Weight Training Definitions

Hypertrophy:
That's an increase in size.

Atrophy:
A decrease in size.

Isometric Contraction:
The muscle develops tension without change in the external length of the muscle ( static contraction ). Eg pushing against a solid wall.

Concentric Contraction:
The muscle shortens while developing tension. Eg the biceps muscle in the up phase of a barbell curl.

Eccentric Contraction:
The muscle lengthens while developing tension. Eg the quadriceps absorbing the impact of a jump landing.

Isotonic Contraction:
The muscle contracts with constant tension. In general terms free weights and pin loaded gym machines are referred to as isotonic equipment. Technically however this is a misnomer because in these exercises the muscle shortens with varying tension while lifting a constant load. It has been suggested that such training would perhaps be more suitably termed " dynamic constant resistance training".

Isokinetic Contraction:
Tension is developed by a muscle shortening at constant speed. Specialized equipment is required for isokinetic training eg. Cybex Isokinetic Dynomometer. The trainer exerts force against the machine which moves at a pre set velocity. Isokinetic equipment is also often termed accommodating resistance equipment because in using it it is possible for muscular tension to be maximal over the full range of movement.

Isolation:
Exercise using ONE JOINT and ONE MAJOR MUSCLE GROUP.

Compound:
Exercise using MORE THAN ONE JOINT and MORE THAN ONE MAJOR MUSCLE GROUP.

Repetition Maximum (RM):
Repetition Maximums (or RMs) indicate the Maximum Number of repetitions that can be completed with a given load, for example:

1 RM indicates a load that can only be lifted once. (Heavy Load).
5 RM indicates a load that can be lifted 5 times but not 6.
10 RM indicates a load that can be lifted 10 times but not 11 (Lighter Load).