Mi análisis sobre las fotografías digitales de Eric

Skilful movement results from the forceful application of correct technique at the correct time. Besides controlling external forces, the

athlete needs to be able to create and control internal forces through the action of the mus- cles and the myotendinous structures (figure 4.12), as explained by the sliding filament theory of muscular contraction.6 _{Practitioners}

need to understand this mechanism of muscu- lar contraction to conceptualize how muscles work to exert forces. As identified in chapter 2, motor units within the muscles contract in response to a stimulus from the motor nerve; these motor units are either contracted or they aren’t (the all-or-nothing principle).

The more motor units that are activated, the greater the force that is generated along the length of the muscle, creating a pull on the bones to which the muscles are attached. If this pull is greater than the mass of the bone (the example in figure 4.13 is the lower arm, with the hand holding a weight), then the bone will move. The role of the athletic development professional is to help the athlete develop the skill of sequencing the timing of the motor action potentials within the requisite number

E5649/Brewer/fig 04.12/542752/HR/mh-R2 Thin filament

(actin)

Thick filament (myosin)

Myofibril Thinfilament Thick filament M line Sarcomere Z Z Z Z H H

zone lineZ bandA bandI

H zone Z line lineZ A band I band Muscle fibre

Muscle fibres (cells) Myofibrils

Muscle

of motor units to ensure successful coactivation of the muscles and thereby achieve the desired action. The physical qualities associated with timing and coordination cannot be separated from the decision-making aspects in analysing the effectiveness of a technique in an applied context.

Not all muscle force is caused by shortening the muscles. Indeed, many of the most import- ant muscular actions within sport are brought about by lengthening the muscle under ten- sion. These eccentric muscle actions (not con- tractions, because the muscle length doesn’t contract or shorten) are essential in controlling movements and all deceleration actions and in producing high-force, high-velocity actions through the stretch-shortening or plyometric actions described in chapter 2.

As illustrated in figure 4.13, eccentric actions can be caused only by an opposing force, because a muscle cannot actively lengthen

through conscious control. In figure 4.13, the opposing force of gravity acting on the dumbbell is greater than can be resisted by the biceps brachii and other assistance muscles responsible for elbow flexion. The load is still being resisted by the architecture of the muscle at a myofibril level (i.e., it is still actively trying to contract), but the muscle cannot generate enough internal force to oppose the action of gravity. Besides ensuring that the muscle is working hard, this type of eccentric loading causes greater damage to muscle tissues. This damage occurs because the heads of the myosin filament are actively pulled off the actin fil- ament before reattaching further along the protein strand. (In a concentric contraction, these would attach and reattach to cause the muscle to shorten progressively.) This process continues until the load is no longer being lowered (i.e., the arm is fully extended). The rip and reattach process causes damage, inflam- mation and swelling to the protein filaments, experienced as delayed onset muscle soreness (DOMS). This soreness is the aching stiffness often felt after strenuous exercise, often (incor- rectly) attributed to lactic acid in the muscles. Coaches should be aware that eccentric actions do not occur only when an external mass greater than the muscle’s maximal con- centric capabilities forces a muscle to lengthen. Indeed, being able to perform strong eccentric actions is as vital as being able to control the movements required in sport. Consider this first in a training context. In the fundamen- tal movement pattern known as the squat, the athlete lowers the body until the hip is below the knee before returning to standing. Typically, this exercise is performed with an external load on the back, but the single-leg squat (figure 4.14) is a variation (small base of support, body mass through a single limb) that doesn’t necessarily require an external mass to overload the musculature.

The squat is an important movement pattern in both assessing (chapter 6) and progressing (chapter 10) the athlete’s functional strength and control. As with any resistance exercise, lowering and raising actions are involved in this movement. Rising (returning to standing) is achieved through the concentric contraction E5649/Brewer/fig 04.13/542753/HR/mh-R2 Concentric action Shoulder Biceps brachii Elbow Wrist 20 kg dumbbell = 20 x 9.81 = force of 196.2 N/m2 40 kg dumbbell = 40 x 9.81 = force of 392.4 N/m2 30 kg dumbbell = 30 x 9.81 = force of 294.3 N/m2 Eccentric action Isometric action

Contractile force exerted by muscle less than 196.2 N/m2_{. Muscle}

shortens during contraction.

Contractile force exerted by muscle greater than 392.4 N/m2_{. Muscle} lengthens despite active contraction to resist mass of dumbbell.

Contractile force exerted by muscle= 294.3N/m2_{. Muscle does}

not change length while actively resisting mass of dumbbell.

20 kg 40 kg 30 kg 20 kg 40 kg

Figure 4.13 Muscle actions: concentric, eccentric

of the hip and knee extensors, which con- tract to exert a force into the ground (action), resulting in a reaction force that overcomes gravity. During the lowering (descent) phase, the flexion movements in the hip and knee are assisted by gravity, which means they need to be resisted (or controlled) by the muscles that contract to cause hip extension. These muscles work eccentrically, that is, they are lengthened under tension, even though this action is not maximal. Note also that during this movement, the gluteus medius contracts isometrically to maintain the level pelvic position.

Eccentric control function is extremely important in controlling dynamic movement in sport. Indeed, any athlete who must decel- erate and change direction needs to be strong eccentrically in the muscles that oppose the joint action. For example, in figure 4.15, the right leg of the tennis player is acting to brake (decelerate) forward momentum. This action involves eccentric contraction of the quadriceps, hamstrings, gluteus maximus and gastrocsoleus (calf) muscles to establish a low, flexed position in the hip and knee and prevent momentum from carrying the athlete’s trunk forward and over the planted foot.

Similar muscle mechanics are required in any deceleration position. If an athlete is moving from an acceleration position and is required to stop or slow, these muscles need to work eccentrically to enable this action to E5649/Brewer/fig 04.14/547883/HR/R1

Eccentric

Isometric Concentric

Figure 4.14 Muscle actions: single-leg squat.

E5649/Brewer/fig 04.15/542757/HR/mh-R2

occur. Deceleration requires a great deal of strength to prevent the muscles from being damaged too much as they resist the forces exerted on them.

Eccentric actions are used in various ways to achieve different training objectives. Sub- maximal loads can be lifted or lowered slowly. This approach may be extremely useful from a rehabilitation training perspective, such as when a muscle has atrophied after disuse fol- lowing injury or when the goal is to increase connective tissue strength, such as in manag- ing a tendinopathy. Slow lowering increases the time during which the tissues are under tension, which is thought to stimulate fibre hypertrophy.

Training slowly, however, has a negative effect on the athlete’s neuromuscular system because training with slow movements leads to slowed movement speeds. From a functional perspective (see chapter 10), the development of eccentric strength may be better achieved by using resisting supramaximal loads or undertaking plyometric actions (chapter 9), depending on the overall balance of the ath- lete’s training programme.

The stretch reflex also requires high eccentric strength to store elastic energy and transfer it into a forceful reflex contraction. The majority of effective movement patterns across sport require the athlete to create and use the stretch-reflex mechanism and the stored elastic energy benefits to move efficiently. This concept can be illustrated in reactive jumping actions. In preparation, the ankle dorsiflexes (toes are pulled towards the knees) to prestretch the gastrocsoleus muscles and place them under tension. This action also creates tension within the Achilles tendon, which will be maintained because the ankle remains dorsiflexed. This tension and the accompanying storing of potential energy are responsible for much of the ankle stiffness that is a feature of the reactive jump, as detailed in chapter 9.

As the athlete makes active flat-foot contact with the floor, gravity exerts a large vertical force through the ankle, knee and hip joints, which is resisted by the simultaneous actions

of the hip (gluteus maximus, hamstrings), knee (quadriceps) and ankle extensors (gastroc- soleus complex) working rapidly and eccen- trically to maintain stiffness in these joints. Gravity exerts a force equivalent to many times body weight (magnitude depends on the height or distance jumped).

The vertical force of gravity lengthens the muscles resisting the force, which in turn activates the muscle spindle fibres to initiate a strong reflex contraction in the opposite direction. The greater the athlete’s eccentric strength is, the more rapidly this response occurs. Simultaneously, or indeed maybe even quicker than the neuromuscular response, the Achilles tendon recoils; the tendon is non- elastic, already stretched and under tension because of ankle dorsiflexion. In an athlete who is not eccentrically strong, the golgi tendon organ also activates, effectively inhib- iting the eccentric muscle action as a protective mechanism, meaning that the forceful reflex action will not occur.

In chapter 9, specialist strength-training methods known as plyometrics are detailed. Plyometrics develops reactive strength in the neuromuscular system, strengthens and max- imizes the nonelastic recoil potential of the tendon, maximizes the response of the muscle spindle fibres and aids in the inhibition of the golgi tendon organs to maximize the elastic properties of the muscle tissue.

The final muscle action in the neuromus- cular system is an isometric action. Isometric contractions occur when a muscular force is exerted but no resulting change occurs in muscle length (i.e., the force generated by the muscle is equal to the opposing mass resisting it); therefore, no change occurs in the joint angle of the attached muscles. Relatively few performances in sport require the performer to hold a static position. The classic exam- ple is in gymnastics, shown in skills such as the crucifix on the rings or a handstand on a beam.

Although static hold positions are rare in sport, the coach should not underestimate the importance of isometric muscle actions, because these are key in the maintenance

of posture. The importance of maintaining posture for the sport performer is explored in detail in chapter 5. The importance of isometric muscle actions in sporting move- ments can be illustrated through the analysis of the pelvis in running (see figure 4.16). As shown in chapter 8, the pelvis must not have an anterior or posterior tilt in the running action or any form of lateral deviation (i.e., a side-to-side sway). Either of these errors can result from ground reaction forces caused as the legs drive forward, lack of hip mobility as the thigh is lifted towards a horizontal posi- tion to accommodate leg swing or rotations in the trunk because of arm-driving action. Rotations caused by ground reaction should be balanced by the opposing arm action, and hips should be mobile enough to enable the leg lift. But if the pelvis is to act as a stable platform through which forces can be trans- ferred, the trunk must be maintained in a stable upright position. The arrangement of the ribs, spine and pelvis must be maintained by the isometric actions of the trunk (external oblique, rectus abdominis), pelvic muscles

(transversus abdominis, multifidus) and hip (predominantly gluteus medius).

Athletic development practitioners may hear about other types of muscle actions such as isotonics (constant tension throughout the movement range) or isokinetics (constant velocity of muscle shortening). These terms are often used to describe various types of machine-based exercise protocols. If these actions can be achieved only with machines and contractions of this nature do not occur naturally in sport, why would coaches want to incorporate them into routines?

Muscle Structure and