What is the correct way for the leg to be at the top of the pedal stroke; from the hip to the knee should be straight, or declined towards the knee? Or it doesn’t matter for efficiency, it’s just a personal comfort thing?
Leg slightly bent at knee when at the bottom of the stroke is roughly best. A better approximation is that the seat should go up if the front of the knee gets sore before the back does, and down if it’s the other way. If you can adjust the crank arm length too, you want it as long as possible without causing knee pain.
The leg should be slightly bent when the crankarm is at the bottom position (with the ball of the foot over the pedal spindle). Basically, your hips should not rock back and forth while you are riding, if they are the seat is too high.
This site is a good starting point.
In watching people ride, I have come to the conclusion that most recreational riders have their seats too low. Their legs come nowhere near straight on the down stroke.
As I raised my seat, I found I got more power.
YMMV
This is true. It’s understandable though; conventional bicycles are designed so that if the seat height is correct, you can’t put your feet flat on the ground. You have to learn to support yourself on your toes while stopped. (Or buy a “crank-forward” bike.)
Another “mistake” I see often is to push the pedal with the middle (arch) of the foot. You should use the ball of your feet instead; that’s the part of your foot designed/evolved for pushing and kicking.
The measurement that really matters is the one described above, leg almost but not quite straight at the bottom of the stroke, and no sideways rocking in the saddle. The angle of the leg at the top of the stroke is a byproduct of this (and of the crankarm length), and is not important in and of itself. Generally, the less bend in the knee, the more powerfully and efficiently you can pedal, and the proper adjustment minimizes said bend as much as possible throughout the stroke. For an extreme example of too much bend, try riding a little kid’s bike or trike where your knees approach your chest. Damn inefficient and awkward as hell, to boot.
I’ll bet it’s because they never adjust the seat after they buy the bike.
That’s because you’re taking advantage of the quadriceps’ full range of motion. Those are powerful muscle groups and if they’re only being partially contracted during a pedal push, their full ability won’t be realized.
I agree. For me, seat height is a trade-off between better power and efficiency (higher) and comfortable stopping (lower). Since I ride mostly urban, the seat is a little lower than optimum for power.
Yeah, and a too-low seat is the ticket to knee problems, too…
I bought a new bike a couple of weeks ago and didn’t have the seat set correctly, then went for a hard ride. I’m still paying in pain.
Another common newbie mistake is using too high a gear. The thinking seems to be that more peddling is more work. Actually, it’s the other way 'round. All other things being equal, more-easy-pedals is easier than fewer-harder-pedals. Ideal cadence is a subject over which reasonable minds differ, but for a newbie I’d say 40 to 60 rpm is about right. Main thing is that you want a happy medium between spinning like a hamster in a wheel and doing the functional equivalent of leg lifts on a universal gym.
As for the OP, I agree with everyone else. Leg slightly bent, but not so high the hips rock. As for stopping, the best solution is to get used to dropping forward off the saddle; then you can easily straddle the bar with your foot/feet flat on the ground; if not, you got the wrong size frame.
I think your RPM range is a bit low. About 60 RPM is about right for a recreational rider. 40 is pretty slow IMHO
Of course what the hell do I know, I am always the last up the hill. 
As I said, reasonable minds can differ. 
But, try this. Make a cranking motion with your fists (as if they were pedals) and do it in cadence to one-elephant, two-elephant, three-elephant, etc. To my mind, that’s pretty fast for a recreational rider. In any event, what I often see is in the 20 to 30 rpm range, and I’m talking about level ground. Holding cadence on hills can be tougher, but most people have the sense at least to shift to their lowest gear for that. It’s on level ground that I mostly see people riding in too high a gear.
I don’t think 60rpm is too fast for a recreational rider. It’s only 2 steps per second, pretty close to normal walking pace. Maybe more of a brisk walk than a stroll, but nothing unusual.
For anything more than leisurely bike path riding, I’d recommend 90rpm as a goal. That’s 3 steps per second. Jogging pace, if you will.
Rather than counting one-elephant, two-elephant can I just look at the cadence displayed on my bike computer instead? Yes I am a geek.
When I am fat and haven’t been riding much 62 is very comfortable for me. If I have been training I can hold almost 80 for long periods.
No arguement about people using too high a gear.
Well, I spend 80% or more of my time in my bike’s highest possible gear. Even uphill, if I get a good run at it.
What I actually see a lot of is people on level ground using way too low a gear. They’re not moving forward very fast, but their feet are going like crazy. But I think these are usually people who simply haven’t learned how to shift yet.
Cranksets are generally available in 2 lengths: 170 mm and 175 mm. There are probably others, but I’m pretty sure these are the most common.
Sure about that? That’s only a difference of about .2 inches, hardly enough to bother with. Also, don’t you have to worry about the pedals hitting the ground in turns when putting a longer crankset on a bike that might not have been designed with that in mind?
If the length of the arm (each arm) changes by 5mm, the difference between the top and bottom of the pedal stroke changes by 10mm. The difference may not be immediately obvious, but it makes a noticeable difference in leg stress (i.e. “My legs are unusually sore after riding this bike; I wonder if it has longer cranks than my usual bike?”)
170 and 175 are indeed the most common crank lengths. 165 is a little less common - something your local shop probably won’t have in stock, but can order for you. Anything ouside this range, you need to buy from a specialty shop.
I don’t agree with the “long as possible” part. As you say, if the arms are too long it causes knee pain. There is no obvious down side to cranks that are too short, as long as you use appropriate (low enough) gears and keep a high cadence. I use 165mm on crank-forward bikes and 160, 155 and 153 on my recunbent bikes. (I’m 5’9" with 31" inseam.)