PART 1: What bench press widths are the best for injury prevention and performance? Muscle activation will be in PART 2. 

WIDTH & INJURY PREVENTION:
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Fees et al. (1998) has reported that a hand spacing of ≥2 biacromial width (shoulder width as defined by the distance between acromion processes) increases shoulder abduction above 75°, whereas hand spacing <1.5 biacromial width maintains shoulder abduction below 45° 

This is important as Gross et al. (1993) state that 90° of abduction combined with end-range external rotation (Kenyon & Kenyon (2009) state normal ROM = 80-100°) has been defined as the “at-risk position” that may increase the risk of shoulder injuries; external rotation is minimal during the flat bench press, but increases in proportion to the angle of inclination during the incline bench press (Green and Comfort, 2007). 

Acute injuries (rupture of pectoralis major) and chronic over-use injuries (anterior instability and atraumatic osteolysis of the distal clavicle - `weightlifter's shoulder`) are common (Green and Comfort, 2007). The risk of both acute and chronic shoulder injury may be increased by repetitive movements performed with the shoulder close to the 90° of abduction, as seen during the bench press when performed with a grip >1.5 times bi-acromial width (Green and Comfort, 2007). Increased external rotation further increases the risk of injury. 

Further reading: https://www.researchgate.net/…/232096734_The_Affect_of_Grip…

WIDTH & PERFORMANCE:
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Research by Barnett et al. (1995) and Lehman (2005) has demonstrated a nonsignificant difference (p > 0.05) in one repetition maximum with a grip width of 100% and 200% biacromial width. 

Electromyographic results showed that grip width did not significantly affect activity of the sternocostal head of the pectoralis major (p > 0.05). However, the narrow grip significantly in-creased the activity of the clavicular head (p < 0.01) and the activity of the triceps brachii (p < 0.05) compared to the wide grip.

However, there is conflicting evidence to counter the findings of Barnett et al. (1995) and Lehman (2005), for example research conducted by Clemons and Aaron, (1997) concluded that increasing grip width increased the activity of all of the muscles analysed. Research by Gilbert & Lees (2003) and Wagner et al. (1992) provide additional support for Clemons and Aaron, (1997) as both studies concluded optimum grip width is between 180 and 200% of biacromial width for maximal bench press performance. 

Further reading: http://dbperformance.weebly.com/effects-of-grip-width-on-be…
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PART 2 will include EMG studies on the muscles below and how they are best activated (bench angle, grip width, etc.):

- PECTORALIS MAJOR
- DELTOIDS
- TRICEPS
- LATISSIMUS DORSI
- BICEPS BRACHII
- ERECTOR SPINAE
- ABDOMINALS
- THE ROTATOR CUFF

REFERENCES:
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BARNETT, C., V. KIPPERS, AND P. TURNER . Effects of variations of the bench press exercise on EMG activity of five shoulder muscles. J. Strength Cond. Res. 9:222–227. 1995.

Clemons, J. and Aaron, C. (1997) Effect of grip width on the myoelectric activity of the prime movers in the bench press. Journal of Strength and Conditioning Research. Vol. 11, No. 2: 82-87. 

Green, C. and Comfort, P. (2007). The Affect of Grip Width on Bench Press Performance and Risk of Injury. Strength and Conditioning Journal, 29(5), pp.10-14.

Gilbert, G. and Lees, A. (2003) Maximum grip width regulations discriminate against larger athletes. Journal of Sports Sciences. Vol. 21, No. 4: 299-300. 

GROSS , M.L., S.L. BRENNER , I. ESFORMES, AND J.J. SONZOGNI. Anterior shoulder instability in weight lifters. Am. J. Sports Med. 21:599–603. 1993. 

FEES , M., T. DECKER , L. SNYDER-MACKLER, AND M.J. AXE . Upper extremity weight-training modifications for the injured athlete: A clinical perspective. Am. J. Sports. Med. 26:732– 742. 1998. 

Kenyon, J. and Kenyon, K. (2009). The physiotherapist's pocket book. Edinburgh: Churchill Livingstone, p.97.

LEHMAN , G.J. The influence of grip width and forearm pronation/supination on upper-body myoelectrical ac-tivity during the flat bench press. J. Strength Cond. Res. 19:587–591. 2005. 

Wagner, L., Evans, S., Weir, J., Housh, T. and Johnson, G. (1992) The effect of grip width on bench press performance. International Journal of Sport Biomechanics. Vol. 8: 1-10.