, and adults. Thereafter, we discuss the importance of ensuring that force absorption qualities are trained throughout youth and how these may be influenced by growth and maturation. In particular, we propose practical methods on how eccentric resistance training methods can be implemented in youth via the inclusion of efficient landing mechanics, eccentric hamstrings strengthening and flywheel inertia training. This article proposes that the use of eccentric resistance training in youth should be considered a necessity to help develop both physical qualities that underpin sporting performance, as well as reducing injury risk. However, as with any other training modality implemented within youth, careful consideration should be given in accordance with an individual's maturity status, training history and technical competency as well as being underpinned by current long-term physical development guidelines.

Surface properties affect the interplay between fascicles and tendinous tissues during landing

PURPOSE

Muscle-tendon units are forcefully stretched during rapid deceleration events such as landing. Consequently, tendons act as shock absorbers by buffering the negative work produced by muscle fascicles likely to prevent muscle damage. Landing surface properties can also modulate the amount of energy dissipated by the body, potentially effecting injury risk. This study aimed to evaluate the influence of three different surfaces on the muscle-tendon interactions of gastrocnemius medialis (GM), and vastus lateralis (VL) during single- and double-leg landings from 50 cm.

METHODS

Ultrasound images, muscle activity and joint kinematics were collected for 12 participants. Surface testing was also performed, revealing large differences in mechanical behavior.

RESULTS

During single-leg landing, stiffer surfaces increased VL fascicle lengthening and velocity, and muscle activity independent of joint kinematics while GM length changes showed no difference between surfaces. Double-leg landing resulted in similar fascicle and tendon behavior despite greater knee flexion angles on stiffer surfaces.

CONCLUSION

This demonstrates that VL fascicle lengthening is greater when the surface stiffness increases, when performing single-leg landing. This is due to the combination of limited knee joint flexion and lower surface absorption ability which resulted in greater mechanical demand mainly withstood by fascicles. GM muscle-tendon interactions remain similar between landing surfaces and types. Together, this suggests that surface damping properties primarily affect the VL muscle-tendon unit with a potentially higher risk of injury as a result of increased surface stiffness when performing single-leg landing tasks.

Kinetic analysis of felines landing from different heights

Background

Kinetic motion analysis has been used in canines and equines as a fundamental objective evaluation measurement. Cats are very capable jumpers, and this ability has biomimetic applications. It is essential to understand movement patterns and physical adaptations of this species, as cats are popular pets for humans. Further to this, motion analysis of a cat's movement patterns may provide potentially valuable information in relation to limb disease and injury. Therefore, the aim of this study was to investigate kinetic differences in cats when landing from varying preselected controlled heights.

Methods

The peak vertical force (PVF) and paw contact area (CA) of both the forelimbs and hindlimbs were collected from seven healthy Chinese domesticated cats while landing from heights of 30 cm, 50 cm, 70 cm and 90 cm respectively. The falling motivation for the cats was facilitated with the use of a flip board. This device provided the basis for the cats to land passively.

Results

The results indicated that the PVF of all examined limbs (fore right, fore left, hind right, hind left) significantly increased as the height increased. When the PVF from the hindlimbs and forelimbs were compared, the forelimbs recorded significantly greater values for all heights examined (P < 0.001). The PVF of the hindlimbs was symmetrical at all heights, but forelimb symmetry only occurred at the lower heights. The hindlimbs demonstrated larger CA than the forelimbs measured from all heights on landing (P < 0.001). Moreover, the paw CA on the left and right limbs were symmetrical.

Discussion

The paw CA of cats may be an effective parameter to evaluate abnormalities or diseases in the limbs of cats. Additionally, these findings highlight how cats land from varying heights, which may also provide reference values for the bionic design of artificial limbs for felines and treatment for limb diseases in this species.

Medial Gastrocnemius Muscle Activity during Single-Leg Hopping to Exhaustion

This study described changes in leg muscle activation characteristics during exhaustive single-leg hopping. Twenty-seven healthy men performed trials (132 hops/min) to exhaustion, without a target height, to a target height with visual feedback and target height with tactile feedback. Mean muscle activation amplitude of the medial gastrocnemius (MG) decreased during the anticipatory period while duration of MG activity was maintained when hopping to a target height and contrasted the changes during hopping without a target height. Changes to MG activity were specific to whether the hopping height had been maintained or not. Changes during the anticipatory period of MG activity, indicative of adaptation in descending motor pathways, implicate utility of a motor learning strategy to allow completion of an exhaustive task.

Relationship between electromyographic activity of knee joint muscles with vertical and posterior ground reaction forces in anterior cruciate ligament reconstructed patients during a single leg vertical drop landing task

The objective of the present study is to examine the relation between activity patterns of knee joint muscles with vertical and posterior ground reaction forces (VGRF and PGRF, respectively) in patients after anterior cruciate ligament reconstruction (ACLR). Twenty males post-ACLR participated in this cross-sectional study. The association between muscle activity with VGRF and PGRF was assessed during a single leg vertical drop-landing task. There were strong negative associations between preparatory VL, VM and MG activity and PGRF (P< 0.05). Strong positive associations were found between reactive VM and LH activity with PGRF (P< 0.05). Preparatory co-activation of VM: MH had significant negative associations with VGRF (P< 0.05) and reactive co-activation of VL:LH had a significant positive relation with PGRF(P< 0.05). Greater preparatory activity and co-activation of knee muscles were associated with lower peak PGRF and VGRF, whereas greater reactive activity and co-activation of knee muscles was associated with greater peak PGRF and VGRF. According to our findings, both activity and co-activation of knee muscles during the preparatory phase were associated with reduced PGRF and VGRF, respectively in ACLR patients; thus, incorporating exercises in order to increase preparatory activity and co-activation of knee joint muscles into rehabilitation programs in ACLR patients seems necessary.

Forefoot and heel take-off strategies result in different distribution of lower extremity work during landings

Previous research suggests that landing mechanics may be affected by the mechanics of the preceding jump take-off. The purpose of the present study was to investigate whether jump take-off mechanics influence the subsequent landing mechanics. Female volleyball (n = 17) and ice hockey (n = 19) players performed maximal vertical jumps with forefoot and heel take-off strategies. During forefoot and heel jumps, participants were instructed to shift their weight to their forefoot or heel, respectively, and push through this portion of the foot throughout the jump. Jump mechanics were examined using 3D motion analysis, where lower extremity net joint moment (NJM) work, NJM, and segment angles were compared between forefoot and heel jumps using multivariate ANOVA. During jump take-off, participants performed more positive ankle plantar flexor and knee extensor NJM work in forefoot compared to heel jumps (P < 0.05). From initial foot contact to foot flat, participants performed more negative ankle plantar flexor and hip extensor NJM work during heel compared to forefoot jumps (P < 0.05). The present results demonstrate that using a heel take-off strategy results in a different distribution of lower extremity NJM work and NJM during landing compared to landings following forefoot jumps.

Changes in Lower-Limb Biomechanics, Soft Tissue Vibrations, and Muscle Activation During Unanticipated Bipedal Landings

We aimed to explore the biomechanical differences between the anticipated drop jump and unanticipated drop landing. Twelve male collegiate basketball players completed an anticipated drop jump and unanticipated drop landing with double legs from a height of 30 cm. Kinematics, impact force, soft tissue vibrations, and electromyographic (EMG) amplitudes of the dominant leg were collected simultaneously. The anticipated drop jump showed more flexed lower limbs during landing and increased range of motion compared to the unanticipated drop landing. The anticipated drop jump also had lower impact force, lesser soft tissue vibration, and a greater damp coefficient at the thigh muscles compared with the unanticipated drop landing. Significant increases in the EMG amplitudes of the tibialis anterior, lateral gastrocnemius, rectus femoris, and biceps femoris were observed in the anticipated drop jump during the pre/post-activation and downward phases. The anticipated drop jump presented more optimized landing posture control with more joint flexion, lower impact force, less soft tissue vibrations, and full preparation of muscle activations compared with the unanticipated drop landing.

A Mixed-Methods Approach to Evaluating the Internal Validity of the Reactive Strength Index

The reactive capacity of the muscle-tendon complex is commonly assessed using the reactive strength index (RSI). Conventionally, the RSI is a ratio of rebound jump height to ground contact time in depth jumping. Several assumptions regarding the linear mechanics acting through the whole-body center of gravity may threaten the internal validity of computation and interpretation of RSI scores. First, it is common for rebound jump height to be predicted from rebound jump flight time. This assumes that the angular positioning of body segments is equivalent at the time instances of rebound jump take-off and landing. Prior literature supports a mixed-methods approach for computing the RSI that is void of this assumption. The mixed-methods approach gives a more valid estimation of rebound jump height. In this approach, rebound jump height is estimated from rebound jump take-off velocity of the whole-body center of mass. This is accomplished by subtracting an estimate of impact velocity, acquired using videography, from change in whole-body center of mass velocity estimated from integrated vertical ground reaction force data. Second, it is often assumed that vertical displacement of the whole-body center of mass during the drop phase of the depth jump is predicted perfectly from the height of the platform used to perform the drop. This assumption may affect the internal validity of comparing RSI scores across individuals and within individuals performing depth jumps from varied heights. The purpose of the present study was to investigate the internal validity of RSI scores computed using the conventional approach and impact velocity variability, which may affect the interpretation of RSI scores. Seventy physically active young adults performed depth jumps from drop heights of 0.51, 0.66, and 0.81 m. RSI was computed using the conventional approach and a mixed-methods approach featuring the use of 2-dimensional videography, body segment parameters, and force platform dynamometry. The two computational methods were compared using linear regression performed on data from each drop height. In addition, a 2 (computational method) by 3 (drop height) Analysis of Variance (ANOVA) was performed to evaluate for main effects and interactions in RSI data. Multiple one sample t-tests were performed to compare estimated and theoretical impact velocities. The ANOVA revealed no main effect or interactions between computational approaches (p = 0.467–0.938). Linear regression revealed moderately strong associations between RSI scores computed using the conventional and mixed-methods approaches (R² = 0.685–0.741). Moreover, linear regressions revealed that the conventional approach tends to overestimate the mixed methods approach for RSI scores below 1.0 and underestimate the mixed methods approach for RSI scores above 1.0. Lastly, estimated impact velocities were observed to be as much as 13% lower versus theoretical (p < 0.001). Researchers with access to motion capture and force platform technology may consider using a mixed-methods approach for computing the RSI, which likely maximizes the internal validity of scores. In addition, results suggest for practitioners to practice caution when comparing conventional RSI scores across individuals.

The Effect of Load Uncertainty on Neuromotor Control in Catching: Gender Differences and Short Foreperiods

To reveal how the CNS copes with load uncertainty in catching, electromyography (EMG) was recorded in 15 females and 14 males while catching visually identical balls of known and unknown weights under varied (1-10 s) and constant (1 s) foreperiods (warning time). EMG integrals, which represented total muscle activity, were computed for three time intervals prior to the catch (anticipatory), and one interval after (compensatory). Load uncertainty caused the CNS to utilize an anticipatory strategy in several muscles, primarily during the ball-flight interval, characterized by preparation to catch balls of unknown weight by utilizing an average of 99.7% of the muscle activation used to catch the heaviest ball under the known weight condition. The constant 1 s foreperiod, which permitted precise temporal anticipation of ball release, did not influence the anticipatory strategy adopted by the CNS to cope with load uncertainty. There were no observed differences in the neuromotor control used by men and women to manage load uncertainty in catching, although there was an interesting difference in the way men and women employed the triceps to prepare to catch balls of a known weight.

Performance Function Tests in Assessing Ankle Fitness

Introduction:

A challenge for any physician caring for athletes is determining readiness for return to competition after an injury. A wide variety of performance function tests (PFTs) have been described, but no norms or minimum performance levels exist for any of them. In this study, healthy athletes were given a series of PFTs to complete. We propose that there will be a minimum performance level for each of the PFTs that all athletes can complete. We also propose, for tests that assess the right and left legs independently, that performance of the right leg will consistently be within 10% of the left. Finally, we propose that performance on one of the functional tests will be predictive of function on all the tests.

Methods:

Athletes were put through a testing protocol, beginning with range of motion and progressing through a series of functional ankle tests of increasing difficulty. Right and left leg data were recorded separately for the first five tests. For each test, mean values, ranges, and SDs were calculated.

Results:

Eighty-one athletes completed the protocol. A wide variation existed in performance ability between athletes; the SD for any of the tests was too high to determine a minimum performance threshold. However, when comparing right to left leg in any one athlete, the difference in performance testing was always less than 10%. Furthermore, performance on the side hop test was predictive of performance on the other tests.

Discussion:

A wide range of performance was noted in all the PFTs, so it is not possible to define a minimum threshold. However, performance of an injured leg to within 10% of the opposite (uninjured) leg suggests achievement of normal function. The side hop test might be a good test by itself to represent overall ankle readiness.

The Influence of Visual, Vestibular, and Hindlimb Proprioceptive Ablations on Landing Preparation in Cane Toads

Coordinated landing from a jump requires preparation, which must include appropriate positioning and configuration of the landing limbs and body to be successful. While well studied in mammals, our lab has been using the cane toad (Rhinella marinus) as a model for understanding the biomechanics of controlled landing in anurans, animals that use jumping or bounding as their dominant mode of locomotion. In this article, we report new results from experiments designed to explore how different modes of sensory feedback contribute to previously identified features of coordinated landing in toads. More specifically, animals in which vision, hindlimb proprioception, or vestibular feedback were removed, underwent a series of hopping trials while high-speed video was used to record and characterize limb movements and electromyographic (EMG) activity was recorded from a major elbow extensor (anconeus). Results demonstrate that altering any sensory system impacts landing behavior, though loss of vision had the least effect. Blind animals showed significant differences in anconeus EMG timing relative to controls, but forelimb and hindlimb movements as well as the ability to successfully decelerate the body using the forelimbs were not affected. Compromising hindlimb proprioception led to distinctly different forelimb kinematics. Though EMG patterns were disrupted, animals in this condition were also able to decelerate after impact, though with less control, regularly allowing their trunks to make ground contact during landing. Animals with compromised vestibular systems showed the greatest deficits, both in takeoff and landing behavior, which were highly variable and rarely coordinated. Nevertheless, animals in this condition demonstrated EMG patterns and forelimb kinematics similar to those in control animals. The fact that no ablation entirely eliminates all aspects of landing preparation suggests that its underpinnings are complex and that there is no single sensory trigger for its initiation.

Vision fine-tunes preparation for landing in the cane toad, Rhinella marina

In toad hopping, the hindlimbs generate the propulsive force for take-off while the forelimbs resist the impact forces associated with landing. Preparing to perform a safe landing, in which impact forces are managed appropriately, likely involves the integration of multiple types of sensory feedback. In toads, vestibular and/or proprioceptive feedback is critical for coordinated landing; however, the role of vision remains unclear. To clarify this, we compare pre-landing forelimb muscle activation patterns before and after removing vision. Specifically, we recorded EMG activity from two antagonistic forelimb muscles, the anconeus and coracoradialis, which demonstrate distance-dependent onset timing and recruitment intensity, respectively. Toads were first recorded hopping normally and then again after their optic nerves were severed to remove visual feedback. When blind, toads exhibited hop kinematics and pre-landing muscle activity similar to when sighted. However, distance-dependent relationships for muscle activity patterns were more variable, if present at all. This study demonstrates that blind toads are still able to perform coordinated landings, reinforcing the importance of proprioceptive and/or vestibular feedback during hopping. But the increased variability in distance-dependent activity patterns indicates that vision is more responsible for fine-tuning the motor control strategy for landing.