. 2022 Jul 1;60(8):2133–2157. doi: 10.1007/s11517-022-02609-w

Table 2.

Data extraction table for studies investigating flexion/extension

Author and reference	Participants	Methods for stiffness measurements	Stiffness results
Beach et al. [2]	• 12 participants • 6 female and 6 male • Female (averages): ○ Age: 23.3 (1.8) years ○ Height: 1.62 (0.06) m ○ Mass: 58.6 (7.0) kg • Male (averages): ○ Age: 24.5 (1.9) years ○ Height: 1.77 (0.07) m ○ Mass: 76.8 (15.0) kg	• Assessing flexion and extension in side-lying • Participant lying on side on two separate platforms • Legs (pelvis down) fixed to static platform and torso on ‘frictionless’ moveable platform • Lumbar spine movements measured using electromagnetic tracking system with the source placed on the sacrum and the sensor placed over the spinous process of L1 • Electromyography (EMG) electrodes placed on erector spinae muscle bellies bilaterally at T9 and L3 level • Passivity considered EMG amplitude less than 5% of maximum voluntary contraction (MVC) • Participant’s torso was pulled using a perpendicular force to the line of the body applied to the top of the torso platform, measured by a force transducer attached to the pulling cable • Time synchronised force and lumbar spine motion data used to calculate stiffness through range	Low: 0.0 (0.1) Nm/%ROM Transition: 0.1 (0.1) Nm/%ROM High: 0.7 (0.3) Nm/%ROM %ROM = percent of ROM achieved in first trial on measurement rig
De Carvalho and Callaghan [4]	• 20 participants • 10 females and 10 males • Female (averages): ○ Age: 25.2 (3.2) years ○ Height: 1.70 (0.04) cm ○ Mass: 67.1 (7.6) kg ○ BMI: 23.1 (2.3) kg/m² • Male (averages): ○ Age: 26.4 (3.5) years ○ Height: 1.79 (0.10) m ○ Mass: 82.7 (15.8) kg ○ BMI: 25.7 (3.4) kg/m²	• Assessing flexion in side-lying • Participant lying on side on two separate platforms • Legs fixed to static platform and torso on ‘frictionless’ moveable platform • Lumbar spine movements measured using two accelerometers on L1 and S1 spinous processes respectively •EMG electrodes placed on erector spinae muscle bellies bilaterally at T9 and L3 level • Passivity considered EMG amplitude less than 5% of MVC • A participant’s torso was pulled using a perpendicular force to the line of the body applied to the top of the torso platform, measured by a force transducer attached to the pulling cable • Time synchronised force and lumbar spine motion data used to calculate stiffness through range	Males: Low: 2.42 (2.07) Nm/° Transition: 0.76 (0.17) Nm/° High: 1.90 (0.67) Nm/° Females: Low: 0.96 (0.68) Nm/° Transition: 0.51 (0.17) Nm/° High: 1.98 (1.65) Nm/° Male and female combined into quartiles (calculated for present review) 1st quartile: 1.69 Nm/° 2nd quartile: 0.64 Nm/° 3rd quartile: 0.64 Nm/° 4th quartile: 1.94 Nm/°
Fewster et al. [5]	• 24 participants • 12 females and 12 males • Age: 26.3 (3.7) • Height: 1.71 (0.08) m • Mass: 76.2 (12.3) kg	• Assessing flexion and extension in side-lying • Participant lying on side on two separate platforms • Legs (pelvis down) fixed to static platform and torso on ‘frictionless’ moveable platform • Lumbar spine movements were measured using a optoelectronic motion capture system with lumbar spine defined by sensors on L1 and sacrum • EMG electrodes placed bilaterally on erector spinae muscle bellies at T9 and L3 level, the rectus abdominis and external obliques • Passivity considered EMG amplitude less than or equal to 5% of MVC • A participant’s torso was pulled using a perpendicular force to the line of the body applied to the top of the torso platform, measured by a force transducer attached to the pulling cable • Time synchronised force and lumbar spine motion data used to calculate stiffness through range	Male: Low: 0.33 (0.10) Nm/%Flexion Transition: 0.44 (0.23) Nm/%Flexion High: 1.42 (0.17) Nm/%Flexion Female: Low: 0.50 (0.13) Nm/%Flexion Transition: 0.38 (0.31) Nm/%Flexion High: 1.56 (0.33) Nm/%Flexion Average across genders: Low: 0.42 Nm/%Flexion Transition: 0.41 Nm/%Flexion High: 1.49 Nm/%Flexion %Flexion = percent of maximum flexion ROM achieved
Gruevski and Callaghan [7]	• 34 participants • 17 mature and 17 young participants • Mature participants average age: 63.7 (3.9) years • Young group baseline average data: ○ Age: 25.8 (5.0) ○ Female: ▪ Height: 1.62 (0.07) m ▪ BMI: 22.8 (1.8) kg/m² ▪ Waist circumference: 72.0 (8.2) cm ▪ Physical activity: 2309 (566) MET-minutes/week ○ Male: ▪ Height: 1.78 (0.07) cm ▪ BMI: 26.1 (4.1) kg/m² ▪ Waist circumference: 88.3 (10.5) cm ▪ Physical activity: 1858 (578) MET-minutes/week	• Assessing flexion and extension in side-lying • Participant lying on side on two separate platforms • Legs (pelvis down) fixed to static platform and torso on ‘frictionless’ moveable platform • Lumbar spine movements were measured using a four-camera optoelectronic motion capture system with lumbar spine defined by sensors on L1 and sacrum • EMG electrodes placed bilaterally on erector spinae muscle bellies at T9 and L3 level, the rectus abdominis and external obliques • Passivity considered EMG amplitude less than or equal to 5% of MVC • A participant’s torso was pulled using a perpendicular force to the line of the body applied to the top of the torso platform, measured by a force transducer attached to the pulling cable • Time synchronised force and lumbar spine motion data used to calculate stiffness through range	Passive stiffness at 40% total flexion: 0.10 (0.05) Nm/° Interpreted to represent second quartile stiffness for present review
Lee and McGill [9]	• 24 participants • All male • Baseline average data: ○ Age: 22.9 (2.7) years ○ Height: 1.79 (0.06) m ○ Mass: 77.5 (10.8) kg	• Assessed flexion/extension in side-lying • Legs (pelvis down) fixed to static platform and torso on ‘frictionless’ moveable platform • Three-dimensional lumbar spine movements measured using electromagnetic tracking system with the source placed on the sacrum and the sensor over T12 • EMG electrodes placed on rectus abdominis, external obliques, internal obliques, latissimus dorsi, upper erector spinae and lower erector spinae • Passivity considered EMG amplitude less than 5% of MVC • A participant’s torso was pulled using a perpendicular force to the line of the body applied to the top of the torso platform, measured by a force transducer	Results divided into two groups, one inexperienced in core strengthening training and one experienced in it Flexion: Inexperienced group: ROM achieved at varying percentages of total applied moment: 50%: 19.7 (8.1)° 65%: 22.1 (10.4)° 80%: 25.9 (12.7)° 90%: 31.8 (11.4)° 95%: 34.2 (11.2)° 100%: 35.6 (11.2)° Experienced group: ROM achieved at varying percentages of total applied moment: 50%: 19.5 (8.0)° 65%: 23.3 (8.5)° 80%: 27.6 (9.7)° 90%: 28.4 (11.7)° 95%: 30.1 (11.1)° 100%: 31.1 (10.9)° Extension: Inexperienced group: ROM achieved at varying percentages of total applied moment: 50%: 12.4 (8.2)° 65%: 16.0 (9.8)° 80%: 20.0 (11.4)° 90%: 21.0 (11.8)° 95%: 23.9 (11.2)° 100%: 25.6 (11.1)° Experienced group: ROM achieved at varying percentages of total applied moment: 50%: 15.9 (8.0)° 65%: 19.6 (8.4)° 80%: 18.6 (9.1)° 90%: 25.5 (7.7)° 95%: 28.7 (8.0)° 100%: 30.5 (8.4)°
McGill et al. [10]	• 37 participants • 15 females and 22 males • Female (average): ○ Age: 20.8 (1.8) years ○ Mass: 62.3 (8.8) kg ○ Height: 1.65 (0.05) m • Male (averages): ○ Age: 21.1 (1.2) years ○ Mass: 74.9 (7.6) kg ○ Height: 1.77 (0.06) m	• Assessed flexion/extension in side-lying • Legs (pelvis down) fixed to static platform and torso on ‘frictionless’ moveable platform • Three-dimensional lumbar spine movements measured using electromagnetic tracking system with the source placed over the pelvis and the sensor over the xiphoid process • EMG electrodes placed on the spine extensors at the L3 level and abdominal obliques • EMG signal presented as audio feedback and passivity considered when myoelectric silence was achieved throughout • A participant’s torso was pulled using a perpendicular force to the line of the body applied to the top of the torso platform, measured by a force transducer • Torque and rotational displacement data were time synchronised and were plotted against each other • Exponential curves were fit, and the derivative equation of this curve was used to calculate the stiffness	Passive stiffness (Nm/°) associated with degree through ROM: Flexion: 2 - 0.29 4 - 0.36 6 - 0.45 8 - 0.56 10 - 0.69 12 - 0.86 14 - 1.08 16 - 1.34 Extension: 2 - 0.17 4 - 0.20 6 - 0.23 8 - 0.28 10 - 0.32 12 - 0.38 14 - 0.45 16 - 0.53 18 - 0.62 20 - 0.73 22 - 0.86 Quartiles calculated for present review for flexion: 1st: 0.36 Nm/° 2nd: 0.76 Nm/° 3rd: 1.46 Nm/° 4th: 2.78 Nm/°
Shojaei et al. [15]	• 20 participants aged between 18 and 30 years old • 10 males and 10 females • Female (averages): ○ Height: 1.65 (0.05) cm ○ Mass: 67.1 (7.0) kg ○ BMI 24.7 (3.8) kg/m² • Male (averages): ○ Height: 1.78 (0.06) m ○ Mass: 78.9 (12.0) kg ○ BMI 24.8 (3.5) kg/m²	• Assessed flexion in standing • Participants stood in a rigid metal hinged frame with their torso fixed in an upright position using a harness connected to a rigid rod • The participants legs and pelvis were constrained to the bottom half of the frame • The frame was adjusted such that the frame’s hinge was aligned with the participant’s S1 spinal level • EMG electrodes placed bilaterally on erector spinae muscle bellies at L3 and L5 level, the rectus abdominis and external obliques • EMG signal was monitored throughout to ensure their level of activity did not change • The participants’ legs were lifted forward/upwards by the frame, and the angle through which they moved was measured using a protractor attached to the leg of the frame • An inline load cell on the harness connecting rod assembly was used to measure the trunks response to the lumbar flexion • Stiffness was calculated from the change in moment and change of range • Participants only taken to 70% of total available passive range	Mean stiffness for quartiles of 70% total range 1st: 0.84 (0.53) Nm/° 2nd: 0.99 (0.66) Nm/° 3rd: 0.95 (0.81) Nm/° 4th: 1.60 (0.70) Nm/° 0–10% of ROM: 0.87 (0.50) Nm/° Quartiles of stiffness for total available range calculated from this study and converted to Nm/° for the current review: 1st: 0.92 2nd: 0.95 3rd: 1.60
Tennant et al. [17]	• 71 participants • 37 females and 34 males • Baseline average data for all participants: ○ Age: 26.2 (7.1) years ○ Height: 1.71 (0.82) m ○ Mass: 70.6 (11.3) kg ○ BMI: 24.0 (2.6) kg/m² • Female (average) ○ Age: 27.4 (8.4) years ○ Height: 1.66 (7.22) m ○ Mass: 64.4 (9.2) kg ○ BMI: 23.2 (2.4) kg/m² • Male (averages) ○ Age: 25.0 (5.1) years ○ Height: 1.76 (6.20) m ○ Mass: 77.4 (9.4) kg ○ BMI: 25.0 (2.5) kg/m²	• Flexion measured in side-lying • Participants’ low body (pelvis and distal) was constrained on an immobile platform with the upper body fixed to a ‘frictionless’ mobile cradle • Range of lumbar movement was measured by a digital camera recording the relative motion of reflective markers placed over the T12 and S2 spinous processes • EMG electrode placed on lumbar erector spinae at L3 level on nondominant side • Passivity assumed if muscle activity was less than 2% of the peak of two trials • The participants were drawn into maximum lumbar flexion via a rigid bar with a uniaxial load cell attached to it • The moment and angle data were time synchronised and plotted with the curves being approximated into three linear regions, the gradients of which represented the stiffness at these different stages	Low: 0.2 (0.1) Nm/° High: 2.26 (1.25) Nm/° For the current review, the low zone is considered to represent the first quartile and the high zone is considered to represent the fourth quartile
Toosizadeh et al. [18]	• 10 participants • 5 females and 5 males • Female (averages): ○ Age: 23.8 (2.6) years ○ Height: 1.64 (0.04) m ○ Mass 57.9 (5.1) kg • Male (averages): ○ Age: 24.4 (4.2) years ○ Height: 1.80 (0.07) m ○ Mass: 71.0 (7.3) kg	• Flexion measured in standing • Participants stood in a rigid metal hinged frame with their torso fixed in an upright position using a harness connected to a rigid rod • The participants legs and pelvis were constrained to the bottom half of the frame • The frame was adjusted such that the frame’s hinge was aligned with the L5/S1 joint of the participant • EMG electrodes were placed on longissimus at the L3 level and rectus abdominis at the level of the umbilicus • EMG signal was monitored to minimize voluntary muscle activation throughout; however, no specific passive threshold was specified • The participants’ legs were lifted forward/upwards by the frame, and the angle through which they moved was measured using inertial measurement units placed over the spinous processes of T12 and S1 • An inline load cell on the harness connecting rod assembly was used to measure the trunk’s response to the lumbar flexion • Stiffness was calculated using four different models: standard linear solid, Prony series, Schapery’s theory and the modified superposition method	Stiffness (Nm/°) for specific percentages of the flexion relaxation angle calculated by two different means Prony series: 30%: 0.07 (0.14) 40%: 0.19 (0.12) 60%: 0.19 (0.23) 80%: 0.24 (0.28) 100%: 0.23 (0.25) Schapery’s theory: 30%: 0.15 (0.01) 40%: 0.15 (0.01) 60%: 0.26 (0.01) 80%: 0.23 (0.01) 100%: 0.54 (0.01)
Voinier et al. [20]	• 31 participants • Control group baseline average data ○ Age: 30.00 (9.32) years ○ Female: 47% ○ Height: 1.75 (0.10) m ○ Mass: 74.0 (9.9) kg ○ BMI 24.1 (2.0) kg/m²	• Assessed flexion/extension in side-lying • Legs (pelvis and distal) fixed to static platform and torso on ‘frictionless’ moveable platform • Angular displacement was measured using inertial measurement units, one placed on the pelvis and one on the torso • A load cell was attached to the moveable platform via a ball and socket joint with an inertial measurement unit attached to it to monitor the angle of pull applied • EMG electrodes were placed over the external obliques and lumbar erector spinae muscles • Baseline EMG levels were established once a subject has been placed within the apparatus • Non-passive trials were identified if the EMG level surpassed the level of two standard deviations of this baseline level for more than 10% of the trial • Participants were pulled into the respective movement to be measured while the displacement and force were recorded • The time synchronised torque and displacement data were fit to a double sigmoid curve, the derivative of which described the inverse of stiffness	Mean neutral zone stiffness (Nm/°) in flexion/extension: 0.18 (0.06) Considered to represent the first quartile for the current review

Author and reference

Participants

Methods for stiffness measurements

Stiffness results

Beach et al. [2]

• 12 participants

• 6 female and 6 male

• Female (averages):

○ Age: 23.3 (1.8) years

○ Height: 1.62 (0.06) m

○ Mass: 58.6 (7.0) kg

• Male (averages):

○ Age: 24.5 (1.9) years

○ Height: 1.77 (0.07) m

○ Mass: 76.8 (15.0) kg

• Assessing flexion and extension in side-lying

• Participant lying on side on two separate platforms

• Legs (pelvis down) fixed to static platform and torso on ‘frictionless’ moveable platform

• Lumbar spine movements measured using electromagnetic tracking system with the source placed on the sacrum and the sensor placed over the spinous process of L1

• Electromyography (EMG) electrodes placed on erector spinae muscle bellies bilaterally at T9 and L3 level

• Passivity considered EMG amplitude less than 5% of maximum voluntary contraction (MVC)

• Participant’s torso was pulled using a perpendicular force to the line of the body applied to the top of the torso platform, measured by a force transducer attached to the pulling cable

• Time synchronised force and lumbar spine motion data used to calculate stiffness through range

Low: 0.0 (0.1) Nm/%ROM

Transition: 0.1 (0.1) Nm/%ROM

High: 0.7 (0.3) Nm/%ROM

%ROM = percent of ROM achieved in first trial on measurement rig

De Carvalho and Callaghan [4]

• 20 participants

• 10 females and 10 males

• Female (averages):

○ Age: 25.2 (3.2) years

○ Height: 1.70 (0.04) cm

○ Mass: 67.1 (7.6) kg

○ BMI: 23.1 (2.3) kg/m²

• Male (averages):

○ Age: 26.4 (3.5) years

○ Height: 1.79 (0.10) m

○ Mass: 82.7 (15.8) kg

○ BMI: 25.7 (3.4) kg/m²

• Assessing flexion in side-lying

• Participant lying on side on two separate platforms

• Legs fixed to static platform and torso on ‘frictionless’ moveable platform

• Lumbar spine movements measured using two accelerometers on L1 and S1 spinous processes respectively

•EMG electrodes placed on erector spinae muscle bellies bilaterally at T9 and L3 level

• Passivity considered EMG amplitude less than 5% of MVC

• A participant’s torso was pulled using a perpendicular force to the line of the body applied to the top of the torso platform, measured by a force transducer attached to the pulling cable

• Time synchronised force and lumbar spine motion data used to calculate stiffness through range

Males:

Low: 2.42 (2.07) Nm/°

Transition: 0.76 (0.17) Nm/°

High: 1.90 (0.67) Nm/°

Females:

Low: 0.96 (0.68) Nm/°

Transition: 0.51 (0.17) Nm/°

High: 1.98 (1.65) Nm/°

Male and female combined into quartiles (calculated for present review)

1st quartile: 1.69 Nm/°

2nd quartile: 0.64 Nm/°

3rd quartile: 0.64 Nm/°

4th quartile: 1.94 Nm/°

Fewster et al. [5]

• 24 participants

• 12 females and 12 males

• Age: 26.3 (3.7)

• Height: 1.71 (0.08) m

• Mass: 76.2 (12.3) kg

• Assessing flexion and extension in side-lying

• Participant lying on side on two separate platforms

• Legs (pelvis down) fixed to static platform and torso on ‘frictionless’ moveable platform

• Lumbar spine movements were measured using a optoelectronic motion capture system with lumbar spine defined by sensors on L1 and sacrum

• EMG electrodes placed bilaterally on erector spinae muscle bellies at T9 and L3 level, the rectus abdominis and external obliques

• Passivity considered EMG amplitude less than or equal to 5% of MVC

• A participant’s torso was pulled using a perpendicular force to the line of the body applied to the top of the torso platform, measured by a force transducer attached to the pulling cable

• Time synchronised force and lumbar spine motion data used to calculate stiffness through range

Male:

Low: 0.33 (0.10) Nm/%Flexion

Transition: 0.44 (0.23) Nm/%Flexion

High: 1.42 (0.17) Nm/%Flexion

Female:

Low: 0.50 (0.13) Nm/%Flexion

Transition: 0.38 (0.31) Nm/%Flexion

High: 1.56 (0.33) Nm/%Flexion

Average across genders:

Low: 0.42 Nm/%Flexion

Transition: 0.41 Nm/%Flexion

High: 1.49 Nm/%Flexion

%Flexion = percent of maximum flexion ROM achieved

Gruevski and Callaghan [7]

• 34 participants

• 17 mature and 17 young participants

• Mature participants average age: 63.7 (3.9) years

• Young group baseline average data:

○ Age: 25.8 (5.0)

○ Female:

▪ Height: 1.62 (0.07) m

▪ BMI: 22.8 (1.8) kg/m²

▪ Waist circumference: 72.0 (8.2) cm

▪ Physical activity: 2309 (566) MET-minutes/week

○ Male:

▪ Height: 1.78 (0.07) cm

▪ BMI: 26.1 (4.1) kg/m²

▪ Waist circumference: 88.3 (10.5) cm

▪ Physical activity: 1858 (578) MET-minutes/week

• Assessing flexion and extension in side-lying

• Participant lying on side on two separate platforms

• Legs (pelvis down) fixed to static platform and torso on ‘frictionless’ moveable platform

• Lumbar spine movements were measured using a four-camera optoelectronic motion capture system with lumbar spine defined by sensors on L1 and sacrum

• EMG electrodes placed bilaterally on erector spinae muscle bellies at T9 and L3 level, the rectus abdominis and external obliques

• Passivity considered EMG amplitude less than or equal to 5% of MVC

• A participant’s torso was pulled using a perpendicular force to the line of the body applied to the top of the torso platform, measured by a force transducer attached to the pulling cable

• Time synchronised force and lumbar spine motion data used to calculate stiffness through range

Passive stiffness at 40% total flexion:

0.10 (0.05) Nm/°

Interpreted to represent second quartile stiffness for present review

Lee and McGill [9]

• 24 participants

• All male

• Baseline average data:

○ Age: 22.9 (2.7) years

○ Height: 1.79 (0.06) m

○ Mass: 77.5 (10.8) kg

• Assessed flexion/extension in side-lying

• Legs (pelvis down) fixed to static platform and torso on ‘frictionless’ moveable platform

• Three-dimensional lumbar spine movements measured using electromagnetic tracking system with the source placed on the sacrum and the sensor over T12

• EMG electrodes placed on rectus abdominis, external obliques, internal obliques, latissimus dorsi, upper erector spinae and lower erector spinae

• Passivity considered EMG amplitude less than 5% of MVC

• A participant’s torso was pulled using a perpendicular force to the line of the body applied to the top of the torso platform, measured by a force transducer

Results divided into two groups, one inexperienced in core strengthening training and one experienced in it

Flexion:

Inexperienced group:

ROM achieved at varying percentages of total applied moment:

50%: 19.7 (8.1)°

65%: 22.1 (10.4)°

80%: 25.9 (12.7)°

90%: 31.8 (11.4)°

95%: 34.2 (11.2)°

100%: 35.6 (11.2)°

Experienced group:

ROM achieved at varying percentages of total applied moment:

50%: 19.5 (8.0)°

65%: 23.3 (8.5)°

80%: 27.6 (9.7)°

90%: 28.4 (11.7)°

95%: 30.1 (11.1)°

100%: 31.1 (10.9)°

Extension:

Inexperienced group:

ROM achieved at varying percentages of total applied moment:

50%: 12.4 (8.2)°

65%: 16.0 (9.8)°

80%: 20.0 (11.4)°

90%: 21.0 (11.8)°

95%: 23.9 (11.2)°

100%: 25.6 (11.1)°

Experienced group:

ROM achieved at varying percentages of total applied moment:

50%: 15.9 (8.0)°

65%: 19.6 (8.4)°

80%: 18.6 (9.1)°

90%: 25.5 (7.7)°

95%: 28.7 (8.0)°

100%: 30.5 (8.4)°

McGill et al. [10]

• 37 participants

• 15 females and 22 males

• Female (average):

○ Age: 20.8 (1.8) years

○ Mass: 62.3 (8.8) kg

○ Height: 1.65 (0.05) m

• Male (averages):

○ Age: 21.1 (1.2) years

○ Mass: 74.9 (7.6) kg

○ Height: 1.77 (0.06) m

• Assessed flexion/extension in side-lying

• Legs (pelvis down) fixed to static platform and torso on ‘frictionless’ moveable platform

• Three-dimensional lumbar spine movements measured using electromagnetic tracking system with the source placed over the pelvis and the sensor over the xiphoid process

• EMG electrodes placed on the spine extensors at the L3 level and abdominal obliques

• EMG signal presented as audio feedback and passivity considered when myoelectric silence was achieved throughout

• A participant’s torso was pulled using a perpendicular force to the line of the body applied to the top of the torso platform, measured by a force transducer

• Torque and rotational displacement data were time synchronised and were plotted against each other

• Exponential curves were fit, and the derivative equation of this curve was used to calculate the stiffness

Passive stiffness (Nm/°) associated with degree through ROM:

Flexion:

2 - 0.29

4 - 0.36

6 - 0.45

8 - 0.56

10 - 0.69

12 - 0.86

14 - 1.08

16 - 1.34

Extension:

2 - 0.17

4 - 0.20

6 - 0.23

8 - 0.28

10 - 0.32

12 - 0.38

14 - 0.45

16 - 0.53

18 - 0.62

20 - 0.73

22 - 0.86

Quartiles calculated for present review for flexion:

1st: 0.36 Nm/°

2nd: 0.76 Nm/°

3rd: 1.46 Nm/°

4th: 2.78 Nm/°

Shojaei et al. [15]

• 20 participants aged between 18 and 30 years old

• 10 males and 10 females

• Female (averages):

○ Height: 1.65 (0.05) cm

○ Mass: 67.1 (7.0) kg

○ BMI 24.7 (3.8) kg/m²

• Male (averages):

○ Height: 1.78 (0.06) m

○ Mass: 78.9 (12.0) kg

○ BMI 24.8 (3.5) kg/m²

• Assessed flexion in standing

• Participants stood in a rigid metal hinged frame with their torso fixed in an upright position using a harness connected to a rigid rod

• The participants legs and pelvis were constrained to the bottom half of the frame

• The frame was adjusted such that the frame’s hinge was aligned with the participant’s S1 spinal level

• EMG electrodes placed bilaterally on erector spinae muscle bellies at L3 and L5 level, the rectus abdominis and external obliques

• EMG signal was monitored throughout to ensure their level of activity did not change

• The participants’ legs were lifted forward/upwards by the frame, and the angle through which they moved was measured using a protractor attached to the leg of the frame

• An inline load cell on the harness connecting rod assembly was used to measure the trunks response to the lumbar flexion

• Stiffness was calculated from the change in moment and change of range

• Participants only taken to 70% of total available passive range

Mean stiffness for quartiles of 70% total range

1st: 0.84 (0.53) Nm/°

2nd: 0.99 (0.66) Nm/°

3rd: 0.95 (0.81) Nm/°

4th: 1.60 (0.70) Nm/°

0–10% of ROM: 0.87 (0.50) Nm/°

Quartiles of stiffness for total available range calculated from this study and converted to Nm/° for the current review:

1st: 0.92

2nd: 0.95

3rd: 1.60

Tennant et al. [17]

• 71 participants

• 37 females and 34 males

• Baseline average data for all participants:

○ Age: 26.2 (7.1) years

○ Height: 1.71 (0.82) m

○ Mass: 70.6 (11.3) kg

○ BMI: 24.0 (2.6) kg/m²

• Female (average)

○ Age: 27.4 (8.4) years

○ Height: 1.66 (7.22) m

○ Mass: 64.4 (9.2) kg

○ BMI: 23.2 (2.4) kg/m²

• Male (averages)

○ Age: 25.0 (5.1) years

○ Height: 1.76 (6.20) m

○ Mass: 77.4 (9.4) kg

○ BMI: 25.0 (2.5) kg/m²

• Flexion measured in side-lying

• Participants’ low body (pelvis and distal) was constrained on an immobile platform with the upper body fixed to a ‘frictionless’ mobile cradle

• Range of lumbar movement was measured by a digital camera recording the relative motion of reflective markers placed over the T12 and S2 spinous processes

• EMG electrode placed on lumbar erector spinae at L3 level on nondominant side

• Passivity assumed if muscle activity was less than 2% of the peak of two trials

• The participants were drawn into maximum lumbar flexion via a rigid bar with a uniaxial load cell attached to it

• The moment and angle data were time synchronised and plotted with the curves being approximated into three linear regions, the gradients of which represented the stiffness at these different stages

Low: 0.2 (0.1) Nm/°

High: 2.26 (1.25) Nm/°

For the current review, the low zone is considered to represent the first quartile and the high zone is considered to represent the fourth quartile

Toosizadeh et al. [18]

• 10 participants

• 5 females and 5 males

• Female (averages):

○ Age: 23.8 (2.6) years

○ Height: 1.64 (0.04) m

○ Mass 57.9 (5.1) kg

• Male (averages):

○ Age: 24.4 (4.2) years

○ Height: 1.80 (0.07) m

○ Mass: 71.0 (7.3) kg

• Flexion measured in standing

• Participants stood in a rigid metal hinged frame with their torso fixed in an upright position using a harness connected to a rigid rod

• The participants legs and pelvis were constrained to the bottom half of the frame

• The frame was adjusted such that the frame’s hinge was aligned with the L5/S1 joint of the participant

• EMG electrodes were placed on longissimus at the L3 level and rectus abdominis at the level of the umbilicus

• EMG signal was monitored to minimize voluntary muscle activation throughout; however, no specific passive threshold was specified

• The participants’ legs were lifted forward/upwards by the frame, and the angle through which they moved was measured using inertial measurement units placed over the spinous processes of T12 and S1

• An inline load cell on the harness connecting rod assembly was used to measure the trunk’s response to the lumbar flexion

• Stiffness was calculated using four different models: standard linear solid, Prony series, Schapery’s theory and the modified superposition method

Stiffness (Nm/°) for specific percentages of the flexion relaxation angle calculated by two different means

Prony series:

30%: 0.07 (0.14)

40%: 0.19 (0.12)

60%: 0.19 (0.23)

80%: 0.24 (0.28)

100%: 0.23 (0.25)

Schapery’s theory:

30%: 0.15 (0.01)

40%: 0.15 (0.01)

60%: 0.26 (0.01)

80%: 0.23 (0.01)

100%: 0.54 (0.01)

Voinier et al. [20]

• 31 participants

• Control group baseline average data

○ Age: 30.00 (9.32) years

○ Female: 47%

○ Height: 1.75 (0.10) m

○ Mass: 74.0 (9.9) kg

○ BMI 24.1 (2.0) kg/m²

• Assessed flexion/extension in side-lying

• Legs (pelvis and distal) fixed to static platform and torso on ‘frictionless’ moveable platform

• Angular displacement was measured using inertial measurement units, one placed on the pelvis and one on the torso

• A load cell was attached to the moveable platform via a ball and socket joint with an inertial measurement unit attached to it to monitor the angle of pull applied

• EMG electrodes were placed over the external obliques and lumbar erector spinae muscles

• Baseline EMG levels were established once a subject has been placed within the apparatus

• Non-passive trials were identified if the EMG level surpassed the level of two standard deviations of this baseline level for more than 10% of the trial

• Participants were pulled into the respective movement to be measured while the displacement and force were recorded

• The time synchronised torque and displacement data were fit to a double sigmoid curve, the derivative of which described the inverse of stiffness

Mean neutral zone stiffness (Nm/°) in flexion/extension: 0.18 (0.06)

Considered to represent the first quartile for the current review

Where possible, results were converted to Newton metres per degree. Participant data was only presented for those participants relevant to the current review. Stiffness values calculated specifically for this review are clearly identified in the stiffness results column. Numbers in brackets represent standard deviations.

m metres, kg kilograms, Nm Newton metre, ROM range of movement, T9 ninth thoracic vertebrae, T12 twelfth thoracic vertebrae, L1 first lumbar vertebrae, L3 third lumbar vertebrae, L5 fifth lumbar vertebrae, S1 first sacral vertebrae, S2 second sacral vertebrae, MET metabolic equivalent, BMI body mass index.