Sensorimotor training has been incorporated into rehabilitation programs in athletic populations and can improve static postural sway and dynamic balance in both athlete and non-athlete populations. However, the rehabilitative exercises performed often represent relatively slow perturbations to human sensorimotor function (>350 ms) while empirical observations suggest that injuries occur in a time span of between 17 and 50 ms after initial ground contact.

Moreover, most of these exercises occur in a controlled and predictable manner whereas, when competing, athletes are required to perform in an unpredictable environment, with a majority of anterior cruciate ligament injuries occurring during non-contact or indirect contact situations involving sudden and unexpected dynamic perturbations.

An ‘Unexpected Disturbance Program’ (UDP) has been demonstrated to be effective in promoting positive outcomes during the final phase of a rehabilitation program in high-performance athletes. In addition, UDP has been shown to be more effective than traditional rehabilitation in returning athletes to high-level physical activities and promoting the maintenance of performance gains following rehabilitation of anterior cruciate ligament (ACL) injuries.

Specifically, measures of maximal leg strength and power were improved with a three week UDP in athletes that had been cleared to return to training following knee injury after having completed four phases of a rehabilitation program, namely: mobility; stability; muscular strength; and sports-specific movement patterns.

These results are in-line with previous studies showing sensorimotor training to be “highly efficient” in enhancing explosive muscle strength.

Kean and colleagues demonstrated that six weeks of wobble board training increased vertical jump height in recreationally trained women, while potentially increasing knee joint protection due to increased activation of the rectus femoris. Furthermore, in elite athletes, six weeks of sensorimotor training has been shown to increase jump performance without a concomitant increase in maximal voluntary isometric strength or rate of force development.

Unexpected Disturbance Program (UDP)

The UDP exercises prescribed in the current study consist of three exercises whereby the athlete was required to react to an unexpected disturbance applied either manually or visually while executing specific exercises.

The first exercise involved three sets of eight drop jumps from a 24 cm box. An investigator standing behind the athlete would randomly push them lightly between the scapulae to provide the unexpected disturbance stimulus.

The second exercise was three sets of eight 20 cm hurdle jumps with a 15 m run-up. An investigator would stand on the far side of the hurdle (~3 m) and give a hand gesture to indicate to the athlete which foot they were required to land on while the athlete was mid-flight clearing the hurdle.

The third exercise consisted of two sets of five 20 m running sprints with five minutes rest between sets. An investigator stood at the halfway point of the sprint and would randomly stick out an arm or leg that the athlete was required to avoid.

The aim of these exercises was to create a stimulus that required rapid, reactionary movements with a short latency period of <200 ms. As non-UDP control group performed all of the above exercises as well but were not perturbed by the investigators during the exercises.

Physical Performance Tests

Within one week prior to the commencement of and within one week after the completion of the unexpected disturbance program (UDP), the following physical performance tests were administered in the following order: countermovement jump; squat jump; 24 cm drop jump; standing broad jump; maximal landing impact force; a time-to-stability balance test; and leg strength (one repetition maximum back squat). A team of elite female field hockey athletes was used for the tests.

A minimum of 5 min rest was provided between each test.

The following day, a 20 m running sprint and 20 m repeated running sprint was assessed with at least 10 min rest between the tests.

Countermovement Jump Test

The test procedure started with a standard 20 min warm-up inclusive of dynamic stretching and 5 min cycling on a stationary bike. Subsequently, three maximal effort unloaded vertical countermovement jumps were performed on a contact mat, arms akimbo throughout the jump.

The athlete lowered themselves into a self-selected half-squat position and utilized the Stretch-Shortening Cycle (SSC) to jump with the instruction to achieve maximal height. The best jump height based on flight time was recorded and lower body instantaneous power was calculated. Each jump attempt was separated by one minute of passive recovery.

Squat Jump Test

The squat jump test replicated the countermovement jump test with the exception that the athlete was unable to utilize the SSC as a 90˚ knee angle in the starting position was enforced. Prior to the explosive jump effort, the athlete was required to hold the squat position for 2 to 3 seconds. One minute of passive recovery was allowed between jumps.

Drop Jump Test

The athlete stepped off a 24-cm box on to the contact mat and instructed to jump explosively immediately upon landing in an effort to achieve maximal height. The use of a 24cm box has been suggested by Behm and Kibele stating that it would be high enough to stress the SSC, whilst allowing participants to emphasize a short ground contact time.

As above, three jump efforts were performed separated by one minute of passive recovery and the greatest jump height was recorded.

Standing Broad Jump Test

The standing broad jump test was performed on a purposely designed rubber mat with 1-cm increments marked on to the non-slip surface. Using both feet, the athletes were instructed to jump forward as far as possible, with an arm-swing action permitted. The distance between the start line and the rearmost heel was recorded and the best of three trials was used for analysis.

Time-to-Stability and Landing Impact Force Tests

For the stability and landing force tests, a 20-cm hurdle was placed 2 cm in front of a force plate sampling at 200 Hz. From a two-leg standing position with hands-on-hips, the athlete was asked to jump over the hurdle and land on one leg on the force plate.

The time-to-stability and ratio of impact force to body weight were automatically calculated by InnerBalance software (Innervations, version 2013.01). Time-to-stability was defined as the difference between the landing time and the time at which stability within 5% of body weight was achieved.

Maximal Leg Strength Test

The one-repetition maximum (1-RM) back squat test was performed using a squat rack, Olympic bar, and plates. After an initial warm-up with a 20 kg load, the athlete was instructed to do a light set of 8 to 10 repetitions with a load of approximately 100% of bodyweight (±2.5 kg) which was measured prior to the test session on a previously calibrated weighing scale (Tanita, USA).

The athletes then followed standardized procedures with incremental increases in weight until they were unable to lift the weight. Each athlete was allowed a maximum of five sets to achieve their 1-RM. In those cases where 1-RM was not achieved after the fifth set, the Brzycki formula was used to predict the 1-RM using the set with the fewest repetitions.

20-m Running Sprint Test

The test procedure on the second day started with a standard 20 min warm-up inclusive of dynamic stretching and 5 min light jogging. The 20-m sprint test was assessed in a gymnastics hall using a dual-beam infrared timing system whereby four pairs of timing gates were set up at 0, 5, 10 and 20 m. The athletes performed three sub-maximal efforts at a perceived 50, 70, and 90% intensity before performing three maximal efforts.

From a standing start position, the athletes were asked to sprint as fast as possible through the gates and to slow down only after they passed the final pair of timing gates. Each running sprint effort was separated by three minutes of passive recovery.

20-m Repeated Running Sprint Test

The same equipment from the 20-m running sprint test described above was used to assess repeated sprint performance. At least 10 min after the maximal running sprint test, each subject performed six 20-m running sprint efforts with 15 s of passive recovery between each sprint effort.

A work-to-rest ratio lower than 1:5 was implemented to minimize the contribution of the aerobic system through adenosine triphosphate resynthesis. Each sprint was timed and the cumulative sprint time was recorded.

Discussion

In confirmation of our hypothesis, the three-week unexpected disturbance program showed improvements in physical performance measures in elite female field hockey athletes. Specifically, the six sessions of UDP were responsible for enhancements in 5-m, 10-m and 20-m running sprint speed, concentric-only jump performance, leg strength and time to stability.

There have been multiple studies and research reporting that sensorimotor training demonstrates elicit effects on neuromuscular function. It is noteworthy that strength gains were observed in both training groups in the current study, despite only three resistance training sessions being performed over the experimental period (i.e., one strength session per week).

The enhanced rate of force development (RFD) following sensorimotor training has been attributed to improved extra faciliatory drive from the afferent system. Changes in motoneuron recruitment, firing frequency, and synchronization have also been proposed to explain improved RFD.

The importance of RFD in athletic performance is demonstrated by the observed relationships between RFD and both running speed and jump performance. Here we report substantial enhancement of running sprint speed and concentric-only jump performance in elite female athletes performing a UDP.

The performance enhancement was particularly evident in the 5m sprint running data. As the initial acceleration phase is dominated by concentric muscle actions, here we suggest that the UDP-mediated improvements in both running sprint time and the concentric-only jump performance are indicative of enhanced RFD.

Given the results obtained in this study, it is important to take note that improvements observed are attributed to the fact that the duration of the intervention was short (6 session performed over three weeks) and the subjects were of elite training status.

It is known that sensorimotor training on a stability platform can induce changes in the brain regions known to be associated with complex motor skill acquisition and integration of vestibular signals for postural control. These changes have been positively correlated with performance in a complex balancing task, as observed by other researches.

Conclusion

The substantial improvements in running sprint speed and concentric-only jump performance lead us to concur with the statement by Gruber and colleagues that sensorimotor training is a “highly efficient” modality for improving explosive strength. The quantitatively large improvement in 5-m running sprint performance represents a highly desirable adaptation with respect to athletic performance.

These results in elite female field hockey athletes may have important implications for injury prevention in this cohort and extend on our previous work whereby an unexpected disturbance program (UDP) was demonstrated to be effective in enhancing performance outcomes in a rehabilitative setting.

Thus, both rehabilitative specialists and strength and conditioning practitioners should be aware that incorporating exercises that specifically target the sensorimotor system through unexpected disturbances can be effective in improving physical performance measures.

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