Background Biceps femoris long head (BFlh) proximal aponeurosis (PA) size appears to be highly variable between individuals (Handsfield et al., 2010) and may not be proportional to muscle size and strength. Individuals with a relatively small aponeurosis may be at higher risk of strain injury (Fiorentino et al., 2012), and this could be an important intrinsic risk factor.

Objective To examine the relationship of BFlh PA area with BFlh muscle size (volume and maximal anatomical cross-sectional area, ACSAmax) and hamstrings voluntary maximal isometric strength (MIS).

Design Cross-sectional study.

Setting Laboratory.

Participants Thirty healthy, recreationally active participants (age: 20.7±2.6 yrs, height: 179.3±7.0 cm, body mass: 72.2±7.2 kg, mean±s) with no history of hamstrings injury.

Risk factor assessment The dominant thigh was scanned (1.5T MRI scanner) and axial plane images (slice thickness: 5 mm) were used to measure BFlh muscle ACSAmax and volume. PA area was calculated as the sum of aponeurosis to muscle contact distance within each image (external and internal) multiplied by the slice thickness. MIS was measured in a prone position (hip and knee joint angles of 150°;full extension=180°).

Main outcome measurements BFlh muscle volume, BFlh ACSAmax, BFlh PA area, MIS.

Results BFlh PA area varied considerably across participants (mean=20.4±5.4 cm², range=7.5-33.5 cm², CV=26.6%) and it was not related to BFlh ACSAmax (r=0.041, P=.830) or volume (r=0.354, P=.055). Also, BFlh PA area was not related to MIS (r=0.178, P=.347). BFlh PA relative size (ratio to ACSAmax) was >4 times smaller in some individuals than others (CV=38%).

Conclusions BFlh PA area was not related to BFlh muscle size or hamstrings strength. Our results confirm the large aponeurosis size variability previously reported (Handsfield et al., 2010) and support the notion that this could be a risk factor for hamstrings strain injury.