question: Why are sit-ups harder to do with your hands behind your head, rather then free?
comment: Have you ever looked at a teeter-totter before? If one person on the teeter-totter is sitting at the very end and person on other side is sitting closer to the middle, then there's an imbalance and that imbalance spreads outward from the middle, which is called the fulcrum. Even though the two people may weigh the same amount, it's almost as if the person that's further away from the middle is heavier. When doing a sit-up, the middle of your body (your hips, waist, and butt) would be like a fulcrum. By placing your hands behind your head, you're putting more weight at the very end and that impacts the amount of effort you have to expend when you do your sit-up. This is caused by gravity. The further out on the end the weight is located, the more it has to fight against gravity and rise above the ground.
passages: It begins with lying with the back on the floor, typically with the arms across the chest or hands behind the head and the knees bent in an attempt to reduce stress on the back muscles and spine, and then elevating both the upper and lower vertebrae from the floor until everything superior to the buttocks is not touching the ground. Some argue that situps can be dangerous due to high compressive lumbar load and may be replaced with the crunch in exercise programs.
   Strength exercises such as sit-ups and push-ups do not cause the spot reduction of fat. Gaining a "six pack" requires both abdominal muscle hypertrophy training and fat loss over the abdomen—which can only be done by losing fat from the body as a whole.
  
  Section::::Form.:Variations.
  
  The movement can be made easier by placing the arms further down away from the head. Typical variations to 
   The position of the hands and arms is also important. The crossed-arms crunch is easier, so it is suitable for beginners. Another option with the hands placed behind the head is more difficult and is usually used to increase the load. One more complicated option is performed with the straight arms extended forward above the head and the wrists crossed.
  
  Section::::Number of Reps.
   An extremely difficult variation is to perform a push-up using only hands, without resting the feet on the floor, i.e., starting from and returning to the planche position. These are known as "planche push-ups". To do this variation, the body's center of gravity must be kept over the hands while performing the push-up by leaning forward while the legs are elevated in the air, which requires great strength and a high level of balance. The entire body weight is lifted in this variation.
  
  Section::::Variations.:Knuckle push-ups.
   A compound variation is the bent over row done with a wide overhand grip that allows the elbows out, as opposed to the elbows-in lat-targeting variation. This is also referred to as a "rear delt row" as opposed to a "raise". Bodyweight/supine rows (or "australian pullups") can also be done with a wide overhand grip to target the rear delts. It requires less work by the lower back and legs to stabilize.
   The width of the grip may also differ. When grabbing and holding the bar during the pull-up, the hands can be apart at shoulder-width, or wider, or narrower enough to touch each other. This may make the pull-up more difficult and may limit the range of motion compared to the shoulder-width grip.
  
  Section::::Safety.
   According to the study published in Journal of Strength and Conditioning Research, the test subjects supported with their hands, on average, 69.16% of their body mass in the up position, and 75.04% in the down position during the traditional push-ups. In modified push-ups, where knees are used as the pivot point, subjects supported 53.56% and 61.80% of their body mass in up and down positions, respectively.
  
  Section::::Muscles worked.
  
  While the push-up primarily targets the muscles of the chest, arms, and shoulders, support required from other muscles results in a wider range of muscles integrated into the exercise.
  
  Section::::Muscles worked.:Abdominals.
   BULLET::::- Situps come straight after the press-ups. 85 are needed for maximum points, again 5 points, as with every other gym exercise sit ups are conducted to an audible bleep. Carried out with partner holding feet, elbows must touch top of knees and then the shoulders and elbows must touch the floor on the way down for a repetition to count. Knees must remain together or else reps will be deducted.
   A variation is to lean forward and to place the hands, palms up, on the ground in front of the body.
  
  Section::::Claimed benefits.
   In the 70s there are examples of this exercise being referred to as pullups. This is no longer common, and this term is now mostly used to refer to the exercise involving pulling the body up to a bar.
  
  Section::::Contraindication.
  
  Due to the amount of internal rotation of the humerus during this movement, many trainers and organizations (such as the ACSM and NFPT) consider this a contraindicated exercise for all trainees. Most will at least advise those with 
   The movement can be considered a bodyweight exercise similar to the military press, while the regular push-up is similar to the bench press.
  
  Section::::Muscles.
   Inner muscles that support the operation of the fingers, wrists, forearms and elbows are also worked isometrically. Some push-up modifications that require to have the arms at different heights effectively engage the rotator cuff.
  
  Section::::Variations.
   The above variation is most difficult when the muscle is maximally contracted, and allows both arms to be worked out at once. To emphasize the contraction in a stretched position using a weight, the arms must be worked unilaterally. The exerciser rotates the side he is working toward the ceiling and lies on the opposite side. The resistance meets its peak when the humerus is parallel with the ground. To attain a similar effect, it is common to use a cable crossover variation to the rear lateral raise, which allows the angle to be done on both sides simultaneously and without lying on one's side, which can be uncomfortable for some people.
   The pulldown usually uses a weight machine with a seat and brace for the thighs. The starting position involves sitting at the machine with the thighs braced, back straight and feet flat on the floor. The arms are held overhead at full extension, grasping a bar connected to the weight stack. The movement is initiated by pulling the elbows down and back, lowering the bar to the neck, and completed by returning to the initial position.
  
  Section::::Form.:Variations.
   Some have associated a "pull-up" with utilizing an overhand (pronated; palms facing away) grip. This includes by the Boy Scouts of America, Guinness World Records "makes a difference between" pull-ups and chin-ups according to grip. Organizations such as the United States Marine Corps however, see pull-ups including both the overhand and underhand grips.
   As the momentum is initiated by the legs, the bottom portion of the range of motion may be under-developed compared to the middle and end portions. The entire range of motion is more reliably trained in a shoulder press. One method of training is to initiate lifting with a shoulder press, and then gradually turning it into a push press when the lifter no longer has the strength to lift it with his shoulders alone and does not want to train with a lighter weight.
   An EEG-study by Melanie A. Hart, Ph.D. Assistant Professor Department of Health, Exercise and Sports Sciences at the Texas Tech University support the claim that cup stacking does utilize both sides of the brain. During the left-hand condition, activity in the right hemisphere was larger than the left, while for the right-hand task, the left hemisphere was greater than the right. Their scientific poster on that topic got awarded by the AAHPERD On the other hand, Hart couldn't get the same results as Udermann when studying improvement on reaction time.
   When doing the pushup care must be taken to ensure perfect form during each rep and full engagement of the scapulae at the top of the pushup. The focus is never on the number of reps but rather the quality of each rep. One quality rep will always be better than 5 poor reps. 
  
  Exercise 2 - The pike pushup
   Powerlifting: Take position on a flat bench with body weight resting on buttocks and upper traps whilst driving feet into the floor. Movement requires the weight to be taken at full arms' length, lowered to upper torso, paused, and then lifted to starting position.
  
  Section::::History.
  
  The bench press has evolved over the years, from floor, bridge, and belly toss variations to the methods used by bodybuilders and powerlifters today.
   At the same time, the arms are brought up with the elbows extended in front of the chest so the bar may now lie across or "rest" across the palms, the front of the shoulder or deltoid muscles, and the clavicles. At this point the lifter should be in a full squat position, with his buttocks on or very close to the heels, sitting erect with the bar resting comfortably across the deltoids and fingers. By keeping a rigid torso and maintaining a deep breath hold the bar bends over the lifter's clavicle.
   Section::::Ergonomics.
   Due to the difficulty of the exercise, it is common to begin training with simpler, related movements. Methods of preparing without equipment (bodyweight exercise) include holding a static handstand position, performing the movement with a reduced range of motion, or performing only the eccentric portion of the movement. Preparing with weight-lifting (overhead press) is also common for strengthening the muscles involved, for those who lack adequate balance or who cannot support their bodyweight on their hands. When one can press their bodyweight, they have developed sufficient strength to do a handstand push up and then need to learn how to balance and exert themselves while inverted.
   With push-ups, many possibilities for customization and increased intensity are possible. Some examples are: One hand can be set on a higher platform than the other or be farther away from the other to give more weight to the opposite arm/side of the body and also exercise many diverse muscles. One can perform push-ups by using only the tips of the fingers and thumb. For increased difficulty, push-ups can be performed on one arm or using weights.
  
  Push-ups between chairs form an integral part of the "Dynamic Tension" Course devised by Charles Atlas, and similar systems.
  
  Section::::Record breakers and attempts.
   To execute the exercise, the weightlifter attains a prone rib cage position. This is usually done by standing and bending over, or by laying face-down on a bench. Flat is ideal, though slight inclines can also be used. The arms hang straight down. The goal is to bring the arms to the side away from the body at a 90 degree angle. While leverage is most difficult with fully extended elbows, a slight bend is useful as it allows the lifter to maintain awareness of this.
   Section::::Variations.:Weighted push-ups.
  
  Progressively overloading classic push-ups using barbell plates, resistance band (or any form of weight). The load is positioned on the upper back. A very effective exercise not commonly performed because of the difficulty to load the human body in that position. 
  
  Section::::Variations.:Planche push-ups.
  