Increased risk of lower back injury can occur during hip flexion, extension, stabilization and back extension activities. Erector Spinae muscles can hyperextend lower back more than usual if abdominal muscles are weak. The abdominal muscles tilt the pelvis forward, improving the mechanical positioning of the Erector Spinae, specifically when the lumbar spine becomes straight. When abdominal strength/endurance is not adequate to counter the pull of the antagonist Erector Spinae under load, these low back muscles are put at a mechanical disadvantage (active insufficiency) further placing additional stresses on these very same lower back muscles. Iliopsoas can pull on the spine during hip flexor activities if the abdominal muscles are weak. Risk is compounded when abdominal weakness is combined with hip flexor inflexibility.
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Increased risk of knee injury (instability) occur during knee extension activities, specifically when knees are flexed more than 90°. When hip and knee are simultaneously extending during a compound movement, hamstrings counter the anteriorly directed forces of Quadriceps. Also see Knee Stability and Angle of Pull for force vector explanation. Hamstrings / Quadriceps strength ratios should be greater than 56% to 80% depending on the population tested.
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- Increased risk of shoulder injury during shoulder flexion and abduction activities, specifically when the elbow travels below the shoulder during shoulder abduction. Risk is compounded with a winged scapula condition. Paradoxically, avoiding full range of motion (i.e. not initiating deltoid exercises from a fully adducted position) may not allow the Supraspinatus to be fully strengthened since it is more fully activated at these initial degrees of shoulder abduction/flexion. Once an injury has occurred, however, range of motion is typically restricted on the shoulder press. See shoulder abduction force vector diagram.
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