Handgrip strength (HS) and knee extensor muscle strength (KEMS) showed a negative correlation with the Disabilities of the Arm, Shoulder, and Hand (DASH) score at one month following cardiac surgery. We performed a longitudinal study to examine changes in HS/KEMS and DASH score during phase II cardiac rehabilitation (CR) in patients after cardiac surgery. We measured and assessed HS, KEMS, and DASH score in 41 consecutive patients at one and three months following cardiac surgery and examined the relation between these factors at three months following cardiac surgery. Wilcoxon signed-rank test and Spearman correlation coefficients were used to analyze the results. Finally, 26 patients (63.2 years, 73.1% male) were analyzed. There were significant differences from one month to three months following cardiac surgery in HS (26.78 ± 8.26 to 31.35 ± 9.41 kgf, p < 0.001), KEMS (1.53 ± 0.42 to 1.72 ± 0.46 Nm/kg, p = 0.001), and DASH score (14.76 ± 12.58 to 7.62 ± 9.29, p < 0.001). DASH score correlated negatively with HS (r = −0.41, p = 0.01) but not with KEMS (r = −0.32, p = 0.09) after three months of phase II CR. Although HS, KEMS, and DASH scores changed significantly from one to three months following cardiac surgery during phase II CR, only HS correlated negatively with DASH score at three months following cardiac surgery.