There's overlap between strength & size, but also some variance. The best combination for growth is strength gained using moderate reps and moderately heavy weights. Below a certain threshold around 6 reps, the increased weight puts inordinately more stress on the connective tissues like ligaments and joints, and shifts the priority of muscle fiber recruitement. Muscle has both high and low twitch fiber; extreme rep ranges enlist these two extremes.
A moderate number of reps engages a wider variety of muscle fiber, and generates higher production of lactic acid. Intuitively you might notice that there is less of a burn from doing lower reps, which is symptomatic of that. That then leads to differences in testosterone secretions that are associated with tissue growth.
Specifically, from studies:
Although heavy resistance does innervate high threshold motor units, serum testosterone levels are increased through moderate to high volume of exercises. This is achieved through multiple sets, exercises, and a moderate repetition range (around 8-10 repetitions), with short rest intervals (between 30 seconds to 1 minute).
For gains in muscular size, smaller motor units need to be recruited first in each set of exercise. As the set progresses in intensity, larger units will then be recruited. If the low threshold motor units are inhibited to recruit the high threshold motor units for explosive movements (as in powerlifting), the low threshold units that are not activated will not undergo hormonal adaptations.
This is because of the size principle of muscle fiber recruitment. Since motor units are recruited in an orderly fashion (from low threshold to high) and can span a range of muscle fiber types (Type I and Type II), then a moderate range of repetitions must be used to recruit the entire spectrum of fibers. This recruitment pattern allows the full spectrum of fibers to adapt to the training by increasing sensitivity to circulating anabolic hormones.
After a muscle has been subjected to intense stress through maximal force contractions over a moderate repetition range, hormones begin the growth process and muscle remodeling. Growth hormone plays a vital role in adapting to the stress of resistance training. Growth hormone levels can be increased through resistance training through high intensity (10 repetitions coupled with heavy resistance) with three sets of each exercise (high total workload) and short, one minute rest periods. Once the levels are elevated, a cascade of events occur; decreased glucose utilization, increased amino acid transport across cell membranes, increased protein synthesis, increased utilization of fatty acids, increased lipolysis (fat breakdown), enhanced immune functions, and a promotion of compensatory renal hypertrophy.
Why Moderate Reps Stimulate Optimal Hypertrophy
Evidence suggests that moderate repetition sets provide an optimal stimulus for growth in the fast twitch fibers, while high repetition sets may optimize the hypertrophy process in slower twitch fibers. Reasons follow. First, sets which fall within a 1 to 5 repetition continuum will most likely cause the participant to fail due to neurological signaling problems before an optimal stimulus for muscle growth can be induced (1, 2). Secondly moderate repetition sets (6-12) take full advantage of human recruiting systems. In general, the nervous system will recruit lower threshold fibers first and enlist high threshold fibers progressively as the set continues on. By the end of a set all available muscle cells have been brought into play (3). Thirdly the release of anabolic hormones is highest after these types of sets (4, 5, 6, 7,
.
A greater circulation of anabolic hormones in the body, results in greater adaptations to imposed demands. Interestingly enough, much evidence supports the postulate that lactic acid production can be very conducive to the release of hormones such as testosterone and growth hormone ( 3, 4, 5 ). Lactic acid is a by-product of glycolysis (6). This is the energy system that is used most heavily during 30-90 seconds of work. Glycolysis is directed by a series of enzymes, which comprise what is known as a chemical pathway. The enzyme lactate dehydrogenase is more active than any enzyme in the pathway. Therefore the more the participant relies on this pathway, the greater the build up of lactate will be. Hakkinen et al. found that blood lactate concentration during exercise correlated significantly (P < 0.01) with the increase in serum GH concentration. While Lu et al. in the Journal of Sports medicine found that increased plasma testosterone levels in males during exercise is at least partially a result of a direct stimulatory effect of lactate on the secretion of testosterone by increasing testicular cAMP production. Consequently blood lactate levels rise highest in moderate sets, with moderate rest. As opposed to low repetition, relatively high rest sets.
Power lifting type movements have a greater reliance on the creatine phosphate (PC) system, which is used for, extremely low repetition, high intensity sets. Note that intensity in this light is in reference to percentage of the athlete’s one repetition maximum. The PC system does not result in lactic acid production and as a result is frequently referred to as the A Lactic anaerobic system.
