The association of reactive balance control and spinal curvature under lumbar muscle fatigue

Background

Although low back fatigue is an important intervening factor for physical functioning among sedentary people, little is known about its possible significance in relation to the spinal posture and compensatory postural responses to unpredictable stimuli. This study investigates the effect of lumbar muscle fatigue on spinal curvature and reactive balance control in response to externally induced perturbations.

Methods

A group of 38 young sedentary individuals underwent a perturbation-based balance test by applying a 2 kg load release. Sagittal spinal curvature and pelvic tilt was measured in both a normal and Matthiass standing posture both with and without a hand-held 2 kg load, and before and after the Sørensen fatigue test.

Results

Both the peak anterior and peak posterior center of pressure (CoP) displacements and the corresponding time to peak anterior and peak posterior CoP displacements significantly increased after the Sørensen fatigue test (all at p < 0.001). A lumbar muscle fatigue led to a decrease of the lumbar lordosis in the Matthiass posture while holding a 2 kg load in front of the body when compared to pre-fatigue conditions both without a load (p = 0.011, d = 0.35) and with a 2 kg load (p = 0.000, d = 0.51). Also the sacral inclination in the Matthiass posture with a 2 kg additional load significantly decreased under fatigue when compared to all postures in pre-fatigue conditions (p = 0.01, d = 0.48). Contrary to pre-fatigue conditions, variables of the perturbation-based balance test were closely associated with those of lumbar curvature while standing in the Matthiass posture with a 2 kg additional load after the Sørensen fatigue test (r values in range from −0.520 to −0.631, all at p < 0.05).

Conclusion

These findings indicate that lumbar muscle fatigue causes changes in the lumbar spinal curvature and this is functionally relevant in explaining the impaired ability to maintain balance after externally induced perturbations. This emphasizes the importance for assessing both spinal posture and reactive balance control under fatigue in order to reveal their interrelations in young sedentary adults and predict any significant deterioration in later years.

Differentiation of the Strength of Back Muscle Contraction Under Fatigue: Does Force Feedback Play a Role?

OBJECTIVE

This study investigates the ability of subjects to differentiate the strength of back muscle contraction with and without feedback information on force produced under fatigue and nonfatigue conditions.

DESIGN

Controlled laboratory study.

SETTING

Research laboratory environment.

PARTICIPANTS

A group of 52 healthy young men participated in the study.

INTERVENTION

Subjects self-estimated 50% of the maximal voluntary isometric contraction of back muscles either on their own volition or on the basis of information about the actual force, before and after the Sørensen fatigue test.

MAIN OUTCOME MEASURES

The force was measured by means of the FiTRO Back Dynamometer.

RESULTS

The self-estimated 50% maximal voluntary isometric contraction was significantly higher than the one calculated from maximal voluntary isometric contraction during 10 trials in 2 repeated sessions (8.3% and 10.0%, P < .05). However, when feedback on the force produced was provided, significantly higher values were observed during an initial trial in both sessions (8.5%, P = .04 and 12.1%, P = .01). Subjects were able to estimate the target force during the following trials. Fatigue induced a decrease in peak force (7.7%, P = .04), whereas the ability to regulate the prescribed force was not compromised. Constant error was lower with than without force feedback during both measurements (2.15% and 6.85%; 3.06% and 8.56%). However, constant and variable errors were greater under fatigue than nonfatigue conditions (8.43% and 5.55%; 0.41% and 0.37%). Similarly, root mean square error decreased with force feedback (from 6.88% to 3.48% and from 8.74% to 5.09%) and increased under fatigue (from 5.87% to 8.67%).

CONCLUSIONS

These findings indicate that force feedback plays a role in the differentiation of the strength of back muscle contraction, regardless of fatigue. It contributes to a more precise regulation of force produced during voluntary isometric contraction of back muscles. This promising method awaits further experimentation to be applied for individuals with low back pain.