🎾 The - Lateral - Reaction - Step - In - Tennis - Footwork¶
Giới Thiệu¶
The - Lateral - Reaction - Step - In - Tennis - Footwork — tài liệu 4 trang từ thư viện sách tennis.
Chủ đề chính: Biomechanic, Cơ sinh học, Bộ pháp
Tóm tắt nội dung (trích từ tài liệu gốc): Biomechanics Symposia 2001 / University of San Francisco THE LATERAL REACTION STEP IN TENNIS FOOTWORK Richard W. Bragg and Thomas P. Andriacchi Stanford Biomotion Lab, Stanford University, Stanford, CA, USA Anecdotal evidence suggests that the key to quickness in tennis is the "reaction step", or the first step in response to the ball. There are three possible lateral reaction steps towards a wide and difficult shot: 1) step first with the outside foot towards the ball (jab step); 2) pivot on the outside foot while turning the hips towards the ball (pivot step); 3) bring the outside foot towar
Lưu ý: Nội dung dưới đây được trích xuất tự động từ PDF gốc tiếng Anh, giữ nguyên ngôn ngữ để bảo toàn độ chính xác kỹ thuật.
Nội Dung Gốc (Tiếng Anh)¶
Biomechanics Symposia 2001 / University of San Francisco
THE LATERAL REACTION STEP IN TENNIS FOOTWORK
Richard W. Bragg and Thomas P. Andriacchi
Stanford Biomotion Lab, Stanford University, Stanford, CA, USA
Anecdotal evidence suggests that the key to quickness in tennis is the "reaction step", or the
first step in response to the ball. There are three possible lateral reaction steps towards a
wide and difficult shot: 1) step first with the outside foot towards the ball (jab step); 2) pivot on
the outside foot while turning the hips towards the ball (pivot step); 3) bring the outside foot
towards the body away from the ball, allowing gravity to assist the motion (drop step or
gravity step). A controlled study of 10 participants showed that when a player used the gravity
step to reach a lateral shot requiring a quick response, the player was 52% more likely to
reach the ball (p < 0.05) and 100% more likely to control the return shot (p < 0.05) when
compared to the jab step. A simple experiment utilizing a 9-marker motion analysis system
provided further evidence to support these findings.
KEY WORDS: tennis, footwork, quickness, reaction, biomotion, motion analysis
INTRODUCTION: Quickness in tennis often seems to be an elusive attribute obscured by terms
such as agility and grace. This elusiveness is apparent in the scientific literature in that there
has not been a quantitative scientific study to date on the biomechanics of tennis footwork and
its relationship to quickness. It has been anecdotally suggested by McLennan (1993, 1995) that
the key to quickness in tennis is the first step in response to the ball, or the "reaction step"
(Figure 1).
Stance Knee Moves Out Foot Follows Out Step Complete
A) Jab Step
Stance Start Inward Heel Turn Full Pivot of Foot Moving Right
B) Pivot Step
Foot Moves In Foot Moves Completely Moving Right
Under & CM Moves Right
Stance
C) Gravity Step
Figure 1 - The three lateral reaction steps.
There are three possible lateral reaction steps (Figure 1) that can be used to move towards a
wide and difficult shot, each distinguished by the initial movement of the foot on the side of the
34
Biomechanics Symposia 2001 / University of San Francisco
body that is closest to the ball (McLennan, 1993). The first is the jab step, and is the most
intuitive of the three. It consists of stepping first with the lead foot in the direction of the ball
(Figure 1A). The second is the pivot step, and involves pivoting on the lead foot while turning
the hips towards the ball and then making the first step towards the ball with the opposite leg
(Figure 1B). The third is the gravity step, and involves bringing the lead foot in towards the body
away from the ball as the hips turn (Figure 1C). This moves the center of mass (CM) of the body
outside the base of support, allowing gravity to assist the motion towards the ball (Figure 1C).
While the jab step is most often taught as "proper" footwork, it has been suggested that the
gravity step represents the quickest and easiest way to get to a wide and difficult shot
(McLennan, 1993, 1995). Boris Becker, Stefan Edberg, and Monica Seles, who are particularly
known for their quickness, have been observed to use the gravity step.
Despite the lack of research on tennis footwork in the scientific literature, studies on the
mechanics of starting motions in other sports may still serve to provide relevant insight. For
example, there have been many studies on the biomechanics, economy, and efficiency of the
explosive start in sprinting. Two of the keys to a good sprint start are reaction time and body
positioning. While reaction time can be improved by performance feedback (Martinez & Ona,
1999), body positioning can be improved with a greater forward lean, which moves the center of
gravity of the body to a position in front of the feet (Mero et al., 1992; Harland & Steele, 1997).
This imbalance may be similar to that exhibited at the beginning of the tennis gravity step
presented here.
The purpose of this study was to determine the relative effectiveness of each type of reaction
step using two separate approaches. The first approach was to examine the footwork of ten
tennis players on the court to look for a correlation between their footwork and their ability to
both reach and return the ball on wide and difficult shots. The second approach was to compare
the kinematics of the reaction steps using a quantitative motion analysis system.
METHODS: Performance Study: Ten participants were recruited from a local tennis club, all of
whom had been previously coached to use the gravity step (unrelated to the study). They were
not informed of the purpose of the study, and they were instructed to use whatever means
necessary to return each shot. Each participant was asked to return a total of 30 shots,
consisting of 10 shots from each of three court positions: 1) standing centered with both heels
on the baseline, 2) standing centered at the "T" formed by the two service boxes, and 3) running
in from the baseline to the "T". The tennis balls were shot from a Playmate tennis ball machine
located on the opposite side of the net from the participant at a centered position approximately
3 m inside the baseline. The ball speed was calibrated such that a ball shot from the machine
would bounce twice before reaching the back fence behind the participant. The ball direction
was calibrated such that a shot to either side of the participant would pass directly over the
corresponding back corner of the singles court (if not struck by the participant). The specific
corner to which each successive ball was shot was random. A video camera was positioned
directly behind the participant to record his/her footwork.
The video data were then analyzed by recording four observations of each shot: 1) whether the
shot was to the forehand or the backhand of the participant, 2) whether the participant was able
to reach the ball with their racket, 3) whether the participant was able to return the ball in a
"controlled" manner, and 4) what type of reaction step was used to reach the ball. A participant
was said to be able to return the ball in a "controlled" manner when their return either went into
or over the net. Shots that went forward but failed to reach the net, or shots that hit the
participant's racket but went to the side or behind the participant were not considered to be
"controlled" returns. This classification of a return being "controlled" is admittedly ad-hoc, but
was necessary to objectively distinguish between shots in which the participant could barely
reach the ball from shots in which the participant reached the ball with time to spare. The type
of reaction step was classified by examining the position on the court of the lead foot at the
instant the ball was shot from the machine and comparing it to the position of the lead foot on
the court when the opposite foot left the ground. If the second position of the lead foot was
closer to the ball than the first, the step was recorded as a jab step. If the second position was
35
Biomechanics Symposia 2001 / University of San Francisco
farther from the ball, it was recorded as a gravity step. If the second position was within
approximately 3 cm of the first position, it was considered a pivot step (although some times this
was not exactly a pivot, but instead a situation in which the participant picked up the lead foot
and placed it back down in the same position). The results were then analyzed to determine the
likelihood of reaching or returning a given shot by using a given reaction step and whether the
shot was a backhand or forehand. Statistical significance between groups was determined using
the test of binomial proportions.
Motion Analysis Study: To provide a biomechanical insight into the different reaction steps
independent of the tennis environment, a single participant familiar with all three types of
footwork was analyzed in a motion analysis laboratory and used as his own control. The
participant was marked with nine retro-reflective markers that were tracked in three-dimensional
space using a Qualisys motion analysis system. The nine markers were placed over relevant
anatomical landmarks that completely specified the 3-D position of the right foot and
approximated the 3-D positions of the right knee and hip joints, the torso, and the left foot. The
center of mass of the entire body was approximated from the 3-D position of the torso. Each of
the reaction steps were then performed moving to the right, the motion data were recorded, and
the procedure was repeated a total of six times for each type of reaction step. Motion to the left
was assumed to be biomechanically similar, and was not recorded.
RESULTS AND DISCUSSION: Performance Study: Of the 100 shots that were attempted by
the participants starting at the baseline, all 100 were reached and 96 were controlled. With
these high success rates, the reaction step clearly made little difference. However, those returns
attempted from the other two court positions (starting at the "T" of the service box and charging
from the baseline) were more difficult, requiring a significant effort and a quick initial response to
reach the shot. Table 1 shows the combined results of these shots.
Table 1 Returns Requiring Quick Initial Movement
Attempted Reached Controlled
Gravity Step 85 44 0.518* 34 0.400**
Pivot Step 65 32 0.492 23 0.354
Jab Step 50 17 0.340* 10 0.200**
TOTALS 200 93 0.465 67 0.335
Significant differences (p < 0.05) between groups are marked with * and **.
These findings show that when a player used the gravity 150
Lateral Position of CM (cm)
step, (s)he was 52% more likely to reach the ball, and
100% more likely to control the return, when compared to 125 GravityStep
the jab step. Interestingly, whether a shot was a forehand 100 Jab Step
or a backhand had no significant effect on the results (data
not shown). 75 Pivot
Motion Analysis Study: Figure 2 shows the position of the 50
center of mass of the participant with respect to its starting
position as a function of time for a typical trial of each type of 25
reaction step. These results demonstrate that during both the
pivot step and jab step, motion towards the ball starts rather 0
slowly, but increases more rapidly around 0.3 seconds, with
the jab step increasing more rapidly than the pivot step. The 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
jab step resulted in about 20 cm greater distance moved Time (seconds)
towards the ball over the time period shown. Using the Figure 2. Lateral motion of
gravity step technique, it seems that although it starts with body center of mass (CM).
the lead foot moving away from the ball, significant motion of
the overall body center of mass towards the ball occurs almost immediately. This progression
towards the ball then remains relatively constant for the duration of the first 0.7 seconds, resulting in
about 10 cm greater distance covered than for the jab step, and almost 30 cm more than for the
36
Biomechanics Symposia 2001 / University of San Francisco
pivot step over the time period shown.
These results can be further explained by taking into consideration the positions of the feet
relative to the body center of mass during the different reaction steps (Figure 3). In the jab step
(Figure 3A), the center of mass of the body remains between the feet, and thus above the base
of support, for about 0.5 seconds. Since the sharp increase in overall lateral motion occurs at
around 0.3 seconds, this motion must be driven almost entirely by forces in the legs. In the pivot
step (Figure 3B), the center of mass passes outside Right Heel (Ball Side)
150 Left Heel (Opposite Side)
the base of support at about 0.3 seconds. In this Lat Center of Mass
era 125
case, the decreased performance may be explained l
Po 100
by the fact that the lead leg must generate forces in
the direction of motion over a smaller range of siti 75
on
(c 50
motion than in the jab step. Finally, in the gravity m)
25
step (Figure 3C), the center of mass moves outside 0
the base of support much sooner (~0.15 seconds). It -25 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
seems likely that, in the latter case, the participant -50 Time (seconds)
quickly moves into a position of "dynamic
imbalance", in which motion towards the ball is aided A) Jab
by gravity as well as the muscle forces in the 150 Right Heel (Ball Side)
Left Heel (Opposite Side)
opposite leg. In addition to the immediate positive eLraa1t25 Center of Mass
effects that the gravity step footwork may have in l 100
Po
getting to the ball most quickly, the possibility that it siti75
on
may also require less muscle action than the other (c 50
methods may also be significant in terms of m) 25
endurance. If a player were to consistently use the 0
gravity step footwork over the course of a long
match, it is possible that the decrease in muscle use -25 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
-50
Time (seconds)
would delay the onset of fatigue and the resulting B) Pivot
decrease in performance.
150 Right Heel (Ball Side)
CONCLUSION: The most important implication of Left Heel (Opposite Side)
these findings is that, contrary to conventional Lat Center of Mass
wisdom and accepted practice, it seems that the
gravity step should be taught by coaches as the era 125
preferred reaction step to reach a wide and difficult
tennis shot. Ultimately, these results could also be l 100
applied to other sports where quick, lateral Po
movement is critical, such as baseball (base stealing
siti 75
on
(c 50
m)
25
0
-250.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
-50
Time (seconds)
and fielding) and soccer (goaltending). C) Gravity
REFERENCES: Figure 3. Relative position of feet
respect to body center of mass for
Harland, M.J., & Steele, J.R. (1997). Biomechanics of the three types of reaction
of the sprint start. Sports Med, 23,11-20.
Martinez, M., and Ona, A. (1999). Influence of increased feedback on temporal parameters of
the athletic sprint start. J Human Movement Studies, 36, 23-36.
McLennan, J. (1993). The first step: "covering the court with gravity motion". USTA Sport
Science for Tennis, Fall 1993, 4-5.
McLennan, J. (1995). Footwork that works: 'gravity footwork' will allow you to reach shots
quicker and easier. USTA Sport Science Quarterly, Fall 1995, 12.
Mero, A., Komi, P.V., & Gregor, R.J. (1992). Biomechanics of sprint running. A review. Sports
Med, 13, 376-392.
ACKNOWLEDGEMENTS: We would like to acknowledge the assistance of Jim McLennan and
the Fremont Hills Country Club, Gene Alexander, David Camarillo, Ajit Chaudhari, Chris Dyrby,
Michael Holzbaur, John MacMahon and Tammy Tam.
37