Impaired Trunk Flexor Strength, Fatigability, and Steadiness in Postpartum Women

Abstract

Purpose: To determine whether postpartum women (vaginal and cesarean delivery) have deficits in trunk flexor strength, fatigability and steadiness, compared with nulligravid women, up to 26 wk postpartum. We hypothesized that postpartum women would be weaker, more fatigable, and have greater torque fluctuations than controls, with cesarean delivery showing greater deficits than vaginal delivery.

Methods: Twenty-two control women (nulligravid) and 29 postpartum women (20-40 yr, 19 who delivered via vaginal birth, 13 via Caesarian section) participated. Postpartum women were tested 8 to 10 wk and 24 to 26 wk postpartum. Control women were tested 16 to 18 wk apart. Maximal voluntary isometric contractions (MVC) were performed at multiple trunk positions with the trunk flexor muscles. To determine trunk flexor fatigability, subjects performed intermittent isometric contractions at 50% MVC (6-s contraction, 4-s rest) in upright sitting until task failure. An MVC was performed during the fatiguing task (one per minute) and at 10 and 20 min of recovery.

Results: At 8 and 26 wk, postpartum women (groups pooled) were weaker at all trunk angles (38% and 44% respectively, P < 0.05) than controls despite no differences in handgrip strength. Postpartum women were more fatigable (71% and 52% respectively) and had greater torque fluctuations than controls (P < 0.05). At 8 wk postpartum, women who had a cesarean delivery, were 59% more fatigable (P = 0.004) than the vaginal delivery group, with no difference between delivery types at 26 wk postpartum.

Conclusions: Musculoskeletal recovery, including trunk flexor muscle strength and fatigability, is incomplete at 26 wk postpartum. These findings provide a rationale for future studies to address outcomes of rehabilitation programs specifically targeted at improving strength and fatigability of the trunk flexor muscles after pregnancy and childbirth.

Figure 1. Torque-Angle Curve

Torque-angle curve for the trunk flexor muscles at the initial and follow up time points for postpartum vs control [A & B] and vaginal vs Cesarean delivery [C & D]. Negative numbers indicate positions of trunk extension, upright sitting is 0°, and positive numbers indicate positions of trunk flexion. Postpartum women had lower maximal trunk flexion torque across all positions at both time points compared to control. However, the torque-angle curve shape was similar for all groups. There were no differences between delivery types (vaginal vs Cesarean). *= group difference at the time point (postpartum vs control).

Figure 2. Fatigability and Recovery

A-B: Time-to-task failure for the intermittent isometric submaximal trunk flexion fatiguing exercise at time points for postpartum (pooled), control, vaginal delivery, and cesarean delivery at 8-10 weeks postpartum [A] and 24-26 weeks postpartum[B]. *=group difference at the time point (postpartum vs control); γ=improvement compared to initial timepoint; #=delivery group difference at the time point (vaginal vs Cesarean). C-D: MVC torque before (baseline), immediately after (task failure, TF), and 10 minutes (R10) and 20 minutes (R20) after fatiguing trunk flexor exercise for controls vs postpartum women at 8-10 weeks [C] and 24-26 weeks [D]. Postpartum women generate lower maximal torque than control women at all time points. Postpartum women demonstrate impaired recovery of MVC strength up to 10 weeks postpartum [C], but show a similar recovery pattern to control women at 26 weeks postpartum [D]. *=group difference at the time point (postpartum vs control); δ=different from baseline MVC. E-F: No statistical difference is present between modes of delivery at either time point [E & F]. Postpartum women demonstrate a decline in baseline MVC strength from 8-26 weeks, driven by a loss of strength for women in the vaginal delivery group. δ=different from baseline MVC.

Figure 3. Steadiness

A-B: Steadiness (CV of torque) of trunk flexor contractions at four intensities (5, 10, 20, & 70% MVC) for control vs postpartum women at 8-10 weeks [A] and 24-26 weeks [B] postpartum. Postpartum women have greater torque fluctuations than controls at both time points, particularly at low contraction intensities. *=group difference at the time point (postpartum vs control). C-D: CV of torque during submaximal (50% MVC) contractions during fatiguing exercise is represented for controls vs postpartum women at 8-10 weeks [C] and 24-26 weeks [D]. Both groups demonstrated increased force fluctuations from the beginning to end of fatiguing exercise at both time points. CV of torque is similar between groups at 8-10 weeks postpartum [C], but postpartum women have greater fluctuations in torque than controls at 24-26 weeks postpartum [D]. *=group difference at the time point (postpartum vs control).