Common Muscular Weaknesses

Abdominal | Hamstrings | Supraspinatus | Infraspinatus | Erector Spinae | Vastus Medialis | Hip Abductor

Abdominal Weakness

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.

  • Examples of affected exercises:

Hamstrings Weakness

  • Example preventative/corrective exercise:

Hamstrings Weakness

Increased risk of knee injury (instability) occurs 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.

Supraspinatus Weakness


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 (ie: 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.

Serratus Anterior Weakness

A weak or fatigued Serratus Anterior may not allow for adequate Scapulohumeral Rhythm compromising proper shoulder mechanics. Weak or fatigued serratus anterior can fail to adequately protract and upwardly rotate the scapula which may not allow for adequate subacromial space for the biceps tendon and rotator cuff and maintaining ideal spacing between the humeral head and the glenoid fossa (Tovin 2006).

Also see Swimming Injuries (shoulder/overuse and fatigue/example).

A Winged Scapula condition is indicative of a serratus anterior weakness.

Infraspinatus Weakness

Lady Lying With Dumbbell

Increased risk of shoulder injury occurs during throwing and shoulder transverse flexion and transverse adduction activities, particularly when the elbow travels behind shoulder. Risk is compounded with a protracted shoulder girdle. The strong stabilizing and dislocating forces of the Pectoralis Major (Sternal and Clavical) is counteracted by the Infraspinatus, Teres Minor, and to a lesser extent, the rear deltoid and long head of the triceps brachii. This counter force is most crucial during:

  • initiation of a transverse adduction/flexion
  • during the end of a throwing movement
    • high deceleration dislocating forces required of the posterior cuff can cause breakdown in their tendons near their humeral attachment.

External rotation-muscular endurance/internal rotation-muscular endurance should be greater than 70%.

  • Examples of affected exercises with suggestions for high risk individuals:
    • Bench Press: Bring bar lower on chest, keeping elbows closer to sides.
    • Chest Press: Elevate seat so elbows are closer to sides
    • Range of motion may need to be limited, so elbows do not go behind shoulders
  • Example preventative/corrective exercises:

Erector Spinae Weakness

PC-Kits Illustrated Exercise Prescription Software

Increased risk of lower back injury occurs during lumbar spine extension or stabilization activities. Back extension exercises involving complete lumbar spine range of motion have demonstrated primarily excellent or good results for those with chronic lower back pain. Excellent or good results by diagnosis: 76% Mechanical / Strain, 72% Degenerative, 78% Disc Syndrome, 75% Spondylo (Nelson 1995). In contrast, McGill (2002) condemns the use of isolated lumbar spine exercise apparatuses and argues erector spinae endurance is more important than strength. See Low Back Debate.

Vastus Medialis Weakness


Increased risk of knee injury (PFPS or chondromalacia) may occur due to knee extension overuse. The patella becomes laterally displaced with the pull of the vastus lateralis. This patella tracking problem can produce wear on the inferior patellar surface. Greater pain is usually experienced during leg extension activities in which the knee is a greater than a 20 to 30 degree angle. Avoiding full range of motion (ie: not locking out) during Quadricep exercise may not allow the Vastus Medialis to be fully strengthened since it is more fully activated at these final degrees of knee extension.

Hip Abductor Weakness

One hip can sag when weight is shifted to one leg. Possible increase risk of Iliotibial band friction syndrome (ITBFS) when combined with Gluteus Maximus and/or Tensor Fascia Latea Inflexibility (Fredericson, et. al. 2000). Relative weakness of the Gluteus Maximus and Gluteus Minumus may also contribute to functional vargus during heavy squats.

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