Protocols Targeting Afferent Pathways via Neuromuscular Electrical Stimulation for the Plantar Flexors: A Systematic Review.

This systematic review documents the protocol characteristics of studies that used neuromuscular electrical stimulation protocols (NMES) on the plantar flexors [through triceps surae (TS) or tibial nerve (TN) stimulation] to stimulate afferent pathways. The review was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) statement, was registered to PROSPERO (ID: CRD42022345194) and was funded by the Greek General Secretariat for Research and Technology (ERA-NET NEURON JTC 2020). Included were original research articles on healthy adults, with NMES interventions applied on TN or TS or both. Four databases (Cochrane Library, PubMed, Scopus, and Web of Science) were systematically searched, in addition to a manual search using the citations of included studies. Quality assessment was conducted on 32 eligible studies by estimating the risk of bias with the checklist of the Effective Public Health Practice Project Quality Assessment Tool. Eighty-seven protocols were analyzed, with descriptive statistics. Compared to TS, TN stimulation has been reported in a wider range of frequencies (5-100, vs. 20-200 Hz) and normalization methods for the contraction intensity. The pulse duration ranged from 0.2 to 1 ms for both TS and TN protocols. It is concluded that with increasing popularity of NMES protocols in intervention and rehabilitation, future studies may use a wider range of stimulation attributes, to stimulate motor neurons via afferent pathways, but, on the other hand, additional studies may explore new protocols, targeting for more optimal effectiveness. Furthermore, future studies should consider methodological issues, such as stimulation efficacy (e.g., positioning over the motor point) and reporting of level of discomfort during the application of NMES protocols to reduce the inherent variability of the results.

Electromyographic responses to unexpected Achilles tendon vibration-induced perturbations during standing in young and older people.

This study aimed to investigate age-related differences in electromyographic (EMG) responses to unexpected Achilles tendon vibration (ATV) perturbations while standing blindfold. ATV with variable and random duration (12-15 s) and rest periods (20-24 s) was applied on 18 young and 16 older volunteers. The anterior/posterior center of pressure (CoP) and the soleus (SOL) and tibialis anterior (TA) EMG were analyzed for 1 s before and 8 s after the ATV onset and offset. ATV induced a posterior shift of CoP in both groups, with more pronounced shift in the older group. During ATV onset, the older group demonstrated less SOL and more TA EMG increase compared to the young group. During the first 0.5 s of ATV offset, SOL EMG was decreased in both age groups, while TA showed a burst of EMG activity that was greater in the older group. No difference in the latencies of EMG peaks or valleys was observed between the groups. It is concluded that ATV induces greater posterior CoP shift in older adults, and they adopt a recovery strategy, characterized by a decreased SOL activation and an increased TA activation. These differences are possibly attributed to the increased fear of falling, decreased limits of stability and reduced capacity of older people to reweight their sensory inflow when proprioception is distorted.

Kinetic and Kinematic Changes in Vertical Jump in Prepubescent Girls After 10 Weeks of Plyometric Training.

PURPOSE: To examine the effect of a 10-week plyometric training (PT) on the kinematic and kinetic properties of prepubescent girls during squat jump, countermovement jump, and drop jumps. METHODS: Twenty-four untrained girls (aged 9-11 y) were assigned to a training group (TG) and a control group. The TG followed twice a week PT for 10 weeks. Squat jump, countermovement jump, and drop jumps performed from heights of 20, 35, and 50 cm were tested before and after PT. Jump height, kinematic, and kinetic parameters were evaluated using a motion analysis system and a force plate. RESULTS: Jumping height in all jump types increased significantly after PT for the TG (P < .001). After training, the TG presented increased power (P < .001) and knee angular velocity (P < .001), higher knee flexion at the deepest point during the braking phase (P < .001), longer contact time (P < .001), and unchanged stiffness and reaction strength index (P > .05). No differences were observed in the control group (P > .05). CONCLUSION: These findings indicate that a 10-week PT positively affected jumping performance in prepubescent girls who improved their drop jump performance after training not by adopting a stiff/bouncing jumping style of short contact time and increased stiffness, but a compliant/absorbing style of prolonged contact time.

Exercise-induced fatigue in young people: advances and future perspectives.

PURPOSE: In recent decades, the interest for exercise-induced fatigue in youth has substantially increased, and the effects of growth on the peripheral (muscular) and central (neural) mechanisms underpinning differences in neuromuscular fatigue between healthy children and adults have been described more extensively. The purpose of this review is to retrieve, report, and analyse the findings of studies comparing neuromuscular fatigue between children and adults. Objective measures of the evaluation of the physiological mechanisms are discussed. METHOD: Major databases (PubMed, Ovid, Scopus and Web of Science) were systematically searched and limited to English language from inception to September 2017. RESULT: Collectively, the analyzed studies indicate that children experience less muscular and potentially more neural fatigue than adults. However, there are still many unknown aspects of fatigue regarding neural (supraspinal and spinal) and peripheral mechanisms that should be more thoroughly examined in children. CONCLUSION: Suitable methods, such as transcranial magnetic stimulation, transcranial electrical stimulation, functional magnetic resonance imaging, near-infrared spectroscopy, tendon vibration, H-reflex, and ultrasound are recommended in the research field of fatigue in youth. By designing studies that test the fatigue effects in movements that replicate daily activities, new knowledge will be acquired. The linkage and interaction between physiological, cognitive, and psychological aspects of human performance remain to be resolved in young people. This can only be successful if research is based on a foundation of basic research focused on the mechanisms of fatigue while measuring all three above aspects.

The acute effects of an intense stretch-shortening cycle fatigue protocol on the neuromechanical parameters of lower limbs in men and prepubescent boys.

The study examined the differences between boys and adults after an intense stretch-shortening cycle fatigue protocol on neuromechanical parameters of the lower limb. Thirteen boys (9-11 years old) and 13 adult men (22-28 years old) were tested for maximal isometric voluntary knee extension torque and drop jump (DJ) performance from 30 cm before and immediately after a fatigue protocol, consisted of 10 × 10 maximum effort vertical jumps. Three-dimensional kinematics, kinetics and electromyographic (EMG) parameters of the lower extremities muscles were recorded during DJs before and after the fatigue test. The results indicated that reduction in maximal isometric torque and jumping performance was significantly higher in adults compared to boys. Vertical ground reaction forces, contact time and maximum knee flexion increased in a greater extend in adults than in boys. In addition, preactivation, EMG agonist activity, knee joint stiffness and stretch reflex decreased more in adults than in boys at all the examined phases of jumping tasks. It is concluded that employed fatigue protocol induced acute reduction in performance and altered motor control during jumping in both age groups. However, the differences in the level of fatigue between the 2 groups could be attributed to neuromuscular, mechanical and kinematic parameters observed between groups.

Muscle Twitch Torque During Two Different in Volume Isometric Exercise Protocols: Fatigue Effects on Postactivation Potentiation.

Xenofondos, A, Bassa, E, Vrabas, IS, Kotzamanidis, C, and Patikas, D. Muscle twitch torque during two different in volume isometric exercise protocols: fatigue effects on postactivation potentiation. J Strength Cond Res 32(2): 578-586, 2018-The purpose of this study was to quantify the effect of the contraction duration of 2 isometric exercise protocols on the postactivation potentiation of 14 well-trained men (age: 22.6 ± 2.8 years, height: 180.3 ± 5.9 cm, and body mass: 72.3 ± 37.9 kg). The protocols consisted of 4 × 6 maximal plantar flexions, of 3-second (P3) or 6-second (P6) duration, performed in random order, with a 2-minute and 15-second intervals between the sets and repetitions, respectively. The torque during maximal isometric voluntary contraction (MIVC), the peak twitch torque (TT), and the rate of torque development (RTD) after each MIVC were analyzed for the first and the last trial of each set, the average of all trials of each set, and the trials within each set that had the highest peak TT. The MIVC had an overall greater reduction during P6 compared with P3 (P3: -4.6 ± 2.3 vs. P6: -16.0 ± 1.9%). P6 showed higher potentiation in TT during the initial repetitions of the first 2 sets (p < 0.05) in contrast to the P3, which revealed a lower potentiation but for a longer period along the exercise session. However, both protocols had on average the same potential for potentiation (P3: 81.6 ± 6.1 vs. P6: 79.8 ± 6.3%). The twitch RTD presented no systematic difference between the 2 protocols (p > 0.05). These data demonstrate the dependence of the TT potentiation on the conditioning stimulus and verify the cumulative effect of potentiation, suggesting the implementation of longer contractions to achieve maximal but temporal TT potentiation and shorter contractions for less variable but prolonged potentiation.

Soleus H-reflex modulation during balance recovery after forward falling.

INTRODUCTION: Our purpose was to examine the Hoffmann reflex (H-reflex) during balance recovery after a simulated forward fall from 2 different inclination angles. METHODS: The soleus H-reflex of 15 healthy adults was measured in 2 different leaning positions (exerting a horizontal force at 15% and 30% of body weight, respectively), with no release (Int0) and at 2 different intervals (Int1, Int2) after the release (∼45 and ∼65 ms, respectively). RESULTS: During Int2, the H-reflex, which was evoked before the onset of the soleus electromyography, was significantly higher than the H-reflex induced 20 ms earlier (Int1). No significant difference was observed between Int0 and Int1 and between the 2 leaning positions. CONCLUSIONS: These findings indicate that Ia afferent input is facilitated before muscle activation during forward falling. This could be important for the timely activation and increased rate of force development required during this task. Muscle Nerve 54: 952-958, 2016.

Rectus femoris electromyography signal clustering: Data-driven management of crouch gait in patients with cerebral palsy.

This study aimed to investigate how electromyography (EMG) cluster analysis of the rectus femoris (RF) could help to better interpret gait analysis in patients with cerebral palsy (CP). The retrospective gait data of CP patients were categorized into two groups: initial examination (E1, 881 patients) and subsequent examination (E2, 377 patients). Envelope-formatted EMG data of RF were collected. Using PCA and a combined PSO-K-means algorithm, main clusters were identified. Patients were further classified into crouch, jump, recurvatum, stiff and mild gait for detailed analysis. The clusters (labels) were characterized by a significant peak EMG activity during mid-swing (L1), prolonged EMG activity during stance (L2), and a peak EMG activity during loading response (L3). Notably, L2 contained 76% and 92% of all crouch patients at E1 and E2, respectively. Comparing patients with a crouch gait pattern in L2-E1 and L2-E2, two subgroups emerged: patients with persistent crouch (G1) and patients showing improvement at E2 (G2). The minimum activity of RF during 20-45% of the gait was significantly higher (p = 0.025) in G1 than in G2. A greater chance of improvement from crouch gait might be associated with lower RF activity during the stance phase. Using our findings, we could potentially establish an approach to improve clinical decision-making regarding treatment of patients with CP.

Construct validity of gymnastics-specific assessment on the neuromuscular function of shoulder flexor and extensor muscles.

The present study examined the construct validity of gymnastics-specific measurements of the shoulder flexor and extensor neuromuscular function. Fifteen male gymnasts and 18 male physical education (PE) students participated voluntarily. The electromyogram (EMG) of 3 flexor and 3 extensor shoulder muscles was assessed during isometric (45, 90, and 135º) and isokinetic (concentric and eccentric 60, 180, and 300º/s) shoulder flexion/extension. Peak torque (PT) and neuromuscular function in terms of neuromuscular efficiency (NME, i.e. torque to EMG ratio) and antagonist/agonist coactivation index were evaluated. The construct validity of the measurements was supported by the ability to discriminate between gymnasts and physical education (PE) students, taking into consideration the shoulders' neuromuscular function parameters (37%-57% of the variability explained). After accounting for the limitations of the study, these findings may assist gymnastics coaches and researchers to develop and evaluate more effective training methods for athletes related to shoulder joint physiological characteristics and level of expertise.

Postactivation Potentiation and the Asynchronous Action of Muscular and Neural Responses.

PURPOSE: This study examined the underlying mechanisms of postactivation potentiation and the time course of muscular- and neural-related variables. METHODS: Fourteen trained males executed 4 sets of six 6-second maximum isometric conditioning plantar flexions, with 15 seconds and 2 minutes of interval between the contractions and sets, respectively. Peak twitch torque (TT), rate of torque development, time to peak torque, half relaxation time, and the neural-related variables of H-reflex and electromyogram, normalized to the maximum M-wave (H/M and RMS/M, respectively), were evaluated, as well as the level of the voluntary activation, assessed by the twitch interpolation technique. All neural-related variables were analyzed for the trial within each set when TT was maximal and for the trial within each set when the neural-related variable itself was maximal. RESULTS: Compared with the baseline measures, TT and rate of torque development significantly increased in all sets (P < .001), whereas time to peak torque and half relaxation time significantly decreased in sets 1 to 4 and 2 to 4, respectively (P < .001). However, H/M and the RMS/M did not change for the repetition of each set for which the TT was maximal (P > .05). Interestingly, the within-set maximum H/M ratio of the lateral gastrocnemius muscle revealed a significant increase in all sets (P < .05), compared with the baseline measures. CONCLUSION: One set of 4 contractions with 6-second duration is sufficient to cause postactivation potentiation for most participants, whereas peak TT augmentation does not coincide with changes in the examined neural-related variables. Further experiments should consider the time lag on their maximal values and their inherent between-participants variability.

The effect of dropping height on jumping performance in trained and untrained prepubertal boys and girls.

Plyometric training in children, including different types of jumps, has become common practice during the last few years in different sports, although there is limited information about the adaptability of children with respect to different loads and the differences in performance between various jump types. The purpose of this study was to examine the effect of gender and training background on the optimal drop jump height of 9- to 11-year-old children. Sixty prepubertal (untrained and track and field athletes, boys and girls, equally distributed in each group [n = 15]), performed the following in random order: 3 squat jumps, 3 countermovement jumps (CMJs) and 3 drop jumps from heights of 10, 20, 30, 40, and 50 cm. The trial with the best performance in jump height of each test was used for further analysis. The jump type significantly affected the jump height. The jump height during the CMJ was the highest among all other jump types, resulting in advanced performance for both trained and untrained prepubertal boys and girls. However, increasing the dropping height did not change the jumping height or contact time during the drop jump. This possibly indicates an inability of prepubertal children to use their stored elastic energy to increase jumping height during drop jumps, irrespective of their gender or training status. This indicates that children, independent of gender and training status, have no performance gain during drop jumps from heights up to 50 cm, and therefore, it is recommended that only low drop jump heights be included in plyometric training to limit the probability of sustaining injuries.

Temporal modulation of H-reflex in young and older people: Acute effects during Achilles tendon vibration while standing.

PURPOSE: Examining how timely is sensory input processed and regulated after a perturbation while standing, is a key element to understand postural control, especially in people with balance deficits, such as older adults. In this study, we investigated the age-related temporal modulations in spinal excitability, by measuring the soleus H-reflex within the first 250 ms after switching on (V-ON) and off (V-OFF) a pair of vibrators, placed over the Achilles tendons. METHODS: Twenty young (25.1 ± 5.1 yrs) and 16 older (71.8 ± 6.1 yrs) adults stood blindfolded, while 12-15 s periods of Achilles tendon vibration (ATV) were applied with 20-24 s intervals. Anterior/posterior center of pressure and electromyographic (EMG) responses [soleus (SOL) and tibialis anterior (TA)] were assessed whereas SOL H-reflex normalized to maximum M-wave (H/Mmax) and SOL EMG (H/SOL) were evaluated before ATV (Pre) and at 50, 100, 150, 200 and 250 ms after the V-ON and V-OFF. RESULTS: Only in young adults the TA/SOL EMG ratio decreased 100 (p = 0.032) and 150 ms (p < 0.001) after V-ON and the H/Mmax and H/SOL decreased 150-250 ms after V-ON (p < 0.001). At 50-250 ms after V-OFF, H/Mmax was reduced compared to Pre values (p < 0.001), with no differences between the age groups (p > 0.05). H/SOL was decreased for the young adults (p < 0.001) and remained reduced at least for the first 250 ms after V-OFF. At 150 ms after V-OFF, SOL/Mmax was decreased only for the older adults (p < 0.001), whereas TA/SOL EMG gradually increased for both groups (p < 0.001). CONCLUSION: When ATV is introduced while standing, the spinal excitability of older people is reduced later and to a lesser extent compared to young adults. Their limited capacity to down-regulate the "noisy" sensory input generated by ATV gives further evidence of a possible mechanism for their inefficient postural control.

Resilience of visually guided weight shifting to a proprioceptive perturbation depends on the complexity of the guidance stimulus.

BACKGROUND: Whole-body tracking of visual motion cues is used in balance training to improve weight shifting ability in old age and sports. RESEARCH QUESTION: How tracking of a complex (pink noise) and a periodic visual target motion during anteroposterior weight shifting affects postural and muscle responses to unilateral hip vibration. METHODS: Twenty-six participants performed 160 anteroposterior weight shifting cycles while tracking the vertical motion of a visual target, concurrently receiving Center of Pressure (CoP) feedback. They were randomly divided to groups; (a) the Constant group tracked a visual target motion constructed by 3 sinusoids of different amplitude, and (b) the Pink group tracked a complex visual target motion constructed by a pink noise generation process. Between the 60th and the 120th cycle, vibration was applied to the right gluteus medius, introducing a sideways CoP deviation. CoP displacement and electromyographic (EMG) responses of soleus, tibialis anterior and peroneus longus were recorded and summarized in blocks of 3 cycles. RESULTS: Sideways CoP deviation induced at the onset/offset of unilateral hip vibration was smaller for the Pink than the Constant group. The Pink group demonstrated greater tibialis anterior and peroneus longus EMG activity around the most anterior sway peak while soleus EMG was similar for the two groups. Both groups successfully coupled weight shifting amplitude to the target motion, but the Pink group tracked the target motion with a greater delay compared to the Constant group. SIGNIFICANCE: Whole body tracking of complex visual motions evokes perception-based action and increases ankle muscle co-activation making sway more resilient to a proprioceptive perturbation induced by unilateral hip vibration. Complex visual guidance motions should be considered when designing balance rehabilitation regimes, aiming at improving weight shifting ability and dynamic balance control.

Neuromuscular, kinetic and kinematic differences in drop jumping between male adolescents with and without intellectual disability.

BACKGROUND: Individuals with intellectual disability (ID) have lower performance in vertical jumps compared to individuals without ID. Training programmes including drop jump (DJ) exercises to gap this deficit are required. However, there is a lack of knowledge on biomechanical variables may influence DJ performance in young people with ID. RESEARCH QUESTION: Are there differences in the neuromuscular, kinetic and kinematic parameters in DJ between male adolescents with and without ID (WID and NID, respectively). METHODS: Thirteen male adolescents WID (Wisk III intelligence quotient: 55.6 ± 11.2) and thirteen age-matched boys NID executed DJ from 30 cm on a force-plate. The vastus lateralis (VL) and biceps femoris (BF) electromyogram (EMG) as well as the knee kinematics and kinetics were obtained. RESULTS: WID had significantly (p < .05) lower DJ height, knee power, peak angular velocity and stiffness compared to NID. VL activity was higher and BF/VL co-activation was lower in WID than NID. SIGNIFICANCE: The deficit in DJ performance shown in WID was accompanied with lower power and stiffness, as well as lack of coordination in the lower limb muscles. Improving these variables, with specific and targeted training interventions, may diminish the performance gap with their age-matched counterparts.

Optimal Drop Height in Prepubertal Boys Is Revealed by the Performance in Squat Jump.

Drop jump (DJ) performance gain with increasing drop height is well documented in adults, but there is still no clear evidence of such gain in children. This study aimed to examine the differences in DJ performance gain in male adults and prepubescent boys by comparing drop heights tailored to each individual's performance and expressed as a percentage of their squat jump (SJ) performance. Fifteen boys (9-11 y) and 15 men (19-27 y) executed DJs from drop heights that were set at 75%, 100%, 125%, and 150% of their best performance in SJ (DJ75, DJ100, DJ125, and DJ150, respectively). Vertical ground reaction force (vGRF), contact time and kinematics of the lower extremities were captured. The results showed that boys jumped significantly lower than adults in DJs, and both age groups presented jumping gain with increasing drop height, up to DJ125. Boys demonstrated longer total contact time, lower angular velocity and vGRF during the propulsive phase, as well as smaller knee flexion at touchdown and lower reactive strength index. vGRF in DJ75 and DJ100 was lower than in DJ125 and DJ150. The highest value for maximum knee flexion was also presented at DJ150. It is concluded that in prepubescent boys, the appropriate drop height for an effective DJ is linked to their performance in SJ and might be between 75% and 125% of their maximum SJ performance.

Standing on unstable surface challenges postural control of tracking tasks and modulates neuromuscular adjustments specific to task complexity.

Understanding the modulations of motor control in the presence of perturbations in task conditions of varying complexity is a key element towards the design of effective perturbation-based balance exercise programs. In this study we investigated the effect of mechanical perturbations, induced by an unstable surface, on muscle activation and visuo-postural coupling, when actively tracking target motion cues of different complexity. Four postural tasks following a visual oscillating target of varying target complexity (periodic-sinusoidal vs. chaotic-Lorenz) and surface (stable-floor vs. unstable-foam) were performed. The electromyographic activity of the main plantarflexor and dorsiflexor muscles was captured. The coupling between sway and target was assessed through spectral analysis and the system's local dynamic stability through the short-term maximum Lyapunov exponent. We found that external perturbations increased local instability and deteriorated visuo-motor coupling. Visuo-motor deterioration was greater for the chaotic target, implying that the effect of the induced perturbations depends on target complexity. There was a modulation of the neuromotor system towards amplification of muscle activity and coactivation to compensate surface-related perturbations and to ensure robust motor control. Our findings provide evidence that, in the presence of perturbations, target complexity induces specific modulations in the neuromotor system while controlling balance and posture.

Altered Drop Jump Landing Biomechanics Following Eccentric Exercise-Induced Muscle Damage.

Limited research exists in the literature regarding the biomechanics of the jump-landing sequence in individuals that experience symptoms of muscle damage. The present study investigated the effects of knee localized muscle damage on sagittal plane landing biomechanics during drop vertical jump (DVJ). Thirteen regional level athletes performed five sets of 15 maximal eccentric voluntary contractions of the knee extensors of both legs at 60°/s. Pelvic and lower body kinematics and kinetics were measured pre- and 48 h post-eccentric exercise. The examination of muscle damage indicators included isometric torque, muscle soreness, and serum creatine kinase (CK) activity. The results revealed that all indicators changed significantly following eccentric exercise (p < 0.05). Peak knee and hip joint flexion as well as peak anterior pelvic tilt significantly increased, whereas vertical ground reaction force (GRF), internal knee extension moment, and knee joint stiffness significantly decreased during landing (p < 0.05). Therefore, the participants displayed a softer landing pattern following knee-localized eccentric exercise while being in a muscle-damaged state. This observation provides new insights on how the DVJ landing kinematics and kinetics alter to compensate the impaired function of the knee extensors following exercise-induced muscle damage (EIMD) and residual muscle soreness 48 h post-exercise.

Age induced modifications in the persistency of voluntary sway when actively tracking the complex motion of a visual target.

Movement persistency, reflected in systematic cycle to cycle fluctuations of a rhythmical task such as walking or voluntary sway, is compromised with increasing age, making older adults more susceptible to falls. In the present study, we tested whether it is possible to improve rhythmic voluntary sway persistency in old age by actively tracking the complex (i.e. persistent) motion of a visual target. Twenty healthy young and 20 older adults performed 132 cycles of anterior-posterior sway under two conditions: a) self-paced sway and b) sway while tracking the vertical motion of a complex visual target. The persistency of sway cycle amplitude and duration, detected from the center of pressure displacement, was quantified using the Fractal exponent α. We also recorded body kinematics in order to assess the intersegmental coordination that was quantified in the Mean Absolute Relative Phase (MARP) and the Deviation Phase (DPh) between the trunk and the lower limbs. In self-paced sway, older adults showed a lower persistency of cycle duration and a higher MARP and DPh between the trunk and the lower limbs compared to young adults. Tracking the complex visual target motion increased the persistency of cycle amplitude, in young but not in older adults, when compared to the self-paced sway while it decreased the persistency of cycle duration in both groups. The relative phase measures showed a moderate to strong relationship with the persistency of cycle amplitude and duration when older adults swayed in their self-pace. These findings suggest older adults cannot exploit active tracking of the complex visual motion cue to improve voluntary sway persistency. This could be related to the less stable and out of phase intersegmental coordination characterizing rhythmic voluntary sway in old age.

Backward Running: Acute Effects on Sprint Performance in Preadolescent Boys.

The aim of this study was to examine the acute effect of backward running (BwR) during warm-up on a 20-m sprint of boys' performance, compared to forward running (FwR). Fourteen recreationally active preadolescent boys (aged 12.5 ± 0.5 years) were examined in 3 protocols: warm-up (control condition), warm-up with 3 × 10 m additional BwR sprints and warm-up with 3 × 10 m additional FwR sprints. Participants were evaluated 4 minutes after each protocol on a 20-m sprint and intermediate distances, as well as the rate of perceived exertion (RPE). Sprint speed across 10-20 m was significantly higher for the BwR warm-up compared to the regular warm-up (p < 0.05) and a significantly higher RPE after the BwR and FwR protocols compared to the control condition was recorded (p < 0.05). No significant difference was detected across the distances 0-5, 5-10, 0-10 and 0-20 m. Although adding 3 × 10-m sprints of BwR or FwR after the warm-up did not enhance performance in a 20 m sprint of preadolescent boys, the positive effect of BwR across 10-20 m distance suggests that BwR could be an alternative means for enhancing performance for certain phases of a sprint for this age. However, preadolescent boys' response to different sprint conditioning exercise stimuli and the optimization of rest time to maximize performance remain to be determined.

Swaying slower reduces the destabilizing effects of a compliant surface on voluntary sway dynamics.

The ability to control weight shifting (voluntary sway) is a crucial factor for stability during standing. Postural tracking of an oscillating visual target when standing on a compliant surface (e.g. foam) is a challenging weight shifting task that may alter the stability of the system and the muscle activation patterns needed to compensate for the perturbed state. The purpose of this study was to examine the effects of surface stability and sway frequency on the muscle activation of the lower limb, during visually guided voluntary postural sway. Seventeen volunteers performed a 2-min voluntary sway task in the anterior-posterior direction following with their projected center of pressure (CoPAP) a periodically oscillating visual target on a screen. The target oscillated at a frequency of 0.25 Hz or 0.125 Hz, while the participants swayed on solid ground (stable surface) or on a foam pad (unstable surface), resulting in four experimental conditions. The electromyogram (EMG) of 13 lower limb muscles was measured and the target-CoPAP coupling was evaluated with coherence analysis, whereas the difference in the stability of the system between the conditions was estimated by the maximum Lyapunov exponent (MLE). The results showed that slower oscillations outperformed the faster in terms of coherence and revealed greater stability. On the other hand, unstable ground resulted in an undershooting of the CoPAP to the target and greater MLE. Regarding the EMG data, a decreased triceps surae muscle activation at the low sway frequency compared to the higher was observed, whereas swaying on foam induced higher activation on the tibialis anterior as well. It is concluded that swaying voluntarily on an unstable surface results in reduced CoPAP and joint kinematics stability, that is accomplished by increasing the activation of the distal leg muscles, in order to compensate for this perturbation. The reduction of the sway frequency limits the effect of the unstable surface, on the head and upper body, improves the temporal component of coherence between CoP and target, whereas EMG activity is decreased. These findings might have implications in rehabilitation programs.