question: When doing body weight exercises like lunges, why does putting my hands on my knee make it easier to stand up? My legs are still lifting my body weight, so I’m confused as to why such a drastic difference.
comment: It's important to distinguish that the work a leg bone does versus the work that leg muscle does. If you are pushing down on your knee with your hands, then it is your arm muscles that are lifting the upper part of your body. It's true that the load goes through your shin bone, from the knee down to the ankle, but the leg *muscles* didn't have to do any work for that lifting. That leaves less weight for the leg muscles to lift. The bone, at least in most people, is way, way strong enough to easily hold your body weight, so that's not the issue. It's the muscle that is the weak link. But, you might ask, aren't my leg muscles way stronger than my arm muscles? I still don't see how they could help so much. Problem is, when you are down in a lunge, your legs are at a funny angle which makes it harder for them to do their job. Like a lunge half-way down is way harder than a squat half way down. The arm muscles right on your knee are at a better angle. Source: studied mechanical engineering in college, so have a sense of forces and angles.
passages: With resistance training it's not recommended to do forward lunges in place, this is due to the shear force it can place on the rear knee once the front leg no longer provides support.
  
  Section::::Yoga.
   Lunge (exercise)
  
  A lunge can refer to any position of the human body where one leg is positioned forward with knee bent and foot flat on the ground while the other leg is positioned behind. It is used by athletes in cross-training for sports, by weight-trainers as a fitness exercise, and by yogis as part of an asana regimen.
  
  In difference to the Split squat exercise, during the lunge the rear leg is also activated.
  
  Section::::Strength training.
   Thigh weights are the most reasonable form of resistance. The location of the mass more readily duplicates the natural fat-storage mechanism of the human body and being closer to the core. In leg raise exercises, it allows more activation of the hip flexors (and abdominals) without putting more strain on the quadriceps muscles for extension, making it good for sports-specific training on movements like knees and jumping. The greater area and safe location allow it to handle much more weight. For those with wide thighs, such as bodybuilders with large quadriceps, or people with large amounts of fat stores on the inner thigh, it may cause chafing. If worn on both legs, however, the chafing would be between the weights and only damage them, possibly only chafing with a lack of tightness.
   Side-lying leg raises target the hip adductors within the inner thigh. They are performed by lying one the side with support coming from one hand placed on the ground and the opposite foot placed on the ground in front of the lower leg. With the top leg slightly flexed and resting, the bottom leg is adducted upwards against gravity and relaxed down to the ground.
  
  Section::::Angles.:Seated.
   BULLET::::- Stance in volleyball. (See partial squats gallery for image.)
  
  For more partial squatting images see here. 
  
  Section::::Partial squat.:Lunging.
  
  A lunge is a variation of the partial squat where a leg is moved forwards with the knee bent but the other remains straight thus moving the upper body forward in line with the bent knee. For example:
  
  BULLET::::- the snooker playing posture
  
  BULLET::::- the fencing lunge – (see partial squats gallery for image)
  
  BULLET::::- the lunge in figure skating – (see partial squats gallery for image)
  
  BULLET::::- the lunge as a weight training or strength training exercise.
   This movement can also be done with "ab slings" which hold the humeri in ~90 degrees of shoulder flexion. This allows one to do a more traditional crunch by bringing the knees up to touch the elbows. It is however possible to assist in this movement by using the lats and other muscles to perform shoulder extension.
  
  Section::::Force.
  
  The demands of force on the muscles can be increased through altered posture or apparatus.
  
  Section::::Force.:Joints.
   The knee has a more complex movement pattern compared to the hip. At heel contact the knee extensor and flexor muscles co-contract to provide stability for the knee joint since it is almost maximally extended at that point in time. Shortly after, as the foot becomes flat on the ground, the knee gradually flexes approximately 10-15 degrees reaching the maximum at about 15% of the gait cycle. This small amount of knee flexion is controlled eccentrically by the knee extensor muscles which serve the purpose of cushioning the rate of loading on the lower limb and preventing excessive knee flexion. Following through to mid-and terminal stance, the knee gradually extends with concentric activity of the knee extensor muscles and approaches near full extension as heel off occurs (30-40% of gait cycle). At this point the knee flexor muscles concentrically flex the knee again for swing phase. The maximum knee flexion that occurs during swing is about 60 degrees.
   Bridging exercises are done with a flexed knee to lessen the stretch on the hamstring (a knee flexor) and focus the hip extension work on the gluteus maximus. In that same respect, the reduced knee flexion makes plantar flexion work comparable to a seated calf raise, due to the lessened stretch on the gastrocnemius (like the hamstring, also a knee flexor).
  
  Since the pelvis is in the air, its weight can be shifted onto the feet allowing greater resistance.
   Bent-knee calf raises are frequently done in a seated position for comfort. Since the weight of the upper body is rested on the seat, resistance is frequently added. Using bodyweight, one leg could be draped across the other (through external rotation) to exercise unilaterally and double the weight lifted.
   It can also be executed by using a weight, such as a barbell, to provide resistance to the action of the calf muscles. The exercise is performed from a seated position while the weight rests on the upper leg, just above the knee. The person engaged in this exercise lifts the weight by pushing down on the balls of the feet.
   Only the muscles on the front and on the back of the thighs can rotate the pelvis forward or backward while in a standing position because they can discharge the force on the ground through the legs and feet. Abdominal muscles and erector spinae can't discharge force on an anchor point while standing, unless one is holding his hands somewhere, hence their function will be to flex or extend the torso, not the hip.
   Human knee joints are enlarged for the same reason as the hip – to better support an increased amount of body weight. The degree of knee extension (the angle between the thigh and shank in a walking cycle) has decreased. The changing pattern of the knee joint angle of humans shows a small extension peak, called the “double knee action,” in the midstance phase. Double knee action decreases energy lost by vertical movement of the center of gravity. Humans walk with their knees kept straight and the thighs bent inward so that the knees are almost directly under the body, rather than out to the side, as is the case in ancestral hominids. This type of gait also aids balance.
   A lunge can be performed using bodyweight alone. However, weight trainers may seek to increase the difficulty using either dumbbells or kettlebells held in each hand, or a barbell held atop the neck and shoulders. Grip strength may be an issue with the dumbbell lunge so practitioners may prefer the barbell lunge.
  
  As a variation, plyometric lunges (also known as split squat jumps) can be performed by jumping explosively between lunge positions.
   An analogy would be a ball that volleys back and forth between two players without touching the ground. The muscle exertion required to maintain an aligned standing posture is crucial but minimal. Electromyography has detected slight activity in the muscles of the calves, hips and lower back.
  
  Section::::Control.:Core muscles.
   Thousands of people suffer from knee injuries, and because the knee supports the body weight during the stance phase while walking, more active orthoses are being made to relieve weight off the knee while in the stance phase.
   This is an awkward exercise due to the reduced stability and difficulty in adding resistance. If one were to raise a leg, the use of the arms to avoid falling sideways would often by necessary. If one were to place a padded barbell on the upper quadriceps to add resistance, this would make the arms less able to stabilize, requiring core stabilization.
  
  Section::::Straight-knee.
  
  Pushing with the foot with a straighter knee stretches the gastrocnemius more, these movements incorporate it better. The soleus still contributes, usually allowing people to lift more weight.
  
  Section::::Straight-knee.:Standing.
   Leg raises may also be performed with the addition of a weight. This is usually held between the feet and may be a small dumbbell or medicine ball. A large amount of weight can be lifted if the "knee raise" style is used in a standing position. This exercise is also known as a thigh raise and involves the holding of a barbell, dumbbell or sandbag upon the thigh. The exerciser raises their thigh and lifts the weight before placing their foot on the ground again. A similar range of movement against resistance can be achieved with a multi-hip machine. As it engages and strengthens the hip flexors it is a useful exercise for sprinters and jumpers.
   Supination is the opposite, and occurs when the foot impacts the ground and there is not enough of an “inward roll” in the foot’s motion. The weight of the body isn’t transferred at all to the big toe, forcing the outside of the foot and the smaller toes which can't handle the stress as well to take the majority of the overweight instead.
   There are a number of exercises that can be done to strengthen the lower leg. For example, in order to activate plantar flexors in the deep plantar flexors one can sit on the floor with the hips flexed, the ankle neutral with knees fully extended as they alternate pushing their foot against a wall or platform. This kind of exercise is beneficial as it hardly causes any fatigue. Another form of isometric exercise for the gastrocnemius would be seated calf raises which can be done with or without equipment. One can be seated at a table with their feet flat on the ground, and then plantar flex both ankles so that the heels are raised off the floor and the gastrocnemius flexed. An alternate movement could be heel drop exercises with the toes being propped on an elevated surface—as an opposing movement this would improve the range of motion. One-legged toe raises for the gastrocnemius muscle can be performed by holding one dumbbell in one hand while using the other for balance, and then standing with one foot on a plate. The next step would be to plantar flex and keep the knee joint straight or flexed slightly. The triceps surae is contracted during this exercise. Stabilization exercises like the BOSU ball squat are also important especially as they assist in the ankles having to adjust to the ball’s form in order to balance.
   Knee-on-stomach
  
  Knee-on-stomach, or knee-on-belly, knee-on-chest, knee-ride, knee mount ("uki-gatame", 浮固, "floating hold" in budō), is a dominant ground grappling position where the top combatant places a knee on the bottom combatant's torso, and usually extends the other leg to the side for balance. This position is typically obtained from side control, simply by rising up slightly and putting a knee on the opponent's stomach or chest.
  
  Section::::Use.
   In Chaturaṅga Daṇḍāsana the hands and feet are on the floor, supporting the body, which is parallel to and lowered toward, but not touching, the floor. It looks much like a push up, but with the hands quite low (just above the pelvis), and the elbows kept in along the sides of the body.
  
  When performed correctly, it can help prepare the body for arm balance asanas by strengthening important muscles and promoting good form.
   Balance may become a difficulty with free-standing calf raises, especially with one-legged variations. Due to this, it is common to hang on to something or lean the hand against a wall for stability. They are also performed using Smith machines or using machines specially designed for calf raises with padded anchors for the weight that rest on the shoulders.
  
  Section::::Straight-knee.:Leg press.
   Unweighted stances are those where half of one's bodyweight is on each foot. Forward-weighted or backward-weighted stances place most of body weight on either the rear or the lead foot, respectively. Forward-weighted stances are more aggressive, allowing stronger punches and kicks because one's weight is already forward. Alternatively, backward-weighted stances are more defensive, allowing one to fall back more easily.
   A downward force applied by the torso to an already bent leg can help to generate a powerful springing potential in that leg. To activate this potential a sudden upward action which incorporates the straightening of the leg is undertaken. For instance, in a boxing stance where the left foot is positioned in front of the right foot, the boxer can position himself so his bodyweight is positioned forward over his lead left leg. If he then throws a lead left hook this will cause a fast and powerful straightening of the left leg which increases the punches overall power. If there was no downward pressure on the lead left leg prior to throwing the hook then this spring effect would be substantially reduced. This method of generating leg spring in order to increase power production can also be incorporated into jumps and this is usually done so instinctively. The jumper bends their legs, applys a downward pressure on their bent legs, and then powers upwards into the jump. Due to the spring potential generated prior to actually taking the jump, the leg straightening which is incorporated into the jump is more powerful than it would be otherwise. For rapid and repetitive sports, such as sprinting, it may be difficult for an athlete to consciously employ leg spring for every action. However, the instinctive usage of a leg spring action may be encouraged by specific training of the associated musculature, such as that of the legs, hips, and core. In this case, the athlete would be trained to emphasise the downward pressure on the bent leg prior to its straightening as part of the next step. 
   The muscles responsible for the movement of the knee joint belong to either the anterior, medial or posterior compartment of the thigh. The extensors generally belong to the anterior compartment and the flexors to the posterior. The two exceptions to this is gracilis, a flexor, which belongs to the medial compartment and sartorius, a flexor, in the anterior compartment.
  
  Section::::Muscles.:Flexors.
  
  Posterior compartment
  
  Medial compartment:
  
  Section::::Muscles.:Blood supply.
  