ALAN W. ARATA, Lt. Col., Ph.D.
Air Force Academy
Colorado Springs, Colorado USA
In football, for example, a defensive back employing BR can keep both the receiver and the quarterback in his field of vision. Once the player turns to run forward, he loses sight of one if not both of these players, placing him at a disadvantage since both the quarterback and the receiver know where the ball is going. Athletes in sports like soccer and basketball will often run forward on offense and backward on defense. Superior BR speed is an advantage for a player in any of these sports, because speed allows them to better defend attacks.
Since high level performance in sports is lucrative business, one might suppose the BR aspect of sport would be thoroughly investigated. Backward running kinematics that have been investigated involved moderate velocities for the rehabilitative aspects of BR. There has been no research aimed at improving BR performance in highly skilled athletes who use BR. The purpose of this study was to investigate the kinematic parameters of maximum velocity BR and compare them to FR.
Velocity was calculated as the average velocity during one stride. Stride length was calculated by measuring the horizontal change in hip marker position between two sequential ground contacts of the designated foot. Stride frequency was determined by measuring the time from first foot contact to the subsequent contact of the same foot. Range of motion was measured from the joint's position of maximum extension to its position of maximum flexion during a stride. Intrinsic support length (ISL) was divided into two portions, hip to toe distance at ground contact (GCSL) and hip to toe distance at toe-off (TOSL). Both ISL measurements were calculated as a percentage of the subject's leg length. Stance time was expressed as a percentage of stride time.
Statistical procedures (analyses of variance) were used to compare differences between: 1) the two maximum velocity conditions (BRmax to FRmax) and 2) BRmax to FRequal in order to investigate similarities and differences between equal velocities of BR and FR.
Hip, knee and ankle ROM were significantly greater during both FR conditions than during BRmax (see Table 1). Ground contact support length (CGSL) was significantly greater for FRmax than BRmax (40% vs. 26%). FRmax TOSL distances were also significantly greater than the BRmax distances (54% vs. 46%). The comparison of GCSL revealed significantly longer FRequal distances than BRmax (42% vs. 26%). BRmax to FRequal comparison of TOSL showed no condition differences (46% vs. 47%). BRmax stance time was significantly greater than both FRequal and FRmax stance times (31% vs. 27% & 26%).
Comparison of Hip, Knee and Ankle Range of Motion (ROM) Between Conditions
All units in degrees; Mean followed by standard deviation
|Hip ROM||41.8 / 9.6||62.7 / 5.3||69.3 / 4.9|
|Knee ROM||83.1 / 12.5||118.7 / 9.1||117.3 / 14.4|
|Ankle ROM||45.0 / 5.0||54.2 / 11.7||51.4 / 9.4|
Anatomical constraints of BR versus FR likely contribute to the shorter BR stride length. For example, during BR extension prior to ground contact, the hip joint is constrained by the anterior musculo-tendinous units spanning the hip joint. Also, the knee may be constrained in extension at or near toe-off. BR also requires muscular work from the hamstring muscle group to slow the knee prior to full extension to prevent hyperextension and injury. Knee joint proprioceptors sense joint extension and send neurological signals to activate antagonistic muscles to avoid damage to the knee joint structure. This antagonistic muscular force is counterproductive to BR velocity. Ankle ROM is constrained during the BR stance phase. As the runner moves backward, the ankle angle increases as opposed to decreasing as in FR, lessening the amount of plantar flexion available and thus limiting propulsive potential since the dorsiflexors are not capable of producing as much torque as the plantar flexors. Ground contact support length (GCSL) were significantly longer for FR than BR. This finding may have been due to the hip joint's anatomical constraint in extension during BR. The FR findings showed FRmax had lower values than FRequal. A shorter hip to toe distance at ground contact could mean less braking force. Kunz and Kaufmann's (1981) FR research with elite decathletes and sprinters supports this concept. The comparison of hip to toe distances at toe-off showed no condition differences, suggesting that this variable might be related more to velocity than direction and supporting the idea of a "longer hip to toe distance at toe-off-- greater velocity" relationship. Since a runner can only produce force while his foot is in contact with the ground, it is logically assumed that a greater velocity can be generated from a longer hip to toe distance at toe-off. Nilsson, Thorstensson, and Halbertsma (1985) research supported this concept when they found that the distance from the center of gravity to the toe increased as velocity increased. Analysis of ground reaction forces could add valuable information on the differences and similarities between high speed BR and FR with respect to differences in ISL.
Relative stance time in FR did not change across the two FR velocities (FRmax and FRequal). Runners performing BR spent a greater amount of time on the ground compared to both FRmax and FRequal. Threlkeld et al. (1989) determined that actual stance time for BR was 10% shorter than for FR. This study's results indicated that actual BRmax stance time was 18% shorter than FRequal stance time. The velocities in this study averaged nearly 2 m s-1 greater than Threlkeld et al. and given the trend of decreased stance time with velocity, the findings appear to be consistent.
For athletes wanting to increase their BR velocity, it appears that stride length, not stride frequency is the factor in the velocity equation that needs to be addressed since stride frequency is already significantly greater for BR than FR. The human has greater anatomical constraints with respect to the hip, knee and ankle in BR than FR. It is unknown what BR stride length can be obtained with training. A kinetic analysis of these three conditions may provide further guidance to individuals and coaches wanting to improve BR velocity. Future kinematic studies should investigate the differences between elite and non-elite backward runners at percentages of maximum BR velocities.
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