Conditioning and work capacity: the evidence

CO-P-01Work window on a counted timed set

Prescription20–60 seconds of work, default 40.

Evidence gradeB

EffectThe included circuit trials clustered work intervals at 20 to 30 seconds with 10 to 30 seconds of recovery between them, across 45 studies and 897 trained participants [1]. MVIII's windows sit at the top of that range and above it, because this app's rests are longer than the meta-analysis median and a longer rest buys a longer leg of work.

PopulationTrained and recreationally trained adults.

Sources[1]

CaveatsThe band is programmed as a constant, not as a range, and that is the point. Every goal's window is a single value that no training phase, no member feedback and no equipment cap is permitted to move. A band the program climbed would make this week's rep count incomparable to last week's, which would destroy the only thing the member is being asked to track. See CO-F-01.

CO-P-02Rest between counted timed sets

Prescription20–120 seconds, default 40.

Evidence gradeC

EffectThe circuit literature used 10 to 30 seconds between exercises and found that low to moderate intensity with short rest produced the largest change in fat mass [1].

PopulationTrained and recreationally trained adults.

Sources[1]

CaveatsWider than the source, deliberately. The floor is 20 seconds, for the one goal built on denying recovery. The ceiling is 120 because two goals in this app answer to their own published rest entries, and a 20-second rest would put them outside a band the rest of their session sits inside: BB-P-05 puts hypertrophy rest at 60 to 300 seconds and PL-P-06 puts trained lifters at a 120-second floor. The spread is a scheduling decision rather than a finding, and the grade says so.

CO-P-03Steady conditioning finisher

Prescription10–20 minutes, default 15.

Evidence gradeC

EffectNo source prescribes a finisher length. What the evidence supports is the choice of steady continuous work over intervals for this outcome [4], and that circuit and continuous conditioning are low-cost additions to a lifting week [1].

PopulationAdults with excess weight, in an energy deficit.

Sources[4] [6]

CaveatsBoth ends are reasoned and neither is measured. The 10-minute floor is a judgement that anything shorter is not worth the member's attention. The 20-minute ceiling is a judgement that somebody already in a deficit is under-recovered, and that more work would spend the lean mass the deficit exists to protect [6].

CO-P-04Rate of loss above which supporting conditioning is programmed

Prescription0.7–1.4 % of bodyweight per week, default 0.7.

Evidence gradeB

EffectElite athletes losing at 0.7 %/wk gained 2.1% lean body mass across the intervention. A matched group losing at 1.4 %/wk gained none (−0.2%), at the same total weight lost and the same protein intake of about 1.6 g/kg/day [6].

PopulationElite athletes in an energy deficit doing strength training.

Sources[6]

CaveatsMVIII uses these two measured figures as its band boundaries, and computes the member's rate as a percentage of bodyweight rather than using the pounds per week they typed. Two pounds a week is 1.3 %/wk for a 150 lb member and 0.8 %/wk for a 250 lb one: one stored number describing two different situations. What the tier changes is the supporting conditioning and nothing else. It never makes the lifting easier, because the lifting is the intervention protecting the lean mass in the first place. # PART B — Findings

CO-F-01A rep count inside a fixed window is not self-evidently a comparison

FindingReps performed inside a work interval drift toward failure, and intra-set fatigue is not neutral. Across 37 studies and 735 participants, higher velocity loss was associated with greater hypertrophy (β = 0.006, 95% CI 0.001 to 0.012) but worse jump performance (β = −0.040, 95% CI −0.079 to −0.001), worse sprint times, and worse velocity against submaximal loads. Maximal strength was unaffected (β = −0.004, 95% CI −0.008 to 0.001) [2].

Evidence gradeC

Population735 resistance-trained participants (656 male, 69 female).

Sources[2]

CaveatsThe row this whole feature is arranged around. The reps at the end of a window are not the same reps as the ones at the start, so two conditions have to hold before this week's count can be set beside last week's: the window has to be identical, and the range of motion has to be identical. MVIII can guarantee the first and cannot observe the second, because reps and load are recorded and depth is not. So it does both of the things that follow: it freezes the window in code, and it tells the member before the set that a shorter rep is not a better score. Graded C rather than B on purpose: the velocity-loss literature concerns prescribed thresholds in traditional sets, not counted reps in a timed window, and applying it here is an inference.

CO-F-02Adding reps at a fixed load is a real progression, not a consolation

FindingForty-three participants were randomised to progress either by adding load at a fixed rep target or by adding reps at a fixed load, over eight weeks and two supervised sessions a week. Hypertrophy differences were negligible in most muscles (under 1 mm), with a modest edge to rep progression in rectus femoris (2.8 mm, 90% CI −0.5 to 5.8). Squat 1RM rose about 20 kg in both, with a slight and non-significant edge to load progression (2.0 kg, 90% CI −2.4 to 7.8) [3].

Evidence gradeB

Population43 resistance-trained participants, 38 completing.

Sources[3]

CaveatsThis is what makes a density set's rep count worth logging rather than merely worth counting. A member whose window count climbs at the same load has progressed in a way the literature treats as equivalent to adding weight, and MVIII's progression rule reads it that way: matching or beating last session's count at the same load earns the next load increment.

CO-F-03The format changes the stimulus, even at identical movements and loads

FindingTwelve participants performed the same three movements at the same loads under three configurations. The continuous as-many-rounds format produced the highest heart rate but significantly lower mean propulsive velocity; the every-minute-on-the-minute format produced the lowest heart rate and the least velocity loss, with bar speed improving across sets; the for-time format produced the greatest intra-set velocity loss [5].

Evidence gradeC

Population12 CrossFit-experienced adults (10 men, 2 women, mean age 31.5).

Sources[5]

CaveatsTwelve participants is twelve participants and the grade says so. It is included because it is directional evidence for something MVIII acts on: a fixed window with structured rest is a different thing from a continuous effort, and treating them as interchangeable conditioning would be wrong.

CO-F-04Interval work is not superior to steady work for fat loss

FindingEleven randomised controlled trials, 379 participants (194 interval, 185 continuous). Body-fat percentage: mean difference −0.55% favouring intervals, 95% CI −1.42 to 0.31, not significant. Abdominal visceral fat: standardised mean difference −0.05, 95% CI −0.29 to 0.19. Intervals were superior for VO₂max (SMD 0.30), fasting blood glucose (SMD −0.67) and total cholesterol (SMD −0.42) [4].

Evidence gradeB

PopulationAdults characterised by excess weight.

Sources[4]

CaveatsThe authors' conclusion, verbatim: "HIIT is not superior to CAT in reducing %BF or abdominal visceral fat in individuals characterized by excess weight." This is why a member with a lot to lose is programmed steady continuous work rather than more intervals, and why the app does not present the steady block as the harder or better option. It is the one they are more likely to finish, and for this outcome the published difference is not significant.

CO-F-05Circuit-based resistance training improves aerobic capacity as well as strength

FindingForty-five studies, 58 experimental groups (n=897) and 34 control groups (n=474). VO₂max +2.93 mL/kg/min (+6.3%), fat mass −1.04 kg, muscle mass +1.18 kg, bench press SMD 1.16 (+20.0%), leg press SMD 1.83 (+23.0%). Programs averaged 10 weeks (range 4–28) at 2–3 sessions per week and 6–14 exercises [1].

Evidence gradeA

Population897 trained and recreationally trained adults.

Sources[1]

CaveatsThe strongest row in this document and the reason the format exists in the app. Note what it does and does not license: it supports programming resistance work in short timed intervals, and it says nothing about whether a member counting their own reps produces a comparable number week to week. That question is CO-F-01.