Br J Sports Med. 2013 Apr;47(6):351-8. doi: 10.1136/bjsports-2011-090664. Epub 2012 Jul 4.
Risk factors for hamstring muscle strain injury in sport: a systematic review and meta-analysis.
Freckleton G1, Pizzari T.
http://bjsm.bmj.com/content/47/6/351.long
What is already know about HMSI?
Hamstring muscle strain injuries (HMSI) are common in sport and there is a high rate of recurrence. Many papers have examined potential risk factors for HMSI, with age and previous hamstring strain being accepted as risk factors, and many others inconsistently identified as contributing to the injury. Qualitative reviews have been undertaken on this topic, but a meta-analysis of risk factor studies has not been performed.
What this study adds?
Meta-analysis identified age, previous history of hamstring strain and increased quadriceps peak torque as risk factors for HMSI. Other strength measures such as hamstring:quadriceps ratio, commonly perceived to be predictive of injury, were not associated with HMSI. This study highlights risk factors that require further investigation to confirm their involvement in HMSI and risk factors that have no association and need not be tested in future studies.
So, it is not because Kareem wasn't "in-shape" that he had a hamstring strain. The science doesn't link many factors consistently to an increased risk of hamstring strains. As the meta-analysis above concluded, previous hamstring strain, being older and having really strong quads are the only consistent factors that increase hamstring strain risk. The fact that he strained his hamstring does put him at higher risk to repeatedly strain it. (Hopefully not like Ken Griffey Jr)