Resolving the weight vs. reps issue depends on some practical concerns and understanding fatigue. Fatigue is the inability to maintain force during repeated muscle contractions and occurs inevitably. Although some state to avoid fatigue during lifting, this makes no sense.
Instead we should reach the type of fatigue that leads to more strength and size. The limiting factor must be tension, the main stimulus for size and strength. This does puts an upper limit on the reps, but this is more liberal than some experts may have you believe. For reasons discussed here, most should have a balance between weight and reps. Once you achieve this, how hard you work is most important.
Fatigue does not come from a single factor but is complex. It can come from…
- energy system failure.
- accumulation of metabolic by-products.
- the nervous system no longer firing signals.
- muscle fiber contractile failure.
- a desire to stop.
Each one of these areas can be drilled down further and this makes it difficult to determine the exact causes for fatigue.
Heavy and light are relative. Heavy for one trainee may feel light for another trainee. You need to gauge how heavy it feels relative to what that person could achieve from one maximum rep. Too close to this figure or too far away can each bring risks. It make sense then to avoid either extreme, especially as a beginner.
For most trainees, too light would be any rep count over 30. Even much beyond 20 may start to bring problems unless longer pauses between reps occur. Too light of a weight will invite fatigue that competes with strength and size development.
Even with a duration much past a minute, the buildup of fatigue by-products could cause task failure. During glycolysis, lactic acid builds up that disrupts muscle contraction. This could end a set before the muscle produces enough or any tension in the fast-twitch fibers meant for heavy or explosive actions.
A light weight will recruit the slow-twitch fibers into service. With too light of a weight, they recycle back into the process as some activate and others recover. This prevents engagement of the fast-twitch fibers and usually occurs during activities such as long distance running that stress the aerobic system. Higher reps also makes it difficult to remain tense in the stabilizer muscles if the set lasts too long.
Too heavy would be any rep count below 3. Although carrying some risks, you likely could go below this figure and still experience more success versus going too high. The fatigue would remain proper for building size and strength.
A minimum time frame seems required because the body cannot recruit the maximum amount of fibers in a very brief instance. The Golgi tendon organ, a monitor located in our muscles, may prevent too much tension at once. This may impose a minimum time for contraction. Try pushing against a wall for just a second. Even if done with all of your might this will fail to build much strength.
Some time allows the tissue to warm-up. Heavy loads also carry much more risk due to the high force per rep. This can place more stress on the body and allow less room for error with form. A form issue that ends the set would also reduce the stimulus more greatly versus a high rep set.
The Repetition Maximum Continuum
Although there are some advantages for higher reps and lower reps, most will do well to strike a balance, at least at first. Instead, many authorities support the rep max continuum, or a range of reps meant to give specific benefits. It typically appears like this:
- Strength and Power: very low reps (1-6)
- Hypertrophy (Size): moderate reps (6-15)
- Endurance: higher reps (15+)
This makes strength gains appear to occur in a narrow range but you can actually build the same amount of strength and size in all of these ranges.
Charts exist for converting multiple rep sets to singles and vice versa. These predict performance reasonably well. The strongest athletes can almost always lift a lighter weight for the most reps. This would not occur unless a relationship between low and high reps existed.
This simple test can allow you to know if a given rep range works for you. Remember that tension is the main stimulus for more strength and size. If you feel that you can no longer continue an exercise due to a lack of squeezing or tensing the active muscles, then you have performed the exercise for this purpose. If becoming out-of-breath or feeling too great of a burning sensation in the muscles instead limits you, then these factors interfered.
Although any exact time frames remain hard to give, the time required to complete the number of reps likely should not exceed 60 seconds. You may self-select any rep goal among 3-30 and likely achieve the same benefits toward strength and size, although higher reps may require longer pauses between reps. I recommend a beginner self-select within a tighter range between 6-15. After choosing this, the focus should change to increasing the weight.
After the trainee masters form on all exercises, the rep range can reduce or increase based on personal preference. Keep in mind that the body always changes to better fight fatigue. Although higher reps may invite less relevant fatigue, the body develops adaptations to handle this better. So use whatever rep range works for you. The more important factor once you choose within the liberal limits discussed is to work hard by training to failure. Motor unit recruitment shows this as correct.
Weight vs. Number of Reps
Once again, I suggest ending the set in less than 60 seconds, unless allowing for longer breaks between reps and therefore more time as well. Self-select and perform 3-30 reps (6-15 for beginners) to the best of your ability and seek to progress in weight. Modify this if necessary to suit your personal preference over time. Keep in mind that diet, sleep, stress, motivation, and other factors affect fatigue and results. These likely matter much more than any weight vs. reps issue.