NASA-STD-3001 Technical Brief

Relevant Technical 

Requirements

Exercise Overview

OCHMO-TB-031

NASA-STD-3001 Volume 1, Rev C

[V1 3017] Post-Mission Reconditioning

[V1 4001] Microgravity EVA Aerobic 

Capacity

[V1 4002] Extraterrestrial Surface EVA

Aerobic Capacity

[V1 4003] In-Mission Aerobic Capacity

[V1 4004] Post-Mission Aerobic Capacity

[V1 4023] Pre-Mission Muscle Strength and 

Function

[V1 4024] In-Mission Skeletal Muscle 

Strength

NASA-STD-3001 Volume 2, Rev D

[V2 7038] Physiological Countermeasures 

Capability 

[V2 7040] Physiological Countermeasure 

Operations

[V2 7043] Medical Capability

[V2 8001] Volume Allocation

Executive Summary

The human cardiovascular and skeletal muscle 

systems have evolved to meet the challenges of 

upright posture in the Earth’s gravitational 

environment. During spaceflight, astronauts 

experience altered gravity environments that lead 

to physiological decrements in aerobic capacity, 

muscle strength, bone strength, vision changes, 

and altered vascular motility, which can lead to a 

decrease in crew performance. Exercise is 

prescribed to astronauts as a countermeasure to 

altered gravity and is vital to maintaining optimal 

crew health and performance. It addresses these 

decrements and is also used as a countermeasure 

for orthostatic intolerance and immune and 

sensorimotor functioning.

The extent of physiological deconditioning varies 

per individual and is dependent on a multitude of 

factors such as starting fitness level, age, mission 

duration, and gravity level. Without intervention, 

acute deconditioning begins immediately upon 

exposure to an altered gravity environment and is 

measurable within a few days. As mission 

duration increases, the decrements will continue 

to increase without intervention. NASA has 

generated standards/thresholds to protect health 

outcomes and enable performance for mission 

tasks. 

Houston We Have a Podcast:

The Body in Space with Gary Jordan Victor Glover and 

Tom Cruise

Ep. 229: The Body in Space | NASA
NASA-STD-3001 Technical Brief

OCHMO-TB-031

Exercise Overview

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Health Technical Requirements

These health technical requirements from the NASA Space Flight Human-Systems Standards 

were chosen based on the ability to mitigate short and long-term health and performance 

effects, and the ability for crewmembers to return to preflight values for aerobic capacity 

and muscle strength during the rehabilitation period within 45 days postflight. For bone 

mineral density (BMD), NASA crewmembers are initially selected with normal bone mineral 

density and for many crewmembers, it requires a year to recover back to preflight values. If 

crew are selected with lower BMD, long term consequences need to be considered.

From NASA-STD-3001 Volume 1, Rev C

4.1.3 In-Mission Aerobic Capacity 

[V1 4003] The in-mission aerobic capacity shall be maintained, either through 

countermeasures or work performance, at or above 80% of the pre-mission capacity 

determined by either direct or indirect measures

4.6.2 In-Mission Skeletal Muscle Strength

[V1 4024] Countermeasures shall maintain in-mission skeletal muscle strength at or above 

80% of baseline values.

4.7.2 In-Mission Bone Countermeasures

[V1 4027] Countermeasures shall maintain bone mineral density (BMD) of the hip and spine 

at or above 95% of pre-mission values and at or above 90% for the femoral neck.

3.1.16 Post-Mission Reconditioning 

[V1 3017] All programs shall provide the planning, coordination, and resources for an 

individualized post-mission reconditioning program, specific to each crewmember, mission 

type, and mission duration. The post-mission reconditioning starts with crew egress at 

landing and includes a guided, phased reconditioning protocol. The goals of the 

reconditioning program include the following:

a. To ensure the health and safety of returning crew.

b. To actively assist the crew's return to full functional abilities and return-to-flight 

status. 

c. To actively assist in the crew’s return to pre-mission fitness

Background

10/02/2023 2
NASA-STD-3001 Technical Brief

OCHMO-TB-031

Exercise Overview

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Fitness for Duty/Performance Requirements

The following health technical requirements from the NASA Space Flight Human-Systems 

Standards were generated based on past EVA experience for NASA astronauts and research 

data for muscle strength. These requirements are not absolute thresholds, crewmembers 

may fall below these levels and potentially require more time and resources to accomplish 

mission tasks. Each operational activity should factor in crewmember fitness and be planned 

accordingly.

From NASA-STD-3001 Volume 1, Rev C

4.1.1 Microgravity EVA Aerobic Capacity 

[V1 4001] Crewmembers shall maintain an in-mission VO2max at or above 32.9 ml⁄(min/kg) 

for missions with microgravity EVAs as determined by either direct or indirect measures. 

4.1.2 Extraterrestrial Surface EVA Aerobic Capacity

[V1 4002] Crewmembers shall maintain an in-mission VO2max at or above 36.5 ml⁄(min/kg ) 

for missions with extraterrestrial surface EVAs as determined by either direct or indirect 

measures. 

4.6.1 Pre-Mission Muscle Strength and Function 

[V1 4023] Pre-mission muscle strength and function shall be per the values in Table 4.6-1—

Pre-Mission Muscle Strength Technical Requirements.

Background

10/02/2023 3
NASA-STD-3001 Technical Brief

OCHMO-TB-031

Exercise Overview

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Factors that affect delivery of O2 to the active muscles include heart rate (HR), stroke volume (SV, the 

volume of blood expelled from the heart with each beat), cardiac output (Qc, product of HR and SV), and 

extraction of O2 from the arterial blood vessels by the muscles (measured as arteriovenous O2, A-V O2, 

difference). The relationship between VO2, Qc, and A-V O2 difference is quantified by the Fick equation 

(Rowell 1986).

Aerobic Capacity – VO2max

VO2max measures the maximum rate a body can absorb and use 

oxygen. During exercise, cells take oxygen (O2) and create energy 

which fuels exercise and allows performance of activities. The 

higher the VO2max, the greater the ability to complete activities. 

VO2max is most accurately tested using a gas analyzer (see right) 

that measures the volume of O2 inhaled and exhaled, as the 

intensity of the exercise increases until the O2 consumption 

levels off indicating a max capacity for using O2 to fuel the work 

being performed, beyond which the body will have to rely on 

anaerobic metabolism. Aerobic exercise helps to maintain VO2max

levels. 

Note: To Convert l/min to ml⁄(min/kg):

(l/min x 1000 ml/min)/body weight in kg Example: 1.17 x 1000ml/min)/80 kg = 14.63

VO2 = Qc × A-V O2 difference

Reference Data

10/02/2023 4

Figure above: Cardiovascular Effects on performance and Operational Limitations. 

VO2max aerobic capacity = Heart Rate + Stroke Volume + arteriovenous oxygen difference [a-VO2]

Encompasses the transport and utilization of oxygen
NASA-STD-3001 Technical Brief

OCHMO-TB-031

Exercise Overview

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Crewmember data from ISS increments 

20-32,28-51, 37-56 Lunar Surface/Apollo VO2 Data (1/6th g)

Apollo Lunar Surface Experiments Package (ALSEP)

Geology (utilizing tools for collecting geological samples)

Overhead activities (ingressing and egressing the vehicle)

Rover (driving and maneuvering extraterrestrial surface vehicle)

ISS Crewmember Data

VO2 during ISS EVAs (microgravity)

Astronaut aerobic characteristics and in-mission VO2 activity data 

• All crewmember height range: 146.6 cm to 194.6 cm 

(58.5 inches to 76.6 inches)

• All crewmember weight mean: 72.4kg/170 lbs std deviation 

8kg/17.7 lbs Range, 95 percentile 90.6 kg/199 lbs

In mission activities require relatively low VO2 levels but crewmember initial fitness level and in 

mission projected losses need to be accounted for to ensure successful mission activities.

Note: To Convert l/min to ml⁄(min/kg):

(l/min x 1000 ml/min)/body weight in kg 

Example: 1.17 x 1000ml/min)/80 kg = 14.63 for 80 kg crewmember

From Biomedical Results of Apollo (1975) 

https://ntrs.nasa.gov/citations/19760005580

0-G EVA MET rates from EVAs 15-46. Average VO2 = 0.77 L/min. Each colored line 

represents VO2 levels (y axis) across time (x axis). Average VO2 for all EVAs (black 

line) was approximately 10 ml/kg/min.

From: HSRB Proposed Muscle Strength 

and Aerobic Capacity-Related updates 

to NASA-STD-3001 Standard 

Recommendation (2018)

Background

10/02/2023 5

Standard
NASA-STD-3001 Technical Brief

OCHMO-TB-031

Exercise Overview

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

• Short duration bedrest without exercise may show comparable decrements to spaceflight and provides 

insight into possible magnitude of decrements due to microgravity exposure.

• Significant changes in knee extension strength and aerobic capacity occur early. 

• Short duration bedrest without exercise may show comparable decrements to spaceflight. 

Bedrest Studies

Research utilizing head-down bedrest (HDBR) methods have been implemented extensively to gather 

physiological data that can be translated to spaceflight practices. These methods are the closest terrestrially 

based research strategies that simulate the physiological changes the human body endures from the 

elimination of gravity during spaceflight, including fluid shifts to the head and upper torso, BMD loss, 

changes to the musculoskeletal system, cardiovascular changes, and sensorimotor degradation*. Subjects 

are placed on bedrest with specific degree of head-down tilt for short to long durations during which time 

countermeasures including exercise protocols are used to mitigate bone loss, strength, and aerobic 

changes. 

*(Belavy et al., 2010; Cromwell et al., 2019; Hargens et al., 2006; Pavy-Le Traon et al., 2007; Schneider et al., 2009; Stuempfle & Drury, 2007)

NASA Bedrest Treadmill

NASA Bedrest Strength Horizontal Squat Device

Background

10/02/2023 6

Benefits of exercise during bedrest studies

NASA uses exercise during bedrest as an analog to 

see how well humans benefit from and/or tolerate 

exercise during microgravity/spaceflight. 
NASA-STD-3001 Technical Brief

OCHMO-TB-031

Exercise Overview

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

This study demonstrates the potential benefit of high-load resistance exercise coupled with high-intensity 

interval training using conventional equipment in a spaceflight analog. Results included increased or maintained 

aerobic and muscle performance immediately after bed rest. Previous bed rest study showed 14% decline after 

10-day bed rest (no exercise).

Integrated resistance and aerobic exercise protects fitness during 14-day bed rest

(Ploutz-Snyder et al., 2014)

• Evaluated the effectiveness of exercise using ISS-like hardware during short duration bedrest to 

maintain cardiovascular and muscular fitness during 14 days of bedrest. 

• Nine subjects participated in 14-21 days of pre-bedrest training, 14 days of bedrest + exercise, and 7 

days of ambulatory recovery.

• Aerobic-VO2peak, ventilatory threshold (VT), and isokinetic/leg press tests performed before/after 

bedrest to evaluate cardiorespiratory and muscular functions.

• Subjects exercised 6 days/week, 3 days of resistance and 6 days aerobic.

• Used vertical treadmill, supine cycle ergometer, and horizontal exercise fixture.

• High-intensity interval and continuous aerobic exercise were performed on alternating days.

• Training based on review of effective training programs for skeletal, muscle, bone, and cardiovascular 

health.

• Exercise alone in this spaceflight analog can preserve cardiorespiratory and muscular fitness for a 14-

day period.

• This test was followed by a 70-day bedrest study with similar conditions.

Bedrest effects of exercise on aerobic capacity

Bedrest effects of exercise on muscle size

NASA bedrest exercise equipment

Right: Centrifuge used at the German 

Aerospace Center’s “envihab” facility to 

study simulated microgravity state.

Left: Horizontal exercise fixture. 

Background

10/02/2023 7

BR = bedrest; VO2peak = oxygen consumption; VCO2peak = carbon dioxide production; VE = rate of ventilation; VT = ventilatory 

threshold SAT = skeletal muscle, subcutaneous; IMAT = intermuscular adipose tissue
NASA-STD-3001 Technical Brief

OCHMO-TB-031

Exercise Overview

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

4/16/20 

70 Day Bedrest Intervention groups

• Diet - Control Group: no exercise

• Ex + T - Traditional ISS equipment + 

testosterone 

• Ex - Traditional ISS exercise equipment

• FLY - Single Flywheel rowing and 

resistance exercise device

1) The exercise protocol mitigated bed rest induced muscle and cardiac deconditioning regardless of 

exercise device used.

2) Peak aerobic capacity was maintained from pre to post bed rest in all exercise groups compared to a 

10% decline in controlled group. 

3) FLY training was effective in mitigating multisystem deconditioning relative to the exercise performed 

on traditional (e.g., resistance machines, treadmill, cycle ergometer) exercise equipment.

Exercise Training Mitigates Multisystem Deconditioning during Bed Rest

(Ploutz-Snyder et al., 2014)

This study evaluated safety and effectiveness of aerobic and resistance exercise using ISS-like exercise 

capability in the spaceflight analog of bedrest. 34 subjects completed 70 days of 6-degree head down tilt. 

The exercise countermeasures consisted of high-intensity interval and continuous aerobic training 

combined with resistive strength training 6 days per week. High-intensity interval aerobic exercise was 

completed every other day alternating with continuous aerobic exercise performed along with resistance 

exercise separated by 4–6 hours. 

Specialized treadmill (left) was custom 

built to allow subjects to remain in the 

supine position during 

exercise, continuous aerobic exercise was 

performed on a supine electronic cycle 

ergometer (right)

CONT = no exercise, EX=traditional exercise equipment, EXT=traditional ISS equipment + testosterone, FLY= single flywheel equipment. BR-5 is 5 

days before bedrest, BR+0 is day bedrest is complete

Background

10/02/2023 8
NASA-STD-3001 Technical Brief

OCHMO-TB-031

Exercise Overview

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Health Technical 

Requirement

4.1.3 In-Mission Aerobic 

Capacity 

[V1 4003] The in-mission 

aerobic capacity shall be 

maintained, either 

through countermeasures 

or work performance, at 

or above 80% of the premission capacity 

determined by either 

direct or indirect 

measures. From NASASTD-3001 Vol 1, Rev C

Results:

Subjects who exercised 

three or more times per 

week during flight, at a HR 

>70% of their age-predicted 

maximum HR and for more 

than 20 minutes per 

session, experienced 

smaller decrements in VO2

at test termination on 

landing day than subjects 

who exercised less 

frequently or at lower 

intensities.

Aerobic Exercise on Space Shuttle: 6-15 days

Extended Duration Orbiter Medical Project Functional Performance Evaluation Greenisen et al., (1999)

Studies were performed on the Space Shuttle to evaluate the usefulness of sub-maximal aerobic exercise 

during flight in reducing the severity of postflight deconditioning. One of the studies (below) tested effects 

of exercise using different modalities including treadmill, rower, and cycle on aerobic deconditioning 

compared to no exercise. 

Study evaluated 

pre-flight to postflight effect of 

exercise on 40 crew 

comparing exercise 

modalities (rowing, 

treadmill, and 

cycle). All modalities 

had some protective 

effects (right).

Study evaluated effect of exercise 

duration and intensity on 25 subjects 

(left). 

Flight duration and exercise volume

Background

10/02/2023 9

VO2peak liters/min

VO2 L*min
NASA-STD-3001 Technical Brief

OCHMO-TB-031

Exercise Overview

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

ISS Preflight Training Exercise Prescription – 180 Days

• Prepare for flight

• Muscle Conditioning (strength, endurance, flexibility, power, coordination, and stamina)

• Aerobic Fitness

• Preflight training begins approximately 2 years before a scheduled launch

• Familiarize astronauts with inflight exercise and hardware operation

• Teach proper safety and technique

Cycle/Phase Sets Reps

Preparation/Hypertrophy 3-4 10-12

Basic Strength 3-4 8-10

Strength & Power 4 4-6

Peaking Power 4 1-3

Stamina 3-4 15+

Example preflight periodization workout

Inflight Training Exercise Prescription (Long-Duration Missions ~180 days/ISS)

Maintain crew health and fitness

• Protect functionality and capability

• Minimize losses: Strength, endurance, flexibility, power, coordination, stamina, aerobic fitness, and 

bone

• Injury Prevention

Astronaut Chris Cassidy using the Advanced 

Resistive Exercise Device (ARED)

Reference Data

10/02/2023 10

Adapted from: Physical Training for Long-Duration Spaceflight (Loehr et al., 2015) 

Inflight Training Time Prescriptions

Metabolic:

Approximately 30 minutes of interval or 

steady-state exercise

Resistive/Strength:

30-60 minutes load-bearing training
NASA-STD-3001 Technical Brief

OCHMO-TB-031

Exercise Overview

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Aerobic Exercise in Space - ISS Long Duration ~ 180 Days

Aerobic training on ISS consists of interval or continuous steady state exercise on either Cycle Ergometer with 

Vibration Isolation and Stabilization System (CEVIS) or second-generation treadmill (T2). CEVIS protocols were 

developed using the pre-flight VO2peak test with prescribed work rates between 70-100% VO2peak. T2 protocols were 

based on pre-flight training and prescribed at 70–100% HRmax. The following data was obtained during ISS 

increments 26S-50S (April 2011 – September 2017). Forty-six astronauts (37 males, 9 females; age: 46.8 ± 6.1 years, 

height: 176 ± 7.1 cm, weight: 79.2 ± 9.9 kg [mean ± SD]) were assigned to missions of 178 ± 48 d. Ten (22%) 

astronauts had previously completed long-duration ISS missions. The following data is based on “Effects of exercise 

countermeasures on multisystem function in long duration spaceflight astronauts”, Scott, J. et al. (2023).

ISS Treadmill – T2

Loading System 

Average load 117 lbs

Range 107- 127 lbs

68% body mass loading

Exercise Session Duration

27 minutes (range 23-29 min)

Heart Rate

131 bpm average

Peak 162 bpm

Heart Rate

>70 % peak heart rate 

69%

> 90% peak heat rate 

11%

**The % maximum HR parameter was calculated as the average HR of the individual exercise sessions for that crew member, divided by the 

crew member’s HRmax (determined preflight as part of Peak Aerobic performance testing), then multiplied by 100%.

Speed mph

All 7 mph +/- 1

Peak 8 mph =/-1

Frequency

84 (range 59-109)/178 days

47% of total days on orbit

ISS - CEVIS

Load (W)

Average load 137 W

Peak 191 W

Exercise Session Duration

26 minutes (range 23-29 min)

Speed rpm

All 78 -7/+19

Peak 92 +/- 5

Application

Heart Rate

135 bpm average

Peak 163 bpm

Heart Rate

>70 % peak heart rate 

76%

> 90% peak heat rate 

13%

Frequency

65 (range 47-76)/178 days

37% of total days on orbit

10/02/2023 11
NASA-STD-3001 Technical Brief

OCHMO-TB-031

Exercise Overview

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

After 178 days, crewmembers in this study lost 

on average 7% of their VO2peak 

Health Technical Requirement

4.1.3 In-Mission Aerobic Capacity 

[V1 4003] The in-mission aerobic capacity shall be 

maintained, either through countermeasures or work 

performance, at or above 80% of the pre-mission capacity 

determined by either direct or indirect measures.

From NASA-STD-3001 Vol 1, Rev C

Post Mission Reconditioning

Crewmembers participate in a reconditioning program post-landing. After 30 days, crewmembers regain their preflight baseline VO2peak values.

Estimated in-mission aerobic capacity declines and 

required in-mission aerobic capacity levels should be 

considered when setting the pre-mission aerobic capacity 

requirement. See table below from [V1 4001] 

Microgravity EVA Aerobic Capacity from NASA-STD-3001 

Vol 1, Rev C for an example.

Aerobic Exercise in Space ISS Long Duration ~ 180 Days Physiological Outcomes

Effects of exercise countermeasures on multisystem function in long duration spaceflight astronauts (Scott et al., 

2023). Evaluation of the effects of ISS exercise countermeasures on multisystem function using latest exercise (T2, 

ARED, CEVIS) devices and exercise prescription processes with average 600 min/week/crewmember; and estimated 

the proportion of astronauts that would experience performance-limiting deconditioning including cardiorespiratory 

(aerobic), bone, and body composition. Testing was performed on NASA and international astronauts from April 

2011-Sept 2017: 46 missions min. of 178 days, all meeting standard physiologic tests prior to flight. 

Crew data from 2012-2022 found that 12-21% of crewmembers fall below the -20% requirement during in-flight time 

points. 20% or greater aerobic reduction is associated with significant performance decrements in ground based 

analog study that evaluated simulated EVA and egress task performance. 

Background

10/02/2023 12
NASA-STD-3001 Technical Brief

OCHMO-TB-031

Exercise Overview

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Muscle Strength & Resistive Exercise

Exposure to weightlessness can cause muscle deconditioning as a result of chronic disuse, insufficient 

functional loads, reduced muscle mass, volume, and performance. Crewmember muscle strength is 

important for the completion of necessary tasks including in-flight activities, EVAs, successful egress of the 

vehicle, and assisting an incapacitated crewmember. Wearing a spacesuit adds additional strength 

demands. Exercise is one of the most important countermeasures used to minimize loss of strength. Goals 

for in-flight training include maintaining crew health and fitness and protecting functionality and EVA 

capability while minimizing losses of strength, endurance, flexibility, power, aerobic fitness, and injury 

protection. 

Pre-flight muscle 

evaluation leg press 

tests maximal 

isometric force and 

power/endurance. 

Knee extension tests 

force control and 

neuromuscular drive. 

MRI - Muscles can be evaluated 

by MRI which shows reductions 

in muscle size. Loss of muscle 

can be directly related to 

movement control – less mass 

available/less tension produced 

when muscles are maximally 

activated and lower functional 

performance

Muscle size can 

be monitored 

for atrophy inflight using a 

guided 

ultrasound 

device (right)

NASA Astronaut Strength, Conditioning, and Rehabilitation Specialists (ACERS) monitor crew fitness inflight and develop exercise prescriptions based on crew ability and performance. Crew data is monitored 

from the ground, including physiologic markers and crew performance such as time and intensity of 

exercise preformed. Strength exercise prescriptions depend on equipment available 

Functional Performance Evaluation Michael C. Greenisen, Judith C. Hayes, Steven F. Siconolfi, and Alan D. Moore of the Johnson Space Center, Houston, TX

Muscle strength is measured as torque and endurance

Torque is the force that drives biomechanical 

movement.

Total work is a measure of muscle endurance.

Concentric contractions - muscles shorten and generate 

force. Eccentric contractions - muscles lengthen in 

response to force. Isometric contractions - generate 

force without changing shape. 

Isokinetic testing measures strength at concentric, 

eccentric, and isometric modes.

Isokinetic testing provides a baseline of muscle strength 

and endurance in select muscle groups. 

Crewmembers have an isokinetic assessment before 

and after flight per NASA AMMERD 6.1

Measurements can be used for evaluation of in-flight 

countermeasures and post-flight rehabilitation.

Background

10/02/2023 13
NASA-STD-3001 Technical Brief

OCHMO-TB-031

Exercise Overview

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Integrated resistance and aerobic exercise protects fitness during 70-days bed rest

This study evaluated safety and effectiveness of aerobic and resistance exercise using ISS-like exercise 

capability in the spaceflight analog of bed rest. 34 subjects completed 70 days of 6-degree head down tilt. 

The exercise countermeasure consisted of high-intensity interval and continuous aerobic training combined 

with resistive strength training 6 d∙wk−1. High-intensity interval aerobic exercise was completed every other 

day alternating with continuous aerobic exercise performed along with resistance exercise separated by 4–

6 h. 

Significant benefits shown from 

all exercise groups compared to 

non-exercise (cont) group. All 

exercise groups were observed 

for leg press total work, 

isokinetic upper and lower leg 

strength, and vertical jump 

power and maximal jump 

height.

BR-5 is measured 5 days before 

flight, BR+0 measured day 

bedrest is complete.

Resistance exercise was performed 

using a horizontal leg press. Custombuilt flywheel device was used for 

aerobic and resistive strength 

exercise.

Muscle size is significantly 

maintained by exercise.

Background

10/02/2023 14
NASA-STD-3001 Technical Brief

OCHMO-TB-031

Exercise Overview

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Extended Duration Orbiter Medical Project Functional Performance Evaluation Greenisen et al. (1999) 

Short-Duration Studies were performed on Shuttle crewmembers to evaluate the usefulness of submaximal skeletal muscle and endurance exercise during flight to optimize crew ability to complete mission 

tasks including those essential to entry, landing, and crew egress. One study evaluated functional changes 

in peak torque and muscle endurance before/after flight, one defined the morphologic and biochemical 

effects of spaceflight on skeletal muscle fibers, a third used MRI to test for leg muscle atrophy.

Study 477 and 617 evaluated functional changes in 

concentric/eccentric strength (peak torque) and endurance 

(workload) of the trunk, upper, and lower limbs.

• Shuttle missions 5-13 days, pre-training began L-30

• Exercisers had protocols of continuous and interval training, 

treadmill prescriptions 60-85% of pre-flight VO2max

Results

• Torque decreased in back and abdomen, quadriceps 15% 

hamstrings 12%

• Total work decreased in quadriceps at R+0, small difference in 

endurance at R+7

• Exercisers had greater maintenance of leg strength (left)

• Both exercisers and non-exercisers lost strength in abdomen 

and trunk suggesting that treadmill exercise may best preserve 

muscle integrity to muscles exercised. 

Study 475 defined the morphologic and biochemical effects of 

spaceflight on skeletal muscle fibers.

• Biopsies taken 21 days before flight and on landing day

• 8 crew 3–5 day mission and 5–11 day missions

• Tested Type 1 slow-twitch fibers (endurance activities), and 

type 2- fast twitch (quick powerful movements)

Results

• Cross section area (CSA) showed 15% loss of type 1 fibers and 

22% decrease of type 2 fibers after flight

Study 606 quantified changes in size, water, lipid composition, and

leg muscles after spaceflight.

• 8 crew 5-7 days and 3-9 days

• Tested MRI of leg L-30, L-16,R+2, R+7

• MRI scan of leg (soleus and gastrocnemius muscles)

• CSA evaluated to assess muscle atrophy 

Results

• 5.8% lost in the soleus, 4.0% lost in the gastrocnemius, 4.3% total

• Loss represents atrophy (loss of muscle mass)

Functional Performance Evaluation Michael C. Greenisen, Judith C. Hayes, Steven F. Siconolfi, and Alan D. Moore

MRI image

Application

10/02/2023 15

Change (percent)
NASA-STD-3001 Technical Brief

OCHMO-TB-031

Exercise Overview

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Resistive/Strength Exercise in Space – ISS Long Duration ~180 Days

For resistance exercise, total volume was calculated for each subject for the categories of squat, heel raise, and 

deadlift exercises utilizing the ARED (600lb. total resistance), then normalized to mission duration (total 

volume/mission duration in days). Warmup exercises were not included in the data set. The 3 exercise categories 

included the following variations: “squat”: back squat, single leg squat, sumo squat; “heel raise”: heel raise and single 

leg heel raise; “deadlift”: deadlift, Romanian deadlift, and sumo deadlift, and bench press. For each exercise category, 

total volume was calculated for each subject by summing the volume (load x reps) across the entire mission. In 

addition, average load (kg), average relative load (kg·kg bodyweight-1), average repetitions per session, and average

repetitions per week were calculated for each subject for the 3 exercise categories. The following data is based on the 

“Effects of exercise countermeasures on multisystem function in long duration spaceflight astronauts”, Scott, J. et al. 

(2023).

ARED – Bench Press ARED - Squat Load

Average load 122 Ibs

Range 98 – 153 Ib

Peak Load

131 Ibs

Range 102 – 160 Ibs

Load Volume

4928 Ibs

Sessions

N/A

Repetitions

44 (range 38 -

81)/178 days

Load

Average load 192 lbs

Range 156 - 215 lbs

Peak Load

220 Ibs

Range 180 – 248 Ibs

Load Volume

38,300 Ibs

Sessions

121

Range (82 to 146)

Repetitions

230 (range 177-

313)/178 days

Load

Average load 239 lbs

Range 198 – 293 lbs

Peak Load

249 Ibs

Range 202 – 306 Ibs

Load Volume

45,855 Ibs

Sessions

85

Range 65 to 105

Repetitions

207 (range 131-256)/178 

days

ARED - Heel Raise ARED - Deadlift

Load

Average load 181 lbs

Range 150 – 223 lbs

Peak Load

229 Ibs

Range 194 – 270 Ibs

Load Volume

31,659

Sessions

117

Range (78-148)

Repetitions

189 (range 135-

223)/178 days

*All Crewmember Height range: 146.6 cm to 194.6 cm (58.5 inches to 76.6 inches). All crew weight mean: 

72.4kg/170 lbs std deviation 8kg/17.7 lbs Range, 95 percentile 90.6 kg/199 lbs

**The % maximum HR parameter was calculated as the average HR of the individual exercise sessions for that 

crewmember, divided by the crewmember’s HRmax (determined pre-flight as part of Peak Aerobic performance 

testing), then multiplied by 100%.

Application

10/02/2023 16
NASA-STD-3001 Technical Brief

OCHMO-TB-031

Exercise Overview

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Summary of Bedrest Data with No Exercise

The following graphs summarize bedrest data collected over studies conducted between 1 to 120 days. The 

bedrest data in combination with the flight data presented in the following slide is used to inform exercise 

requirements and protocols for missions of various durations.

-20.1%

Effect of bedrest on aerobic capacity without exercise

Background

10/02/2023 17

Data from: Strock N, Varanoske A, Spector E, Frisco D, Young M, Prejean B, Fincke R, Marshall-Goebel 

(2023) Exercise Requirements for Gateway and Orion: Human Physiology, Performance, Protection, 

and Operations (H-3PO) Laboratory Review of Muscle and Aerobic Outcomes in relation to Exercise 

Prescription Design. 

Aerobic Capacity: Relationship between change in 

aerobic capacity (VO2peak) and bed rest duration in 

control participants with no exercise. Individual 

points on the scatterplot indicate aggregated data 

from individual studies employing 6° head down 

tilt and horizontal bed rest with no exercise (n=443 

subjects; 48 data points). Some studies had 

measures at multiple times throughout bed rest or 

employed additional research interventions – each 

of these are presented as an individual dot at the 

corresponding timepoint. Some points for 

individual studies may be overlayed by other 

studies. The estimated 30-day decrement in knee 

extension strength is -20.1%. 

-17.6%

Knee extension: Relationship between change in knee 

extension strength (from pre-post bed rest) and duration of 

bed rest, in control participants with no exercise. Individual 

points on the scatterplot indicate aggregated data from 

individual studies employing 6° head down tilt and 

horizontal bed rest with no exercise (n=283 subjects; 34 

data points). Some studies had measures at multiple times 

throughout bed rest or employed additional research 

interventions – each of these are presented as an individual 

dot at the corresponding timepoint. Some points for 

individual studies may be overlayed by other studies. The 

estimated 30-day decrement in knee extension strength is -

17.6%.
NASA-STD-3001 Technical Brief

OCHMO-TB-031

Exercise Overview

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Health Technical Requirement

4.6.2 In-Mission Skeletal Muscle 

Strength

[V1 4024] Countermeasures shall

maintain in-mission skeletal muscle 

strength at or above 80% of baseline 

values. From NASA-STD-3001 Vol 1, Rev C

Fitness for Duty Requirements

Primary goals is to protect astronaut health and performance and to safely and efficiently complete mission 

tasks such as extravehicular activity (EVA) and vehicle egress after landing back on Earth or on a partial 

gravity surface. Specifically, we define high risk as a 20% or greater reduction in an endpoint because that 

threshold is associated with significant performance decrements in a ground-based analog study that 

evaluated simulated EVA and egress task performance.

Post-Mission Reconditioning

Crewmembers participate in a reconditioning program post-landing. After 30 days post-landing, 

crewmembers regain their pre-flight baseline force, work, and power values.

Crewmembers experienced deficits (some with gains) in their lower extremities but did not exceed the 

requirement and were able to be reconditioned within 30 to 45 days post-mission.

Physiological Outcomes for Resistive Exercise on Long Duration (178 

days) Mission on ISS Scott et al., 2019

Inflight resistance exercise training

Background

10/02/2023 18
NASA-STD-3001 Technical Brief

OCHMO-TB-031

Exercise Overview

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Effects of exercise countermeasures on multisystem function in long duration spaceflight astronauts –

Bone Scott et al., 2019

Bone loss is a known risk of spaceflight with estimates ranging from potential loss of 1-1.5 % loss per 

month. Determining the extent of bone recovery after prolonged spaceflight is important for understanding 

risks to astronaut long-term skeletal health.

Bisphosphonates as a supplement to exercise to 

protect bone during long duration spaceflight Study 

of 7 ISS astronauts (left) who spent mean of 5.5 

months on ISS with regular exercise using treadmill, 

cycle ergometer, and ARED with Bisphosphonate, 

compared with 18 astronauts using iRED or ARED only. 

ARED alone provided significant prevention of bone 

loss compared to iRED, and the combination of ARED 

and bisphosphonates showed the greatest protection

Changes in mean bone 

mineral density (BMD) 

were moderate, ranging 

from −2.1% ± 0.7% to 

−3.7% ± 0.6%; however 

much greater declines 

were observed for bone 

content in the trabecular 

regions (−5.3% ± 1.6% to 

−8.5% ± 2.5%)

Many bone endpoints were vulnerable to increasing 

mission duration. Resistance and treadmill volume 

loads were the key countermeasure factors associated 

with improved strength, bone, body composition, and 

cardiorespiratory endpoints.

Post Mission Reconditioning

Findings for BMD loss varied between the HDBR studies evaluated. In some, there were no changes in BMD observed 

which was attributed to the duration of the studies being too short to capture changes to BMD (Ploutz-Snyder et al., 

2018; Wang et al., 2023). Other studies observed minor changes in BMD during HDBR studies, with exercise 

countermeasures providing some level of protection to these changes (Belavy et al., 2011; Zwart et al., 2008). 

Exercise as a countermeasure to bone loss can also 

help prevent renal stones which are associated 

with increased calcium loss from bone. 

4.7.2 In-Mission Bone Countermeasures

[V1 4027] Countermeasures shall maintain bone mineral density of the hip and spine at or above 95% of premission values and at or above 90% for the femoral neck. From NASA-STD-3001 Vol 1, Rev C

10/02/2023 19

TotHip = total hip, Troch = trochanter, FNeck = femoral neck, 

LSpine = lumbar spine

Background
NASA-STD-3001 Technical Brief

OCHMO-TB-031

Exercise Overview

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Post-flight Training Exercise Prescription (Reconditioning) 

Focus on all deficits and return each crewmember back to preflight status as quickly as possible

• Originally, postflight reconditioning was approached very conservatively

• Process has evolved and become much more aggressive which decreased recovery time

• Since the introduction of improved exercise protocols and hardware on ISS, crewmembers are 

returning in much better physical condition

Post-flight Reconditioning Timeline

• Re-adaption to gravity begin upon re-entry and landing (R+0)

• Reconditioning program lasts 45 days starting R+1

• Reconditioning is scheduled for 2 hours per day/7 days a week, flexible if needed

• Crewmember, Flight Surgeon and ACSR can request additional time past 45 days if needed

Reconditioning is a Dynamic Program

• Uses functional movement patterns

• Uses multiple planes at multiple joints

• Emphasis is tailored to individual deficits

Functional Fitness Assessment

• Data is used to determine conditioning status as compared to preflight levels

• Pre-flight baseline performed at L-60

• Post-flight performed at R+7 and R+30

• Crewmembers are reassessed across the span of their lifetime

Application

10/02/2023 20
NASA-STD-3001 Technical Brief

OCHMO-TB-031

Exercise Overview

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Design Considerations

Program managers and vehicle developers must consider the system limitations and capabilities when 

designing and building exercise equipment and hardware for various spacecrafts. Some of these 

considerations include:

• The vibration produced when exercise equipment is in use, such as the vibration isolation and 

stabilization (VIS) system.

• The needed resistive force, mass, and power required for the vehicle to operate the exercise system.

• The required data collection capability to track crewmember performance (i.e., heart rate, VO2max, etc.).

• The mass and volume requirements for the vehicle to accommodate the exercise system and human 

movement (see example graphics below). 

(Not to scale)

L

Passive

or 

Active 

VIS

H

W

Exercise 

Device

The vehicle must be designed to accommodate all 

aspects of the exercise system (i.e., mass, volume, 

vibration) as well as the volume envelope required for 

human movement while exercising.

Vehicle structural fatigue must be considered when 

designing the vibration isolation and stabilization 

system (VIS).

10/02/2023 21
NASA-STD-3001 Technical Brief

OCHMO-TB-031

Exercise Overview

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Summary

Data from this table can be used to determine exercise hardware, capabilities, and protocols for varying mission 

duration lengths.

22 10/02/2023
NASA-STD-3001 Technical Brief

OCHMO-TB-031

Exercise Overview

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Back-Up

10/02/2023 23
NASA-STD-3001 Technical Brief

OCHMO-TB-031

Exercise Overview

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

NASA-STD-3001 Volume 1, Rev C

[V1 3017] Post-Mission Reconditioning Post-mission reconditioning shall be aimed at returning to 

baseline muscle strength.

[V1 4001] Microgravity EVA Aerobic Capacity Crewmembers shall maintain an in-mission VO2max at or 

above 32.9 ml/(min/kg) for missions with microgravity EVAs as determined by either direct or indirect 

measures. 

[V1 4002] Extraterrestrial Surface EVA Aerobic Capacity Crewmembers shall maintain an in-mission 

maximum aerobic capacity (VO2max) at or above 36.5 ml/(min/kg) for missions with celestial surface EVAs 

as determined by either direct or indirect measures.

[V1 4003] In-Mission Aerobic Capacity The in-mission aerobic capacity shall be maintained, either 

through countermeasures or work performance, at or above 80% of the pre-mission capacity determined 

by either direct or indirect measures.

[V1 4004] Post-Mission Aerobic Capacity The post-mission reconditioning shall be aimed at achieving a 

VO2max at or above the crewmember’s pre-mission values.

[V1 4023] Pre-Mission Muscle Strength and Function Pre-mission muscle strength and function shall be 

per the values in Table 2, Pre-Mission Muscle Strength Requirements.

[V1 4024] In-Mission Skeletal and Muscle Strength Countermeasures shall maintain in-mission skeletal 

muscle strength at or above 80% of baseline values.

NASA-STD-3001 Volume 2, Rev D

[V2 7038] Physiological Countermeasures Capability The system shall provide countermeasures to meet 

crew bone, muscle, sensorimotor, thermoregulation, and aerobic/cardiovascular requirements defined in 

NASA-STD-3001, Volume 1

[V2 7040] Physiological Countermeasure Operations The physiological countermeasure system design 

shall allow the crew to unstow supplies, perform operations, and stow items within the allotted 

countermeasure schedule.

[V2 7043] Medical Capability A medical system shall be provided to the crew to meet the medical 

requirements of NASA-STD-3001, Volume 1.

[V2 8001] Volume Allocation The system shall provide the defined habitable volume and layout to 

physically accommodate crew operations and living

Referenced Technical Requirements

10/02/2023 24

View the current versions of NASA-STD3001 Volume 1 & Volume 2 on the 

OCHMO Standards website
NASA-STD-3001 Technical Brief

OCHMO-TB-031

Exercise Overview

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Reference List

1. NASA-STD-3001 Volume 1 Revision C. (2023). https://www.nasa.gov/wpcontent/uploads/2023/03/nasa-std-3001-vol-1-rev-c-with-signature.pdf

2. NASA-STD-3001 Volume 2 Revision D. (2023). https://www.nasa.gov/wpcontent/uploads/2023/03/nasa-std-3001-vol-2-rev-d-with-signature.pdf

3. Johnston, RS, Dietlein, LF, Berry, CA, Parker, JF, West, V. (1975). Biomedical Results of Apollo. 

https://ntrs.nasa.gov/search.jsp?R=19760005580

4. Downs, ME, Ade, C, Barstow, TJ, Ryder, J, Martin, D, Abercromby, AF...& Feiveson, A. Temporal Changes 

in Astronauts’ Muscle and Cardiorespiratory Physiology Pre-, In-, and Post-Spaceflight. NASA. 

https://ntrs.nasa.gov/api/citations/20200001305/downloads/20200001305.pdf

5. Hackney, KJ, Scott, JM, Hanson, AM, English, KL, Downs, ME, & Ploutz-Snyder, JL. (2015). The 

Astronaut-Athlete: Optimizing Human Performance in Space. Journal of Strength and Conditioning 

Research, 29(12): 3531-45.

6. Loehr, JA, Guilliams, ME, Petersen, N, Hirsch, N, Kawashima, S, & Ohshima, H. (2015). Physical Training 

for Long-Duration Spaceflight. Aerospace Medicine and Human Performance, 86(12): A14-A23.

7. Ploutz-Snyder, LL, Downs, M, Goetchius, E, Crowell, B, English, KL, Ploutz-Snyder, R...& Scott, JM. 

(2018). Exercise Training Mitigates Multisystem Deconditioning during Bed Rest. Med Sci Sports Exerc, 

50(9): 1920-1928.

8. Abadie, LJ, Cranford, N, Lloyd, CW, Shelhamer, MJ, & Turner, JL. (2021). The Human Body in Space. 

NASA Human Research Program. https://www.nasa.gov/hrp/bodyinspace

9. Dr. Jessica Scott, Memorial Sloan Kettering. From NASA to MSK: Exercise Oncology. 

https://www.youtube.com/watch?v=F5zqUEJpx-g

10. Gulliams, ME, Nieschwiz, B, & Hoellen, D. Pre, In, and Postflight Exercise Conditioning for U.S. 

International Space Station Astronauts. NASA. https://www.youtube.com/watch?v=WL-ArtIrjLw

11. Scott, JM, Feiveson, AH, English, KL, Spector, ER, Sibonga, JD, Lichar Dillon, E...& Everett, ME. 

(2023). Effects of exercise countermeasures on multisystem function in long duration spaceflight 

astronauts. npj Microgravity, 9(11).

12. Hughson, RL. (2016. Studying Cardiovascular Health in Microgravity. NASA A Lab Aloft (International 

Space Station Research). https://blogs.nasa.gov/ISS_Science_Blog/2016/05/04/studyingcardiovascular-health-in-microgravity/

13. Graham, ZA, Lavin, KM, O'Bryan, SM, Thalacker-Mercer, AE, Buford, TW, Ford, KM, Broderick, TJ, & 

Bamman, MM. (2021). Mechanisms of exercise as a preventative measure to muscle wasting. Am J 

Physiol Cell Physiol, 321(1): C40-C57. 

14. Moore, AD, Downs, ME, Lee, SMC, Feiveson, AH, Knudsen, P, & Ploutz-Snyder, L. (2014). Peak exercise 

oxygen uptake during and following long-duration spaceflight. Journal of Applied Physiology, 117(3): 

199-343.

15. Levine, BD, Lane, LD, Watenpaugh, DE, Gaffney, FA, Buckey, JC, & Blomqvist, CG. (1985). Maximal 

exercise performance after adaptation to microgravity. Journal of Applied Physiology, 81(2): 686-94.

10/02/2023 25
NASA-STD-3001 Technical Brief

OCHMO-TB-031

Exercise Overview

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Reference List

16. Lorenz, D, and Morrison, S. (2015). Current concepts in periodization of strength and conditioning for 

the sports physical therapist. International Journal of Sports Physical Therapy, 10(6): 737-747.

17. Tesch, PA, Berg, HE, Bring, D, Evans, HJ, & LeBlanc, AD. (2004). Effects of 17-day spaceflight on knee 

extensor muscle function and size. European Journal of Applied Physiology, 93, 463-468.

18. Apollo Inflight Exerciser. (2016). National Air and Space Museum. 

https://airandspace.si.edu/stories/editorial/apollo-inflight-exerciser

19. Davis, SA, and Davis, BL. (2012). Exercise Equipment Used in Microgravity: Challenges and 

Opportunities. Current Sports Medicine Reports, 11(3): 142-147.

20. Ploutz-Snyder, L, Ryder, J, English, K, Haddad, F, & Baldwin, K. (2015). Evidence Report: Risk of Impaired 

Performance Due to Reduced Muscle Mass, Strength, and Endurance. NASA Human Research 

Program: Human Health Countermeasures Element. 

https://humanresearchroadmap.nasa.gov/Evidence/reports/Muscle.pdf

21. Ploutz-Synder, LL, Downs, M, Ryder, J, Hackney, K, Scott, J, Buxton, R, Goetchius, E, & Crowell, B. 

(2014). Integrated resistance and aerobic exercise protects fitness during bed rest. Medicine and 

Science in Sports and Exercise, 46(2): 358-368.

10/02/2023 26