Lower limb muscle strength profiles and injury associations: a two-season prospective cohort study in men's professional rugby union

Objectives: To profile professional rugby union players using longitudinal hip, groin and hamstring strength measures and identify characteristics associated with non-contact lower limb injury and reinjury. 

Design: Prospective cohort study. Methods: 248 players were involved in the study. Over 24 months, injury surveillance data was collected, and a battery of strength tests were conducted at the start of each pre-season and mid-season. The strength tests included: Nordic hamstring exercise (Nordic); hip abduction and adduction at 60° (ABD60, ADD60); and 90° of hip flexion (ABD90 and ADD90). 

Results: Strength profiles are presented. For the 55 players who featured at the first and last test of the study; Nordic, ADD90 and ABD90 increased in the forwards only (P = 0.002 to 0.01). Strength changes at the end of the study were correlated with measures obtained at the start (r = − 0.38 to − 0.49), and amongst the test battery measures (r = 0.42 to r = 0.68). Hamstring, and hip/groin made up 43 % of lower limb non-contact injuries. Those who sustained a hip/groin injury had greater odds (OR = 4.0, P = 0.0016) of also suffering a hamstring injury. For injured players, only ABD60 reduced post-injury (P = 0.0021). Players who reinjured were characterised by lower pre-injury ADD90; and either higher pre-injury Nordic or an ADD90 that did not improve post-index injury. Pre-season strength profiles remained unchanged for uninjured players. 

Conclusions: Strength changes were observed overtime, and post-injury. Players who were weaker from the outset improved whilst those who were stronger struggled to maintain strength. Changes from pre-season to in-season are important since most prospective injury studies only measure strength during pre-season. Strong knee flexor strength alone may not protect from reinjury, and further investigations are required to further support the observations that adductor strength is a protective factor in lower limb non-contact reinjury in rugby union players.