KINEMATIC COMPARISON OF HIGH SPEED BACKWARD AND FORWARD RUNNING

ALAN W. ARATA, Lt. Col., Ph.D.
Air Force Academy
Colorado Springs, Colorado USA

Abstract

Backward running (BR) has been used for strength gain and rehabilitation over the past several years, but no study has investigated BR as it is used in competitive sporting events, a maximum velocity. The purpose of this study was to compare the kinematic parameters of maximum velocity BR (BRmax) to the parameters of two forward running velocities (FRmax & FRequal). Thirty runners volunteered for the study. Each participant completed BR trials at maximum velocity, FR trials at maximum velocity (FRmax) and FR trials at BRmax velocity ñ5% (FRequal). Variables that were compared included velocities, stride lengths, stride frequencies, ranges of motion (ROM) of the hip, knee and ankle, intrinsic support lengths (ISL) and stance times. The FR conditions had greater stride lengths, great ROMs at each joint, and greater ISLs than did BRmax. Subjects spent a greater percentage of time in stance during BR than FR and stride frequencies were greater for BRmax than FR. The results indicated that there are major kinematic differences between FR and high speed BR. The shorter stride length of BR may be the most notable difference between BR and FR velocity.

Introduction

With the advent of competition, humans have been looking for ways to enhance performance and increase chances of victory. Because of this, forward running (FR) has benefitted from a great deal of investigation since it is the basis for a number of competitive sports as well as health and fitness activities. With topics covering cardiovascular fitness to injury mechanisms to proper mechanics for fast running, an enormous amount of effort has been invested in FR research. Backward running (BR), on the other hand, has not received this kind of attention, although the rehabilitative aspects of BR have recently been under investigation (Devita & Stribling, 1991; Flynn et al., 1994; Flynn & Soutas-Little, 1995; Flynn & Soutas-Little, 1993; Threlkeld et al., 1989). However, no investigations have been conducted with sports performance in mind. Only Bates, Morrison and Hamill (1986) have mentioned the importance of BR in sport, noting that it was done in quick bursts on athletic fields or courts. It is curious that BR has seen so little research, as it plays an important role in a number of highly competitive sports, including football, basketball, soccer and other team competitions played in similar settings.

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.

Methods

Thirty healthy male volunteers, 18 - 28 years of age, served as subjects for the study. All were active in a running / athletic program. Each subject was first given a suitable practice period on the 20m runway prior to performing three successful BR trials at maximum BR velocity followed by a rest period of at least one minute or longer if requested. This was followed by three successful, FR trials at maximum FR velocity (FRmax). Following these trials, each subject performed three FR trials at his average maximum BR trial velocity (within a ñ 5% target range) (Frequal).

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.

Results

There was no significant difference between BRmax and FRequal velocities (5.1 m s-1 vs. 5.2 m s-1). This statistical test did not indicate that the velocities were the same, but for the comparisons in this study, these two condition velocities (BRmax and FRequal) were considered equal. FRmax velocities were significantly faster than BRmax velocities (7.3 m s-1 vs. 5.1 m s-1). Both FRmax and FRequal stride lengths were significantly longer than BRmax stride length (3.95 m & 3.60 m vs. 2.50 m). BRmax stride frequency was significantly greater than both FRequal and FRmax stride frequencies (124 strides*min-1 vs. 86 strides*min-1 & 118 strides*min-1).

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%).

Table 1.

Comparison of Hip, Knee and Ankle Range of Motion (ROM) Between Conditions (n=30)
All units in degrees; Mean followed by standard deviation

Parameter	BRmax	FRequal	FRmax
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

Discussion

There has been no published literature on maximum velocity BR. A general consensus is that FR is faster than BR. The results from this study confirm that widespread belief. Overall, BRmax velocity was 69.8% of FRmax velocity. The individual range for BR to FR was 60.3% - 79.2%. Like several BR variables, the maximum length of a BR stride was unknown. Since the product of stride length and stride frequency equals velocity, and since it was assumed velocity would be lower in BR than FR, it was hypothesized that stride length would be greater in FR. The results of this study confirmed this hypothesis. It was also hypothesized that stride frequency would not differ between BR and FR and that individuals have a motor program (set of directions in the brain) for stride frequency independent of running direction. This study's results did not support this hypothesis since BR had a significantly greater stride frequency than FR.

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.

Conclusion

Researchers have investigated backward walking and compared it to forward walking and concluded there is very little temporal difference (Vilensky, Gankiewicz, & Gehlsen, 1987; Winter, Pluck, & Yang, 1989). This study's comparisons of high speed BR to FR indicate that BR is kinematically and temporally different than FR. This is most noticeable in FR's greater ROM of the hip, knee and ankle, FR's greater stride length, BR's greater stride frequency and BR's longer stance time as a percentage of stride time.

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.

References