This has to do with specificity.
The body will respond with a mix of strength and size (also hypertrophy) to any training regimen that represents a progression in stimuli for the particular goal. However, if you use 5x5, your strength gains will be more significant than size and the size gains will not accumulate for very long. Meaning the hypertrophy gains (size gains) will plateau long before the strength gains on this program.
Training specificity refers to the point that if you want a specific result, the body is most responsive to training that is specific to the goal.
If you want to run faster, you practice and train specific to the gaol for best results.
Any time you change your routine and the body has to adapt, you are likely to experience muscle gains (not all the time, the progressions do have to meet a few rules), even if the change is to a strength routine.
Below I include some specific research on the topic if you want to understand exactly what happens.
Any questions?
There is a HUGE amount of research that has replicated this over and over again.
These will highlight 2 recommendations I commonly make. The first being that the various ranges of sets/reps give different responses. The second being that one should cycle through training, beginning with endurance work, then strength, then hypertrophy... (as one form of progression)
First some other notes: Training upper body only (no lower body progressive hypertrophy/strength training) resulted in greatly diminished muscle hypertrophy.
Resting 2 or 5 minutes showed no change in results of strength training.
In hypertrophy training, shorter duration rests resulted in increased hypertrophy compared with longer rests.
In hypertrophy training high set volume (6) did not result in additional muscle growth or hormonal response over moderate set volume (4). Though it was higher than for low set volume (2).
In strength training with 5 reps/3 minute rests, 2, 4, or 6 sets resulted in no change in hormonal response.
Frequent program variation (daily) causes more lean mass gains than infrequent (4-week) program variation.
Faster movements do stimulate more muscle, demonstrate increased rate of fiber recruitment (power), and strength. However, as there is the potential for adaptation, variance in training pace can alleviate the adaptation.
Research Highlights: Muscular adaptations in response to three different resistance-training regimens: specificity of repetition maximum training zones
European Journal of Applied Physiology, 2002 Nov;88(1-2):50-60. Epub 2002 Aug 15.
Subjects were divided into four groups:
- a low repetition group performing 3-5 repetitions maximum (RM) for four sets of each exercise with 3 min rest between sets and exercises,
- an intermediate repetition group performing 9-11 RM for three sets with 2 min rest,
- a high repetition group performing 20-28 RM for two sets with 1 min rest,
- a non-exercising control group
Maximal strength improved significantly more for group 1 compared to the other training groups.
Maximal Endurance significantly increased at the end of the study for only group 3.
All three major fiber types (types I, IIA, and IIB) hypertrophied for Group 1 and Group 2.
No significant increases were demonstrated for either Group 3 or 4.
The percentage of type IIAB fibers increased for all three resistance-trained groups 1-3.
Muscular adaptations to combinations of high- and low-intensity resistance exercises
The Journal of Strength and Conditioning Research, 2004 Nov;18(4):730-7
Tested long-term effects of resistance-training regimens with 3 types of regimens on serum growth hormone (GH) concentration and muscular size cross sectional area:
- hypertrophy type medium intensity (approximately 10 repetition maximum [RM]) short interset rest period (30 s)
- strength type - 5 sets of high-intensity (90% of 1RM) and low-repetition exercise
- combi-type - and a single set of low-intensity and high-repetition exercise added immediately after the strength-type regimen
Postexercise increases in serum GH concentration showed a significant regimen dependence:
hypertrophy-type was greater than combi-type which was greater than strength-type
Muscular size cross sectional area showed a significant regimen dependence:
hypertrophy-type was greater than combi-type which was greater than strength-type
Skeletal muscle fiber area alterations in two opposing modes of resistance-exercise training in the same individual.
European Journal of Applied Physiology, 1990;61(1-2):37-41.
Two groups took turns participating in both of two 7.5-week regimens. Muscle samples were analyzed for area changes.
- performing a muscular strength program (high-resistance, low-repetition)
- performing a muscular endurance program (low-resistance, high-repetition)
Group A did Program-1 followed by Program-2
Group B did Program-2 followed by Program-1
The results showed that the Group-B progression maximized muscle fiber area growth continually, while Group-A increased then decreased muscle fiber area.