Vertical Leg Stiffness Research Articles

A-33 Clinical Case Slide - Head and Neck I

Anterior cruciate ligament injuries have become commonplace among female athletes, with more than 70% of injuries resulting from noncontact mechanisms. Muscular fatigue likely contributes to knee injury risk due to adaptations that occur during intense exercise. There is limited research on high intensity fatigue protocols occurring on consecutive days that may better mimic competitive sport. PURPOSE: To compare knee flexion angle, knee flexion moment, and vertical leg stiffness during single-leg jump landing measured immediately after high intensity bout of fatiguing exercise. METHODS: 12 female varsity soccer players (Age = 19.25 ± 1.36 years, Height = 170 ± 6.96cm, Weight = 62.36 ± 8.53kg) were randomly assigned to control and fatigue groups. Both groups completed five single-leg drop jumps from a 30cm height, landing on their dominant leg, defined as the kicking leg, every day for four days. The first day was completed without intervention and the fatigue group completed consecutive fatigue protocols before the drop jumps on days two through four. The fatigue protocols included 100 squat jumps, 100 lunge jumps, and 100 burpees, on days two, three, and four, respectively. Exercises were completed in five sets, 20 repetitions in each, with a 50m sprint between sets. Motion capture and force plate data were used to calculate percentage of change in knee flexion angle (°), knee flexion moment (Nm/kg), and vertical stiffness (kN/m) compared to pre-fatigue values. A Mann-Whitney U Test (data were not normally distributed) was used to compare differences between groups. RESULTS: There were no statistically significant differences between groups. Knee flexion angle remained constant to pre-fatigue values (4.24 ± 14.6%, -1.37 ± 15.96%, 4.17 ± 19.77%) on days two through four, respectively. No significant changes in knee moment were noted after three days of exercise (37.95 ± 38.37%, -12.69 ± 59.52%, 52.35 ± 124.22%). Increases in stiffness were not statistically significant after three days of exercise (38.34 ± 66.84%, 6.32 ± 54.43%, 25.79 ± 30.81%). CONCLUSION: A short bout of high intensity exercises did not alter dominant leg landing mechanics during single leg landing. The duration and mode of exercise used in this experiment did not cause a significant alteration in landing biomechanics on any of the testing sessions.

Neuromuscular Characteristics of Drop and Hurdle Jumps With Different Types of Landings

The objective of this study was to compare drop (DJ) and hurdle jumps using a preferred, flat foot (FLAT) and forefoot (FORE) landing technique. Countermovement jump height was used to establish the hurdle and the DJ heights. The subjects performed forward hurdles and vertical DJs on a force plate. Measures included vertical ground reaction force (VGRF), contact time, leg stiffness, and rate of force development (RFD). Electromyographic (EMG) activity was measured in the rectus femoris, biceps femoris, tibialis anterior, and gastrocnemius during 3 phases: preactivity, eccentric phase, and concentric phase. All the kinetic variables favored hurdles over DJs. Specifically, hurdle-preferred technique and FORE exhibited the shortest contact time and DJ FLAT the longest. The VGRF was higher in hurdle preferred and FORE than in DJ preferred, FLAT, and FORE. For stiffness and RFD, hurdle preferred and FORE were higher than DJ preferred and FLAT. Hurdle jumps showed higher rectus femoris EMG activity than DJ did during preactivity and eccentric phases but lower activity during the concentric phase. Considering the type of landing, FLAT generally demonstrated the greatest EMG activity. During the concentric phase, DJ exhibited higher rectus femoris EMG activity. Biceps femoris activity was higher with hurdles in all the phases. Gastrocnemius showed the highest EMG activity during the concentric phase, and during the eccentric phase, hurdle preferred and FORE showed the highest results. In conclusion, the hurdle FORE technique was the most powerful type of jump.

Spring Mass Characteristics of the Fastest Men on Earth

The spring mass model has widely been used to characterize the whole body during running and sprinting. However the spring mass characteristics of the world's fastest men are still unknown. Thus the aim of this study was to model these characteristics for currently the 3 fastest men on earth (Usain Bolt, Tyson Gay and Asafa Powell). This was done by using data collected during the 2009 World championships in Berlin and the modelling method of Morin et al. 21. Even though Bolt achieved the greatest velocity (12.3 m.s - 1) over the 60-80 m split compared to his competitors, his estimated vertical stiffness (355.8 kN.m - 1) and leg stiffness (21.0 kN.m - 1) were significantly lower than his competitors. This reduction in stiffness is a consequence of Bolt's longer contact time (0.091 s) [corrected] and lower step frequency (4.49 Hz).Thus Bolt is able to run at a greater velocity but with lower stiffness compared to his competitors.

Reliability and validity of field-based measures of leg stiffness and reactive strength index in youths

The aim of the study was to assess the reliability of a mobile contact mat in measuring a range of stretch–shortening cycle parameters in young adolescents. Additionally, vertical leg stiffness using contact mat data was validated against a criterion method using force–time data. The reliability study involved 18 youths completing a habituation and three separate test sessions, while 20 youths completed a single test session for the validity study. Participants completed three trials of a squat jump, countermovement jump, and maximal hopping test and a single trial of repeated sub-maximal hopping at 2.0 Hz and 2.5 Hz. All tests were performed on the contact mat. Reliability statistics included repeated-measures analysis of variance, intraclass correlation coefficient, and coefficient of variation (CV), while the correlation coefficient (r) and typical error of estimate (TEE) were reported for the validity study. Squat jump height was the most reliable measure (CV = 8.64%), while leg stiffness during sub-maximal hopping, and reactive strength index produced moderate reliability (CV = 10.17–13.93% and 13.98% respectively). Measures of leg stiffness obtained from contact mat data during sub-maximal hopping were in agreement with the criterion measure (r = 0.92–0.95; TEE = 6.5–7.5%), but not during maximal hopping (r = 0.59; TEE = 41.9%). The contact mat was deemed a valid tool for measuring stretch–shortening cycle ability in sub-maximal but not maximal hopping. Although reliability of performance was generally moderate, the tests offer a replicable assessment method for use with paediatric populations.

Continuous change in spring-mass characteristics during a 400 m sprint

The purpose of the present study was to utilise a spring-mass model to (1) continuously measure vertical stiffness ( K vert) and leg stiffness ( K leg) over an entire 400 m sprint, and (2) investigate the relationship between leg spring stiffness ( K vert and K vert) and the performance characteristics of mean forward running velocity ( V forwad), mean stride frequency ( f stride), and mean stride length ( L stride). Eight well-trained male athletes performed a 400 m sprint with maximal effort on an outdoor field track. K vert was calculated from the subjects’ body mass, ground contact time and flight time at each step. V forwad, f stride and L stride were determined from video images. K vert and V forwad peaked at the 50–100 m interval, and consistently decreased from the middle to the later part of the sprint. K leg peaked at first 50 m interval, and remained constant from next 50 m interval to finish. As compared with peak values, K vert and V forward in the last 50 m decreased by about 40% and 25%, respectively. A significant positive linear relationship existed between the K vert and V forward. While K vert was significantly correlated with f stride, it had no correlation with L stride. Further, no significant positive linear relationship was found between K leg and V forward, f stride, or L stride. This result indicates that in order to keep V forward at later stage of a 400 m sprint, maintaining the higher f stride through retaining a higher K vert would be necessary.

Vertical Leg Stiffness Following Ankle Taping and Bracing

This study sought to determine if prophylactic ankle taping and bracing influenced vertical leg stiffness during hopping. Twenty healthy and physically active participants completed testing under three ankle stabilizer conditions, ankle tape, semi-rigid ankle brace or control (no stabilizer) during three separate sessions. Immediately following stabilizer application, participants completed an exercise bout prior to completing the hopping. Vertical ground reaction forces were collected during the hopping and were used to calculate vertical leg stiffness. No significant changes in vertical leg stiffness were revealed. These results are likely attributable to the exercise bout causing sufficient loosening of the stabilizers, concurrent proximal joint compensations and/or the possibility that the effect of ankle stabilizers on shock absorption during hopping is more subtle than the effects revealed during drop landings.

Gender Comparisons of Dynamic Restraint and Motor Skill in Children

To assess differences in neuromuscular dynamic restraint between high-skilled and low-skilled prepubescent girls and boys. To determine the contribution of sport experience and physical characteristics to motor skill. Nineteen girls and 17 boys (8.89-9.40 y) participated. Isometric hamstring and quadriceps muscle strength was assessed. Subjects performed 3 landing trials for measurement of preparatory EMG and vertical leg stiffness. Motor skill was assessed through analysis of 12 fundamental tasks. Sport experience was reported as hours per week and total years in organized and nonorganized activity. Dynamic restraint variables of isometric strength, preparatory EMG activity, and vertical leg stiffness were measured between groups. The contributions of time in sport, type of sport, and physical characteristics on skill were analyzed. No significant gender or skill differences were found in quadriceps strength (P = 0.73), hamstring strength (P = 0.96), hamstring-to-quadriceps ratio (P = 0.71), or vertical leg stiffness (P = 0.38). Low-skilled children exhibited significantly greater (47.8%) preparatory hamstring-quadriceps coactivation than high-skilled subjects (P = 0.03). Participation in organized and nonorganized sport accounted for 29% of the variance in motor skill. Neuromuscular differences between genders were not observed, but dynamic restraint EMG measures differed between skill levels. The factors predisposing females to noncontact injuries may develop prepuberty to postpuberty from a combination of variables. Greater coactivation in the low-skilled group appears consistent with immature feedforward neuromuscular control strategies. These unrefined motor skills are less economical, may compromise dynamic restraint, and appear partially determined by sport experience.

Fatigue, Vertical Leg Stiffness, and Stiffness Control Strategies in Males and Females

Fatigue, Vertical Leg Stiffness, and Stiffness Control Strategies in Males and Females

DOMS-associated changes in ankle and knee joint dynamics during running.

The purpose of this study was to determine whether leg mechanics change due to DOMS by examining ankle and knee joint kinematics and stiffness before and after a down hill run. Sagittal plane kinematics were recorded with high-speed (120 Hz) video at a speed representing 75% of VO2peak of nine well-trained male runners before (RE1) and 48 h after (RE2) a 30-min downhill run. From the recorded video, 10-12 consecutive strides were digitized, and the following variables were calculated for each stride: ankle and knee range of motion (ROM), ankle and knee peak angular velocity, ankle and knee stiffness, and leg vertical stiffness. A repeated measures ANOVA was calculated for each variable (alpha = 0.05). Both knee and ankle ROM during stance decreased with DOMS, but otherwise there were few changes in ankle mechanics with DOMS. Knee stiffness tended to increase during the early portion of stance (from initial stance to maximum angular velocity of flexion) with DOMS, immediately followed by a decrease (to maximum knee flexion) in stiffness. Changes in knee stiffness caused vertical leg stiffness to increase for the initial portion of stance with DOMS. Knee mechanics changed such that the knee stiffness increased at initial stance, resulting in an increase in vertical leg stiffness. This change in knee stiffness possibly serves as a protective mechanism to prevent further damage or pain in the knee extensor musculature.