**4. Discussion**

To understand the peculiarities of the MTU work in athletes of four different sport specializations, it is necessary to briefly present specific features of those sport disciplines.

#### **4.1 Alpine skiing**

Alpine skiing includes four disciplines, which are divided into technical disciplines (slalom SL, giant slalom GS) and speed disciplines (super giant SG, downhill DH). In slalom (SL) the peak ground reaction force (GRF) being a measure of the external load on the skier and equipment, can reach five times the body weight of an athlete [41]. In three other alpine skiing disciplines the highest GRF values are observed in giant slalom [42]. The main muscles involved in alpine skiing are the thigh muscles, hamstrings, quads, calves, and foot muscles [43]. Most of the muscles involved in alpine skiing contract eccentrically during the race. This means that the muscle lengthens as it contracts. The contraction rate during downhill skiing is relatively slow compared to running or jumping. This is due to the fact that, for example, the angle of the hip does not change significantly during a ski turn, so the rate of contraction is relatively low. Alpine skiing requires fast movements with multiple maximum contractions. Compared to other athletes, alpine skiers have high isometric leg strength [44] and isokinetic leg strength, measured as maximum torque during knee extension [45]. In technical disciplines, a "quasi-static" component of skiing has already been proposed because of the evidence for coactivation during contraction of the thigh muscles [46]. Evidence from elite Swiss riders confirms that eccentric muscular work prevailed over concentric work in the SL, GS and SG disciplines [47]. Authors [47] claimed that alpine skiing is "the only sport in which eccentric muscular activity predominates" [47]. Higher levels of force allow the skier to work with less relative strength, which can lead to more control in turns (less energy dissipation) and less fatigue when racing.

#### **4.2 Mogul skiing**

Mogul skiers have to overcome irregularities in the form of bumps and pits when descending from the mountain. On a mogul course, the turns to the left and right of

*Peculiarities of Muscle-Tendon Mechanics and Energetics in Elite Athletes in Various Sports DOI: http://dx.doi.org/10.5772/intechopen.97000*

the downhill line are always made cyclically, without any separate transition phase between individual turns. An athlete goes around the hillocks on the descent. He/she makes a turn at each bump that forms the so-called "swinging rhythm". Mogul skiing is characterized by clearly identifiable ski loading and unloading phases, and alteration of ski bending and unbending. The load, being almost evenly distributed between both skis, reaches the maximum value of about 150% of the skier's body weight on each ski. The technique obliges athletes to move along the track with an angle at the knee joints close to 90° to ensure the minimum deviation of the CoM trajectory. This stance allows a skier to cushion the height differences of bumps and pits, while maintaining and controlling the speed. According to the data of [48, 49], it was shown that in mogul skiers, in contrast to alpine skiers, the ratio of eccentric/concentric patterns is close to those in running.

### **4.3 Bobsleighing**

A standard bobsled race may be divided into three phases: start, race and finish [50]. Acceleration and high speed at the end of the start phase are crucial for high bob speed during a race and a better overall result [50, 51]. At the start phase, athletes must start the race by pushing the bob within 60 seconds after the start signal and run at full speed about 55 m. The start and push phases last for only 6 seconds, but they are the most important for the final result [52]. To perform a push-start, leg extensors must generate high mechanical power in the hip, knee and ankle joints to create significant horizontal propulsive force that propels the athlete's body and bob forward. Previous studies reporting the electromyographic (EMG) activity in sprinters in the acceleration phase have shown that biceps femoris (RF) is one of the most important muscles for developing maximum speed [53]. Similar as at the start of a sprint run, the longer running distance increased workload of the plantar ankle flexors [52].

#### **4.4 Ski jumping**

Take-off in ski jumping is a very important phase [54, 55], since it affects the initial flight phase. Three parameters indicating a change in body position during a take-off are important for the overall performance: (1) generation of angular momentum during a take-off [54, 56, 57]; (2) high speed of leg extension [54, 56, 57]; and (3) direction of a take-off [58–60]. Athletes perform a take-off in a quasi-static crouch position [57]. This position is necessary to maintain an effective aerodynamic posture and to resist the pressing external force acting on the body at a speed of about 90 km/h. According to [61], time from the start of the lower extremities extension and the rise of the athlete's center of gravity (COG) is about 0.22 ± 0.03 sec. The data presented in [62] indicate the time range of 0.25–0.30 sec. The study [59] examined the activity of the vastus lateralis muscle, vastus medialis, gastrocnemius, tibialis anterior and gluteus maximus. During acceleration on the jumping ramp the activity of all those muscles is very low. When an athlete enters the inner curve of the jumping ramp, the activity of the vastus lateralis, tibialis anterior and the gastrocnemius muscles grows that stabilizes the knee and ankle joints. Stable knees resist additional stress caused by the centrifugal force. The gluteus maximus remains inactive. During the take-off activity of the vastus lateral and the gastrocnemius muscles significantly increase. The tibialis anterior and gastrocnemius muscles are active to stabilize the ankle, but their activity is not greater than that registered at the curve of the jumping ramp. Greater activity of Gl at the end of the take-off results in an increase of moments in hip joints. Involvement of GA in the final phase of the take-off is considerably lower than in vertical jumps, in which plantar flexion plays very important role. A ski jumper rises skis quickly that does not allow him/her to use

the gastrocnemius effectively, and knee extensors are the main muscles involved in the take-off. On the other hand, the design of ski jumping boots limit efficient plantar flexion during the take-off [62].

The data presented in the results section show that all groups of athletes chose different strategies for producing muscle power in order to achieve the maximum jump height in drop jumps. We will try to explain the mechanisms of work of lower extremities' muscles in athletes, who demonstrate different organization of movement and interaction of muscles and tendons in drop jumps. Let us examine ways of enhancing MTU contraction, which employ elastic properties of muscles, in particular, their resilient elements, that results in muscle potentiation. Besides that, enhancement of muscle contraction occurs due to stretch reflex, which manifests itself when the muscle is stretched for 20–50 ms [63] and even up to 60 ms [64]. There is no disagreement in the literature that the preliminary stretching of the MTU enhances its subsequent contraction, and this fact is described by many authors. However, with regard to the mechanisms underlying this phenomenon, many authors express doubts about the unambiguity of conclusions in favor of each of them, in particular, regarding the use of the energy of elastic deformation [65]. At the same time the use of the energy of elastic deformation in running and hopping is beyond any doubt. All doubts arise from the difficulty to examine human MTU behavior in vivo, in particular in complex multi-joint movements. Without inquiring into the mechanisms of MTU pre-stretching, let us discuss if the athletes participating in our study use the positive effect of MTU pre-stretch.

In the groups of alpine skiers and mogul skiers, duration of the take-off went far beyond the time limits where it was possible to use the positive effect of the stretch reflex. However, the subjects of these two groups could enhance voluntary contraction of their leg muscles due to elastic strain energy, even though the shock absorption phase was very long. Although an increase in the MTU stretching time reduced efficiency of elastic strain energy use, this mechanism was preserved [66]. We might be confused by the fact that the optimal ratio of muscle length and speed of their stretching is disturbed in alpine and mogul skiers because of large angular amplitudes of movements in leg joints. However, the use of elastic energy is still possible for any muscle length [67]. The question arises with regard to the use of the positive effect of shock absorption in drop jumps performed by alpine and mogul skiers. Question: Did the MTU work differ in these groups of athletes, since the technique of sport exercise was different? The answer is obvious - yes, it differed. This is confirmed by the data related to the transfer of elastic deformation energy between the lower extremities links. First of all, let us examine results of the drop jump from the height of 0.3 m, in which the load on the leg muscles is optimal for achieving the maximum jump height [68]. In the drop jump from 0.3 m the possible use of elastic energy in the take-off phase was: 23 ± 1.6% (Hip-Knee) and 15 ± 1.1% (Knee-Ankle) in alpine skiers, and 12 ± 0.6% (for Hip-Knee) and 8 ± 0.5% (for Knee-Ankle) in mogul skiers. Alpine skiers used energy transformation more effective than mogul skiers that was due to the higher stiffness of their leg muscles. The difference in the utilization of stored elastic energy between the subjects was visible when comparing metabolic costs in RF and VAS and tendon strength in these muscles, which were significantly higher in mogul skiers. In a drop jump from the height of 0.5 m the external load on the leg muscles was almost critically high. Mogul skiers showed a drop in mechanical power as well as an increase in RF metabolic costs and RF and VAS\_L tendon strength. In both groups we found a decrease of elastic energy use. Referring to the difference in the requirements of a sport exercise, we may assume that there are no mandatory requirements for mogul skiers to generate maximum power during a mogul race, unlike for alpine skiers.

#### *Peculiarities of Muscle-Tendon Mechanics and Energetics in Elite Athletes in Various Sports DOI: http://dx.doi.org/10.5772/intechopen.97000*

In alpine skiers, the decrease in efficiency of energy transfer might be associated with the decrease in stiffness in order to increase the role of muscle activity.

Bobsledders demonstrated the highest percentage of mechanical energy transfer. As the height of a drop jump increased, the metabolic costs of activation of the GA and SOL muscles decreased and the peak strength of tendons increased. These results indicate the use of the mechanism of transfer of mechanical energy generated during muscles pre-stretch. This mechanism can only be activated if the extension starts in the hip joint, which generates the greatest power. The energy transfer cannot occur instantaneously, because the bicarticular muscles need time to stretch and gain stiffness in order to reduce energy loss caused by dissipation [69]. When the hip joint extended due to Gl contraction, the energy was transferred to the shin via the RF, then it took time for the GA to be activated after the start of the knee extension. Elastic energy was stored in the tendons of the ankle extensors and then it was released quickly. The MTUs of the muscles acted as an adjustable spring, and the contractile component modulated the energy required for fast limbs extension. Thus, leg extension occurred with successive achievement of peak power values in each of the joints and peak values of the linear velocity of the leg links in the direction from the proximal joint to the distal one. In fact, energy transfer in squat jump and counter movement jump occurs exactly this way [3, 70–72]. As a result, the ankle joint being controlled by the weakest muscle group, can develop greater power due to the energy received from the proximal joint [3]. Thus, to achieve maximum muscle power, bobsledders used conjoint work of both muscles and tendons that permitted them to make use of the mechanism of energy transfer based on the pre-stretch effect. The group of bobsledders included former track-and-field athletes. To push the bob, they extended the lower extremities using similar range of articular angles as in drop jumps. According to our data, it can be assumed that ski jumpers have no technical skills to increase muscle power in a take-off using additional mechanisms besides muscle activity. The ski jumpers showed the longest take-off time in drop jumps, and it corresponded to the take-off time from the take-off table in their sport exercise. In accordance with the temporal characteristics of the take-off, we may assume that ski jumpers do not use the effect of stretch reflex. High metabolic costs of muscle work and small forces in the tendons indicated that the ski jumpers used only muscle strength to perform drop jumps from different height and did not employ any additional mechanisms to increase the power of the movement. Prestretch in the MTU in the shock absorption phase in drop jumps led to utilization of energy stored in tendons and muscles into heat in the take-off phase. Athletes performed the take-off in drop jumps using the technique they are accustomed to, i.e. the technique of their sport exercise. This was confirmed by a decrease in legs stiffness and an increase in mechanical power as the drop jump height increased.
