Methods

Participants

Eighty-two introductory psychology undergraduate students (mean age 19.9 years) participated, mid-second semester, for course credit. Data from eight were excluded from analyses due to failure to follow directions or incomplete data, leaving 38 females and 36 males in the study. All participants provided informed consent, as approved by the University of Delaware Human Subjects Review Board, and indicated that they had no known medical conditions that would prevent them from engaging in a low-impact cardio exercise.

Materials and procedure

Following from previous research showing recruitment of a range of neural regions when learning and remembering name–face pairs (Zeineh, Engel, Thompson, & Bookheimer, 2003), stimuli were paired faces and names. Name–face pairs were presented on a CRT monitor with an 800 × 600 resolution using Psychophysics Toolbox for Matlab (Brainard 1997; Pelli 1997). Face pictures were 15 full-color 258 × 350 pixel photographs of men with neutral expressions, collected from the Karolinska Directed Emotional Faces database (Lundqvist, Flykt, & Öhman 1998). Names were common male names in the United States (Joseph, David, William, Richard, Michael, Robert, John, Thomas, James, Charles, Daniel, Paul, Mark, Donald, and Christopher) and were presented in black, Times New Roman, 120 point font. Each face appeared in the center of the screen above its associated name.

The learning and testing phases were conducted 24 h apart from each other. Thirty-eight participants were assigned to the post-learning activity procedure (ten females and nine males in both the exercise and the non-exercise conditions), and 36 participants were assigned to the pre-learning activity procedure (nine females and nine males in both the exercise and the non-exercise conditions). The groups were run sequentially, starting with the post-learning activity procedure.

Post-learning activity procedure

During the learning phase (day 1), participants sat at a comfortable distance from a computer screen and were told that they would later be tested on their memory for name–face pairs that would appear on the screen. Each name–face pair appeared once for 4 s in each of seven blocks. Presentation order within each block was random, with the constraint that the same pair was not repeated twice in a row (i.e., at the end of one block and the start of the next).

Participants were also asked to rate their emotional state in order to isolate the effects of physical arousal from those of emotional arousal: if emotional arousal were to differ between the exercise and non-exercise groups, the intention was to assess the impact of exercise while controlling for emotional arousal. Immediately after the learning period, participants rated their subjective emotional state in terms of valence (positive versus negative) and arousal (high versus low) by placing a mark within a 9 × 9 “affect grid” (Russell, Weiss, & Mendelsohn, 1989), wherein the horizontal extent represented valence and the vertical extent represented arousal. Baseline heart rate (beats per minute; bpm) was measured via a fingertip pulse oximeter (CMS-50DL). Participants then engaged in one of two post-learning activities. The learning phase and the post-learning activity were separated by approximately 1 minute.

Participants engaged either in 5 minutes of step-exercise (exercise condition) or 5 minutes of quiet activity, during which they built freely with connecting toy pieces (K’Nex; non-exercise condition). The exercise condition involved repeatedly stepping on and off of a 6.5” step, self paced, while counting the number of steps made. The non-exercise condition involved freely joining pieces together while keeping count of the number of pieces used. Counting was used in both conditions as a means to discourage participants from rehearsing information from the learning phase. An experimenter used a stopwatch to time both the exercise and non-exercise activities and told the participant to stop at the end of 5 minutes. The two types of post-learning activity were performed alone in a small room, behind a closed door, and only one participant was run at a time. After engaging in the activity, subjective emotional state and heart rate were measured again via an affect grid and fingertip pulse oximeter.

Twenty-four hours later, participants returned to complete a memory test (day 2). Each participant received a worksheet with the 15 faces from the previous day’s learning phase and was instructed to write the corresponding name beneath each picture. This test was untimed, and participants were encouraged to guess when they were unable to remember a name but could leave an answer blank if they were unable to guess. Participants were then debriefed.

Pre-learning activity procedure

The materials and procedure were the same as in the post-learning activity procedure, with the exception that the exercise versus non-exercise manipulation was administered prior to learning. Participants first provided baseline measures of heart rate and self-report valence and arousal levels, and immediately engaged in their assigned pre-learning activity. Post-activity measures of heart rate and affect were recorded, and participants then viewed the names and faces in the learning portion of the experiment. After the learning phase, heart rate and affect measures were taken and participants performed paper-and-pencil mazes for 5 minutes as a means to equate immediately subsequent activity between groups. Twenty-four hours later, participants returned to the lab to complete the memory test.

Data analysis

Answers were coded as correct when participants recalled the exact name, substituted a common nickname (e.g., “Mike” for “Michael”), or had an obvious misspelling (e.g., “Michal”). All other responses were considered incorrect. Every participant therefore had a score based on number of correct responses, with 0 representing none correct and 15 representing perfect memory performance.

Performance was analyzed using the raw number of correct responses. Mood valence and arousal measures were based on independent 9-point scales (with 1 corresponding with negative emotional valence/low arousal and 9 corresponding with positive emotional valence/high arousal). Percent change in heart rate was calculated by subtracting pre-activity from post-activity heart rate and dividing by pre-activity heart rate, in order to accommodate expected variation in baseline heart rate (e.g., an increase of 10 bpm might have different implications following a baseline heart rate of 60 bpm compared to a baseline of 110 bpm). Change in affect measures were calculated by simply subtracting post-activity measures minus pre-activity measures, as assumptions about the relative meaning of (for example) an increase of 2 units of valence rating may be unwarranted, given the inherently subjective nature of this measure.

Results

A 2 (activity timing: pre- versus post-learning) × 2 (activity type: exercise versus non-exercise) × sex (male versus female) ANOVA on number of correct responses revealed no main effects of activity timing (F(1, 66) = 0.81, p = 0.372, η 2 = 0.009), activity type (F(1, 66) = 0.80, p = 0.375, η 2 = 0.010), or sex (F(1, 66) = 0.03, p = 0.856, η 2 < 0.001). No two-way interactions with sex emerged (ps > 0.34). However, a significant activity timing × activity type interaction emerged (F(1, 66) = 7.38, p = 0.008, η 2 = 0.091), as did the three-way interaction between activity timing, activity type, and sex (F(1, 66) = 4.00, p = 0.05, η 2 = 0.049). These are explored in more detail below, considering first the impact of exercise (versus non-exercise) after learning and then the impact of exercise (versus non-exercise) before learning. In the exercise condition, there was no difference in the number of steps reported taken by men and women (men, mean (M) = 121.7, standard deviation (SD) = 53.8; women, M = 138.3, SD = 51.1; t(33) = 0.93, p = 0.36; data missing from two participants). That said, number of steps constituted the only measure in the study that was both non-subjective and non-verifiable, and—as the intended purpose of the counting task was simply to discourage rehearsal of the learned material—participants’ instructions did not emphasize counting accuracy. Thus, we do not include this factor in our subsequent analyses. (An improvement in future follow-up studies may be to track number of steps more objectively).

Post-learning activity procedure

Participants who performed exercise after the learning phase exhibited significantly better memory for name–face pairs 1 day later (M = 9.58, SD = 3.72) than those who engaged in the non-exercise activity after learning (M = 6.37, SD = 3.37; t(36) = 2.789, p = 0.008, d = 0.90) (Fig. 1).

Fig. 1 Means (and standard errors) of correct items reported in the memory test in the post-learning activity procedure from experiment 1 Full size image

Males versus females

A 2 (activity: exercise versus non-exercise) × 2 (sex: female versus male) ANOVA revealed a significant main effect of activity (F(1, 34) = 7.831, p = 0.008, η 2 = 0.16), no significant main effect of sex (F(1, 34) = 0.276, p = 0.603, η 2 = 0.006), and a significant interaction between them (F(1, 34) = 5.259, p = 0.028, η 2 = 0.11) (Fig. 1).

Women in the exercise group remembered more than women in the non-exercise group (M = 10.5, SD = 3.41 versus M = 4.9, SD = 1.97, t(18) < 0.001, d = 2.01). However, men in the exercise group (M = 8.56, SD = 3.97) performed no differently to men in the non-exercise group (M = 8.0, SD = 3.94) (t(16) = 0.298, p = 0.77, d = 0.14). Women performed worse than men in the non-exercise condition (t(17) = 2.21, p = 0.04, d = 0.995), but not in the exercise condition (t(17) = 1.15, p = 0.27, d = 0.52).

Mood measures

Collapsed across men and women, no differences in self-reported mood measures of valence emerged (exercise, M = +0.58, SD = 1.87; non-exercise, M = +0.05, SD = 1.84; t(36) = 0.876, p = 0.39), nor of arousal (exercise, M = + 0.95, SD = 2.84; non-exercise, M = + 1.05, SD = 1.75; t(36) = −0.138, p = 0.89). There were no between-activity differences in self-reported mood measures of valence or arousal in either women or men (ts < 1.40, ps > 0.18).

Heart rate

Heart rate increased to a greater degree in the exercise condition than in the non-exercise condition when collapsed across participants (t(36) = 3.41, p = 0.002, d = 1.17), when limited only to the women (t(18) = 2.89, p = 0.010, d = 1.28), and marginally when limited only to the men (t(16) = 3.41, p = 0.070, d = 0.97). Following exercise, men and women did not differ in their change of heart rate (t(17) = 1.12, p = 0.28, d = 0.48). Among women, collapsed across activity, increase in heart rate correlated with memory accuracy. Means and standard deviations of heart rate change are reported in Table 1, along with the correlations of heart rate change with memory performance.

Table 1 Means (and standard deviations) of percentage heart rate increase following exercise and non-exercise activities, as well as bivariate correlations between percentage heart rate increase and memory performance, for women, men, and combined Full size table

Pre-learning activity procedure

In contrast to the results from the post-learning activity procedure, in the pre-learning activity procedure those in the exercise group (M = 6.44, SD = 3.91) performed somewhat worse than those in the non-exercise group (M = 8.00, SD = 3.83) on the memory task, although this difference was not significant (t(34) = 1.20, p = 0.24, d = 0.40).

Males versus females

A 2 (activity type) × 2 (sex) ANOVA for memory scores revealed no significant main effect of activity (F = 1.404, p = 0.245, η 2 = 0.04), no main effect of sex (F = 0.459, p = 0.503, η 2 = 0.013), and no interaction between them (F = 0.459, p = 0.503, η 2 = 0.013). In the subsequent planned comparisons, women in the exercise condition (M = 6.44, SD = 3.64) showed a pattern suggesting worse performance than those in the non-exercise condition (M = 8.89, SD = 4.23), although this did not reach statistical significance (t(16) = 1.314, p = 0.21, d = 0.62). A similar non-significant pattern emerged among men (exercise, M = 6.44, SD = 4.39; non-exercise, M = 7.11, SD = 3.41; t(16) = 0.360, p = 0.72, d = 0.17).

Mood measures

Collapsed across men and women, there was no main effect of pre-learning activity on self-reported ratings of change in emotional arousal (exercise, M = +1.18, SD = 1.67; non-exercise, M = +0.50, SD = 1.58; t(33) = 0.1.232, p = 0.23), and the difference in self-reported mood valence was only marginal (exercise, M = −0.06, SD = 1.43; non-exercise, M = +1.00, SD = 1.78; t(33) = 1.929, p = 0.062).Footnote 1 No differences in arousal or valence emerged among the men or women alone (ts < 1.95, ps ≥ 0.07).

Heart rate

Heart rate increased to a greater degree in the exercise condition than in the non-exercise condition when collapsed across participants (t(34) = 4.68, p < 0.001, d = 1.61), when limited only to the women (t(16) = 4.05, p = 0.001, d = 1.94), and when limited only to the men (t(16) = 2.74, p = 0.015, d = 1.38). Following exercise, men and women did not differ in their change of heart rate (t(16) = 1.73, p = 0.10, d = 0.83). Means and standard deviations of heart rate change are reported in Table 1, along with the correlations of heart rate change with memory performance.

Discussion

Five minutes of post-learning exercise appeared to enhance memory for paired associations among women, but an equivalent period of exercise prior to learning yielded no similar benefit. Memorial benefits of acute exercise may stem primarily from its impact on post-learning processes such as consolidation. The exercise-induced benefit in the post-learning activity procedure appeared to be partly driven by women’s lower accuracy (relative to men’s) in the control condition (Fig. 1). Experiment 2, which aimed to replicate and extend the post-learning activity findings, provided an additional opportunity to assess whether this pattern was spurious.