To understand cross-country (X-C) siding it is important to record and identity forces of skis and poles separately and together. They both contribute to the forward progression, but their functional significance may be more complex than that of the ground reaction forces in running and walking. This report presents two methods to record forces on skis and poles during normal X-C skiing. A long force-platform system with four rows of 6-m long plates is placed under the snow track for recording of Fz and Fy forces of each ski and pole separately. This system is suitable especially for the study of diagonal technique under more strict experimental conditions. The second system consists of small lightweight Fz and Fy component force plates which are installed under the boot and binding. These plates can be easily changed from one ski to another, and telemetric recording allows free skiing over long distances and with different skiing techniques, including skating. The presentation emphasizes the integrated use of either system together with simultaneous cinematographic and electromyographic recordings.
Paavo V. Komi
Caroline Nicol and Paavo V. Komi
Magnitude of the reflex contribution to force enhancement was investigated in vivo during passive stretches of the Achilles tendon (AT) of one female subject. Thirty passive (5 × 6) dorsiflexions were induced by a motorized ankle ergometer. Achilles tendon force (ATF) was sensed by a buckle transducer applied surgically around the right AT. Single passive stretches resulted in a low but rather linear ATF increase in the absence of EMG (surface electrodes) activity. In the presence of reflexes, a clear ATF enhancement occurred 13–15 ms after the beginning of the EMG reflex responses. In double dorsiflexions at either 1.2 or 1.9 rad · s-1, which were separated by a maintained stretched position of either 40 or 90 ms, the first stretch resulted in initial linear ATF increase, followed by an additional force enhancement during the plateau phase. This reflexly induced increase represented 94 ± 4 N and 184 ± 1 N, respectively, for the 40 and the 90 ms plateaus, corresponding to 210 ± 85% and 486 ± 177% enhancements as compared to the first passive stretch effect. The results suggest further that timing of the stretch during the twitch response influences the magnitude and rate of force potentiation.
Antti Mero and Paavo V. Komi
This study was undertaken to compare force-time characteristics, muscle power, and electromyographic (EMG) activities of the leg muscles in maximal sprinting and in selected bounding and jumping exercises. Seven male sprinters performed maximal bounding (MB), maximal stepping (MS), maximal hopping with the right (MHR) and left (MHL) legs, and maximal sprint running (MR). These “horizontal” exercises and running were performed on a force platform. EMG activity was telemetered unilaterally from five leg muscles during each trial. The results indicated significant (p < .001) differences among the studied exercises in velocity, stride length, stride rate, flight time, and contact time. Also, significant differences were noticed in reactive forces (p < .01-.001) and power (p < .01) among the performances, whereas only insignificant differences were observed in EMG patterns. The average resultant forces during the braking and propulsion phases in MS, MHR, and MHL were greater (p < .001) than in MR and MB. Stepping and hopping are cyclic and sprint-specific and may be used as strength exercises for sprinters because of great strength demand.
Mikko Virmavirta and Paavo V. Komi
This study measured the takeoff forces exerted by jumpers during the 70-m ski jumping competition of the 1988 Winter Olympics in Calgary. Instrumentation consisted of four force plates installed under the snow of the takeoff platform. The results indicated that the greatest force was already exerted 149±9 ms before the release. The second force peak appeared closer to the edge of the takeoff platform. The correlations between the variables measured in this study were generally weak but some were considered important. The official approach velocity of the first round and relative maximum force as well as the mean relative force during the whole takeoff sequence of the second round correlated to length of jump. The mean relative forces at the end of takeoff and for the whole takeoff sequence were significantly higher among the best jumpers. It is concluded that although the force analyses among the jumpers do not reveal conclusive interrelationships, the fast development of the takeoff forces may be an important prerequisite for successful ski jump performance.
Paavo V. Komi and Albert Gollhofer
Mikko Virmavirta and Paavo V. Komi
Electromyographic (EMG) activities of gluteus maximus (GL), vastus later-alis (VL), vastus medialis (VM), tibialis anterior (TA), and gastrocnemius (GA) were measured telemetrically from four world-class athletes during the entire ski jumping performance. Integrated electromyographic activities (IEMG) were calculated from the different phases of jump. TA and GA showed alternate activation during the curve, suggesting that maintenance of the inrun position is a process requiring continuous active control. VL and VM were observed to contribute mostly to the entire takeoff phase whereas GL became strongly active within the last 4 meters of the takeoff. GA was slightly but continuously active during the inrun and showed only a small increase during takeoff. The quick lifting of the skis, as evidenced by the activation of TA, does not seem to allow effective use of GA at the end of the takeoff. Strong continuous activity of the knee extensors and TA dominated the midflight phase whereas the activation of GL and GA increased toward the end of the flight.
Taija Finni and Paavo V. Komi
During dynamic activities it is difficult to assess in vivo length changes in human tendon and aponeurosis. The present study compared the outcome of two methods during unilateral squat jump and drop jump performances of four volunteers. Tendinous tissue elongation of vastus lateralis muscle was estimated using either (a) direct measurement of in vivo fascicle length change and muscletendon length estimation (kinematic method), or (b) prediction using a quadratic force function in combination with direct tendon force measurement (force method). In the kinematic method the most critical measures contributing to the 10% uncertainty were the fascicle angle and fraction of the estimated fascicle length. The force method was most sensitive to resting length, with 1% error margin. Both methods predicted the same pattern of tendinous elongation because of the monotonic force/length relationship. The magnitude of length change, however, differed considerably between both methods. Based on the force method, the changes were only 20% (absolute values) or 30% (strain values) of those obtained with the kinematic method. On average, the maximum strains were 5% with the force method and 15% with the kinematic method. This difference can be explained by the fact that the kinematic method characterizes not only the changes in tendon length but also includes aponeurosis strain along the muscle belly. In addition, the kinematic method may be affected by non-uniform distribution of fascicle length change along the length of the muscle. When applying either method for estimating the patterns of tendon and tendinous tissue length changes during human locomotion, the given methodological considerations should be acknowledged.
Mikko Virmavirta and Paavo V. Komi
The Paromed Datalogger® with two insole pressure transducers (16 sensors each, 200 Hz) was applied to study the feasibility of the system for measurement of plantar pressure distribution in ski jumping. The specific aim was to test the sensitivity of the Paromed system to the changes in plantar pressure distribution in ski jumping. Three international level ski jumpers served as subjects during the testing of the system. The Datalogger was fixed to the jumpers’ lower back under the jumping suit. A separate pulse was transmitted to the Datalogger and tape recorder in order to synchronize the logger information with photocell signals indicating the location of the jumper on the inrun. Test procedure showed that this system could be used in ski jumping with only minor disturbance to the jumper. The measured relative pressure increase during the inrun curve matched well the calculated relative centrifugal force (mv2 · r‒1), which thus serves a rough estimation of the system validity. Strong increase in pressure under the big toes compared to the heels (225% and 91%, respectively) with large interindividual differences characterized the take-off. These differences may reflect an unstable anteroposterior balance of a jumper while he tries to create a proper forward rotation for a good flight position.
Paavo V. Komi and Antti Mero
A two-dimensional film analysis was performed on five men and six women finalists in the javelin throw of the Los Angeles Olympic Games of 1984. In addition to the great interindividual variation in the measured kinematic and kinetic parameters, the results indicated that in men the winner had the highest release velocity of javelin (29.12 m × s−1), and that a significant (p < .01) relationship could be obtained between the release velocity and throwing distance. Despite great variation in throwing distance in women (55.88 m - 69.56 m) the release velocities were in relatively small range (20.73 m × s−1 - 23.62 m × s−1). High impact loading was specific to the last foot contact on the ground. It was characterized by a short duration (0.032 s) and high velocity knee flexion (12.66 ± 2.11 and 12.27 ± 3.81 rad × s−1, respectively, for men and women). The respective knee extension velocities averaged 5.80 ± 2.00 and 7.60 + 5.17 rad × s−1. Despite the fact that a small number of analyzed world-class throws revealed only some biomechanical differences between good and poorer performance, the results can be used to identify some new criteria for successful performance in javelin throw.
Antti Mero and Paavo V. Komi
The effects of running at supramaximal velocity on biomechanical variables were studied in 13 male and 9 female sprinters. Cinematographical analysis was employed to investigate the biomechanics of the running technique. In supramaximal running the velocity increased by 8.5%, stride rate by 1.7%, and stride length by 6.8% over that of the normal maximal running. The elite male sprinters increased their stride rate significantly but did not increase their stride length. The major biomechanical differences between supramaximal and maximal running occurred during the contact phase. In supramaximal running the inclination of the ground shank at the beginning of eccentric phase was more "braking" and the angle of the ground knee was greater. During the ground contact phase, the maximal horizontal velocity of the swinging thigh was faster. The duration of the contact phase was shorter and the flight phase was longer in the supramaximal run as compared to the maximal run. It was concluded that in supramaximal effort it is possible to run at a higher stride rate than in maximal running. Data suggest that supramaximal sprinting can be beneficial in preparing for competition and as an additional stimulus for the neuromuscular system during training. This may result in adaptation of the neuromuscular system to a higher performance level.