The muscles located at the top and front of your hips are your hip flexors (because they raise your thigh when flexed) and work cooperatively together with the muscles at the back of your calves (gastrocnemius, soleus, tibialis posterior). For this reason when there is a problem with your hip flexors it effects the calves and vice versa.
When you get a hip or calf muscle injury it will usually effect one leg more to begin with and will restrict how long a stride you can take. This happens because when your hip flexors (muscles at the front and top of your thigh) are stiff they are unable to stretch fully when you move your thigh backwards when you are walking. In the case of stiff or injured calf muscles it becomes more difficult to move your leg backwards and make proper contact with your heel on the ground and they will shorten your stride to.
There is an easy way to check and see if this is happening with you if you are unsure. The next time you walk on a straight path with evenly sized and spaced squares of pavement (see postural tips in book) see if your feet land in equal postions on each pavement square as you walk on, a bit like the games kids play when they are trying not to step on the lines.
If you find (as most will to some extent) that it is difficult to keep stepping in an identical place on each paver with your right and left feet, the chances are that the hip and calf muscles are stiffer on one leg than the other. To begin with there maybe no discomfort in your short-stepping leg atall and if you are lucky you may be able to eventually equalize the relative tensions in your right and left legs through daily stretching exercises alone.
If you ignore your uneven stride and do not use corrective stretching exercises each day you will progressively stiffen further until you do start to notice pain when walking in your hip/groin area and your calf (or maybe even injure them). If you are a regular runner or stretcher you are more likely to notice a developing problem early on but if you are not you might not know how stiff your leg muscles are getting until you go to sprint across a busy road during break in the traffic.
I have torn my own calf muscles on 2 occassions doing just this and I have known plenty of others who have done this too. It is painful and inconvenient but preventable.
You can do alot to safe guard your own leg muscles from tearing with regular massage and stretching. Your lower back will benefit from these strategies too because your hip muscles join and move the bones in your thigh, pelvis and lower back, infact you may notice lower back pain before your hip or calf feels stiff.
Give it a go today and see if you can easily land your feet in even positions on pavement squares walking at a normal pace, if you cannot it is time to do something now.