`In 1950´s T. Hettinger and E. Muller (1953, 1955) established that
a daily effort of 2/3 maximum, for a period of six seconds, would increase
strength by approximately 5% per week.
One reason for such improvements is the enhanced activation of motor
units during an isometric muscle action.
One can recruit almost all motor units during a maximal isometric contraction.
As stated by sports scientist Yuri Verkhoshansky, "each 6-second isometric
contraction is in effect equal to many dynamic contractions (ballistic type) in
which maximal force is of a duration no more than 0.1 seconds" (1977).
Recent studies however suggest that strength increases over a range of 15 to 20
degrees on either side of the training angle. This potential limitation is easily
overcome by training several angles (ex. starting point, mid-point, end-point)
of a specific movement.
Verkhoshansky listed the following negative aspects of isometric training:
- Isometrics fatigue the nervous system
- Isometrics have a harmful influence on the cardiovascular system
- Isometrics decrease coordination and speed of movement
The development of absolute strength however has a negative influence on speed.
For this reason, we must focus our attention towards explosive strength.
When training explosive strength, tension should be generated with
maximum speed (Verkhoshansky, 1977).
Shorter sequences (ex. 3 to 6 seconds) are also recommended when training for
explosive strength. Longer sequences are commonly used when training for
muscle mass increases. For example, it is common for individuals to hold
isometric positions for 30 to 60 seconds when training for size. The increased
time under tension is thought to produce greater mass gains.
And with proper sequencing of work and rest, coupled with time for breathing,
relaxation, and stretching, one can eliminate all negative effects.`
From: Infinite intensity by http://www.rossboxing.com/thegym/thegym25.htm
Adding hooks with bands would be a great addition or as combination with the iso holds at different ranges to improve speed.
As the hook is involving the stretch reflex of the pecks and shoulders (stretch by the initial torque/hip rotation prior to the fist and arm beginning to travel), adding some heavy bench pressing would have an additional impact - no bulking just some dense muscles with high sets and low reps.