MUSCULAR ACTIVITY IN THE STRETCH-SHORTENING CYCLE (SSC): NOT ONLY MAXIMIZATION BUT OPTIMIZATION IS NECESSARY

INTRODUCTION: Drop jumping is a very complex skill with a stretch-shortening cycle (SSC) involving several phases – preparation for landing, landing itself, shock absorption and push-off. Only a very short span of time (£ 200ms) is available for the execution of a drop jump. For this reason much is required of the jumper’s neuromuscular system. Especially an EMG reduction of m. gastrocnemius activity just before and at the beginning of landing has been reported several times (Schmidtbleicher/Gollhofer, 1985). This reduction has been interpreted as a mechanism to protect the muscle from excessive stretch loads. Previously, only qualitative results have been reported. METHODS: Reactive capabilities were analyzed in two complex studies of 18 sport students. The following movements were studied: squat jumps, countermovement jumps and drop jumps from drop heights of 16, 24, 32, 40, 48 and 56 cm. EMG-activities of the m. tibialis anterior, m. soleus, m. gastrocnemius and m. rectus femoris were registered to describe muscle activity during vertical jumping. Simultaneously, ground reaction forces and changes in knee angle were registered. A new method was developed for measuring the EMG-reduction of m. gastrocnemius. RESULTS: In these studies it could be observed that EMG reduction of m. gastrocnemius occurs simultaneously with the maximal innervation of m. rectus femoris. Strong action in the knee joint is demonstrated by the activity of the leg extensor m. rectus femoris, which is needed to stop downward movement and to start the push-off. The m. gastrocnemius´ activity with its antagonistic effect on the knee joint was reduced when m. rectus femoris activity started, and reached maximal innervation significantly later than m. rectus femoris during ground contact. This is especially the case for good jumpers. After a training period of four weeks (only reactive movements) this program of innervation could be observed in almost all the participants in these studies. CONCLUSION: The results show that the EMG-reduction of m. gastrocnemius during drop jumping cannot be considered a reliable mechanism to protect the muscle from excessively large stretch loads, but that this EMG activity of the antagonistic muscles makes possible an optimal intermuscular coordination for drop jumping. REFERENCES: Schmidtbleicher, D.; Gollhofer, A. (1985). Einflußgrößen des reaktiven Bewegungsverhaltens und deren Bedeutung für die Sportpraxis. In: Bührle, M. (1985). Grundlagen des Maximal- und Schnellkrafttrainings, 271-281

BIOMECHANICAL CHARACTERISTICS RELATED TO POLING PROPULSIVE EFFECTIVENESS IN CROSS-COUNTRY V2 SKATING TECHNIQUE

The purpose of this study was to investigate whether specific kinetic and kinematic characteristics distinguish the propulsive effectiveness of upper body in V2 cross-country skating technique. Female and male skiers (n=25) performed V2 on a treadmill using roller skis at a 6% incline, while kinetic parameters of plantar pressure and ski pole forces, as well as 3d-kinematic data were collected. The ratio between propulsive and overall impulse (effectiveness) of ski poles was 50% for female and 52% for male athletes and highly correlated with ski pole angles. Male skiers showed smaller pole angles at maximum propulsive force than females (P\u3c0.05). Athletes should consider a more effective ski pole planting angle in order to improve propulsive poling action and consequently their performance

Landing in Ski Jumping: A Review About its Biomechanics and the Connected Injuries

<jats:title>Abstract</jats:title><jats:p>The present review deals with the current scientific knowledge related with ski jump landing. A specific focus is given on the landing biomechanics, the methods utilized for its analysis and the injuries connected to the landing phase. Despite the demonstrated importance for the safety and the performance of ski jumpers, the landing and its preparation are rarely investigated. In this paper, after having firstly described the execution of landing and its preparation and the reason why is important to analyze it, an overview of the current status of the research related to the landing biomechanics is reported (kinetics, kinematics, electromyographic activation, aerodynamics, computer simulation). The third part describes the methods and technologies utilized in literature to analyze the landing and its preparation (video cameras, inertial sensors, force insoles, wind tunnel and computer simulation). After that, an overview of the injuries related to landing is reported. The final section proposes future research in the field of biomechanics of ski jump landing in different fields, such as computer simulations, kinematic analysis, equipment development and biomechanics of female athletes.</jats:p&gt

Ground Reaction Forces and Kinematics of Ski Jump Landing Using Wearable Sensors

In the past, technological issues limited research focused on ski jump landing. Today, thanks to the development of wearable sensors, it is possible to analyze the biomechanics of athletes without interfering with their movements. The aims of this study were twofold. Firstly, the quantification of the kinetic magnitude during landing is performed using wireless force insoles while 22 athletes jumped during summer training on the hill. In the second part, the insoles were combined with inertial motion units (IMUs) to determine the possible correlation between kinematics and kinetics during landing. The maximal normal ground reaction force (GRFmax) ranged between 1.1 and 5.3 body weight per foot independently when landing using the telemark or parallel leg technique. The GRFmax and impulse were correlated with flying time (p < 0.001). The hip flexions/extensions and the knee and hip rotations of the telemark front leg correlated with GRFmax (r = 0.689, p = 0.040; r = −0.670, p = 0.048; r = 0.820, p = 0.007; respectively). The force insoles and their combination with IMUs resulted in promising setups to analyze landing biomechanics and to provide in-field feedback to the athletes, being quick to place and light, without limiting movement

Analysis of Landing in Ski Jumping by Means of Inertial Sensors and Force Insoles

Landing and its preparation are important phases for performance and safety of ski jumpers. A correct ski positioning could influence the jump length as also the cushioning effect of the aerodynamic forces that permits the reduction of landing impacts. Consequently, the detection of ski angles during landing preparation could allow for analyzing landing techniques that result in reduced impact forces for the athletes. In this study, two athletes performed with force insoles and inertial sensors positioned on the ski during training conditions on the ski jumping hill. The results confirmed previous studies, showing that impact forces can reach more than four times body weight. In the analyzed cases, the force distribution resulted to be more concentrated on the forefoot and the main movement influencing the impact was the pitch. The combination of inertial sensors, in particular gyroscopes, plus force insoles demonstrated to be an interesting set up for ski jumping movement analysis

Random Whole Body Vibration over 5 Weeks Leads to Effects Similar to Placebo: A Controlled Study in Parkinson’s Disease

Background. Random whole body vibration (WBV) training leads to beneficial short-term effects in patients with Parkinson’s disease (PD). However, the effect of WBV lasting several weeks is not clear. Objectives. The aim of this study was to assess a random WBV training over 5 weeks in PD. Methods. Twenty-one participants with PD were allocated to either an experimental or a placebo group matched by age, gender, and Hoehn&Yahr stage. The WBV training consisted of 5 series, 60 s each. In the placebo group, vibration was simulated. The primary outcome was the change of performance in Functional reach test (FRT), step-walk-turn task, biomechanical Gait Analysis, Timed up and go test (TUG), and one leg stance. Findings. In most of the parameters, there was no significant interaction of “timegroup.” Both groups improved significantly in Gait parameters, TUG, and one leg stance. Only in the FRT [; ] and in the TUG [; ] the experimental group performed significantly better than the placebo group. Conclusions. Random WBV training over 5 weeks seems to be less effective than reported in previous studies performing short-term training. The slight improvements in the FRT and TUG are not clinically relevant

Relationships between strength and endurance parameters and air depletion rates in professional firefighters

The aim of this study was to quantify the physical demands of a simulated firefighting circuit and to establish the relationship between job performance and endurance and strength fitness measurements. On four separate days 41 professional firefighters (39 ± 9 yr, 179.6 ± 2.3 cm, 84.4 ± 9.2 kg, BMI 26.1 ± 2.8 kg/m) performed treadmill testing, fitness testing (strength, balance and flexibility) and a simulated firefighting exercise. The firefighting exercise included ladder climbing (20 m), treadmill walking (200 m), pulling a wire rope hoist (15 times) and crawling an orientation section (50 m). Firefighting performance during the simulated exercise was evaluated by a simple time-strain-air depletion model (TSA) taking the sum of z-transformed parameters of time to finish the exercise, strain in terms of mean heart rate, and air depletion from the breathing apparatus. Multiple regression analysis based on the TSA-model served for the identification of the physiological determinants most relevant for professional firefighting. Three main factors with great influence on firefighting performance were identified (70.1% of total explained variance): VO, the time firefighter exercised below their individual ventilatory threshold and mean breathing frequency. Based on the identified main factors influencing firefighting performance we recommend a periodic preventive health screening for incumbents to monitor peak VO and individual ventilatory threshold

Physiological responses to firefighting in extreme temperatures do not compare to firefighting in temperate conditions

Purpose: The aim of this study was to examine physiological responses to two different simulated firefighting exercises: a firefighting exercise with flashovers, smoke, poor visibility and extreme temperatures (300°) in a burning container and a standard firefighting exercise in temperate conditions. Furthermore, a second purpose of the study was to find out if the contribution of strength and endurance capacities to firefighting performance changes when the demands of the firefighting exercise change. Methods: Sixteen professional firefighters performed a maximum treadmill test, strength testing, a standard simulated firefighting exercise (SFE) without heat and flashovers and a firefighting exercise with a simulation of the flashover phenomenon in a burning container (FOT). The treadmill testing was used to determine peak oxygen uptake (VO), ventilatory threshold (VT1) and respiratory compensation point (RCP). Three intensity zones were identified according to heart rate (HR) values corresponding to VT1 and RCP: zone 1-HR below VT1, zone 2-HR between VT1 and RCP, zone 3-HR above RCP. Firefighting performance was determined by a simple time-strain-air depletion model (TSA) taking the sum of z-transformed parameters of time to finish the exercise, strain in terms of mean heart rate, and air depletion from the breathing apparatus. Correlations were then established between TSA based firefighting performance parameters and fitness variables representing strength and endurance. Results: HR was significantly lower during SFE (79.9 ± 6.9%HR) compared to FOT (85.4 ± 5.2%HR). During SFE subjects spent 24.6 ± 30.2% of time in zone 1, 65.8 ± 28.1% in zone 2 and 9.7 ± 16.6% in zone 3. During FOT subjects spent 16.3 ± 12.8% in zone 1, 50.4 ± 13.2% in zone 2 and 33.3 ± 16.6% in zone 3. Out of all correlations, relative VO showed the highest relation to mean HR during SFE (-0.593) as well as FOT (-0.693). Conclusions: Endurance in terms of VO is an important prerequisite for both firefighting exercises. However, for standard simulated firefighting exercises it is important to work below VT1. For firefighting exercises in extreme temperatures with smoke, poor visibility and unexpected flashovers a high fitness level is required in order to keep the time spent above RCP as short as possible