KINEMATICS PRIOR TO CONTACT IN LANDINGS PRECEDED BY ROTATION

Barry A. Munkasy, Jill L. McNitt-Gray and Michele D. Welch
USC Biomechanics Research Laboratory,
Department of Exercise Sciences, University of Southern California,
Los Angeles, CA 90089-0652

Presented at the 20th Annual Meeting of the American Society of Biomechanics
Atlanta, Georgia. October 17-19, 1996

INTRODUCTION

Evaluation of gymnastics performance is largely defined by a gymnast's ability to successfully land complex forward and backward rotating skills. Unfortunately, a high incidence of injury has been associated with load experienced during the landing phase of these skills (Linder & Caine, 1990). During optional competition, gymnasts have chosen to perform backward rotating skills more often than forward rotating ones. Preference towards backward rotating skills may be attributable to the higher landing success rate for backward rotating skills than for forward rotating skills (McNitt-Gray, 1992). In addition, this preference towards performance of backward rotating skills may also be associated with the ability of the gymnast to generate momentum prior to the skill while facing backward, the ability of the gymnast to view the surface for a longer period when preparing for contact, or may be associated with the differences in load distribution within the lower extremity during contact observed between landings of forward and backward rotating skills (Nigg 1985; McNitt-Gray & Nelson, 1988; McNitt-Gray et al., 1991; Sidaway, McNitt-Gray, & Davis, 1989; McNitt-Gray, 1992; Wilkerson & Smith, 1992).

The purpose of this study was to examine mechanisms gymnasts use to prepare for landings preceded by forward and backward rotation. We hypothesized the ability of the gymnast to prepare for contact may be compromised by the differences in visual conditions and segment between landings preceded by forward rotation as compared to backward rotation. Since critical actions for attenuation of forces experienced within 20 to 40 ms after contact must occur prior to contact (Lees 1981; Melvill-Jones et al., 1971; Viitasalo et al., 1987) we've chosen to examine absolute joint angular velocities and relative segment velocities of the lower extremity during front and back salto landings. This approach has proven to be useful in providing insight regarding preparation for contact strategies used during simple drop landings (McKinley, 1992; McKinley et al., 1992) and reduced force landings (Munkasy et al., 1992).

PROCEDURES

Twelve healthy male gymnasts, who were members of the US Men's Junior National Team, volunteered to serve as subjects (mean (SD) mass 61.3 (9.4) kg., height 1.67 (0.05) m, age 15.75 (0.87) yr., experience 8.9 (1.9) yr.., practice time 19.6 (5.1) hr./week). During data collection, each subject successfully landed two front tucked saltos and two back tucked saltos using their normal competitive landing style. Both tasks were initiated from a height of 0.72 meter above the top surface of a regulation gymnastics landing mat (0.1 m thick, 100 ILD) fully supported by a Kistler force plate (0.6 x 0.9 m). A successful landing was one in which the gymnast landed without taking a step. Segment kinematics were recorded using high speed video (200 fps; NAC Motion Analysis). Segment end points were digitized using a video based data acquisition system (Peak Performance, Inc.). Each coordinate of the digitized body landmarks were digitally filtered using a fourth order Butterworth filter with a cut-off frequency derived by the method of Jackson (1979). Segment centers of mass were computed using the data of Zatsiorsky et al., (1983). Data analyzed were relative segment velocities of the thigh, shank, and foot and joint angular velocities of the hip, knee, and ankle observed 10 ms prior to contact during front and back salto landings. Relative segment velocities were calculated as follows:

vT/H = vT - vH

vS/K = vS - vH - vK/H

vF/A = vF - vH - vK/H - vA/K

where v represents vertical velocity; / means with respect to; T, S, and F represent thigh, shank, and foot centers of mass; and H, K, and A represent the hip, knee, and ankle joints.

RESULTS AND DISCUSSION

In contrast to drops (Munkasy et al., 1992) the lower extremity center of mass fell at a faster rate than the total body center of mass for both front and back saltos. Intuitively, this must occur because the legs must be brought underneath the trunk prior to contact.

For back salto landings absolute joint angular velocity data indicated that the ankle, knee, and hip were all flexing prior to contact. However, in front saltos the knee and hip were extending (see Figure 1) . In terms of relative segment velocity the foot was falling slower than the ankle and the shank was falling slower than the knee for both movements. However, in front saltos the thigh was falling faster than the hip while the antithesis was true for back saltos (see Figure 2) .

Figure 1.

Figure 2

Rotation had a differential effect on preparation for landing. While strategies for drop landings and back saltos are similar in terms of absolute joint angular velocity (McKinley, 1992; McKinley et al., 1992) and relative segment vertical velocity (Munkasy et al., 1992) it appears that fronts are different. The uniqueness of front rotation landings from the kinematic perspective presented here and the kinetic perspectives presented elsewhere (McNitt-Gray et al., 1991; McNitt-Gray, 1992; Wilkerson & Smith, 1992) may partially explain the gymnasts' avoidance of and lack of success with forward rotating skills (McNitt-Gray, 1992). While avoidance may also in part explain the noted kinetic variability (McNitt-Gray et al., 1993; Wilkerson & Smith, 1992), a difference in variability for the kinematics examined was not found.

Vision is thought to play a major role in preparation for contact (Lee et al., 1981). During activities involving rotation (especially in the forward direction) time to view the landing surface may be reduced due to the anatomical orientation of the head and eyes and the need to complete rotations with limited angular momentum and flight time (McNitt-Gray, 1992). The differences found here may be associated with differences in viewing time or biomechanical constraints associated with the need to stop the linear and angular momentum present at contact with the landing surface without stepping.

These results indicate that landing forward rotation skills pose a different challenge and may require a different landing strategy than do drop landings and back saltos. To overcome these differences and the lack of success associated with forward rotation skills landing drills may need to be emphasized.

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ACKNOWLEDGMENTS

The authors wish to acknowledge the support of the US Olympic Committee, the Division of Sports Science at the US Olympic Training Center in Colorado Springs, CO, USA Gymnastics, and the work of the members of the USC Biomechanics Laboratory.