❖ Internal forces :-

🔹Force generated inside the human body as:

1. Forces of ligaments. (tensile force).
2. Muscles, joint capsules.
3.Tendons (musculotendinous junction) (tensile force) .

🔹Muscular contraction :-
Contraction is active shortening of a muscle with the distance between the two muscle attachments.

🔹Muscle produces force in three different muscle-lengthening situations (concentric, isometric, and eccentric).

🔹Concentric (shortening) contraction:

1- The internal forces generated by the muscle are greater than the external force

• في ال concentric ال int.force المتولدة بتكون أكبر من ال External ومثال على دا الـelbow flexion.

2- The muscle produces positive work on the external load.

هنا ال work اللي طالع بيبقى positive ودا عشان ال int.force اتغلبت على ال Ext.force لذلك هنلاقي ان ال movement ضد الجاذبية و في اتجاه ال inner range.

3- Concentric contraction produces the lowest magnitude of the muscular force.

و دا طبقاً للـ Length tension relationship و هي ان فيه علاقه طريه بين ال length و ال tension.

4- Moving the heel closer to the buttocks is an example of a concentric contraction of the hamstring.

🔹Eccentric (lengthening) contraction:

1- The external forces are greater than the internal force.

•زي لما تيجي تعمل Elbow Extension.

2- The external load produces negative work on the muscle.

•ال work المتولد بيكون negative نتيجة تخلى الـ External force على الـ Internal force.

3- Eccentric contraction produces the highest magnitude of the muscular force.

4- Moving the heel away from buttocks is an example of an eccentric contraction of the hamstring.

🔹Isometric contraction:

1- The overall muscle length does not change.

و دا بيحصل أما تيجي تحرك كوعك لفوق او لتحت و تلاقي sudden stop في اي مستوى فـ ميحصلش تغير في الـ muscle length.

2- The internal force generated by the muscle equals the effects of the external
forces.

3- No mechanical work is done as a load is not moved.

بس في Metabolic work بينتج عنه استهلاك ال oxygen اللي موجود في العضلة وبيتكون Lactic acid و يسبب fatigue.

4- Isometric contraction produces intermediate(متوسط )magnitude of the muscular force.

5- No visible change in muscle length in isometric contraction.

6- isometric contraction occurs during :-

- Arm wrestling match when opponents generate equal forces.

- Pushing against a stable wall.