Serve BiomechanicsThe tennis serve is the stroke that puts the ball in play and is often referredto as the most important stroke in the game of tennis. It has become a principleweapon of attack and is used to place the opponent on the defensive by forcing areturn from the weak side or by moving the receiver out of position. A goodstrong serve can sometimes be the basis of winning a game of tennis. I haveincluded eight picture sequences to illustrate the tennis serve. Represented inpicture A is the stance of the serve. In this part of the serve, the personneeds to take a position sideways to the net, about three or four feet to theright center mark behind the baseline. The left foot is two to three inchesbehind the line, the toes pointing toward the net post. The back foot isparallel to the baseline and spread conformably from the front. Pictures B and Crepresent the preparation phase. In these pictures, the execution of the balltoss is performed. The ball toss is the key to a well-executed serve: a goodrelease consistently places the ball in the proper hitting position. A poorrelease can throw off timing and ultimately cause a bad serve. In pictures D-Fthe action phase is represented. In picture D of the action phase, the elbowreaches a position slightly higher than the shoulder, then the elbow bends andthe racket head drops down behind the back into what is called the “backscratching position.” In picture E, the ball should be at its maximumheight of the toss before striking it. In picture F, the last of the actionphase, the movement of striking the ball is explosive in an upward and forwardmotion until contact Pictures G and H represent the follow through. In thefollow through the action is performed up and out, not down, in the direction ofthe intended target area. The follow through is a natural continuation of thestroke. A good follow through will help prepare for the next step in approachingthe net for a return. Kinematics is defined as the study of motion. It iscompiled of different bodily planes and different joint motions. In thebeginning of the serve, during the stance (picture A), the feet are outwardlyrotated. The hips and the trunk are extended. The left shoulder is slightlyflexed along with the right shoulder and the shoulder girdles are slightlyabducted. Both of the wrists are pronated with the elbows slightly flexed.
During the preparation (pictures B and C) the feet are still in an outwardlyrotated position. The hips slightly abduct with the trunk still in fullextension. The shoulders are abducted, with slight elevation of the shouldergirdle. Both elbows are extended, but the right wrist stays in a pronatedposition and the left wrist is supinated. During the action (pictures D-F) theright foot inwardly rotates along with it performing planter flexion but theleft foot stays in an outwardly rotated position. The hips are adducted but thenthey shift to abduction. The trunk starts in hyperextension then get fullyextended and slightly rotate to the left. Both knees flex but the left kneeextends while the right knee stays flexed. The left shoulder goes from flexionto extension while the right shoulder performs high diagonal adduction. The leftelbow goes from extension to flexion and the right elbow goes from flexion toextension. The left hand goes from supination to pronation while the right handstays in a pronated position. Finally, during the follow through (pictures G andH) the left foot inwardly rotates along with some planter flexion. The rightfoot inwardly rotates and goes back to a naturally flat position. Both hips areflexed along with the flexion of the trunk and it’s rotation. The left shoulderremains in an extended position but the right shoulder follows through with thehigh diagonal adduction, while both shoulder girdles perform abduction. Theright elbow slightly flexes but the left elbow extends. The knees go fromflexion to a greater degree of flexion. The kinematics of the tennis serve is acomplicated thing, it consist of many laws and principles. One law is the law ofinertia and the principles deal with motion, force and projectiles. These lawsand principles can be applied to a skill, for example the tennis serve. Thefirst principle deals with stability. This principle consist of smaller groupsdealing with mass, friction, height of the center of gravity, position of thecenter of gravity, and base of support. The mass of the person in pictures Athrough H is of a certain weight. This mass or weight throughout the serve isconsidered to be constant. This observation is made because during the tennisserve the person performing the serve cannot gain or loss mass during the serve.
Friction can be a major factor in a sport or game. The type of footwearavailable can help an althea to the point of better counter force when jumpingor better traction for different surfaces. A tennis shoe does not need to havegreat counter force because there is not a lot of jumping but traction isimportant because of different surfaces like clay, grass, and concrete. Eachsurface performs differently for each athlete so there should be a traction onthe shoe to benefit sprinting forward but also being able to move side to side.
Height of center of gravity is one important factor in good equilibrium. In thestance position of the tennis serve (picture A), the height of gravity is in themiddle around the navel. This remains constant through the prep phase (pictureC), but when the action phase starts (pictures D – F) the height of gravitymoves up with the extension of the racket arm and trunk. Then in the followthrough (picture G, H) height of gravity starts to go down because of thedescending motion of the racket arm and trunk. Position of center of gravity isanother important part in good equilibrium. In the stance position of the tennisserve (picture A) the position of gravity is about two inches above the belt.
Then in the prep phase (picture C) it moves upward about four inches above thenavel. Next, in the action phase (pictures D – F) center of gravity moves threeinches to the right of the navel and about four inches above the navel. In thefollow through (picture G, H) the center of gravity is about four inches outsidethe body parallel to the belt line. Base of support is the area formed by theouter most region of contact between the body and a support surface. During theserve (pictures A – D) the base of support is pretty much constant. It is justenough not to fall but good enough to push off of the surface. In picture E thebase narrows a little because of the force of pushing off with the left foot.
Pictures F and G, the base is wider then narrows and drops down and forward,this is because of momentum pulling the body forward into the follow through. Atthe end the base of support is narrowed to bring the feet under the body forbetter equilibrium because the center of gravity is outside the body, this is tokeep the body from falling. The first law of kinematics is the law of interia.
This law has principles that deal with motion. Translatory motion is defined asmotion moving in a straight line. Rotary motion is defined as motion moving in acircle. Combining translatory and rotary motion in the tennis serve is shown bythe rotary motion of the racket arm and the motion of the trunk in the followthrough. Translatory motion is shown by the stepping forward of the trunk, legs,and partially of the racket arm. Continuity of motion is shown during the actionphase (pictures D – F) with the tossing of the tennis ball while bringing theracket into a striking position and also extended the trunk and legs. This isall performed at the same time with no pause. If there was pause in any of thesemotions there would not be enough momentum to create a very effective serve.
Momentum is the product of a body’s mass and linear velocity. Momentum can bechanged by changing direction. To produce an effect of momentum during theserve, since mass is constant, the velocity of the body must be increased. Thisis done by the extension of the racket arm along with the trunk and moving thecenter of gravity forward to produce a good momentum in striking the tennisball, this intern with the racket striking the ball turns it into a force, thisforce is equal to the momentum of the body. During the action phase of the serve(pictures D – F) transfer of momentum is achieved by extending the legs, trunk,and racket arm. This is done because mass remains constant, so to increase speedthe body must become longer to help contribute to the total body momentum. Inthe tennis serve acceleration is proportional to force because mass in the bodyof the server is constant. So if the server has a great amount of accelerationthen there will be a great amount of force when striking the tennis ball withthe racket. Maximum acceleration is achieved by moving the whole body in aforward motion with continuity and timing. There are really no extraneousmovements because most movements like the extension of the legs, trunk, racketarm, and the moving forward of the body are all extended to create greater bodymomentum. Timing is very important and should be practiced because it isprobably the hardest thing to get down to create maximum acceleration andeffective motion. In the serve the body’s radius is lengthened so according tothe principle the rotation is shortened during the follow through phase, this isbecause with a lengthened radius the body has more area to cover. This issacrificed because greater momentum to where the ball must be hit is moreimportant then rotational speed. This is illustrated in the action phase(pictures D – F) with the extension of the legs, trunk and racket arm. Thisshows the lengthening of the body’s radius. During the action phase of the servenever unsupported as seen in the pictures D – F. Both feet seem to be on theground in constant support of the body, so this principle does not apply to thisparticular serve. Although some people might actually, force a split second,become airborne during the serve, then this principle would apply. There arethree major surface variations in tennis all with different counter force. Clayis the first surface, although it is somewhat soft is does not contain goodcounter force because it has a bad coefficient of restitution, which means itdoes not bounce back to original shape very well. Clay is also somewhat slipperyso players must slide to position to hit the ball. Grass is another form ofsurface variation, it is also somewhat soft and does not have a very goodcoefficient of restitution but it is better then clay. Grass also has a degreeslipperiness and also requires the sliding into position. Finally, concrete isthe last surface variation, it has no counter force because there is no give andno coefficient of restitution but concrete is not as slippery as the other twosurfaces. During the action phase of the tennis serve (pictures D – F), thedirection of counter force is projected down and back which in turn propels theserver up and forward. This is done in a perpendicular manner to the surface sothere will be no slippage. When the racket strikes the ball there are counterforces. When the ball is in contact with the racket, the racket has momentum butthere is one possibility in the make of the racket to help contribute to counterforce. If a player has a stiff racket with loss strings the give of the racketis not as great as the strings but when both their coefficients of restitutionare activated, they perform in created more force for the striking of the ball.
The ball itself also has a coefficient of restitution to help propel itself offthe racket. The player also must have a firm grip at impact to reduce ofeliminate give at the grip. Temporarily stored counter force can be found inmany parts of the tennis serve. For one, the ball itself has it own ability tospring back to original shape, this all depends on the make of the ball. Next isthe racket and the strings, each have a coefficient of restitution which resultsin temporarily stored counter force. A stiff racket like tightly wound stringshave high restitution unlike a flexible racket and loss strings. Finally theshoes an athlete wears can have stored counter force depending on make andmaterial. In the tennis serve as observed in pictures D – F, during the stickingmotion both feet are in contact with the ground to provide maximum to the ball,even though the bodies extended and looks like it could go airborne. Total forceis equal to the sum of the forces of each body segment contributing to the act,if the forces are applied in a single direction and in the proper sequence withcorrect timing. If the variables sequence, timing, and direction are not allapplied correctly together the total force will be minimal. During the actionphase, pictures D-F, in the tennis serve total force is achieved by theextension of the arms, legs and trunk along with the timing of the movement ofthe racket arm in striking the ball, and also the leaning forward of the body.
Force applications should be constant and as even as possible. The forceapplications should be this way so that maximum force can be used to overcomethe resistance of gravity and air or water, and minimum force can be used toovercome inertia. The relationship between constant force over a greaterdistance and resulting velocity is a positive one. When the distance over whicha force is applied increases, so does the velocity. During the prep phase,pictures A-C, the extension of the racket arm is done to create the distance inwhich greater velocity will occur and this resulting in a greater striking forceof the tennis ball. Resulting movement depends on the direction and magnitude ofthe acting forces. If two of these forces act in the same general direction, thedirection of the resulting force is somewhere between the two, and the magnitudeof the resulting force is more than either, but not as much as the total of thetwo contributing forces. During the action phase, pictures D-F, the force of thearm, body and racket hitting the ball forward along with gravity pulling itdownward, the ball’s flight, although going forward is also going down. Thispath of the ball is the between result of the two forces acting upon it. Therelationship of muscle length and resulting force is that, the longer the muscleis, the greater the increase in force of that muscle. During the prep phase,pictures A-C, the muscles in the body and the racket arm are tensed or put onstretch to increase the length of the muscle to produce greater force. Therelationship of linear speed to lever length is positive. This means that when alever gets longer the linear speed gets faster, thus increasing the strikingforce. During the action phase, pictures D-F, the extension of the levers of thebody and racket arm increases the linear speed, resulting in greater strikingforce of the tennis ball. Emphasis on proper follow through eliminates thetendency to decelerate a throwing or striking action prior to its completion.
Some other benefits of a proper follow through would be to maintain balance andto protect the joint by gradually slowing the body parts. In any case, oncecontact is broken with the object, follow through actions has no influence onthe flight of the object. There are some external forces that can be used tobenefit performance, like water resistance, friction, gravity, and airresistance. Having the correct shoe for the correct playing surface can be veryhelpful because it could help reduce friction resulting in greater speed. Usinggravity and air resistance when striking the tennis ball could be used in theplacing of the ball to make it harder for the opponent to hit it. Therelationship of air/water resistance and velocity is that if the velocity atwhich a body travels is increased by two, the air/water resistance against itwill increase by four. During the action phase, pictures D-F, when the tennisball travels at a certain speed the air resistance is squared. This intern canaffect the flight of the ball. Centrifugal force is only experienced during arotational (angular) or curvilinear motion. It results from the tendency for anobject to continue in a straight line instead of a curved path. It iscounteracting by forces (usually muscular) which, if effective, equal or exceedthe centrifugal force and tend to maintain the object to continue in its curvedpath. This counteracting force is centripetal force. In the case of a freelymoving body, as velocity increases, centrifugal force increases. Additionalweight also increases centrifugal force. The smaller the radius of a curvedpath, the greater the centrifugal force with the same velocity. During theaction phase, pictures D-F, the half circle motion of the racket arm, when inthe process of hitting the ball, wants to go in a straight line but the muscleskeep it from doing that. Instead, it brings the racket around to strike the ballat the peak height of the ball toss. A force from a blow can be diminished bydistributing the force over either a greater time (and distance) or area, orboth. During the action phase, pictures D-F, the size of the racket headdetermines the distributing of the force of the tennis ball. The bigger theracket head the more the force of the ball is diminished. In catching an object,the object’s momentum is dissipated by eccentric muscular contractions allowingthe joints to move through controlled flexion; and while momentum is beingreduced, body parts flex to grasp the object securely. This principle is notfound in the tennis serve because there is no object being caught. If theapplication is directly through the projectile’s center of gravity, only linearmotion results from the force. As the projecting force is moved farther from thecenter of gravity, rotary motion of the object increases at the expense oflinear motion. If the force is below the object’s center of gravity, backspinresults. If the force is above the object’s center of gravity, topspin results.
The striking of the ball above or below it’s center of gravity which can internresult in topspin or backspin which can change the direction of the ball when ithits the court and also in the air. The force of gravity on a object starts todiminish it’s vertical velocity as soon as contact is broken. The factors thatdetermine how soon gravity will cause the object to descend are weight, amountof force driving it upward, and the effect of air resistance on the object. Therelationship of speed and air resistance is that as speed increases airresistance plays a more significant role. Objects that are less dense andstreamlined are influenced less by air resistance, and the less surface area anobject presents, the less will be the effect of air resistance on the object.
The tennis ball being somewhat small, round, hollow, and fuzzy, makes it lessdense, more streamlined, and it presents less surface area. This all internmakes the tennis ball a pretty streamlined. The optimal angle for maximumdistance when the beginning and end points are at the same level is 45 degrees.
The effects of a less than optimal angle results in little distance. The effectof a greater then optimal angle can also result in little distance. Whenstarting points are above or below ending points, reduce angle to get maximumdistance. Since the greatest angle for projection is 45 degrees, the tennisserve is struck downward as close to 45 degrees as possible. The relationship ofthe angle of incidence to the angle of reflection is equal. This means that theangle at which the object approaches a surface is equal to the angle at which itleaves that surface. The factors that could change this would be irregularshapes of the two colliding surfaces, the force resulting from elasticity of theobject, and the spin of the object both during and after contact. If all else isconstant , the angle the tennis ball hits the ground is the angle it willproject of the ground. Also in tennis spin will effect the angle. A highlyelastic object will quickly spring back to its original shape after beingcompressed. The compression of the tennis ball is somewhat moderate but there isalso what is called a high compression ball on the market. The elasticity isreally high because the ball is made out of rubber, which has a goodrestitution. The greatest rebound results from a moderately compressed ball withhigh elasticity. For tennis, a stiff racket and loose strings will produce thegreatest elasticity of the ball because the racket if loose does not have greatrestitution as well as the strings do. Also the ball is highly compressed andhas a lot of elasticity to it, to help propel it fast. An object propelledwithout spin tends to waver because of air resistance against the object’sirregular surface. A small amount of spin on an object produces a stabilizingeffect which tends to hold it on its line of flight. Increased spin will tend tocause the object to curve in the same direction as the spin because of unequalair pressure cause by the spinning. During the serve or contact with the ballspin is added to the ball to throw the opponent off guard. The effect of thespin is to make the opponent change direction very quickly. To cause an objectto spin in the desired direction, the striking implement should be drawn acrossthe object in the direction of the desired spin. Topspin is caused by animplement striking forward-upward. Backspin is produced when the strike is madeforward-downward. In the serve topspin is added to draw the ball out or make itjump up. Backspin is added to make the ball fall short, so to make the opponentrun to the net. Topspin causes a lower angle of rebound, a longer bounce, andmore roll. Backspin causes a higher angle of rebound, a shorter bounce, and lessroll. In the serve these spins are used to throw the opponent off guard and tomake them make a quick decision and mess up. The effects on a vertical surfaceare different than on a horizontal surface. Topspin causes a higher rebound ,backspin causes a lower rebound, right spin causes a rebound to the left, andleft spin causes a rebound to the right. Tennis serve does not happen on avertical surface at all. The serve is the most important part of tennis, itstarts the game and sets the tone for the match. Many of the professionals todayhave mastered most of theses principles, and these principles can be seen byjust watching a match by a valid tennis player.