Optimal Rest Between Workouts? Despite Inter-Personal and Exercise-Specific Differences 72h May be a Valid Rule of Thumb - Especially for Compound Movements

As usual, there is no one size fits it all answer when it comes to the "optimal" inter-workout rest times - 72h in-between workouts w/ compound movements does yet seem reasonable.

As Korak et al. point out in their latest research paper, previous studies yielded diverse results, when it comes to the "optimal" recovery time between resistance training sessions: In fact, there is a study for all recommendations ranging from 1-7days between resistance training exercise bouts may be needed for replication of previous performance.

As general guidelines, the National Strength and Conditioning Association (NSCA) states that increased recovery time is needed between heavy lifting days and that upper body musculatures recovers faster than lower body musculature and single joint lifts require less recovery time than multi-joint lifts (NSCA. 2008).
Korak et al.'s "careful review of the literature cited in the NSCA guidelines" does yet reveal that most of the references are based on anecdotal evidence in older review papers or other textbooks and no quantitative evidence of recovery patterns were collected in the investigations cited supporting upper body versus lower body recovery or single versus multi-joint lift recovery.

"Recent investigations have sought to quantitatively determine the number of days needed for recovery to occur. While these investigations have extended the knowledge concerning lifting recovery as a whole, they have not delineated if discrepancies exist between multi-joint, single-joint, upper body, and lower body" (Korak. 2015).

Accordingly, Korak et al. conducted a study that was designed to quantify muscle recovery patterns between single- (SJ) and multi-joint (MJ), and upper body (UB) and lower body (LB) exercises at 24 and 48 h.

Table 1: Overview of the lifting protocol used in the study at hand (Korak. 2015)

A secondary objective was to examine the efficacy to self-evaluate recovery using the classic perceptual subject ratings of perceived exertion scale (RPE)  and the more novel perceived recovery scale (PRS). Based on the data Korak et al. hoped to be able to finally answer the question:

How much recovery is actually needed between two resistance training sessions?

To this ends, 10 recreationally strength trained college age males (26 ± 6 years) were recruited. The subjects had to complete 2 sets of 10 exercises. At an intensity equal to 85% of their 10-RM they had to perform 8 reps for first set of the exercise, which was intended  to induce standardized fatigue before the subsequent set to failure. For the second they performed 10 reps to failure at the predetermined 10-RM.

"The protocol was replicated during two additional sessions with days of rest (either 24 or 48h) between the next two lifting sessions serving as the independent variable. A counter-balanced crossover design was used. Half of the participants repeated their workout 24 h after the baseline session and rested for 48 h before their fourth and final session (e.g. baseline Monday, 24 h session on Tuesday, and 48 h session on Thursday). The other half completed their third session 48 h after baseline testing and their fourth and final session 24 h later (e.g. baseline Monday, 48 h session on Wednesday, and 24 h session on Thursday). Participants were instructed to refrain from other exercise, alcohol, and to maintain regular diet and sleeping patterns from 48 h prior to their baseline testing session until completion of the study" (Korak. 2015).

The same sequence of exercises was incorporated in the baseline and all treatment sessions. All participants completed 10 different exercises. Resistance exercises included: flat barbell bench press (BP), seated dumbbell military press (MP), barbell dead lift (DL), machine leg press (LP), knee extension (KE), machine triceps extension (TE), dumbbell side raises (SR), machine chest fly (CF), and seated machine hip abduction/adduction (HipAB/AD).

Figure 1: Box and whisker plot comparisons of cumulative means for Δ in repetitions from baseline for upper body (UB), lower body (LB), multi-joint (MJ), and single-joint (SJ) exercises at 24 and 48 h (n = 10; middle line = median; top and bottom boxes represent 2nd and 3rd quartiles; error bars represent min and max scores). Red signifies "on average not recovered", grey signifies "on average recovered" and green indicates "supercompensated", i.e. on average participants did a minimal amount of extra reps (Korak. 2015).

The data in Figure 1 shows how the recovery times (24h and 48h)  affected the number of reps the subjects were able to perform. It's easy to see that the short rest period of only 24h lead to significant decreases in the number of reps the subjects were able to perform in almost all subjects.
If you scrutinize the data in Figure 1, you will also notice the following in part surprising revelations..

• the performance decline was most pronounced for the multi-joint movements (BP, MP, DL, LP) and least pronounced if not non-existent for the single joint exercises,
• there was a tendency for the lower body to recover faster than the upper body that stands in contrast to the broscientific advice that training legs would require longer recovery periods,
• the recovery after lower body exercises was in fact so rapid that there was a measurable supercompensation effect (=subjects did more reps than before) even after only 48h of recovery in a handful of subjects.

Due to the large inter-subject variability and in view of the fact that the effects were tested only acutely, the scientists suggestions that "72 h of recovery should be implemented for multi-joint barbell lifts targeting the same muscle groups in slower recovering lifters" does still have to be considered a "rule of thumb".

Figure 2: Percentages of participants classified as recovered A from multi-joint (MJ), single-joint (SJ), upper 2 body (UB), and lower body (LB) exercises at 24 and 48 h when ∆ in reps from baseline was averaged 3 based on exercise type (Korak. 2015).

Furthermore, it's possible that you belong to the lucky 30% of trainees who perform the same number of reps after only 24h of recovery from an intense multi-joint exercise (Figure 2, left) and still need extra time to recover to make progress in terms of strength and size gains. Next to inter-individual variations the significance of the results of the study at hand are thus hampered by the assumption that being able to perform the same number of reps on a given exercise after a certain amount of rest signifies "optimal" recovery.
References:

• Jones, Eric J., et al. "Stability of a practical measure of recovery from resistance training." The Journal of Strength & Conditioning Research 20.4 (2006): 756-759.
• Korak, John A., James M. Green, and Eric K. O'Neal. "Resistance Training Recovery: Considerations for Single vs. Multi-joint Movements and Upper vs. Lower Body Muscles." International Journal of Exercise Science 8.1 (2015): 10. 
• McLester, John R., et al. "A Series of Studies---A Practical Protocol for Testing Muscular Endurance Recovery." The Journal of Strength & Conditioning Research 17.2 (2003): 259-273.
• National Strength and Conditioning Association. Essentials of Strength and Conditioning-3rd Edition. Champaign, IL: Human Kinetics, 2008.