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Athletic groin pain (AGP) is prevalent in field sports with recurrent accelerations, decelerations, and changes of direction. Despite this, the biomechanics contributing to AGP remains poorly understood and under-investigated in comparison with other sporting injuries.

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Stiffness, which is resistance of a body to deformation under a given load, has attracted attention in injury prevention research as a potentially modifiable risk factor. Two types of stiffness are typically measured when examining dynamic athletic tasks, whole-body vertical stiffness, and joint stiffness. Whole-body vertical stiffness reflects the resistance of the centre of mass to vertical displacement under a given vertical ground reaction force. Joint stiffness refers to the resistance of a particular joint to rotation under a given moment of force. 

It has been suggested that abnormal magnitudes of stiffness may lead to an increased risk of injury by increasing peak force and/or rate of force development, or conversely by increasing the energy absorbed by soft tissues in a lengthened position.

With respect to AGP, stiffness may be of particular importance as any alteration in the magnitude of loading or the manner in which loads are absorbed may overload the musculo-tendinous and bony structures surrounding the pubic symphysis region. To date however, no research has examined whether stiffness is affected by AGP.

The aim of this study was to determine if AGP affects whole-body vertical and joint stiffness and if so whether a return to play the following rehabilitation is associated with a change in stiffness.

Sixty-five male subjects with AGP aged 18-35 years and involved in multidirectional field sports who had successfully completed the exercise intervention were examined in this study, along with fifty male matched uninjured controls. Inclusion criteria required all AGP participants to undergo clinical consultation, MRI imaging, and physical examination to confirm the diagnosis of AGP. 

The control group was uninjured but matched to the AGP group based on age, sport, and participation level alongside leg dominance.

Primary sporting participation within both groups was distributed across four sports with the largest proportion of subjects in both groups playing Gaelic football.

AGP subjects completed a three-stage rehabilitation program focusing on intersegmental control and strength, linear running mechanics, and change of direction mechanics. Components of strength, power, and plyometric training were incorporated into the rehabilitation program. 

The Copenhagen Hip And Groin Outcome Score (HAGOS) was examined pre-and post-rehabilitation.

Assessment included a biomechanical examination of stiffness during a lateral hurdle hop test. The hurdle hop involved a lateral hop over a 15-cm hurdle followed by an immediate hop back to the initial starting position. The distance between foot contacts was 40 cm.

The AGP participants were examined on their painful side, contralateral non-weight-bearing foot behind with the knee flexed to approximately 90 degrees, and hands unrestricted for balance. Subjects with AGP were tested pre-(AGP pre) and post-rehabilitation (AGP post), while controls were tested once.

Reflective markers were placed at bony landmarks on the lower limbs, pelvis, and trunk as per the Vicon Plug-in Gait model. Lower extremity kinematics and kinetics were captured. Three-dimensional marker position was tracked using 8 infrared cameras, synchronized with two 40 × 60 cm force platforms collecting ground reaction force data. Motion and force data were captured at a sampling frequency of 200 Hz and 1000 Hz, respectively.

Stiffness was examined during the eccentric phase of the hurdle hop action defined as the period from initial ground contact to peak whole-body negative power.

All AGP subjects completed rehabilitation in a median of 9.14 weeks (5.14-29.0). The AGP subjects also reported significant improvements in 5 subscales (pain, symptoms, function in daily living, function in sport and recreation, quality of life (< .01 for all subscales)) following rehabilitation, with only one subsection [participation in physical activities (= .36)] not changing significantly.

Vertical whole body, ankle plantar flexor, knee extensor, and hip abductor stiffness were significantly less in the AGP pre group in comparison with uninjured control group pre-rehabilitation. When the AGP group was compared pre-and post-rehabilitation, hip abductor stiffness increased significantly and ankle internal rotator stiffness decreased significantly. Post-rehabilitation, vertical whole body, ankle plantar flexor, and knee extensor stiffness remained significantly less in AGP pre group in comparison with uninjured control group, while hip abductor stiffness was no longer significantly different between the two groups.

The main finding from this study was that the AGP group were significantly less stiff in comparison with controls for four of the ten stiffness variables: ankle plantar flexor, knee extensor, hip abductor, and whole-body vertical stiffness. In contrast, the pre-to post-rehabilitation analysis identified that hip abductor stiffness and ankle internal rotator stiffness changed significantly, while the post-rehabilitation vs uninjured comparison indicated that only hip abductor stiffness was no longer significantly different between the AGP and uninjured group. The results suggest that hip abduction stiffness may represent a target for rehabilitation.

Source: Gore SJ et al. (2018) Is stiffness related to athletic groin pain? Scand J Med Sci Sports 28:1681–1690.