Pass training in a Footbonaut: why newcomers should be treated carefully

Objectives
Adductor muscle injuries are the second most common injury in soccer and
accountable for a disproportionate amount of loss from training. The high
incidence can be attributed to the high amount of passes in soccer. A
Footbonaut (FB) enables intensive and isolated training of short passes. It
has been shown, that adductor muscles have to bear high stress during pass
training. But it is unknown if this depends on experience level. Therefore, the
purpose of this study was to compare the kinematics of the hip joint, muscle
forces and shortening velocities of the m. adductor longus and m. gracilis
between players that were experienced (Ex) in playing in the FB and
inexperienced players (NEx). Knowledge on these differences is valuable in
terms of injury prevention.

Methods
Twenty-six subjects were investigated and arranged in two groups regarding
their experience level in using the Footbonaut. Group NEx consisted of ten
players from a semiprofessional level without any experience in playing in a
FB (180,6 cm ± 4,4; 77,7 kg ± 6,4). Sixteen players (179,3 cm ± 8,2; 74,8 kg
± 7,4) from the U23-team of the TSG Hoffenheim with weekly sessions in the
FB form the Ex group. Kinematics of ten passes per player were recorded with 16 infrared cameras (200 Hz, Vicon, Oxford). Swing phase (SP) was defined as toe-off to ball contact. To calculate hip joint kinematics and muscle parameters of adductor longus and gracilis, an anatomical scaled AnyBody model (AnyBody Technology, Aalborg) was used. Statistical differences of time histories were tested with statistical parametric mapping for joint kinematics and muscle parameters and discrete t-tests for anthropometric data.

Result
NEx showed significant higher muscle forces than Ex during the backswing of the SP in the adductor longus and gracilis. Ex showed a shift of the peak muscle forces to the right and therefore higher forces than NEx during the forward swing, prior to ball contact. Furthermore, the analysis showed no statistic differences in muscle shortening velocity, hip joint kinematics and anthropometric data.
The differences in muscle forces show, that experience is an important factor influencing adductor muscle forces during pass training in a FB. The higher forces of the adductor longus during the backswing phase compared to the Ex, generate a higher potential for injury in the NEx because they produce a higher portion of the total forces using eccentric muscle force while the Ex produce more force during the concentric phase of the SP. The difference between the higher eccentric force of the Ex compared to the NEx in the gracilis is comparatively small. Consequently, the time histories of force of the Ex, can be rated as less risky in terms of injury risk because high eccentric loads are a known risk factor for muscle injuries. As a consequence, coaches have to be cautious when using the Footbonaut with unexperienced players for intensive pass training.