Biopac hardware could be an useful tool. This hardware can be used with many types of sensors according to your measurements.

Good luck.

Also take a look at CED equipment, they make amplifiers and digital to analogue converters. Also their Spike2 and Signal software is very good and widely used.

http://www.ced.co.uk/indexu.shtml

http://www.otbioelettronica.it/index.php?lang=en

The company is managed by former academic researchers, who are very helpful in developing hardware and software according to the research needs. There in excellent ratio between quality and price.

There are several different systems available as the previous contributors have mentioned.  Technically the most difficult part may be the sensors.  what type of data do you hope to collect.?  Do you want to record with particular movements in a controlled way or free moving over several hours or days?

Two books on electromyography (human EMG, but may be useful)

J. Basmajian and C. De Luca, Muscles alive: their functions revealed by electromyography, Baltimore: Williams & Wilkins Company, 1985.

C.J. De Luca, Electromyography. Encyclopedia of Medical Devices and Instrumentation: John Wiley, 2006.

Wow, there are plenty of suggestions. Thanks to everyone!

@Darrell we are interested in studying myotonia in the mouse model of myotonic dystrophy. According to 'classic' protocol, it will be a series of single needle insertions into hindlimb, forelimb, and thoracolumbar paraspinal muscles (Kanadia et al., Science 2003, 1978-1980). We want to record myotonic discharges upon electrode insertions, no long-tem monitoring of muscle activity is planned. In the paper authors use a concentric needle electrode. I'd greatly appreciate if you could expand your comment about the sensors in context of our particular problem. Thank you very much!

Recording myotonic discharges is very simple, but hard to quantify. I would suggest a standart comercial clinical EMG machine and concentric EMG needle electrodes. It would be nice to have long "live play" recordings with audio. Miotonic discharges are better heard than seen ("dive bomber").

In our lab we use a combination of Digitimer filters and amplifiers (Neurolog System); and analogue to digital convertor with Signal software (both from CED). CED provides valuable trainings on the use of their equipment and softwares, 

For more details, check the links below

http://www.digitimer.com/index.htm

http://www.ced.co.uk/indexu.shtml