11/11/2025
📌Control of a forward lean in the
👉(A) initial phase and
👉(B) latter phase of the movement.
The lumbar erector spinae, gluteus maximus, and hamstring muscles act eccentrically to control a flexor moment about an axis fixed in the femoral heads. Note that the moment arm of the hamstring muscles (small black arrowhead) increases with increased trunk flexion, allowing them to control a large flexion moment with somewhat reduced motor unit recruitment.
Details📝
◼️ The control of leaning forward
Described as an activity performed every day
The mechanics of this movement involve a shift in muscle activity dependent on the body's center of mass
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◼️ Phases and Muscle Actions in Controlling a Forward Lean
🟩 1. Initiation
The movement begins with the activation of the abdominal muscles, which initiates trunk flexion
🟦 2. Transition of Control
Once the mass of the head/arms/trunk passes anterior to the body's center of mass (COM), the activity of the abdominal muscles ceases
🟧 3. Controlling the Flexion
Following the cessation of abdominal muscle activity, there is activation of the lumbar erector spinae and hip extensors
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◼️ Role of the Hip Extensors in Controlling the Flexor Moment
💠 The lumbar erector spinae, gluteus maximus, and hamstring muscles act eccentrically to control a flexor moment about an axis fixed in the femoral heads
💠 Research has demonstrated that the moment arms of the hamstring muscles make them more effective hip extensors than the gluteus maximus during both phases of a forward lean
💠 The moment arm of the hamstring muscles increases as trunk flexion increases, which allows them to control a large flexion moment with a somewhat reduced motor unit recruitment
💠 If an individual maintains an extended knee position as the forward lean progresses, the increased passive tension in the hamstring-tendon actuators provides additional control while simultaneously sparing some metabolic cost