Technique and ground reaction forces back handspring TIMOTHY J.

KOH,*† MA,

MARK D.

in the

GRABINER,* PhD, AND

GARRON G. WEIKER,‡ MD From the

*

Department of Biomedical Engineering and Applied Therapeutics and &Dag er; Department of Orthopaedic Surgery, The Cleveland Clinic Foundation, Cleveland, Ohio skills should consider the influence that technique may have joint loading patterns. McNitt-Gray,9 for example, conducted a study comparing two-legged landing strategies following drop jumps for recreational athletes and gymnasts. The results suggested that, as impact velocity was increased, the recreational athletes changed landing strategy (i.e., increased hip and knee joint flexion) so as to distribute impact force over a relatively longer time interval. This resulted in a relatively small peak impact force. Gymnasts trained to land without excessive joint motion demonstrated joint flexion patterns that were less sensitive to increases in impact speeds, and were thus subjected to relatively large peak impact forces at high impact velocities. Technique, modification of technique, and joint-loading patterns may be important because of their possible role in the etiology of injury. It has been hypothesized that idiopathic osteoarthrosis is &dquo;a failure of the protective mechanisms that normally function to dampen peak dynamic loads.&dquo;13 The result of this failure is damage to articular cartilage and subchondral bone. In fact, a model for creating osteoarthrosis in animals involves repetitive impact loading while inhibiting the natural mechanism used to attenuate these impact forces (controlled flexion of the joints in-

ABSTRACT

on

ground reaction forces at the hand that produced compression forces and varus/valgus moments at the elbow joint during the double-arm support phase of the back handspring were studied. The relationship of technique, namely elbow joint flexion, to these forces were also studied. Compression forces and forces producing valgus moments have been implicated in overuse injuries to the elbow joint. Video and force plate analysis of six young female gymnasts showed that 1) the elbow joint flexed during the double-arm support phase, and 2) the reaction forces at the hand produced large compression forces (an average of 2.37 times body weight) and sizable valgus moments at the elbow (an average of 0.03 times body weight times body height). The

The combination of these forces may contribute to the occurrence of lateral compression injuries of the elbow joint (e.g., osteochondritis dissecans of the capitellum). Correlations of measures of elbow angle and measures of reaction force showed that large elbow flexions may protect the elbow from large valgus moments.

volved).15 Repetitive, large-amplitude, short-duration impact forces have been implicated in osteoarthrosis injury to joints. 14 The back handspring is likely associated with such impact forces. The competitive requirement of maintaining straight arms during the double-arm support phase inhibits the natural

The back handspring in gymnastics is a transition maneuver that prepares the gymnast for many different complex tumbling maneuvers. Gymnasts thus perform the back handspring numerous times in training and in competition. The competitive performance requirements for the back handspring do not allow for elbow flexion during the doublearm

support phase. During competition,

a

gymnast’s

mechanism used to attenuate these forces (elbow flexion), and thus does not allow for distribution of impact forces over a period of time. Overuse elbow joint injuries, including osteochondritis dissecans of the capitellum, have been identified in young female gymnasts by one of the authors (GGW), and in the literature.8,11,12,16 These injuries are not rare, and they can force the gymnast to discontinue participation.’ These injuries are similar to those found in baseball pitchers,’ which

score

is penalized one-tenth of a point if elbow flexion is detected by the judges. As a result, athletes are coached to maintain straight arms during the support phase. Those who coach developmental and advanced gymnastics t Address reprints and correspondence to: Timothy J. Koh, MA, Department of Biomedical Engineering and Applied Therapeutics (Wb-3), The Cleveland Clinic, 1 Clinic Center, Cleveland, OH 44196. 61

62

assumed to be related to forces that produce a valgus moment at the elbow joint during the pitching motion and are

thus compress the lateral compartment of the joint (the radiohumeral joint). The injuries in the gymnasts were assumed by the present authors to be related, in part, to the ground-reaction forces acting at the hand that produce compression and valgus moments at the elbow joint during maneuvers in which the upper extremity is weightbearing. These injuries in gymnasts previously have been considered to be associated with compression forces and valgus moments at the elbow.8,16 However, a review of the related literature revealed no studies describing the forces transmitted to the upper extremity during the execution of these skills, or the relationships of these forces with technique. In the back handspring, elbow flexion during the double-arm support phase could influence the forces acting at the elbow joint by influencing 1) the magnitude and direction of the ground reaction force itself, and 2) the position of the elbow joint relative to the ground-reaction force (e.g., say the forearm is vertical and a horizontal force acting at the hand produces a valgus moment at the elbow; the same force at the hand will produce a flexion moment at the elbow if the forearm is rotated 90° about its long axis). The rationale underlying this study was that if a strong link between technique and possibly harmful

joint-loading patterns could be defined, then preventive or rehabilitative action may be prescribed. The purposes of this study were 1) to determine the magnitude of the ground reaction forces acting at the hand that produce compression forces and varus/valgus moments at the elbow during the double-arm support phase of the back handspring, and 2) to determine the relationship of the elbow angle, and the changes in the elbow angle during the support phase, with these forces. MATERIALS AND METHODS

Subjects Six female gymnasts, aged 11 to 13, who competed for a local club team volunteered for participation in this study. These were Level 8 gymnasts in the new United States Gymnastics Federation ranking system, which ranks gymnasts on a scale of 1 to 10, with 10 being the highest. None had any history of elbow joint injury. Informed consent was obtained from a parent of each subject. Data collection

landing mat and immediately performed a two-legged &dquo;rebound&dquo; vertical jump. The purpose of the rebound jump was to simulate the initiation of a skill immediately following the back handspring. The back handsprings studied were thus made similar to those most often used in training and competition. The left arm of each subject was marked with reflective bands around the wrist, elbow, and upper arm near the deltoid tuberosity. The reflective bands, when illuminated with spotlights, provided a sharp contrast to the background (i.e., the rest of the gymnast’s body, and the floor and walls), which was necessary for data reduction. Two high-speed shuttered video cameras (model V14B, Nac, Inc, Tokyo, Japan) were placed approximately 3 meters from the force plate, and 3 meters apart to record, at 200 Hz, the three markers during the double-arm support phase of each back handspring. The field of view of each camera was approximately 65 cm high by 75 cm wide. Ground reaction forces at the left hand during each double-arm support phase were sampled at 1000 Hz with a laboratory microcomputer and synchronized with the video data using hardware from the Motion Analysis Corporation (Santa Rosa, CA). A pilot study showed no significant differences at the 0.05 level between the ground-reaction forces at the left and right hands during the double-arm support phase of a back handspring with a short (3 meter) approach. Thus, the forces at the left hand were deemed sufficient to describe the forces during the double-arm support phase of the back handsprings in the present study. Data reduction For each trial, a Motion Analysis system was used to determine the edges (or outlines) of the markers around the wrist, elbow, and middle upper arm for each frame of the video record of the double-arm support phase. The centroid of each marker was then calculated from the edge data for each frame. The two-dimensional paths of the centroids from each camera were then used as input to a direct linear transformation algorithm,’ which determined the three-dimensional paths of the marker centroids. This algorithm required calibration of the volume above the force plate occupied by the markers during the double-arm support phase. Calibration was performed prior to data collection using eighteen 2-cm diameter spheres at known locations distributed over the surface of a 38 cm long, 30 cm wide, and 46 cm high rectangular volume above the force plate. The error associated with this calibration was less than 1 mm.

normal warm-up procedure, each subject performed three trials of a round-off back handspring. Each trial consisted of a 7-meter approach, followed by a roundoff, then a back handspring in which the left hand was placed on a force platform (AMTI model OR6-5-1, Advanced Mechanical Technology, Inc, Newton, MA) during the double-arm support phase. The runway and force plate were covered with a 6-mm thick rubberized mat. The subjects completed the back handspring onto a 5-cm thick

Following

a

The three-dimensional path of each marker was smoothed using a second-order Butterworth filter with a cut-off frequency of 30 Hz. The elbow angle, defined as the included angle between the arm and forearm, was determined for each frame of the double-arm support phase. A separate experiment showed that the error in determining elbow angles from video records of representative static positions of the upper limb was within the error of measuring the corresponding angles with a goniometer. Error in determining

63

elbow angles was estimated at 2° using a theoretic analysis that included the error in locating marker centroids and possible soft tissue movement. The components of the reaction force at the hand that produced 1) compression at the elbow joint and 2) varus/ valgus moments at the joint were extracted from the force plate data. The force that produced compression at the elbow joint was termed compression force, and the forces that produced varus and valgus moments were termed varus and valgus forces. The compression force was defined as the component of the reaction force that acted along the long axis of the forearm, and tended to compress the elbow joint. The varus/valgus force was defined as the component of the reaction force that acted perpendicular to the long axis of the forearm, and tended to produce a varus/valgus motion of the forearm relative to the arm. The direction of the varus/valgus force was determined from the cross product of the long axis of the arm with the long axis of the forearm. Note that varus and valgus forces act along the same line in space, and that varus forces have negative values, and valgus forces have positive values. The instant defining the beginning of the double-arm support phase was taken to be the first frame in which the vertical ground-reaction force rose above 10 N (touchdown). The instant defining the end of the support phase was taken to be the frame in which the vertical ground-reaction force returned and remained below 10 N (takeoff). For each trial, the frame representing touchdown was first identified, and the elbow angle at this instant was determined. Next, the peak varus, peak compression, and peak valgus forces, and the time elapsed from touchdown to these peak forces, were determined. The minimum elbow angle was recorded, as was the amount of flexion from touchdown to the minimum elbow angle and the time elapsed during this interval. Finally, the average compression and varus/valgus forces were determined for the intervals of touchdown to minimum elbow angle, and touchdown to takeoff. The peak and average force measurements for each subject were normalized to

body weight. Data

analysis

Means and standard deviations were computed for the time, elbow angle, and ground reaction force data for each subject, and then for the group. Correlations of measures of elbow angle with measures of force were performed using each trial for each subject. The significance of these &dquo;repeated measures&dquo; correlations was tested using a method that accounted for within subject clustering.’ The 0.05 level was used to indicate statistical significance.

RESULTS AND DISCUSSION

Descriptive data Plots of the elbow angle, and the compression and varus/ valgus forces, versus time for one trial of one subject are shown in Figure 1. The shapes of these plots are typical of

Figure 1. Typical plots of elbow angle versus elapsed support time, and compression force (CF) and varus/valgus force (VRF/VLF) versus elapsed support time. Critical instants in the support phase are labeled. all trials analyzed. Descriptive statistics for measures of the elbow angle and elapsed time at some of the critical instants depicted in Figure 1 are presented in Table 1. The data for one trial of Subject 2 were not suitable for analysis; thus, for this subject, only two trials were analyzed. For all of the subjects, the mean value for the elbow angle at touchdown indicated that the elbow joint was slightly flexed. The elbow joint angle then decreased to a minimum value, which occurred at an average of 43% of the elapsed support time. The elbow angle then increased and, for five of the six subjects, the mean value for the elbow angle at takeoff was greater than at touchdown, indicating that the arm was straighter at takeoff than at touchdown. Straight arms were not maintained during the support phase as is generally coached. The average amount of flexion during the support phase for five of the six subjects (Subject 4 being the possible exception) is considered poor technique, and would likely induce a score penalty during competition. Descriptive statistics for measures of compression and varus/valgus forces and elapsed time at some of the critical instants depicted in Figure 1 are presented in Table 2. The peak varus force generally occurred very soon after touch-

64

TABLE 1

Descriptive statistics for measures of elbow angle (degrees) and elapsed time (milliseconds)a

°

b

N 3 trials for each subject except N 1800 full extension. =

=

2 for

Subject 2.

=

TABLE 2

Descriptive statistics

Q

for

measures

of

ground reaction force (body weight) and elapsed time (milliseconds)°

N 3 trials for each subject, except N 2 for Subject 2. VLF, valgus force; CF, compression force; VRF, varus force (see text for definitions of these forces); t, time from touchdown in preceding column. =

=

down, and before the peak compression and peak valgus forces. The peak varus force for Subject 4 was an exception; it occurred much later in the support phase than any of the peak forces for any of the other subjects. Hence, data for Subject 4 were excluded when computing the grand mean for the time to peak varus force. The peak varus force was

to

peak force

much smaller than the peak compression force, and roughly equivalent to the peak valgus force. The contribution of the average peak varus force (averaged over all trials and subjects) to the varus moment at the elbow was 19 Nm. The largest average value for a given subject (Subject 6) was 31 Nm. These contributions were calculated by multiplying the

65

appropriate average peak varus force by its average moment arm about the elbow joint. The large peak in the compression force generally occurred after the peak valgus force and before the peak varus force. The magnitude of the compression force dominated that of the varus/valgus force throughout the double-arm support phase (Fig. 1, Table 2). The shape of the plot of the compression force versus elapsed time, was similar to that of the vertical force applied to the hand in falls onto an outstretched hand,’ and applied to the feet in the performance of a running forward somersault in gymnastics The peak valgus force occurred, on average, only a very short time after the peak compression force. The contribution of the average peak valgus force to the valgus moment at the elbow for the group was 18 Nm. The largest average value for a given subject (Subject 4) was 40 Nm. When normalized to body weight times body height (to account for intersubject differences), the average value for the group was 3.0% and the largest average value for a given subject (Subject 4) was 5.7%. The absolute values for the contributions of the peak valgus force to the valgus moment at the elbow are much lower than the maximum valgus moment found during the pitching motion of collegiate baseball players (100 Nm).’ However, the normalized values of the present study are close to normalized values reported for the pitching motion of eight healthy subjects (4.65% body weight times body height). 17 This suggests that, on a relative scale, the valgus moments produced by ground reaction forces in the back handspring approach the valgus moments seen in baseball pitching. It should be noted that the values taken from the literature were reported as a varus moment of the arm acting on the forearm, but the equal in magnitude and opposite in direction valgus moment of the forearm acting on the arm (from Newton’s third law of motion) is comparable to the values presented in this study. As in baseball pitching, the peak valgus force in the back handspring tends to compress the lateral compartment of the elbow and to distract the medial compartment. At the time of the peak valgus moment in baseball pitching, there is a large peak force acting along the long axis of the forearm that tends to distract both compartments of the elbow joint.’7 The peak compression force encountered during the back handspring occurred only a short time before the peak valgus force, and by itself may be associated with a larger load on the radiohumeral joint than on the ulnohumeral joint.3 Repetitive loading by this combination of compression and valgus forces in the back handspring may put the gymnast at risk for overuse elbow injuries, especially to the lateral compartment of the elbow (e.g., osteochondritis dissecans of the capitellum). The average varus/valgus force, both from touchdown to the minimum elbow angle and from touchdown to takeoff, were near zero. This is because the sign of the force fluctuated from negative to positive (varus to valgus) and vice versa during the support phase. Thus, this force did not produce an appreciable net moment for the given time

intervals. The average compression force was less than body for each time interval, emphasizing the short duration of the peak compression force and the sharp difference between the magnitude of this peak and that of the compression forces encountered during the rest of the support phase. It should be noted that the forces described above are contributions of the reaction force at the hand to elbow joint forces and moments, and not resultant elbow joint forces and moments. Additional forces contributing to elbow joint forces and moments include the inertial forces of the forearm, and forces provided by the muscles surrounding the elbow. The inertial forces are likely to be small because of the small mass and moment of inertia of the forearm. Muscles surrounding the elbow could contribute substantially to elbow joint forces,’ but the quantification of these contributions is beyond the scope of this study.

weight

Correlations Correlation of measures of elbow angle with measures of peak force yielded the coefficients presented in Table 3. There was a significant correlation of the elbow angle at touchdown with the peak varus force. Since varus forces are negative, this correlation indicates that the larger the elbow angle (or the straighter the arm) at touchdown, the less negative (or smaller) the peak varus force. This correlation must be interpreted with caution since the range for the elbow angle at touchdown was only 10°. There was also a significant correlation of the change in elbow angle from touchdown to the minimum angle with the peak valgus force, indicating that a large amount of elbow flexion during the support phase was associated with a relatively small peak valgus force. These relationships may be a result of the elbow angle at touchdown and elbow flexion during the double-arm support phase influencing 1) the magnitude and direction of the ground reaction force itself, and 2) the position of the elbow joint relative to the ground-reaction force. There was little evidence supporting the first interpretation ; further analysis showed that correlations of the elbow angle at touchdown and the change in elbow angle with the magnitude of the peak resultant ground-reaction force yielded nonsignificant coefficients of r 0.22 and r 0.45, respectively. This suggests that the measures of elbow angle had little, if any, association with the magnitude of the peak resultant ground-reaction force. However, variability within and between subjects in the horizontal and vertical velocity of the center of gravity at touchdown may have been a confounding factor. This is discussed at the end of =

this section. TABLE 3 Correlation coefficients (N

Q

b

=

17)’

See footnote at Table 2 for definitions of abbreviations.

Statistically significant.

=

66

There was some evidence supporting the second interpretation for the significant correlation of the change in elbow angle with the peak valgus force. A large elbow flexion at the time of the peak valgus force may have put the elbow in a position to minimize the contribution of the groundreaction force to the peak valgus force. To investigate this possibility, the relationship of each component of the ground reaction force expressed relative to the direction of motion of the gymnasts with the peak valgus force was examined. The vertical component of the ground-reaction force contributed more to the peak valgus force (68%), than the foreaft and medial-lateral components (32%). The correlation of the change in elbow angle with the contribution of the vertical component to the peak valgus force yielded a significant coefficient of r -0.88, suggesting that the larger the change in elbow angle, the smaller the contribution of the vertical component. Thus, a large change in elbow angle may indeed have put the elbow in a position to minimize the contribution of the ground-reaction force at the time of the peak valgus force. Hence, a large elbow flexion may have been a protective mechanism to help minimize peak valgus forces. A possible, but unmeasured, confounding factor in the present study may have been variability in the forward horizontal and downward vertical velocities of the center of gravity of the subjects at touchdown. A large variability between subjects in the horizontal and vertical velocities at touchdown would influence the subsequent ground-reaction forces independent of the measures of elbow angle. Since the arm alone was marked for analysis, whole body center of gravity measurements could not be determined from the video data. However, vertical velocities at touchdown were estimated from the vertical ground-reaction force data using the vertical impulse from touchdown to the local minimum in the vertical ground-reaction force curve and the mass of the subject.10 The correlation of the vertical velocity at touchdown estimated in this manner, with the peak compression, peak varus, and peak valgus forces, yielded coefficients of r 0.90, r 0.22, and r 0.20, respectively. If the estimate represents the true vertical velocity at touchdown, these correlations suggest that the vertical velocity at touchdown is strongly related to the peak compression force, and not related to the peak varus and peak valgus forces. This strong relationship between the vertical velocity at touchdown and the dominant component of the groundreaction force may have outweighed more subtle relationships between measures of elbow angle and ground-reaction forces. Further study should provide experimental or statistical control for horizontal and vertical velocity to more clearly delineate the relationships of changes in elbow angle with reaction forces, and, thus, the possible protective mechanism of elbow flexion. =

=

=

=

CONCLUSIONS Reaction forces at the hand that produce large compression forces and sizable valgus moments at the elbow joint are encountered during the double-arm support phase of the back handspring. The combination of these forces may contribute to the occurrence of lateral compression injuries of the elbow (e.g., osteochondritis dissecans of the capitellum). Large elbow flexions may protect the elbow from relatively large forces producing valgus moments. Further study on the possible protective mechanism of controlled joint flexion in gymnastics seems warranted.

ACKNOWLEDGEMENTS The authors thank Kevin Campbell, George Miller, Don Metzger, and Dave Forester for their help during the data collection and reduction; Ron Ganim and his gymnasts for their participation in the study; and Jon Feuerbach for his critique of the manuscript. REFERENCES 1. Abdel-Aziz YI, Karara HM: Direct linear transformation from computer coordinates into object space coordinates in close-range photogrammetry. Proceedings of the Symposium on Close-Range Photogrammetry. Falls Church VA: American Society of Photogrammetry, 1971, pp 1-8 2. Amis AA, Dowson D, Wright V: Elbow joint force predictions for some strenuous isometric actions. J Biomech 13: 765-775, 1980 3. An KN, Morrey BF: Biomechanics of the elbow, in Morrey BF (ed): The Elbow and Its Disorders. Philadelphia, WB Saunders, 1985, p 57 4. Andrews JR: Bony injuries about the elbow in the throwing athlete. Instr Course Lect 34: 323-331, 1985 5. Carlsoo S, Johansson O: Stabilization of and load on the elbow joint in some protective movements. Acta Anat 48: 224-231, 1962 6. Donner A, Cunningham DA: Regression analysis in physiological research: Some comments on the problem of repeated measurements. Med Sci Sports Exerc 16: 422-425, 1984 7. Feltner M, Dapena J: Dynamics of the shoulder and elbow joints of the throwing arm during a baseball pitch. Int J Sport Biomech 2: 235-259, 1986 8. Jackson DW, Silvino N, Reiman P: Osteochondritis in the female gymnast’s elbow. Arthroscopy 5: 129-136, 1989 9. McNitt-Gray JL: The influence of impact speed on joint kinematics and impulse characteristics of drop landings. Congress Proceedings, XII International Congress of Biomechanics, Los Angeles, University of California, Los Angeles, 1989, Abstract #159 10. Miller DI, Nissinen MA: Critical examination of ground reaction force in the running forward somersault. Int J Sport Biomech 3: 189-206, 1987 11. Nocini S, Silvij S: Clinical and radiological aspects of gymnast’s elbow. J Sports Med Phys Fitness 22: 54-59, 1982 12. Priest JD, Weise DJ: Elbow injury in women’s gymnastics. Am J Sports

Med 9: 288-295, 1981 13. Radin EL: Osteoarthrosis. What is known about

prevention.

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222: 60-65, 1987 14. Radin EL: Role of muscles in

protecting

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injury. Acta

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Scand, Suppl 711: 143-147, 1985 15. Radin EL, Rose RM: Role of subchondral bone in the initiation and progression of cartilage damage. Clin Orthop 213: 34-40, 1986 16. Simon SR, Radin EL, Paul IL, et al: The response of joints to impact

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II. In vivo behavior of subchondral bone. J Biomech 5:

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Singer KM, Roy SP: Osteochondrosis of the humeral capitellum. Am J Sports Med 12: 351-360, 1984 Werner SL, Dillman CF, Fleisig GS, et al: Elbow kinetics in pitching. Abstracts of the First World Congress of Biomechanics 1990, La Jolla, CA, University of California, San Diego, 1990, p 354

Technique and ground reaction forces in the back handspring.

The ground reaction forces at the hand that produced compression forces and varus/valgus moments at the elbow joint during the double-arm support phas...
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